Teleradiology

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1998; 52: Elsevier.

64-68 Paris

Dossier “Imaging”

Teleradiology H Ducou le Pointe

Summary

- According to the American College of Radiology, teleradiology is an electronic tranumssion of radiological images from one location to another for the purposes of interpretation or consultation. This article provides a historical perspective and discusses both solved and unsolved problems concerning the different elements of a teleradiology system (image acquisition. image transmission. image display, image compression). It conclude\ that teleradiology will in the future be a part of picture archiving and communication systems (PACS). Technical problems have for the most part hecn solved, with the exception ol quality of gray-scale monitors. The characteristics of such a system depends on its goals. 0 1998 Elsevier, Paris.

teleradiology

I digital

radiography

/ data

compression

INTRODUCTION Teleradiology is a part of telemedicine. Telemedicine is defined as combining telecommunications technology with medical expertise for the delivery of medical care or education. Various telemedicine applications have been developed, including teledermatology. telepsychiatry. telecardiology, telepathology, teleradiology. and others. Teleradiology as defined by the American College of Radiology (ACR) is an electronic transmission of radiological images from one location to another for the purposes of interpretation and/or consultation. Teleradiology has to be differentiated from picture archiving and communication systems (PACS). PACS deal with image management of an institution, and are composed of an integrated network of digital devices used to electronically acquire, store, manage. and display information concerning patients. Images in a teleradiology system are digital (or digitized). transmitted to a remote location and then displayed or printed for viewing or reading. Telcradiology does not deal with image management and storage. There are numerous clinical celeradiology trials for the purpose of diagnosing images from remote sites where radiologists are unavailable (telediagnosis). for consulting remote experts (teleconsultation) and for training people at distant sites (tele-education). Numerous varieties of systems exist at present. and technical spe-

cifications and performances vary considerably among vendors. This article provides a historical perspective and discusses what problems are resolved or unresolved concerning each element of a teleradiology system. HISTORICAL

PERSPECTIVE

Kuduvalli et al [23] give an excellent review of the history of teleradiology. The first instance of transmitting picture information for medical diagnosis dates back to 1950 when Gershon-Cohen and Cooley [ 131 used existing telephone lines and a facsimile system adapted to convey medical images between two hospitals 4.5 km apart in Philadelphia (USA). In 1959. Jutra [21] pioneered teleradiology by interlinking two hospitals in Montreal (Canada). 8 km apart, with coaxial cable to transmit videotaped telefluoroscopy examinations. Thus, most of the early experiments consisted of analog transmission of medical images via telephone lines [33]. dedicated coaxial cables, UHF radio [34]. and satellite channels [20, 24. 28). The first use of digitized image data for teleradiology systems began with the availability of television camera video digitizers, generating 512 x 512 x 8 bit digital image data. Gayler et al [ 121 published in I98 I an evaluation of such a system. Despite the fact that radiologists’ scores

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for findings, impressions and confidence levels were significantly lower for radiographic images viewed on the teleradiology system, they concluded: “The quality of images provided by the teleradiology system was high enough to warrant further study”. In 1982, Rasmussenet al [30] concluded that the resolution of digitization (512 x 5 12 x 6 bits) provide satisfactory radiographic images for gross pathologic disorders, but would require higher resolution for subtler findings. The same year, Curtis et al [7] reported the result of field trial based on the review of 4,028 cases, where images were digitized with a video camera for display on a workstation with 5 12 x 5 12 x 8 bit resolution. The high accuracy rate (95%) comparing digital to plain film readings was in part attributed to a high proportion of normal cases. In 1984 Gitlin’s group, and in 1987 DiSantis et al [8] used a 1,024 x 1,024 matrix of pixels and data compression. Gitlin concluded that the relative accuracy is a function of data compression used. DiSantis demonstrated that this moderate resolution is adequate for only certain types of diagnosis. In 1987, Kagetsu et al [22] evaluated images of 897 emergency room cases available for follow-up. digitized them to a 5 12 x 512 x 8 bit image matrix and compressed at a 2.5: I ratio before transmission. Discrepancies in interpretations between the video image and plain film directly resulting from inadequacy of the digital display occurred in only 1.6% of cases. High-resolution digital teleradiology was first reported by Carey et al [4] in 1989. Images were digitized (1,684 x 2,048 x 12 bit) and a hard copy was generated on the receiving end with a laser printer. Diagnostic accuracy was reported to be 98%. In 1992, Yoshimo [35] performed an evaluation of a high resolution teleradiology system. They found that two readers had statistically poorer performances in interpretation of laser printed copy of transmitted images compared with their performance in interpretation of the original films. The same year, Halpern et al [17] performed an evaluation of teleradiology for interpretation of intravenous urograms. Images were digitized with a laser scanner to 2,048 x 1,684 x 12 bit pixel resolution and viewed on a 1,024.line monitor with both zoom and window/level capability. They concluded that “their system demonstrates sufficient sensitivity for the detection of clinically important urographic findings in the emergency setting. However, final reading of the

original films is still necessary to assure appropriate specificity”. In 1993, Goldberg et al [14] evaluated 685 plain film cases transmitted from an outpatient center. Plain film digitizer had a resolution of 1.684 x 2,048 x 12 bits. The diagnostic workstation was interfaced with a high-resolution monitor (2,048 x 2,560 pixels). Discrepancies in interpretation occurred in 2.6% of cases. They concluded that primary diagnosis without review of the original plain films is feasible. The last step is the transition from digitizing to digital. The Medical Diagnostic Imaging Support (MDIS) system, developed by the US military, deploys filmless digital imaging technology throughout the defense medical establishment. The teleradiology system is an integral part of the MDIS program. In Korea. the MDIS teleradiology system can acquire images from computerized tomography (CT), magnetic resonance (MR), ultrasound and computed radiography for the network for transmission [27]. In 1996. Uldal [3 11 reported a teleradiology application with a digital system imaging (OS1 Philips Diagnost 96, Philips Medical Systems International, RV). Radiologists do not feel that the image quality (1,024 x 1,024 x 8. reduced from 1024 x 1024 x 12 at the DSI) is insufficient for diagnosis. Recent developments in image handling technology and in telecommunication networks suggested that the majority of technical problems could be overcome. IMAGE

ACQUISITION

Conventional film still constitutes a large portion of radiology practice; integrating them in a teleradiology system requires film digitization [5, 6, 181. In the earliest teleradiology experiments, images were digitized by video camera. Area digitizers illuminate a film with a uniform light source and view the illuminated film with a vidicon camera or a charge-coupled device (CCD) camera. Film digitization is fast (in a video frame time). This method is relatively low in cost but suffers from limited spatial and contrast resolution. Laser digitizers work with a high luminance laser beam focused on a small spot and scanned across a film. They have very good contrast resolution (8 to 16 bits) and good spatial resolution. Linear detector digitizer moves the film across an illuminated slit or moves the detector over the film with slit or area illumination. Generally the

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detector used is a linear CCD array. Current generation CCD digitizers offer a good spatial resolution (4,096 x 5,120 pixels). The low-cost systems have 8-bit gray scale, and expensive systems 10 to 16 bits. They are less expensive than laser digitizers. In the future conventional radiographs will be acquired digitally. Digital fluorography and photo-stimulable phosphor computed radiography (CR) are the two main methods for accomplishing this. Digital fluorography is becoming the preferred option for the gastrointestinal and urinary tract. It is also the accepted technique in general angiography. CR can be used in any equipment that holds and exposes film-screen cassettes, and utilizes a reusable photostimulable phosphor plate. This plate is read by a laser scanner to extract the image in digital form, up to 4,000 x 4,000 pixels. From the published literature 2,000 x 2,000 is adequate for primary diagnosis except for certain diagnostic tasks such as fine linear lung abnormalities, subperiosteal bone resorption or microcalcification detection. In these cases a 4,000 x 4,000 pixels matrix seems to he necessary. CT, MR and ultrasonography (US) produce digital data. Most of them have actually a proprietary format using video capture. Analog output data are converted into a digital form (generally images can be captured with video cameras). Digital conversion after video capture can lose some data (gray scale conversion is done with g-bits), and radiologists cannot access the full 12-bit data. A standard for transmission is now implemented on new CR, CT, MR and US models. This standard is called DICOM-3 (Digital Imaging and Communications in Medicine). It had been elaborated by the ACR and the National Electronics Manufacturers Association. Unfortunately, the DICOM-3 standard could be incompatible due to flexibility in the standard definition. IMAGE

TRANSMISSION

In a teleradiology system digital image data are transmitted via a wide area network (WAN) [2, lo]. The choice of a WAN depends on the type of image, the number of images, data transfer rate requirements and cost of the communications link. Images can be transmitted by means of common telephone lines, coaxial cables, fiber-optic cables, lasers, and microwave equipment with

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transmission links to towers or communication satellites. Communication links generally used in a WAN include ISDN (integrated services digital network) or DS-1 carrier. ISDN is a network standard that provides end-to-end digital links to support a spectrum of applications, including voice and data. ISDN provides services whose access is through a limited number of standard digital network interfaces (basic or primary rate interfaces). The basic interface uses two B-channels for 128 kbps data, compressed video and digitized voice transfer, and 16 kbps D-channel for out-ofband signaling. For higher transfer rates, primary rate interface is provided (23 B-channels and one D-channel). ISDN is widely available in Europe and the United States. DS-I carrier service is equivalent to 24 voice-grade channels. each with a bandwidth of 4,000 Hz or 8,000 samples per second. Each sample may be 7 bits (56 kbps) or 8 bits (64 kbps). The Ggnaling rate for 24 channels is 64 kbps or 56 kbps. The transmission line facilities provided by the carriers for DS- I service have an identical bandwidth called T-l and the two designations are used interchangeably. DS- I is a service standardi/.ed throughout North America and Japan. It is not identical to any European standard. The next generation for WAN will be perhaps ATM iasynchronous transfer mode) network [ 15, 191. ATM offers high handwidth (at least 155 Mbps). This service is not yet widely available in Europe and in the United States. The Internet will be also another solution in the future for the delivery of teleradiolopy services. Internet protocol allows interconneclion of heterogeneous networks. The Internet is widely accessible and inexpensive and supports multimedia applications. The biggest disadvantage of Internet transfer is the lack of data security protection. IMAGE

DISPLAY

One of the most crucial components in the teleradiology system is the gray-scale display monitor 19, 251. Many radiologists encounter difficulties when they shift from interpreting analog images on a film alternator to interpreting images on an gray-scale display workstation. Spatial resolution needed is 2,000 x 2,000 for most clinical applications. In 1994, ACR standard for teleradiology [I] recommends a minimum resolution of 2,000 x 2.000 x I? bits for digitization

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and 2,000 x 2,000 x 8 bits for display. According to ACR standard a small matrix system (.5K x .5K x 8 bits) is acceptable to digitize or display CT, MR, US, nuclear medicine, and digital fluorography. Spatial resolution of a gray-scale monitor is often given by the specification of the display monitor or frame buffer. Dwyer focused on the fact that the resolvable pixel matrix of a monitor is significantly smaller, the reason being that the pixel on a gray-scale monitor is not square, rather it is formed by an electron beam with a two dimensional Gausian distribution in electron density, and a certain degree of overlapping between pixels is therefore noted; spatial and contrast resolution is reduced. According to ACR standards, luminance of the gray-scale monitors should be at least 50 ft-lamberts (ft-L). Actually most monitors provide between 50 to 150 ft-L. Conventional film alternators provide luminance intensity on the order of 200 to 400 ft-L. Low luminance can affect the radiologist’s perception of contrast and spatial detail. The luminance output of ordinary light boxes and gray-scale monitors is not uniform. For light boxes. variations have little effect on the perceived dynamic range due to their high absolute luminance output. For gray-scale monitors, the effect of luminance variation is more pronounced due to their lower light output. Refresh rate is the number of times per second a screen is re-painted. Slow (60 Hz) refresh rates produces a flickering environment which causes eye strain and operator fatigue. A fresh rate over 70 Hz is required to ensure an image display monitor is flicker free. Characteristics of gray-scale monitor vary with time, so they must be tested periodically [Ill. Parson [29] proposed a protocol which requires only a photometer and the ability to generate the Society of Motion Picture and Television Engineers (SMPTE) test pattern, it takes approximately 5 minutes per monitor per week to gather data. IMAGE

COMPRESSION

Data compression can improve imaging system efficiency by reducing the image transmission time and the required storage space. Digital image compression methods are designed to reduce the amount of image data necessary to represent the original image without compromising diagnostic

accuracy. Several criteria are used to classify image compression methods. The most commonly accepted classification is based on the degree of preservation of the image. Lossless or lossy is the most important distinction between compression methods. Images reconstructed by reversible compression methods (eg, Huffman coding, LempelZiv coding) yield image data identical to the original image, but do not typically achieve high compression ratios 3:l [26]. Compression ratio is defined as the original image file size (numerator) vs the compressed image file size in bytes (denominator). Much higher compression ratios are obtained with irreversible methods. Image data undergo a linear transformation (eg, the discrete Fourier, Hadamard, sine or cosine transforms). The pixels in the transformed image are then quantified and encoded. In the medical field, the discrete cosine transform (DCT) is the orthogonal transform the most used and evaluated for image compression. The joint photographic expert group (JPEG) has defined a general-purpose image compression standard [32]. JPEG is also supported by the DICOM standard [15]. JPEG is based on a DCT transformation. Compression ratio of 10: 1 seems to be acceptable in teleradiology [3, 161. However, JPEG suffers from block artifacts that become increasingly apparent at higher compression ratios. It is one of the reasons of the recent interest of the medical community in wavelet transform. Wavelet compression can provide better performance than JPEG for a given level of image quality. ACR standards do not specify any type of compression, but require that compressed images be clearly labeled as such.

CONCLUSIONS

Teleradiology has come a long way from early attempts. Teleradiology systems will be in the future a part of PACS systems. Technical problems seem to be resolved except perhaps the quality of gray-scale monitors. The characteristics of a system depends of its goals. Primary diagnosis seems to be technically possible with a teleradiology systems, but is not allowed by law in all countries. Research is needed to establish the cost-effectiveness of teleradiology.

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