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A high resolution workstation prototype for diagnosis of digital mammograms
Computer Methods and Programs in Biomedicine 71 (2003) 243 /259 www.elsevier.com/locate/cmpb
A high resolution workstation prototype for diagnosis of digital mammograms Javier Quiles a,b,*, Pablo G. Tahoces c, Miguel Souto a,b, Juan J. Vidal a,b a
Departamento de Radiologı´a, Laboratorio de Investigacio´n en Imagen Radiolo´gica, Universidad de Santiago, C/San Francisco 1, 15705 Santiago de Compostela, Spain b Complejo Hospitalario Universitario de Santiago (C.H.U.S.), Santiago de Compostela, Spain c Department of Electronics and Computer Science, University of Santiago de Compostela, Santiago de Compostela, Spain Received 27 February 2001; received in revised form 30 May 2002; accepted 24 June 2002
Abstract The development of a total digital high resolution mammography display system must meet a number of requirements that remain a challenge nowadays, most probably because of the special nature of breast imaging. In this paper, we discuss our particular approach to address some problems concerning the complexity of soft-copy diagnosis in digital mammography, such as image quality and user interface evaluation. Based on the experience obtained in the previous implementation of a medical image browser, a more ambitious project is being developed at the Department of Radiology of the University of Santiago de Compostela (Spain) in collaboration with the Department of Medical Informatics of INTELSIS, an emerging software company in our country. This new system will provide complete support to display, store and analyze mammographic studies in digital format. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: PACS; User interface; Digital mammography; Diagnostic image quality; DICOM
1. Introduction Digital image modalities offer a number of advantages compared to conventional screen-film format; detection, processing and visualization become independent processes and as a consequence, images can be accessed from several locations, processing algorithms can be applied for improved lesion detection, previous informa* Corresponding author. Tel.: /34-981-570-982; fax: /34981-547-164 E-mail address: [email protected] (J. Quiles).
tion recovered from either hospital information systems (HIS) or radiology information systems (RIS) can be combined with imaging studies, and computer assisted methods are also available in order to improve diagnostic accuracy. Furthermore, another promising field opened by digital technology is the development of teleradiology systems. In this scenario, the field of mammography, considered the most efficient method for early detection of breast cancer [1], has delayed its migration to digital technology. In fact, most of the examinations are being performed to date in
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conventional film format [2]. One important reason for this screen-film dependence is the great volume of data required to obtain digital images with spatial and contrast resolution comparable to those from the conventional format when X-ray dose is maintained. A number of drawbacks concerning image acquisition, transmission and storage had to be overcome by technologists and scientists in the last decade, in order to build a film-independent mammography system. Recent technologies allow the direct acquisition of 4 K /4 K digital images of the breast [3,4]. Gigabit ethernet or ATM protocols, joined to the development of specific compression methods, allow the transmission of digital mammograms in a local area network without remarkable delays. Finally, the continuos advances in storage devices provide affordable solutions for massive storage systems. In spite of these advances, the successful implementation of these advantageous features requires not only the acquisition and storage of digital mammograms, but also a system to visualize the images independently of film support. CRT (Cathode Ray Tube) monitors have been suggested as an acceptable alternative to accomplish this essential function, but several questions remain unsolved about the so-called ‘soft-copy’ systems. First, image quality could be adversely affected by the limited spatial resolution of monitors (2 K and 8 bits per pixel) and the lower luminance compared to that provided by screen-film/viewbox combination. Secondly, the complexity of the image reading process increases: a number of additional functionalities arise and the number of simultaneous high resolution images presented to the reader must be reduced due to the limited surface of the monitors. Thus, the user interface and the automatic mechanisms for image management become a critical part of the system that must compete against the extremely handy format of a film. Finally, it must be pointed out that the field of digital mammography has received an important support from the ACR/NEMA (American College of Radiology/National Engineer and Manufacturer Association) DICOM (Digital Image Communication) Standard. New features have been
recently incorporated that provide an important reference in the designing of soft-copy workstations for digital mammography, particularly with respect to display protocols, image routing and consistent grayscale appearance. This new part of the standard represents an important step towards the integration of digital mammography into the clinical practice. Our department has been working in the field of digital radiography [5 /7] for more than 12 years now. Over this period of time, a limited Picture Archive and Communications System has emerged as a natural response to the needs of the research activity [8]. A number of computerized schemes for the automatic detection of masses and microcalcifications in digital mammograms have been developed. A database has been created with more than 1000 digital mammograms, and it is used to test and design new Computer Assisted Diagnosis algorithms. In the field of soft-copy visualization of digital mammograms our first experience consisted in the development of an X Window application to retrieve images from both a VAX4000 host computer and from Unix workstations, and to visualize individual digital mammograms on the monitors [9]. Based on the experience obtained with this first browser, a more ambitious project is currently under way. In this project, a DICOM based workstation prototype with an enhanced user interface is designed for evaluation of soft-copy diagnosis in digital mammography.
2. Design considerations Due to the special nature of breast imaging, the development of a soft-copy digital mammography system must meet a number of requirements that remain a challenge nowadays. It is worth noting some issues to describe the complexity of this task. 2.1. Desired functionality The standard for Teleradiology from the American College of Radiology [10] indicates that a display application to be used for radiological interpretation of images should provide the following features:
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. . . . . . . . . .
capable of window and level adjustment; pan and zoom of images; rotating and flipping functions; preservation of the correct patient orientation; calculation and display of accurate linear measurements; pixel value determination in appropriate values for the modality; display prior image compression ratio; processing and cropping; provide information of the matrix size and bit depth per pixel; display the total number of images acquired in the study.
Upon this basic functions, a display workstation for radiologic use should provide access to different techniques for image enhancement and analysis that prove to be useful from the medical point of view. CAD schemes have reached encouraging results when used as a second opinion to support diagnostic decisions. Particularly in breast imaging, [11 /14] techniques have been developed for detection and classification of masses and microcalcifications, and they could be incorporated to the workstation functionality as an added value. 2.2. DICOM information model for digital mammography Clinical experience has shown that each image modality presents unique inherent problems that need to be solved before soft-copy images are ready for diagnosis. In the original release of DICOM standard only two image models (Information Objects) were available to introduce digital mammograms into a DICOM based system: the ‘Second Capture’ and ‘Computed Radiography’ modalities. Both of them allow performing all basic image operations (storing, printing and displaying) but they did not address specific needs of mammography [15] and required proprietary capabilities and coding from applications to adequately operate on digital mammograms and studies. The increasing use of digital devices for image acquisition of X-ray mammograms urged the incorporation of a new part of the standard in
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order to support new detector requirements and specific needs for the diagnostic use of single projection modalities including image routing, display arrangement of several images, consistency of appearance on different display systems and matching series of images with corresponding previous studies. As a response to all these required functionalities, the DICOM supplement 32, ‘Digital X-Ray’, became a part of the standard in September 1998. This document defines new information objects with a number of features that allow adequate display of digital mammographic studies (Fig. 1). It is established that image laterality, patient orientation and anatomic region information are mandatory attributes that must be present in any X-ray image file that claims conformance with the standard. With the use of these new features, rotation of images can be automated during the display process and the radiologists would not be disturbed by having to perform continuos rotationing and positioning procedures with images. The correct grayscale appearance is also guaranteed with the mandatory specification of the full transformation process to be performed on any image for display. This new DICOM object is a powerful tool that provides all necessary mechanisms for the successful incorporation of digital mammography into PACS.
2.3. DICOM connectivity A complete application is also expected to be connected into a PACS, and will have to support the handling and management of all digital documents and data of diagnostic relevance, such as image acquisition parameters, image annotations, diagnostic reports and clinical examination requests, patient history, referring physician, actual disease and patient demographic data. All this requested information is likely to be presented in multimedia format in the near future, such as audio or video files. Elaborate schemes including rule-based prefetching or background processes for previous download of images become essential features of the system in order to compensate a possible delay in retrieving images from a long term archive.
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Fig. 1. DICOM mammography information object attributes.
In this scenario, the medical workstation must incorporate several DICOM functionalities that should provide connectivity to a number of devices inside a PACS network. Query and retrieval of images and studies from an image server, sending images to a printer or to storing results back into an image server are essential functions that must be supported.
It is known that the use of digital technology involves a potential loss of information, which might reduce the diagnostic performance of the system. These effects must be evaluated to avoid any possible degradation that may affect the final clinical results. Previous studies have evaluated the suitability of CRT displays for the detection of microcalcifications with encouraging results [16,17].
2.4. Display capabilities and image quality
2.5. The user interface
Digital image workstations should allow the diagnostic interpretation of high-resolution radiographs directly from the display screen and guarantee, at least, the same diagnostic accuracy as that obtained with conventional radiographs.
It seems logical that any user interface design should take into consideration the traditional way of performing film-based radiology [18]. Viewboxes provide a large display surface with a high spatial resolution and dynamic range, as well as
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optimal luminance. Of interest is also that image reading habits have been developed based on the simultaneous presentation of multiple films. If parallel viewing of multiple images were required, as it has been established by traditional work patterns, the solution nowadays would consist of several high-resolution monitors. Some studies have tried to address this issue, but further work needs to be done that specifically deals with mammographic images [19]. Differences between film-based and digital systems in image handling are obvious. Radiologists will have to adapt to new digital reading habits that will differ from those that are used at the present. Although they are not supposed to be skilled computer users, they must perform complex operations to combine the information from several images at the same time. Designers must dedicate special effort for this high complexity and functionality to be available from a simple and easy-to-use interface. For all these reasons, the design of the user interface is one of the most critical components for this type of medical application. An additional problem arises when technical designers have to create user interfaces for applications in an intuitive manner without the logical rules that should be established to achieve an optimum result [20]. Furthermore, the user interface of a digital image workstation must compete against the extremely handy format of conventional films where complex image arrangements and rotations can be quickly provided and modified. Stations for pictorial display, especially those containing multiple display surfaces (i.e. several display monitors), pose separate and unique problems, and many of them have not been formally addressed yet. An accurate model that reflects all significant steps in the diagnostic process must be performed to provide the basis of the implementation of the system.
2.6. Model building and user requirements The first step in the development process has been dedicated to obtain an accurate model that
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reflects both the user needs and the technical requirements of the system [21]. Several approaches can be used to obtain the information model that accurately reflects users demands. Five radiologists were interviewed to obtain precise information about the reading process of mammographic images. We have employed an interview scheme based on a questionnaire that had been answered by final users. From the responses obtained, a number of conclusions were obtained, these being the starting point to describe the activity of the radiologists. The questionnaire is presented in Appendix A. Several important conclusions were extracted about image presentation and workflow: . The first step in the reading process is to obtain a list of cases to be reported. . Almost all conventional studies are comprised of four images. . After one case has been chosen, the most recent images are displayed in front of the reader. The most common distribution depicts two contralateral images of one projection in the two upper positions, and the two images corresponding to the second projection in the lower positions. Images corresponding to the right breast are placed at the left side of the display surface, and images of the left breast are placed at the right side of the display surface. If a previous study of the patient is available, images are placed with the same distribution in a contiguous surface. . A general overview of the study is performed, comparing each image with its corresponding contralateral view and then images are analyzed one by one, being compared to one another. This indicates that . The whole functionality of the system data elements (studies, patients, images and reports) must be accessed from a unique window screen. . Database contents should always be accessible, as well as the main functions to be performed by the user. Thus, the toolbar and the database manager are two elements that should always be present in the application main window. . There is a common way to start the reading sessions. It consists in displaying the most
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recent study of the selected patient and the previous one, if existing any. This feature is expected to be incorporated into the system. Control elements for window and level should always be present in an immediate interface mechanism, as it has been considered a very useful function for soft-copy reading by all the radiologists. Control elements should be kept to a minimum in the image window to reduce disturbances in image analysis. As a consequence, separated screens for image management and image analysis should be employed. A resolution of 2000 /2500 is considered acceptable to provide an overview of the images. This resolution is imposed by the monitors. The full resolution image (4000 / 5000) is only presented on request, when a suspicious region has been detected. The users reached a common way of stablishing comparisons between images for diagnostic analysis.
3. The workstation 3.1. DICOM environment In order to reach a test environment for DICOM based display workstation for mammography, a number of questions appeared. First of all, the workstation had to be able to obtain images from a DICOM based storage node. These storage systems had to include complete mammographic studies that conformed to DICOM supplement 32 for mammography. Thus, an acquisition device that supported mammography image modality had to be connected in the network. Finally, in order to obtain a complete simulated PACS environment, image printing also had to be supported. A complete system was developed that allowed the acquisition and storage of DICOM mammographic images. The diagnostic workstation is integrated in these environment and retrieves studies from the image archive for display. Additionally, it allows hard copy prints of separated images or complete studies. According to the criteria outlined above, the components and
DICOM functionality of the system are shown in Fig. 2. 3.2. Image acquisition Nowadays, two different FDA (Food and Drug Administration) approved technologies are available for direct acquisition of high quality digital mammograms. Both of them firstly convert X-ray photons into light by means of a cesium iodide thallium-doped scintillator. Light photons are then transformed in electrical charge by using either a two-dimensional array of amorphous silicon photodiodes and thin-film transistor [3] or a charge coupled device [4]. For experimental purposes, high quality images can be obtained by digitization of conventional film images. In our prototype, images are acquired by linear digitization of conventional images using a Lumiscan 85 laser scanner [Lumisys Inc., Sunnyvale, CA] which obtains a 4080 /5100 pixel matrix. This can be roughly translated into a pixel size of 50-mm equivalent to 10 lp/mm. The density resolution is scanned in a linear function from 0.3 to 4.1 OD (optical density) in increments of 0.001 OD. The actual pixel value range is from 0 to 4096. These features guarantee a good quality in the raw data which will be used in the system [22]. The output of the scanner is stored in a proprietary file format which is converted in a second step to a DICOM 3.0 file format, following the specifications contained in the specific supplement no. 32 [23]. The DICOM standard provides a suitable frame to the radiologists information model. Both are based on common information objects: patients, studies, series, images and reports. The attributes of these elements in the specification for mammography also constitute an adequate approximation to the radiologist information needs. The DICOM file format is also useful to keep the system opened to connections with different devices such as printers or massive archives. 3.3. Short term image storage A digital image with 4000 /5000 pixel matrix and 12 bits per pixel entiles an image file of 40 MB
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Fig. 2. Description of DICOM environment.
to be stored in the system. Even with the application of lossless compression algorithms, the amount of data can only be reduced to a range of 10/20 MB depending on each image contents. As a consequence, a database containing 50 studies of four mammograms represents a data volume of 40 GB. A feasible solution to the problem of massive storage implemented in our system, is the use of several disk arrays. The main unit [Sun StoreEdge A5100] offers a maximum storage of 500 GB and is connected to the host computer through a fiber channel. The disks are used as short-term storage, with relative fast retrieval times of 4 s/image for final display on high-resolution monitors. Data is stored following the DICOM file format and includes all relevant
information for diagnosis, which can be easily retrieved from the database interface. 3.4. The display workstation 3.4.1. Hardware The kernel of the system consists in a Sun Ultra 80 under Solaris 2.8. The RAM memory has been extended to 1024 MB to provide fast image manipulation. Soft-copy visualization can be configured from one to four high-resolution monitors. The actual configuration for the display of digital mammograms consists of two high resolution monitors (2000 /2500 /10 bits) both being controlled through an onboard graphic card DOME [Md5 DOME Imaging Systems Inc., Waltham,
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MA] and one conventional monitor which is used to perform all functions that are related to data management. 3.4.2. Software architecture The application has been built in a three layer model. One layer is dedicated to communication with the environment using the DICOM protocol. The second layer supports all internal functionalities to be performed with images and data, and the third layer performs all mechanisms related to the interaction with the user. We will focus on the description of the user interface as all other modules have been described elsewhere [12,13,24] (Fig. 3). 3.5. Functionalities supported Regarding the environment, the application is able to communicate with different devices that use the DICOM protocol. . Choose one image server among those existing in the network. . Present to the user query results with accessible studies in the server. Once the storage system has been chosen the system presents information about patients, studies, images and reports that can be retrieved to the local storage system. . Present to the user the data contained in the local storage system. . Acquire new images from a network acquisition device.
Fig. 3. Software architecture.
. Print individual images or complete studies on a DICOM compliant printer existing on the network. Images in a study can be automatedly disposed on the film with a preconfigured layout, or manually selected. . Activate retrieval of selected images to the local storage system device for examination.
3.6. Display functions: study level In the study level, several operations are performed by the system, aimed to ease the difficulties of image positioning and manipulation. The system offers an automated scheme that the users can optionally activate to follow an established sequence of image comparisons with minimal interaction. When the automated mode is activated, the user just selects one study from the main panel. The selected study is displayed in the first high-resolution monitor placed in front of the reader and the previous study is opened the side high-resolution monitor. Images are automatically rotated and mirrored using the DICOM attributes of image laterality and projection to obtain the conventional disposal on the monitors. The user gets a rough overview of the four images and starts the detailed. This is a common step for both the automated and non-automated display methods where the radiologist gets a rough overview of the four images comprising the study and the images from the previous study if any. This is the starting for the detailed revision of the images. The automated method consists of an established sequence of comparisons where images are displayed individually on each monitor to obtain the highest possible resolution. The two craniocaudal images, the two mediolateral-oblique images, the two left breast projections, the two right breast projections and then each image of the current study is compared to its corresponding image of the previous study, if it has been found on the database. If the manual method is chosen, the user is able to select the disposal of each monitor to contain one, two or four images and then select the images to be displayed.
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Any reports associated with previous studies can be displayed and printed using a conventional text editor. 3.7. Display functions: image level The system provides all basic functionalities for image manipulation and visualization. This includes rotation, mirroring, magnifying glass and image polarization. High resolution images of 4000 /5000 pixels can be displayed in partial views of 2000 /2500 pixels. The images can be easily panned to visualize the full mammograms. Annotations included in the image can be visualized as well as all DICOM additional information regarding image acquisition parameters on user demand. Another possibility supported to display the images is to employ a software module which has been developed to permit hard-copy printing on any DICOM compliant printer. 3.8. Critical time response features There are two functions that deserve special mention as they pose critical time constraints on the system: study retrieval and graylevel interactive modification. Fast image retrieval is accomplished by using a fast disk optical interface (fiber channel) to the display workstation, and by determining that the first unreported study should automatically be loaded when the patient folder is chosen by the user. By using this combination, retrieval times are reduced to less than 20 s per study. The second function that requires fast time responses from the system is graylevel interactive modification. Display functions deserve specific consideration: window and level modification is a fundamental operation that allow matching the graylevel range of the stored image, primarily to the narrower range of the display and secondarily to the eye of the viewer. The 4096 intensity levels in the stored image must be displayed as 256 graylevels on the displayed image and a loss of contrast might be produced. This drawback is avoided by allowing the user perform a linear
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conversion from the stored image graylevels to the 256 graylevels displayed on the monitors. The entire stored intensity range is available by narrowing the range of input graylevels (window) and then displacing the graylevel value where the window center is set (level). Graylevels on the screen can be modified by choosing different values for window and level. Then, the image is refreshed according to the new window and level values, providing better contrast definition for a range of values on the stored image, corresponding to one kind of tissue. Window and level are generally set to produce a linear graylevel scale, while the eye operates more like a logarithmic amplifier, and thus, processing can be applied to obtain perceptual linearization of the device [25]. The system offers a non-linear preconfigured transformation to improve the first appearance of the images on the display. 3.9. User interface Because of their flexibility and worldwide acceptance for GUI design in Unix environments, both Motif and Qt toolkits have been selected for the implementation of the graphical elements of the user interface. Motif is by far the most popular framework to guide user interface design on X11 graphical systems and the Qt toolkit has become increasingly popular as many web applications use this alternate style for user interface elements. Due to the increasing presence of internet applications, many users are familiar to this style for interface elements, such as buttons or list presentations. This reduces the learning time required by the user to interact with data, and prompts for acceptance of the system. Following these design specifications and user requirements the user interface consists of these elements: 1) Main window (Fig. 4). This is the starting point to any interaction with the system, from where all functions related to image management on local system and remote devices are performed. In accordance with related works, the disposal of the elements has been established offering the most important area to the presentation of basic objects of the work.
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Fig. 4. User interface. Main window.
The main window contains a toolbar that offers several options to be performed in the reading session: . Acquire new images from an acquisition device. . Print one study on hardcopy DICOM printer. . Connect to an image server (a dialog window is opened to let the user choose from a predefined list of available servers on a network). . Query and retrieve studies or images from an image server. . Send images to an image server. . Visualize study on the high-resolution monitor. The users just clicks on a push button to retrieve the patients, studies and images available in the local image storage. These data is presented on the top window in a hierarchical mode and can be sorted in accordance to several parameters such as alphabetical order, revision state (pending or
already analyzed), acquisition date, etc. The lower part of the window activates after a connection to a remote server and shows data available for retrieval to the local node. Once one study has been chosen for analysis with a mouse double click, the study is opened in the high resolution monitor in the ‘study window’ (Fig. 5). The study window contains two main areas. The first one for managing image depiction on the monitors and a second one for visualization of images. The control area offers four push buttons for direct configuration of images on the screen (four image view, two image view vertical, two image view horizontal and one image). An icon-size view of each image is presented in an image map, where the user is able to recognize all images comprising the study. Images for display are selected on the image map individually grouped. Images can be selected by means of a
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Fig. 5. Study display window.
drag and drop operation: they are selected and moved into the visualization window. Another option is using the automated sequence of comparisons between images that can be activated/ deactivated on a check box beneath the image map. Functionalities related to image display and manipulation have been implemented into a single
proprietary interface element (‘widget class’) called ‘Viewer Widget’ that has been developed at our laboratory. This widget class can be easily integrated into Motif based user interface and implements specific methods to support the visualization of a mammographic image. When an image icon is dragged and dropped on the widget, the image is automatically displayed. Then
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all tools for image processing can be applied: zooming and panning, graylevel modification, rotating and flipping, and magnification of selected regions in the image.
4. Status report 4.1. System architecture The three layer model on which the model is based allows the independent evaluation and improvement of the different aspects of the system: connectivity, functionality and user interaction. 4.2. The communication layer Based on the DICOM standard, enables a high connectivity with existing devices in a PACS. The visualization workstation is able to query and retrieve images and patient information from an image archive that implements DICOM storage services. Printing images or complete studies is also allowed. Finally, image archive management is also available from the workstation. 4.3. The functionality layer This layer of software is the kernel of the system and contains all modules that perform all the actions related with images transparent to the user and independent of the DICOM communication interface. It is composed of three main parts: 1) The database implements the DICOM information model for mammography. This data structure allows an easy management of the information inside the application and according to the standard, provides accurate definitions of images contained in a study, each image laterality, each image projection, and each image predefined window and level or presentation look up table. All these parameters are essential for a correct presentation of images on the monitors. The database is loaded with information obtained from acquisition devices or provided by the DICOM query/retrieve module.
2) Upon the data repository, several modules perform all actions related to image management and visualization. Data manager: This module performs query and retrieve, update or delete functions inside the local database. It provides all information of patients, studies and images to the user interface module. Image tools: This library of functions contains all images and studies manipulation tools. It provides mechanisms to load one complete study in memory in high and low resolution and perform image transformations and positioning operations such as flip, rotate, magnification, filters. CAD tools: This is a special module included to hold the computer-assisted schemes for automatic detection of masses and microcalcifications. These modules have been developed at our laboratory with encouraging results. 4.4. The user interface layer All functions contained in the two layers described above must be available to the user in an easy to use interface. It has been shown that the implication of final users in the design step provides coherent results in the final development. In our design, the distribution of control elements has been aimed to reduce disturbing elements in the image display window by using pop-up menus that can be displayed using the standard mechanisms of Motif based interfaces and do not reduce the viewing area. As Bora¨lv et al. [26] have pointed out, the design of medical user interfaces requires the highest transparency to handle the interface, allowing the user to concentrate on the health care task. The control elements for handling images have also been implemented following the radiologist workflow model to take the maximum profit from the limited surface of the display when several images are simultaneously required. Our prototype design has been evaluated on an heuristic based evaluation in order to detect usability problems. Five radiologists who participated in the design sessions performed, after a short tutorial, a simulated session for diagnostic analysis and scored their degree of satisfaction on
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several features by filling an adapted standardized questionnaire [27]. The values ranged from 1 (very good opinion) to 5 (very bad opinion). The average value obtained on the questionnaires is shown on each evaluated item. The most important conclusions extracted are: . Regarding arrangement of elements in monitors, all users manifested their satisfaction in having one monitor dedicated to perform data management operations, and separately, two monitors to perform image analysis. Average value: 1.5. . The navigational procedures in the database objects (patients, studies and images) have been found to be useful and easy to use. Average value: 2. . Automatic retrieval of previous studies is considered as a useful feature. Average value: 1.2. . The one-click methods for automatic comparison between pairs of images has been well considered by users. Average value: 1.8 . Drag and drop mechanisms to move images from the database into the high-resolution monitors have been well accepted by readers. Average value: 1.6. . Two of the participant radiologists suggested that retrieval and image processing times should be improved. These drawbacks of the system could be improved with an adequate prefetching mechanism and the use of more powerful hardware. Average value: 3.6. . All users complained of the lack of interactive help support. The implementation of this feature had been discussed and finally rejected in the design step, because it was considered as a possible cause of disturbance in the analysis process of the radiologists. Average value: 4.0. . As a desirable feature, the users manifested that a workstation should provide access to the different techniques for image enhancement and analysis that prove to be useful from the medical point of view (commented in the observations free field in the questionnaire). The results of the user interface evaluation are encouraging and the positive overall response
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indicate that users appreciate the line of design and the implementation of the prototype.
5. Discussion Medical images in digital format exhibit a number of advantages over conventional radiographs: they can be adjusted to obtain optimal contrast and brightness for visualization of different kinds of tissue; edge-enhancement algorithms can be applied to improve lesion detectability; computer assisted diagnosis schemes provide a valuable resource to obtain a second opinion and further improve the quality of the final diagnosis; images can be transmitted to remote locations to be reported by experts for either a first or a second opinion; a telemammography system would also possess several advantages, one of the most important of which would be its application for the screening programs for the early detection of breast carcinoma. The examinations could be carried out by mobile units and checked on-line by an expert mammographer to avoid supplementary exposures. Despite these promising possibilities, only a few imaging departments have undertaken the challenge of soft copy digital mammography. This could be attributed in part to the high requirements in image quality and also to the difficulties that arise in the design of an efficient user interface [28]. In fact, several authors have argued about the suitability of digital mammography for a clinical routine use [29]. Regarding image resolution, the ACR Standard for Teleradiology specifies that digitized radiographic films should employ a large matrix format , which corresponds to a spatial resolution of at least 2.5 lp/mm (a pixel size of roughly 200 mm) and a bit depth of 10 bits or greater. Many studies have been carried out that deal with the minimum spatial resolution and pixel depth that a digital mammographic display system must provide to ensure no loss of significant diagnostic information. It is commonly accepted that a meticulous search for the clustered microcalcifications that might herald an early-stage cancer should be done on all mammograms [30]. In fact, clustered micro-
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calcifications are a critical radiological sign in breast imaging for early cancer diagnosis, their visualization and classification requiring not only a high spatial resolution, but also a very high contrast resolution. Existing studies using microcalcifications provide opposite results and controversy still exists: some authors indicate that there is no loss of diagnostic accuracy when using images with a pixel size of 0.1 mm (equivalent to 5 lp/mm) [31], while some other authors indicate that there is a significant decrease of diagnostic performance when the mentioned spatial resolution is used [32]. Nowadays, images with a pixel size of 0.05 mm are available from recent image acquisition systems. This is expected to enhance the detection of microcalcifications, but several technical problems arise, the most important of which is that concerning the resolution capabilities of display monitors, as only a partial view of these images can be reached in existing models. There are some other factors that may affect the final perception of the image that deserve comment. One of most important of them is the lower luminance of the monitors. This feature has been improved in recent models based on active-matrix liquid crystal displays (AMLCD) but they still provide approximately half of the luminance of a film-box. The effect of these changes on the perception of abnormalities is unknown and requires further investigation, although their effect will not be known until these systems have been put into practice [33]. Another crucial subject that needs to be solved is related to the user interface of computer applications for medical imaging. The first question to be solved points at the number of monitors to be employed. The surface is limited and the reader must perform a number of operations to obtain the desired positioning of images for a comfortable analysis. Four monitors offer larger surface in order to emulate the traditional arrangement of images. Two monitors might result in a limited surface for mammographic analysis, but a good design and an automatic display of images might produce an acceptable result at a lower cost. Secondly, the lack of a standard definition of user interface elements for medical imaging application also poses a hard drawback in the final acceptation
of these systems. A number of applications are available but each vendor offers different elements, icons and functionalities that overload the learning time for radiologists. Finally, efficient standard quantitative methods for user interface evaluation have not been developed yet. As a consequence, user interfaces depend on subjective evaluation and thus, a methodological improvement of the system is not possible. For all these reasons, comparison with other systems cannot be stablished in terms of user interface quality. A more thoroughful development and evaluation must be carried out in order to obtain a definite, useful and friendly tool for digital mammogram analysis, as several relevant issues have not been addressed yet. Finally, an unavoidable subject to be incorporated in the future digital mammography system is CAD schemes, aimed to improve diagnostic performance. These systems have reached encouraging results when used as a second opinion to support diagnostic decisions. Particularly, in breast imaging, techniques have been developed for detection and classification of masses and microcalcifications, and they could be incorporated to the workstation functionality as an added value.
6. Future plans One of the immediate objectives is to add a new module to the system, which implements CAD algorithms that have been developed and tested at our laboratory. This will provide a useful tool to enhance the radiologist confidence and consistency in the diagnosis but will also pose a challenge in the final user interface, as the presentation of CAD results might result in added complexity. Although, undoubtedly, further investigation must be carried out, a first approach for a soft-copy display workstation for digital mammography that fulfils the expectations of the final users has been reached.
Acknowledgements The authors of this work are grateful to Roberto Patin˜o for his technical assistance in the edition of this manuscript. This work has been supported by
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Secretarı´a de Estado de Polı´tica Cientı´fica y Tecnolo´gica under project no. TIC2000-0507.
Appendix A: Users activity measurement form
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A high resolution workstation prototype for diagnosis of digital mammograms Javier Quiles a,b,*, Pablo G. Tahoces c, Miguel Souto a,b, Juan J. Vidal a,b a
Departamento de Radiologı´a, Laboratorio de Investigacio´n en Imagen Radiolo´gica, Universidad de Santiago, C/San Francisco 1, 15705 Santiago de Compostela, Spain b Complejo Hospitalario Universitario de Santiago (C.H.U.S.), Santiago de Compostela, Spain c Department of Electronics and Computer Science, University of Santiago de Compostela, Santiago de Compostela, Spain Received 27 February 2001; received in revised form 30 May 2002; accepted 24 June 2002
Abstract The development of a total digital high resolution mammography display system must meet a number of requirements that remain a challenge nowadays, most probably because of the special nature of breast imaging. In this paper, we discuss our particular approach to address some problems concerning the complexity of soft-copy diagnosis in digital mammography, such as image quality and user interface evaluation. Based on the experience obtained in the previous implementation of a medical image browser, a more ambitious project is being developed at the Department of Radiology of the University of Santiago de Compostela (Spain) in collaboration with the Department of Medical Informatics of INTELSIS, an emerging software company in our country. This new system will provide complete support to display, store and analyze mammographic studies in digital format. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: PACS; User interface; Digital mammography; Diagnostic image quality; DICOM
1. Introduction Digital image modalities offer a number of advantages compared to conventional screen-film format; detection, processing and visualization become independent processes and as a consequence, images can be accessed from several locations, processing algorithms can be applied for improved lesion detection, previous informa* Corresponding author. Tel.: /34-981-570-982; fax: /34981-547-164 E-mail address: [email protected] (J. Quiles).
tion recovered from either hospital information systems (HIS) or radiology information systems (RIS) can be combined with imaging studies, and computer assisted methods are also available in order to improve diagnostic accuracy. Furthermore, another promising field opened by digital technology is the development of teleradiology systems. In this scenario, the field of mammography, considered the most efficient method for early detection of breast cancer [1], has delayed its migration to digital technology. In fact, most of the examinations are being performed to date in
0169-2607/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 9 - 2 6 0 7 ( 0 2 ) 0 0 0 9 6 - 2
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conventional film format [2]. One important reason for this screen-film dependence is the great volume of data required to obtain digital images with spatial and contrast resolution comparable to those from the conventional format when X-ray dose is maintained. A number of drawbacks concerning image acquisition, transmission and storage had to be overcome by technologists and scientists in the last decade, in order to build a film-independent mammography system. Recent technologies allow the direct acquisition of 4 K /4 K digital images of the breast [3,4]. Gigabit ethernet or ATM protocols, joined to the development of specific compression methods, allow the transmission of digital mammograms in a local area network without remarkable delays. Finally, the continuos advances in storage devices provide affordable solutions for massive storage systems. In spite of these advances, the successful implementation of these advantageous features requires not only the acquisition and storage of digital mammograms, but also a system to visualize the images independently of film support. CRT (Cathode Ray Tube) monitors have been suggested as an acceptable alternative to accomplish this essential function, but several questions remain unsolved about the so-called ‘soft-copy’ systems. First, image quality could be adversely affected by the limited spatial resolution of monitors (2 K and 8 bits per pixel) and the lower luminance compared to that provided by screen-film/viewbox combination. Secondly, the complexity of the image reading process increases: a number of additional functionalities arise and the number of simultaneous high resolution images presented to the reader must be reduced due to the limited surface of the monitors. Thus, the user interface and the automatic mechanisms for image management become a critical part of the system that must compete against the extremely handy format of a film. Finally, it must be pointed out that the field of digital mammography has received an important support from the ACR/NEMA (American College of Radiology/National Engineer and Manufacturer Association) DICOM (Digital Image Communication) Standard. New features have been
recently incorporated that provide an important reference in the designing of soft-copy workstations for digital mammography, particularly with respect to display protocols, image routing and consistent grayscale appearance. This new part of the standard represents an important step towards the integration of digital mammography into the clinical practice. Our department has been working in the field of digital radiography [5 /7] for more than 12 years now. Over this period of time, a limited Picture Archive and Communications System has emerged as a natural response to the needs of the research activity [8]. A number of computerized schemes for the automatic detection of masses and microcalcifications in digital mammograms have been developed. A database has been created with more than 1000 digital mammograms, and it is used to test and design new Computer Assisted Diagnosis algorithms. In the field of soft-copy visualization of digital mammograms our first experience consisted in the development of an X Window application to retrieve images from both a VAX4000 host computer and from Unix workstations, and to visualize individual digital mammograms on the monitors [9]. Based on the experience obtained with this first browser, a more ambitious project is currently under way. In this project, a DICOM based workstation prototype with an enhanced user interface is designed for evaluation of soft-copy diagnosis in digital mammography.
2. Design considerations Due to the special nature of breast imaging, the development of a soft-copy digital mammography system must meet a number of requirements that remain a challenge nowadays. It is worth noting some issues to describe the complexity of this task. 2.1. Desired functionality The standard for Teleradiology from the American College of Radiology [10] indicates that a display application to be used for radiological interpretation of images should provide the following features:
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. . . . . . . . . .
capable of window and level adjustment; pan and zoom of images; rotating and flipping functions; preservation of the correct patient orientation; calculation and display of accurate linear measurements; pixel value determination in appropriate values for the modality; display prior image compression ratio; processing and cropping; provide information of the matrix size and bit depth per pixel; display the total number of images acquired in the study.
Upon this basic functions, a display workstation for radiologic use should provide access to different techniques for image enhancement and analysis that prove to be useful from the medical point of view. CAD schemes have reached encouraging results when used as a second opinion to support diagnostic decisions. Particularly in breast imaging, [11 /14] techniques have been developed for detection and classification of masses and microcalcifications, and they could be incorporated to the workstation functionality as an added value. 2.2. DICOM information model for digital mammography Clinical experience has shown that each image modality presents unique inherent problems that need to be solved before soft-copy images are ready for diagnosis. In the original release of DICOM standard only two image models (Information Objects) were available to introduce digital mammograms into a DICOM based system: the ‘Second Capture’ and ‘Computed Radiography’ modalities. Both of them allow performing all basic image operations (storing, printing and displaying) but they did not address specific needs of mammography [15] and required proprietary capabilities and coding from applications to adequately operate on digital mammograms and studies. The increasing use of digital devices for image acquisition of X-ray mammograms urged the incorporation of a new part of the standard in
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order to support new detector requirements and specific needs for the diagnostic use of single projection modalities including image routing, display arrangement of several images, consistency of appearance on different display systems and matching series of images with corresponding previous studies. As a response to all these required functionalities, the DICOM supplement 32, ‘Digital X-Ray’, became a part of the standard in September 1998. This document defines new information objects with a number of features that allow adequate display of digital mammographic studies (Fig. 1). It is established that image laterality, patient orientation and anatomic region information are mandatory attributes that must be present in any X-ray image file that claims conformance with the standard. With the use of these new features, rotation of images can be automated during the display process and the radiologists would not be disturbed by having to perform continuos rotationing and positioning procedures with images. The correct grayscale appearance is also guaranteed with the mandatory specification of the full transformation process to be performed on any image for display. This new DICOM object is a powerful tool that provides all necessary mechanisms for the successful incorporation of digital mammography into PACS.
2.3. DICOM connectivity A complete application is also expected to be connected into a PACS, and will have to support the handling and management of all digital documents and data of diagnostic relevance, such as image acquisition parameters, image annotations, diagnostic reports and clinical examination requests, patient history, referring physician, actual disease and patient demographic data. All this requested information is likely to be presented in multimedia format in the near future, such as audio or video files. Elaborate schemes including rule-based prefetching or background processes for previous download of images become essential features of the system in order to compensate a possible delay in retrieving images from a long term archive.
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Fig. 1. DICOM mammography information object attributes.
In this scenario, the medical workstation must incorporate several DICOM functionalities that should provide connectivity to a number of devices inside a PACS network. Query and retrieval of images and studies from an image server, sending images to a printer or to storing results back into an image server are essential functions that must be supported.
It is known that the use of digital technology involves a potential loss of information, which might reduce the diagnostic performance of the system. These effects must be evaluated to avoid any possible degradation that may affect the final clinical results. Previous studies have evaluated the suitability of CRT displays for the detection of microcalcifications with encouraging results [16,17].
2.4. Display capabilities and image quality
2.5. The user interface
Digital image workstations should allow the diagnostic interpretation of high-resolution radiographs directly from the display screen and guarantee, at least, the same diagnostic accuracy as that obtained with conventional radiographs.
It seems logical that any user interface design should take into consideration the traditional way of performing film-based radiology [18]. Viewboxes provide a large display surface with a high spatial resolution and dynamic range, as well as
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optimal luminance. Of interest is also that image reading habits have been developed based on the simultaneous presentation of multiple films. If parallel viewing of multiple images were required, as it has been established by traditional work patterns, the solution nowadays would consist of several high-resolution monitors. Some studies have tried to address this issue, but further work needs to be done that specifically deals with mammographic images [19]. Differences between film-based and digital systems in image handling are obvious. Radiologists will have to adapt to new digital reading habits that will differ from those that are used at the present. Although they are not supposed to be skilled computer users, they must perform complex operations to combine the information from several images at the same time. Designers must dedicate special effort for this high complexity and functionality to be available from a simple and easy-to-use interface. For all these reasons, the design of the user interface is one of the most critical components for this type of medical application. An additional problem arises when technical designers have to create user interfaces for applications in an intuitive manner without the logical rules that should be established to achieve an optimum result [20]. Furthermore, the user interface of a digital image workstation must compete against the extremely handy format of conventional films where complex image arrangements and rotations can be quickly provided and modified. Stations for pictorial display, especially those containing multiple display surfaces (i.e. several display monitors), pose separate and unique problems, and many of them have not been formally addressed yet. An accurate model that reflects all significant steps in the diagnostic process must be performed to provide the basis of the implementation of the system.
2.6. Model building and user requirements The first step in the development process has been dedicated to obtain an accurate model that
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reflects both the user needs and the technical requirements of the system [21]. Several approaches can be used to obtain the information model that accurately reflects users demands. Five radiologists were interviewed to obtain precise information about the reading process of mammographic images. We have employed an interview scheme based on a questionnaire that had been answered by final users. From the responses obtained, a number of conclusions were obtained, these being the starting point to describe the activity of the radiologists. The questionnaire is presented in Appendix A. Several important conclusions were extracted about image presentation and workflow: . The first step in the reading process is to obtain a list of cases to be reported. . Almost all conventional studies are comprised of four images. . After one case has been chosen, the most recent images are displayed in front of the reader. The most common distribution depicts two contralateral images of one projection in the two upper positions, and the two images corresponding to the second projection in the lower positions. Images corresponding to the right breast are placed at the left side of the display surface, and images of the left breast are placed at the right side of the display surface. If a previous study of the patient is available, images are placed with the same distribution in a contiguous surface. . A general overview of the study is performed, comparing each image with its corresponding contralateral view and then images are analyzed one by one, being compared to one another. This indicates that . The whole functionality of the system data elements (studies, patients, images and reports) must be accessed from a unique window screen. . Database contents should always be accessible, as well as the main functions to be performed by the user. Thus, the toolbar and the database manager are two elements that should always be present in the application main window. . There is a common way to start the reading sessions. It consists in displaying the most
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recent study of the selected patient and the previous one, if existing any. This feature is expected to be incorporated into the system. Control elements for window and level should always be present in an immediate interface mechanism, as it has been considered a very useful function for soft-copy reading by all the radiologists. Control elements should be kept to a minimum in the image window to reduce disturbances in image analysis. As a consequence, separated screens for image management and image analysis should be employed. A resolution of 2000 /2500 is considered acceptable to provide an overview of the images. This resolution is imposed by the monitors. The full resolution image (4000 / 5000) is only presented on request, when a suspicious region has been detected. The users reached a common way of stablishing comparisons between images for diagnostic analysis.
3. The workstation 3.1. DICOM environment In order to reach a test environment for DICOM based display workstation for mammography, a number of questions appeared. First of all, the workstation had to be able to obtain images from a DICOM based storage node. These storage systems had to include complete mammographic studies that conformed to DICOM supplement 32 for mammography. Thus, an acquisition device that supported mammography image modality had to be connected in the network. Finally, in order to obtain a complete simulated PACS environment, image printing also had to be supported. A complete system was developed that allowed the acquisition and storage of DICOM mammographic images. The diagnostic workstation is integrated in these environment and retrieves studies from the image archive for display. Additionally, it allows hard copy prints of separated images or complete studies. According to the criteria outlined above, the components and
DICOM functionality of the system are shown in Fig. 2. 3.2. Image acquisition Nowadays, two different FDA (Food and Drug Administration) approved technologies are available for direct acquisition of high quality digital mammograms. Both of them firstly convert X-ray photons into light by means of a cesium iodide thallium-doped scintillator. Light photons are then transformed in electrical charge by using either a two-dimensional array of amorphous silicon photodiodes and thin-film transistor [3] or a charge coupled device [4]. For experimental purposes, high quality images can be obtained by digitization of conventional film images. In our prototype, images are acquired by linear digitization of conventional images using a Lumiscan 85 laser scanner [Lumisys Inc., Sunnyvale, CA] which obtains a 4080 /5100 pixel matrix. This can be roughly translated into a pixel size of 50-mm equivalent to 10 lp/mm. The density resolution is scanned in a linear function from 0.3 to 4.1 OD (optical density) in increments of 0.001 OD. The actual pixel value range is from 0 to 4096. These features guarantee a good quality in the raw data which will be used in the system [22]. The output of the scanner is stored in a proprietary file format which is converted in a second step to a DICOM 3.0 file format, following the specifications contained in the specific supplement no. 32 [23]. The DICOM standard provides a suitable frame to the radiologists information model. Both are based on common information objects: patients, studies, series, images and reports. The attributes of these elements in the specification for mammography also constitute an adequate approximation to the radiologist information needs. The DICOM file format is also useful to keep the system opened to connections with different devices such as printers or massive archives. 3.3. Short term image storage A digital image with 4000 /5000 pixel matrix and 12 bits per pixel entiles an image file of 40 MB
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Fig. 2. Description of DICOM environment.
to be stored in the system. Even with the application of lossless compression algorithms, the amount of data can only be reduced to a range of 10/20 MB depending on each image contents. As a consequence, a database containing 50 studies of four mammograms represents a data volume of 40 GB. A feasible solution to the problem of massive storage implemented in our system, is the use of several disk arrays. The main unit [Sun StoreEdge A5100] offers a maximum storage of 500 GB and is connected to the host computer through a fiber channel. The disks are used as short-term storage, with relative fast retrieval times of 4 s/image for final display on high-resolution monitors. Data is stored following the DICOM file format and includes all relevant
information for diagnosis, which can be easily retrieved from the database interface. 3.4. The display workstation 3.4.1. Hardware The kernel of the system consists in a Sun Ultra 80 under Solaris 2.8. The RAM memory has been extended to 1024 MB to provide fast image manipulation. Soft-copy visualization can be configured from one to four high-resolution monitors. The actual configuration for the display of digital mammograms consists of two high resolution monitors (2000 /2500 /10 bits) both being controlled through an onboard graphic card DOME [Md5 DOME Imaging Systems Inc., Waltham,
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MA] and one conventional monitor which is used to perform all functions that are related to data management. 3.4.2. Software architecture The application has been built in a three layer model. One layer is dedicated to communication with the environment using the DICOM protocol. The second layer supports all internal functionalities to be performed with images and data, and the third layer performs all mechanisms related to the interaction with the user. We will focus on the description of the user interface as all other modules have been described elsewhere [12,13,24] (Fig. 3). 3.5. Functionalities supported Regarding the environment, the application is able to communicate with different devices that use the DICOM protocol. . Choose one image server among those existing in the network. . Present to the user query results with accessible studies in the server. Once the storage system has been chosen the system presents information about patients, studies, images and reports that can be retrieved to the local storage system. . Present to the user the data contained in the local storage system. . Acquire new images from a network acquisition device.
Fig. 3. Software architecture.
. Print individual images or complete studies on a DICOM compliant printer existing on the network. Images in a study can be automatedly disposed on the film with a preconfigured layout, or manually selected. . Activate retrieval of selected images to the local storage system device for examination.
3.6. Display functions: study level In the study level, several operations are performed by the system, aimed to ease the difficulties of image positioning and manipulation. The system offers an automated scheme that the users can optionally activate to follow an established sequence of image comparisons with minimal interaction. When the automated mode is activated, the user just selects one study from the main panel. The selected study is displayed in the first high-resolution monitor placed in front of the reader and the previous study is opened the side high-resolution monitor. Images are automatically rotated and mirrored using the DICOM attributes of image laterality and projection to obtain the conventional disposal on the monitors. The user gets a rough overview of the four images and starts the detailed. This is a common step for both the automated and non-automated display methods where the radiologist gets a rough overview of the four images comprising the study and the images from the previous study if any. This is the starting for the detailed revision of the images. The automated method consists of an established sequence of comparisons where images are displayed individually on each monitor to obtain the highest possible resolution. The two craniocaudal images, the two mediolateral-oblique images, the two left breast projections, the two right breast projections and then each image of the current study is compared to its corresponding image of the previous study, if it has been found on the database. If the manual method is chosen, the user is able to select the disposal of each monitor to contain one, two or four images and then select the images to be displayed.
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Any reports associated with previous studies can be displayed and printed using a conventional text editor. 3.7. Display functions: image level The system provides all basic functionalities for image manipulation and visualization. This includes rotation, mirroring, magnifying glass and image polarization. High resolution images of 4000 /5000 pixels can be displayed in partial views of 2000 /2500 pixels. The images can be easily panned to visualize the full mammograms. Annotations included in the image can be visualized as well as all DICOM additional information regarding image acquisition parameters on user demand. Another possibility supported to display the images is to employ a software module which has been developed to permit hard-copy printing on any DICOM compliant printer. 3.8. Critical time response features There are two functions that deserve special mention as they pose critical time constraints on the system: study retrieval and graylevel interactive modification. Fast image retrieval is accomplished by using a fast disk optical interface (fiber channel) to the display workstation, and by determining that the first unreported study should automatically be loaded when the patient folder is chosen by the user. By using this combination, retrieval times are reduced to less than 20 s per study. The second function that requires fast time responses from the system is graylevel interactive modification. Display functions deserve specific consideration: window and level modification is a fundamental operation that allow matching the graylevel range of the stored image, primarily to the narrower range of the display and secondarily to the eye of the viewer. The 4096 intensity levels in the stored image must be displayed as 256 graylevels on the displayed image and a loss of contrast might be produced. This drawback is avoided by allowing the user perform a linear
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conversion from the stored image graylevels to the 256 graylevels displayed on the monitors. The entire stored intensity range is available by narrowing the range of input graylevels (window) and then displacing the graylevel value where the window center is set (level). Graylevels on the screen can be modified by choosing different values for window and level. Then, the image is refreshed according to the new window and level values, providing better contrast definition for a range of values on the stored image, corresponding to one kind of tissue. Window and level are generally set to produce a linear graylevel scale, while the eye operates more like a logarithmic amplifier, and thus, processing can be applied to obtain perceptual linearization of the device [25]. The system offers a non-linear preconfigured transformation to improve the first appearance of the images on the display. 3.9. User interface Because of their flexibility and worldwide acceptance for GUI design in Unix environments, both Motif and Qt toolkits have been selected for the implementation of the graphical elements of the user interface. Motif is by far the most popular framework to guide user interface design on X11 graphical systems and the Qt toolkit has become increasingly popular as many web applications use this alternate style for user interface elements. Due to the increasing presence of internet applications, many users are familiar to this style for interface elements, such as buttons or list presentations. This reduces the learning time required by the user to interact with data, and prompts for acceptance of the system. Following these design specifications and user requirements the user interface consists of these elements: 1) Main window (Fig. 4). This is the starting point to any interaction with the system, from where all functions related to image management on local system and remote devices are performed. In accordance with related works, the disposal of the elements has been established offering the most important area to the presentation of basic objects of the work.
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Fig. 4. User interface. Main window.
The main window contains a toolbar that offers several options to be performed in the reading session: . Acquire new images from an acquisition device. . Print one study on hardcopy DICOM printer. . Connect to an image server (a dialog window is opened to let the user choose from a predefined list of available servers on a network). . Query and retrieve studies or images from an image server. . Send images to an image server. . Visualize study on the high-resolution monitor. The users just clicks on a push button to retrieve the patients, studies and images available in the local image storage. These data is presented on the top window in a hierarchical mode and can be sorted in accordance to several parameters such as alphabetical order, revision state (pending or
already analyzed), acquisition date, etc. The lower part of the window activates after a connection to a remote server and shows data available for retrieval to the local node. Once one study has been chosen for analysis with a mouse double click, the study is opened in the high resolution monitor in the ‘study window’ (Fig. 5). The study window contains two main areas. The first one for managing image depiction on the monitors and a second one for visualization of images. The control area offers four push buttons for direct configuration of images on the screen (four image view, two image view vertical, two image view horizontal and one image). An icon-size view of each image is presented in an image map, where the user is able to recognize all images comprising the study. Images for display are selected on the image map individually grouped. Images can be selected by means of a
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Fig. 5. Study display window.
drag and drop operation: they are selected and moved into the visualization window. Another option is using the automated sequence of comparisons between images that can be activated/ deactivated on a check box beneath the image map. Functionalities related to image display and manipulation have been implemented into a single
proprietary interface element (‘widget class’) called ‘Viewer Widget’ that has been developed at our laboratory. This widget class can be easily integrated into Motif based user interface and implements specific methods to support the visualization of a mammographic image. When an image icon is dragged and dropped on the widget, the image is automatically displayed. Then
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all tools for image processing can be applied: zooming and panning, graylevel modification, rotating and flipping, and magnification of selected regions in the image.
4. Status report 4.1. System architecture The three layer model on which the model is based allows the independent evaluation and improvement of the different aspects of the system: connectivity, functionality and user interaction. 4.2. The communication layer Based on the DICOM standard, enables a high connectivity with existing devices in a PACS. The visualization workstation is able to query and retrieve images and patient information from an image archive that implements DICOM storage services. Printing images or complete studies is also allowed. Finally, image archive management is also available from the workstation. 4.3. The functionality layer This layer of software is the kernel of the system and contains all modules that perform all the actions related with images transparent to the user and independent of the DICOM communication interface. It is composed of three main parts: 1) The database implements the DICOM information model for mammography. This data structure allows an easy management of the information inside the application and according to the standard, provides accurate definitions of images contained in a study, each image laterality, each image projection, and each image predefined window and level or presentation look up table. All these parameters are essential for a correct presentation of images on the monitors. The database is loaded with information obtained from acquisition devices or provided by the DICOM query/retrieve module.
2) Upon the data repository, several modules perform all actions related to image management and visualization. Data manager: This module performs query and retrieve, update or delete functions inside the local database. It provides all information of patients, studies and images to the user interface module. Image tools: This library of functions contains all images and studies manipulation tools. It provides mechanisms to load one complete study in memory in high and low resolution and perform image transformations and positioning operations such as flip, rotate, magnification, filters. CAD tools: This is a special module included to hold the computer-assisted schemes for automatic detection of masses and microcalcifications. These modules have been developed at our laboratory with encouraging results. 4.4. The user interface layer All functions contained in the two layers described above must be available to the user in an easy to use interface. It has been shown that the implication of final users in the design step provides coherent results in the final development. In our design, the distribution of control elements has been aimed to reduce disturbing elements in the image display window by using pop-up menus that can be displayed using the standard mechanisms of Motif based interfaces and do not reduce the viewing area. As Bora¨lv et al. [26] have pointed out, the design of medical user interfaces requires the highest transparency to handle the interface, allowing the user to concentrate on the health care task. The control elements for handling images have also been implemented following the radiologist workflow model to take the maximum profit from the limited surface of the display when several images are simultaneously required. Our prototype design has been evaluated on an heuristic based evaluation in order to detect usability problems. Five radiologists who participated in the design sessions performed, after a short tutorial, a simulated session for diagnostic analysis and scored their degree of satisfaction on
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several features by filling an adapted standardized questionnaire [27]. The values ranged from 1 (very good opinion) to 5 (very bad opinion). The average value obtained on the questionnaires is shown on each evaluated item. The most important conclusions extracted are: . Regarding arrangement of elements in monitors, all users manifested their satisfaction in having one monitor dedicated to perform data management operations, and separately, two monitors to perform image analysis. Average value: 1.5. . The navigational procedures in the database objects (patients, studies and images) have been found to be useful and easy to use. Average value: 2. . Automatic retrieval of previous studies is considered as a useful feature. Average value: 1.2. . The one-click methods for automatic comparison between pairs of images has been well considered by users. Average value: 1.8 . Drag and drop mechanisms to move images from the database into the high-resolution monitors have been well accepted by readers. Average value: 1.6. . Two of the participant radiologists suggested that retrieval and image processing times should be improved. These drawbacks of the system could be improved with an adequate prefetching mechanism and the use of more powerful hardware. Average value: 3.6. . All users complained of the lack of interactive help support. The implementation of this feature had been discussed and finally rejected in the design step, because it was considered as a possible cause of disturbance in the analysis process of the radiologists. Average value: 4.0. . As a desirable feature, the users manifested that a workstation should provide access to the different techniques for image enhancement and analysis that prove to be useful from the medical point of view (commented in the observations free field in the questionnaire). The results of the user interface evaluation are encouraging and the positive overall response
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indicate that users appreciate the line of design and the implementation of the prototype.
5. Discussion Medical images in digital format exhibit a number of advantages over conventional radiographs: they can be adjusted to obtain optimal contrast and brightness for visualization of different kinds of tissue; edge-enhancement algorithms can be applied to improve lesion detectability; computer assisted diagnosis schemes provide a valuable resource to obtain a second opinion and further improve the quality of the final diagnosis; images can be transmitted to remote locations to be reported by experts for either a first or a second opinion; a telemammography system would also possess several advantages, one of the most important of which would be its application for the screening programs for the early detection of breast carcinoma. The examinations could be carried out by mobile units and checked on-line by an expert mammographer to avoid supplementary exposures. Despite these promising possibilities, only a few imaging departments have undertaken the challenge of soft copy digital mammography. This could be attributed in part to the high requirements in image quality and also to the difficulties that arise in the design of an efficient user interface [28]. In fact, several authors have argued about the suitability of digital mammography for a clinical routine use [29]. Regarding image resolution, the ACR Standard for Teleradiology specifies that digitized radiographic films should employ a large matrix format , which corresponds to a spatial resolution of at least 2.5 lp/mm (a pixel size of roughly 200 mm) and a bit depth of 10 bits or greater. Many studies have been carried out that deal with the minimum spatial resolution and pixel depth that a digital mammographic display system must provide to ensure no loss of significant diagnostic information. It is commonly accepted that a meticulous search for the clustered microcalcifications that might herald an early-stage cancer should be done on all mammograms [30]. In fact, clustered micro-
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calcifications are a critical radiological sign in breast imaging for early cancer diagnosis, their visualization and classification requiring not only a high spatial resolution, but also a very high contrast resolution. Existing studies using microcalcifications provide opposite results and controversy still exists: some authors indicate that there is no loss of diagnostic accuracy when using images with a pixel size of 0.1 mm (equivalent to 5 lp/mm) [31], while some other authors indicate that there is a significant decrease of diagnostic performance when the mentioned spatial resolution is used [32]. Nowadays, images with a pixel size of 0.05 mm are available from recent image acquisition systems. This is expected to enhance the detection of microcalcifications, but several technical problems arise, the most important of which is that concerning the resolution capabilities of display monitors, as only a partial view of these images can be reached in existing models. There are some other factors that may affect the final perception of the image that deserve comment. One of most important of them is the lower luminance of the monitors. This feature has been improved in recent models based on active-matrix liquid crystal displays (AMLCD) but they still provide approximately half of the luminance of a film-box. The effect of these changes on the perception of abnormalities is unknown and requires further investigation, although their effect will not be known until these systems have been put into practice [33]. Another crucial subject that needs to be solved is related to the user interface of computer applications for medical imaging. The first question to be solved points at the number of monitors to be employed. The surface is limited and the reader must perform a number of operations to obtain the desired positioning of images for a comfortable analysis. Four monitors offer larger surface in order to emulate the traditional arrangement of images. Two monitors might result in a limited surface for mammographic analysis, but a good design and an automatic display of images might produce an acceptable result at a lower cost. Secondly, the lack of a standard definition of user interface elements for medical imaging application also poses a hard drawback in the final acceptation
of these systems. A number of applications are available but each vendor offers different elements, icons and functionalities that overload the learning time for radiologists. Finally, efficient standard quantitative methods for user interface evaluation have not been developed yet. As a consequence, user interfaces depend on subjective evaluation and thus, a methodological improvement of the system is not possible. For all these reasons, comparison with other systems cannot be stablished in terms of user interface quality. A more thoroughful development and evaluation must be carried out in order to obtain a definite, useful and friendly tool for digital mammogram analysis, as several relevant issues have not been addressed yet. Finally, an unavoidable subject to be incorporated in the future digital mammography system is CAD schemes, aimed to improve diagnostic performance. These systems have reached encouraging results when used as a second opinion to support diagnostic decisions. Particularly, in breast imaging, techniques have been developed for detection and classification of masses and microcalcifications, and they could be incorporated to the workstation functionality as an added value.
6. Future plans One of the immediate objectives is to add a new module to the system, which implements CAD algorithms that have been developed and tested at our laboratory. This will provide a useful tool to enhance the radiologist confidence and consistency in the diagnosis but will also pose a challenge in the final user interface, as the presentation of CAD results might result in added complexity. Although, undoubtedly, further investigation must be carried out, a first approach for a soft-copy display workstation for digital mammography that fulfils the expectations of the final users has been reached.
Acknowledgements The authors of this work are grateful to Roberto Patin˜o for his technical assistance in the edition of this manuscript. This work has been supported by
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Secretarı´a de Estado de Polı´tica Cientı´fica y Tecnolo´gica under project no. TIC2000-0507.
Appendix A: Users activity measurement form
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