Digital radiography using storage phosphor technology: How computed radiography acquires data

Digital Radiography Using Storage Phosphor Technology: How Computed Radiography Acquires Data By John C. Weiser

HIS ARTICLE discusses the technical background of digital radiography based on storage phosphor technology. The initial x - r a y produced data are captured on an imaging plate, then extracted from this plate by the image plate reader. The extracted data then are converted into digital form so that computer-based image processing can be performed. The transformed image data are then either displayed on a workstation or printed onto film using a laser film printer. These steps in the creation of a digital radiograph are discussed sequentially. Image processing is discussed in greater detail in the article that follows this one.

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makes them suitable for use with either the new solid-state lasers or the existing HeNe scanners. However, the older imaging plates are not well suited for use with the new scanners. The imaging plates fit inside special cassettes that are compatible with existing radiographic equipment, so no modification to the x-ray rooms is necessary. The radiographic exposure is obtained in the same way as is done with a film-screen cassette. The CR cassette is then placed into the CR scanner much the same way that a film-screen cassette is placed into a daylight cassette loader, and the readout scan is begun. THE READOUT PROCESS

CHARACTERISTICS OF THE STORAGE PHOSPHOR IMAGING PLATE

The image receptor used for computed radiography (CR) functions somewhat like a conventional radiographic screen in that it converts the energy absorbed from an x-ray exposure into a visible light emission. However, the storage phosphor emits only a small portion of the absorbed energy immediately as light. The remaining energy is retained for many hours and is not emitted unless stimulated by a second exposure to visible light of the proper wavelength. Thus, the term "storage phosphor" has been applied to this class of materials, and "photostimulated luminescence" has been used to describe the readout process. 1 The amount of energy stored at any point on the image receptor is directly proportional to the x-ray energy absorbed at that point, so a latent image is formed in the storage phosphor. The storage phosphors best suited for use in medical radiography are europium-doped barium fluorohalide crystals, BaFX:Eu 2+, where X can be chlorine, bromine, or iodine. 2 Until recently, bromine was the halide of choice because the photoabsorption peak of the barium fluorobromide crystal was well matched to the emission wavelength of the HeNe lasers that were used to scan the imaging plate during readout. Longer-wavelength solidstate lasers have been introduced in more recent CR scanners along with storage phosphor image receptors using iodine as the halide. The broader absorption peak of the barium fluoroiodide compounds Seminars in Roentgenology, Vol XXXII, No 1 (January), 1997: pp 7-11

For image readout, the plate is placed on a translation stage inside the CR scanner and scanned by the laser. The laser beam is guided through a series of optics to a rapidly moving mirror, which causes it to scan rapidly back and forth in a straight line perpendicular to the direction of the translation stage. Each time one scan of the laser is completed, the translation stage has advanced the imaging plate the appropriate distance to start the next line. As the scanning laser passes over each area of the plate, the stored energy from the x-ray exposure is released. This energy is in the form of a light emission that has a much shorter wavelength, and a much smaller intensity, than the scanning laser light. A separate optical path is used to collect this stimulated light emission and measure its intensity. To avoid being "blinded" by the scattered light

ABBREVIATIONS ADC, analog-to-digital converter; CR, computed radiography; CRT, cathode ray tube; kVp, kilovolt peak.

From the Department of Radiology, Robert C. Byrd Health Sciences Cente~ West Virginia University, Morgantown, WE. Address reprint requests to John C. Weiser, PhD, Associate Professor--Medical Physics, Department of Radiology, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV26506-8150. Copyright © 1997 by IEB. Saunders Company 0037-198X/97/3201-000555. 00/0 7

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from the laser, the collection optics are filtered so that they have a very high transmittance for the stimulated light and a very high absorption for the laser light. The light collection area covers the length of a scan line, and remains fixed while the translation stage moves the plate under it. CR scanners on the market today use one of two methods to collect the stimulated light. One method uses a single photomultiplier tube interfaced to a light guide covering the length of the scan. The other method uses an array of five photomultiplier tubes to obtain the required area of coverage. The analog signal from the photomultiplier tube(s) is amplified and compressed by either a square-root or logarithmic process, sampled, and digitized by an analog-to-digital converter (ADC). After readout, the plate is flooded with an intense light to erase any residual latent image and either placed in an output stacker inside the CR scanner for eventual loading into a new cassette, or placed back into the original cassette that it was in before scanning, ready for reuse. Commercial CR scanners differ in the method they use to transport the storage phosphor plates. Some scanners require a flexible imaging plate that can be sent on a rather tortuous path inside the scanner as it is passed from an input stacker to the readout area to the eraser to an output stacker and finally to a new cassette. This design allows for maximum throughput of images and allows for more compact scanner design, but places limitations on the composition of the substrate that holds the storage phosphor materials, the plate backing materials, and the protective coating. Other designs have a transport mechanism that always keeps the imaging plate flat, thus allowing the use of more rigid backing material, less pliable substrate and protective coating, and the incorporation of backscatter protection into the plate backing rather than into the cassette. IMAGE QUALITY CONSIDERATIONS

The design of the CR scanner up to this point determines the limits of two important factors that affect image quality: spatial resolution and contrast sensitivity.

Spatial Resolution Because a digital image is made up of a twodimensional array of discrete picture elements, or pixels, the smaller the size of each pixel, the greater

the spatial resolution. The spatial resolution of a CR system is asymmetric, because two distinct mechanisms affect its resolution in two perpendicular directions. In the direction of the laser scan, the spatial resolution is determined by the number of samples registered by the ADC during the scan. For example, on one of the common CR scanners, the signal from the photomultiplier tube is sampled 2140 times as the laser scans a distance of 430 ram. This means that the distance the laser beam scans during each sample is approximately 0.2 mm, and it is this sampling distance that defines the spatial resolution in this "fast scan" direction. In the other direction, sometimes called the "slow scan" direction, the spatial resolution is determined by how far the translation stage moves from the start of one line to the start of the next. The movement of the translation stage is usually coordinated with the sampling frequency in the fast scan direction so that the nominal pixel is square. Thus, in the example above, the translation stage would move 0.2 mm between scan lines. Positional information for each pixel is obtained by keeping track of the movement of the translation stage and by clock synchronization between the sampling frequency and the scanning speed of the laser. Because independent factors can contribute to loss of spatial resolution, it is important during quality control testing to measure the resolution in both directions.

Contrast Sensitivity The other important image quality factor determined by the design of the scanner and light collection system is the contrast sensitivity. Contrast sensitivity refers to the ability of the system to distinguish between two objects with similar radiographic densities. In a CR system, this contrast sensitivity is a function of the precision with which the ADC can record the value of the signal coming from the photomultiplier tube. The precision is determined by the number of discrete values which the ADC can use to record the signal. This number is defined as 2n, where n is the number of bits used by the ADC. Most CR scanners introduced into the market over the last 5 years have used a 10-bit ADC, which means that they could record 25°, or 1024 discrete values of signal from the photomultiplier tube. Newer CR scanners now available use 12-bit ADC, which gives them increased contrast sensitivity.

DIGITAL RADIOGRAPHY: HOW CR ACQUIRES DATA

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CONSEQUENCES OF CR SYSTEM DYNAMIC RANGE

the CR system. When properly exposed, the filmscreen cassette will reject the unwanted regions of exposures, because they are in the toe or the shoulder of the characteristic curve, where there is minimal contrast sensitivity. The CR method for identifying and rejecting the unwanted digital data is more complex; however, the improved image quality that results from the flexibility in exposure technique far outweighs the inconvenience of waiting a few moments for the computer to process the data.

The storage phosphor imaging plate is not limited by the response cutoff seen in the toe and shoulder of the characteristic curve of a film-screen system. Although the film-screen system can exhibit a linear response between its toe and shoulder with exposures differing by a factor of approximately 100 to 1, the storage phosphor image receptor has a linear response for exposures differing by a factor of up to 100,000 to 1.3,4 The wide latitude of the storage phosphor response allows the scanner to be designed to accept significant variations in x-ray exposure technique. This high tolerance for inconsistencies in exposure technique has contributed to the early demonstration of advantages using CR in intensive care and emergency medicine applications. Current CR systems are designed to measure and record imaging plate exposures between 0.01 R and 100 R, and they digitize the logarithm of the plate response with 12-bit precision, as shown in Fig 1. The range of measured intensities that make up a typical x-ray image cover an exposure range of 100 to 1 and account for only 2048 of the possible 4096 values that can be recorded by the ADC. Exposures outside of this useful range, such as the small amount of radiation that penetrates through the collimators or scatters outside the collimated field, are also faithfully recorded with equal precision by the ADC. At the other extreme, the strong signal of the unattenuated x-ray beam is also recorded from some areas of the imaging plate. In this respect, a film-screen system has an advantage of a sort over

EXPOSURE DATA ANALYSIS Identification of the useful data recorded from the readout of the storage phosphor plate can be broken down into three general process categories: identification of the anatomic area of interest, identification of the collimation, and identification of the unattenuated beam or image background.

Identification of the Area of Interest The first approach to this problem is to construct a pixel histogram that depicts the relative frequency of occurrence of each digital value over the entire area of the image receptor. For example, the pixel histogram of posteroanterior chest image will show a relatively broad peak of low exposure values accumulated from the mediastinal region, a large peak of midrange values coming from the cardiac area, a distinct peak of high penetration from the lung field, a declining frequency of even higher values representing the air-skin interface, and finally a group of very high exposure values caused by the image background. It should be emphasized

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Fig 1. Theoretical response of a 12-bit CR system.

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that the histogram analysis does not depend on any information about the location of any of the pixels. The operator is usually required to input some information about the type of examination that is being scanned, so that the histogram analysis program can select from a range of expected histogram shapes. Information on the presence of prosthetic devices or contrast media is also important because these will change the histogram.

Identification of Collimation and Background Pixel histogram data alone are not sufficient to reliably predict the range of useful image data. The shape of the pixel histogram is significantly affected by the amount of collimation that is used on the image. Therefore, it is necessary to attempt to gain positional information about the location and shape of the collimation and to reject information based on its position on the image (outside of the collimation) rather than the recorded exposure value. Collimation recognition routines evaluate a series of lines across the image and look for the sharp transitions that occur at the interface between the collimation and background, or between the collimarion and the anatomic region of interest. The position of these transitions is used to estimate the location of the collimators, and exposure values of pixels that are determined to be outside of the area defined by the collimator edges will not be included in the pixel histogram analysis. The identification of the collimated areas of the image improves the reliability of the estimate of the lower exposure limit of the anatomic region of interest. The upper end of the anatomic region of interest, such as the air-skin interface, can also be better defined by using a similar approach to identify the transition between the useful image area and the image background. There are two approaches to the form of the data that is retained at this point, depending on the manufacturer of the CR scanner. One approach is to discard the data that were determined to be outside of the useful image exposure range, and to remap the remaining data into a normalized file with 10-bit precision. This approach simplifies the requirements for further processing of the data and works well as long as the original collimation recognition and pixel histogram analysis routines functioned properly. However, if an error is made

JOHN C, WEISER

in the analysis routine, it is not possible to recover the lost data. The other approach is to retain the entire 12-bit data file and to store information about the estimate of the upper and lower bounds of the useful image data in the image file header. Then, if an error in the image analysis routine occurs, it is possible to go back and establish new upper and lower limits on the original data. The specific approach used in various commercial CR scanners can be found in technical descriptions published by the manufacturers, s,6 DISPLAY PROCESSING

After the limits of the useful image are estimated, further processing of the image data is applied to optimize the final image display, either for hard copy output on a laser imager or soft copy readout on a cathode ray tube (CRT) display. The two general forms of display processing used for CR images are contrast scaling, which determines display contrast as a function of imaging plate exposure, and frequency processing, which bases the amount of contrast on the spatial frequency of the image information.

Contrast Scaling Contrast scaling refers to a series of look-up tables that can be applied to the digital image data to map pixel values to optical density of a laser printed film or luminance of a CRT display. In terms of a film-screen system comparison, contrast scaling gives us the ability to optimize the shape of the characteristic curve to match the type of image being viewed. For example, an abdominal examination has many areas where the tissue interfaces are of low contrast. One therefore would usually choose a high-contrast screen film system. In digital imaging, one would choose a look-up table with an abbreviated toe, followed by a highcontrast area to cover the exposure range of the abdominal organs and their surrounding fat, and ending with a broad shoulder, which provides little contrast for the lung bases. Conversely, a chest radiograph has a wide range of object radiodensities. Chest radiographs are obtained at high kilovolt peak (kVp) levels and with low-contrast film to demonstrate all anatomic structures. The look-up table for a digital radiograph therefore would have a broad toe, providing little contrast to the mediasfihal region, then exhibiting a linear region of higher

DIGITAL RADIOGRAPHY: HOW CR ACQUIRES DATA

contrast through the cardiac and lung areas, and ending with a narrow shoulder. A more advanced variation of tone scaling used by some CR scanners is known as adaptive histogram equalization, or dynamic range compression. This process reduces the differences in display intensities between different parts of the image to increase the contrast within each part. In the case of the chest image, the average display value of the mediastinum would be darker and the average display value of the lungs would be lighter, but the overall contrast of both areas would be increased.

FrequencyProcessing Frequency processing refers to a routine that breaks down a digital image into its spatial frequency components and optimizes the display contrast for a desired range of frequencies. 7 Objects that are small or have a sharp edge contain high spatial frequencies. Larger objects and those with smooth edges contain lower spatial frequencies. Because frequency processing is often used to improve the contrast of the higher spatial frequencies, it is sometimes called edge enhancement. The common method for identifying a particular range of spatial frequency information in an image is known as unsharp masking. In this method, which was developed for use with analog film before the advent of digital imaging, a blurred image is obtained and subtracted from the original image. Because a blurred image has lost most of its

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high-frequency information, the image that remains after the blurred image is subtracted from the original is just the higher-frequency content of the original. In the digital process, the pixels that are identified as contributing to the higher-frequency information in the subtracted image are given boosted contrast. CONCLUSION The computed radiography system is an essential part of a digital radiology department. The wide dynamic range of the storage phosphor image receptor allows the CR system to have a high tolerance for variations in exposure technique. Conventional film-screen systems have a fixed exposure sensitivity and latitude that must be matched by the technologist when selecting the kVp and mAs for each individual patient and study. The CR system has an adjustable exposure sensitivity and latitude, which are matched after the exposure to whatever radiographic technique the technologist selected. The mapping of the scanner processing and display parameters to the appropriate data picked up from the image receptor is not a trivial matter. Although the general concept of how this task is accomplished is the same for all scanners, each manufacturer has developed a unique set of terms and parameters to describe the process. It is important that the strengths and limitations of CR systems be well understood as their incorporation into digital radiology departments increases.

REFERENCES 1. Leverenz HW: An Introduction to Luminescence of Sol5. Nakajima N, Takeo H, Ishida M, et al: Automatic Setting ids. New York, NY, Wiley, 1968 Functions for Image Density and Range in the FCR System-Technical Review No. 3. Tokyo, Japan, Fuji Photo Film 2. Sonoda M, Takano M, Miyahara J, et al: Computed Company, 1995 radiography utilizing scanning laser stimulated luminescence. 6. Bogucki TM, Trauernicht DR Kocher TE: Characteristics Radiology 148:833-838, 1983 of a Storage Phosphor System for Medical Imaging--Technical 3. HillenW, Schiebel U, Zaengel T: Imaging performance of and Scientific Monograph No. 6. Rochester, NY, Eastman a digital storage phosphor system. Med Phys 14:744, 1987 Kodak Company, 1995 4. Fujita H, Katsuhiko U, Morishita J, et al: Basic imaging 7. Ishida M, Kato H, Doi K, et al: Development of a new properties of a computed radiographic system with photostimudigital radiographic image processing system. Proc SPIE 347: lable phosphors. Med Phys 16:52-59, 1989 42, 1982