ACQUISITION HARDWARE FOR DIGITAL IMAGING

Transcription

1 ACQUISITION HARDWARE FOR DIGITAL IMAGING WILLIAM R. WIDMER Use of digital radiography is growing rapidly in veterinary medicine. Two basic digital imaging systems are available, computed radiography (CR) and direct digital radiography (DDR). Computed radiographic detectors use a two-step process for image capture and processing. Image capture is by X-ray sensitive phosphors in the image plate. The image plate reader transforms the latent phosphor image to light photons that are converted to an analog electrical signal. An analog to digital converter is used to digitize the electrical signal before computer analysis. Direct digital detectors provide digital data by direct readout after image capture a reader unnecessary. Types of DDR detectors are flat panel detectors and charge coupled device (CCD) detectors. Flat panel detectors are composed of layers of semiconductors for image capture with transistor and microscopic circuitry embedded in a pixel array. Direct converting flat panel detectors convert incident X-rays directly into electrical charges. Indirect detectors convert X-rays to visible light, then to electrical charges. All flat panel detectors send a digitized electrical signal to a computer using a direct link. Charge coupled device detectors have a small chip similar to those used in digital cameras. A scintillator first converts X-rays to a light signal that is minified by an optical system before reaching the chip. The chip sends a digital signal directly to a computer. Both CR and DDR provide quality diagnostic images. CR is a mature technology while DDR is an emerging technology. Veterinary Radiology & Ultrasound, Vol. 49, No. 1, Supp. 1, 2008, pp S2 S8. Key words: computed radiography, direct conversion, direct digital radiography, flat panel detector, indirect conversion, veterinary. Introduction IGITAL RADIOGRAPHY IS probably the most important Dadvance in veterinary imaging since the advent of diagnostic ultrasound. Over the past two decades, digital radiography has largely replaced conventional radiography in human imaging centers. Digital radiography is now commonly employed in veterinary teaching and referral hospitals, and in many private practices. Eventually, most veterinary practices will use this modality. There are two general types of digital radiography: (1) computed radiography (CR) and (2) direct digital radiography (DDR) (Fig. 1). 1 5 Charge coupled devices (CCD) are a third type of digital imaging system, but are often considered with DDR hardware because their image is sent directly to a computer. Each uses conventional X-ray equipment, including the X-ray machine, table, grid, etc., to acquire an image. However, the procedure is filmless and presents the final image on a computer monitor. Thus, no dark room, film storage, film processing, or light boxes are necessary. The main differences between CR and DDR involve acquisition of the image, not the final result. A major advantage of all digital imaging systems is the ability to send image data to a picture archiving communications From the School of Veterinary Medicine, Purdue University, W. Lafayette, IN (Widmer). Address correspondence and reprint requests to William R. Widmer, at the above address. widwil@purdue.edu doi: /j x system (PACS) for analysis and storage. For more information about PACS, see the article in this Supplement. 6 CR CR was developed by Fuji (Tokyo, Japan) and has been in use since the 1980s. 4 The basic components of a CR system include a detector or image plate (IP) for acquiring the image, a device to read the IP, an analog to digital converter (ADC), and a computer and software to process the digitized image. 4,7,8 The first step with CR is image acquisition and a standard X-ray exposure of the patient is made. A flexible IP containing photosensitive phosphors is used for image capture (Fig. 2). 9 Photosensitive phosphors have a complex crystalline structure containing halogens and activators. Barium fluorohalides doped with an activator such as europium are commonly used phosphors for CR IPs. Image plates used in CR function like conventional film/ screen combinations and are placed in a rigid custom cassette for protection. Sometimes the IP is installed in conventional X-ray cassettes after removal of intensifying screens. Following X-ray exposure, the flourohalide and activator act together to capture a latent image. 8 Events relating to image capture by photosensitive phosphors are complex, involve quantum mechanics and are incompletely understood. 7,9 Briefly, electrons in the crystalline phosphor are excited to a higher energy level where they S2

2 Vol. 49, No. 1, Supplement 1 ACQUISITION HARDWARE FORDIGITAL IMAGING S3 Fig. 1. Concept of digital radiography. CR and DDR both use conventional X-ray equipment and ultimately produce a digital image. are trapped at various sites in the phosphor. Trapping is thought to occur in both the valence band and the conduction band of the electron energy spectrum. 7 9 Trapped electrons form a stored or latent image that is analogous to the latent image acquired on conventional X-ray film. However, the latent image of a CR system decays rapidly compared with a conventional film screen system. Depending on type of phosphor, the latent image in CR system is stable for only minutes to days and must be processed quickly. 7 9 The latent CR image is processed by the reader and displayed on a computer monitor. The reader consists of a laser, optical scanner, photomultiplier tube, and motorized platform (Fig. 3). After the IP is fed into the reader, the latent image stored on the phosphor is scanned by a helium neon laser that releases trapped electrons returning them to a lower energy state. By the process of photostimulable luminescence, the stored energy is released in the form of visible light. 7 The laser beam scans the IP in a series horizontal lines similar to the raster scanner used by Fig. 2. Computed radiographic image plate (IP). The IP is flexible and contains X-ray sensitive phosphors. Because IPs are easily damaged, they are placed in empty conventional X-ray cassettes (minus intensifying screens). Fig. 3. Basic layout of reader for a computed radiography system. The latent image stored on the phosphor of the imaging plate and is scanned by a laser, emitting visible light. The light is gathered by a photomultiplier tube and converted to an electrical signal and sent to an analog to digital converter. The digitized signal is sent to a computer. the electron stream directed at a television screen. The light emitted by each point of the IP is collected by an optical system that is coupled to a photomultiplier tube. An optical filter is used to prevent high intensity light produced by the laser from interfering with the visible light emitted from the phosphor. The photomultipler has a wide dynamic range and converts the various light intensities into corresponding electrical signals. 7,8 Up to this point all image data are in an analog format. Analog electrical signals are amplified and passed through an ADC. Each analog signal is assigned a binary value that corresponds to the brightness of each pixel. Individual data points are sent to a specified pixel in the image matrix. A matrix is often used for CR. The brightness of each pixel ultimately represents the degree of X-ray attenuation of structure that was imaged and is assigned a corresponding shade of gray. As with CT, structures that have high X-ray attenuation are represented by bright pixels while structures with low attenuation produce dark pixels. A 12 bit system (2 12 ) is commonly used and can represent pixel values giving a gray scale range of ,8 Not all of the energy stored in the phosphor is used in the readout process; therefore, the last step in the process is to erase the IP for reuse. This is accomplished by flooding the IP with high intensity white light, which releases residual energy. Like conventional X-ray intensifying screens, IPs can be reused many times before image quality degrades. One report states that a CR plate could be reused up to 10,000 times. 8 However, the exact life time of a CR plate for veterinary use is unknown but depends on the quality of the protective cassette, care during processing, and other general use factors.

3 S4 WIDMER 2008 Further imaging processing is entirely digital and uses computer software. Automated software programs initially manipulate the data, creating a histogram, identifying a gray scale level for each pixel. The raw image data are adjusted for over or underexposure of the IP. A feature of all digital imaging software is enhancement of image contrast. This is done with a look up table (LUT), which is a data set of preset pixel values that can be substituted for pixel values of the raw data matrix. The LUT enhances contrast by increasing the difference between different anatomic structures. For instance, suppose the raw data for the liver were represented by pixels with a gray scale value of 45 and the pixels representing the spleen had a value of 40. The difference in contrast would be or 5, which would be difficult for the human eye to detect. Application of LUT software might assign pixel values of 55 for the liver and 35 for the spleen. Now the difference in contrast would be ¼ 20, making the gray shades of the liver and spleen more obvious to the interpreter. Look up tables are specific for different body regions (e.g., abdomen, thorax, musculoskeletal) and are operator selectable because they are usually part of the imaging algorithm. Algorithms also determine the degree of edge enhancement (edge sharpness) and contrast resolution, brightness and frequency at which data are sampled. The image data are presented on the computer monitor according to default settings dictated by the software. Further manipulation can be made according to the observer s preference. This includes adjusting contrast and brightness, sharpness, size (zoom), and color enhancement. For more information on image processing see the article in this Supplement. 10 DDR DDR, like CR, produces a truly digital image. 2 The principal difference between CR and DDR is the detector (image plate), which has an integrated readout mechanism (Fig. 1). The detector captures incident X-rays from the patient and produces a digital signal that is sent directly to a computer for interpretation. This eliminates the need for an image/plate reader, as with CR, and results in faster image acquisition. In this paper, the term DDR refers to digital systems that can directly readout the image (e.g., those without any processing step requiring a reader). There are two basic categories of direct digital readout detectors flat panel detectors and CCD detectors. Some medical physicists do not consider CCDs to be truly direct digital detectors. However, they are commonly marketed as a type of direct digital equipment and do eliminate the plate reading step of CR. Thus, they will be discussed under the broad category of DDR. A schematic of DDR detectors is presented in Fig. 4. Flat Panel Detectors Flat panel detectors are rigid imaging plates that have a similar appearance to cassettes used in a conventional film screen system (Fig. 5). 2,3,11 15 These detectors classified as either direct converting or indirect converting. 2 Direct converting detectors have an IP that contains a semi conductor that directly converts X-ray energy into an electrical charge. Indirect converting detectors have a two-phase process for transforming X-ray energy into electric signals. X-rays are first converted to light that in turn are converted Fig. 4. Comparison of direct digital detectors flat panel and charge coupled device (CCD) detectors. Flat panel detectors either directly or indirectly convert incident X-rays into an electrical charge. Direct detectors use a photodetector to convert X-rays into an electrical charge while indirect detectors first transform X-rays into visible light via a scintillator. A photodiode converts light to an electrical charge. Both direct and indirect detectors have thin film transistor (TFT) arrays that store and digitize charges before readout. With CCD, a scintillator also converts X-rays to visible light. A reducing lens is used to minify the light beam to the dimensions of the CCD chip. The chip converts light energy into electrical charges and then to a digital format for readout. Red flashes denote conversion of incident energy (light or X-ray) into electrical signals. Note that while conversion to electrical charge varies with type of detector, all have direct readout, unlike computed radiography systems.

4 Vol. 49, No. 1, Supplement 1 ACQUISITION HARDWARE FORDIGITAL IMAGING S5 glass substrate. Readout electronics are at the lowest level and charge collector arrays (storage capacitors) are at the higher levels of the TFT, near the interface with the photoconductive layer. 2 Electrical charges collected by electrodes are stored in capacitors before readout from each TFT array. Readout occurs row by row and is sent to the computer after analog to digital conversion. Fig. 5. Flat panel detector. Both direct and indirect converting detectors are housed in a rigid cassette and connected directly to the imaging computer hardware. to electrical energy. Digital data reaching the computer is processed identically to that described for CR. Direct Converting Detectors Direct conversion should not be confused with direct readout. 2 The former relates to direct conversion to electrical signals at the IP level while the latter pertains to direct transfer of image data from the IP to the computer. All DDR has direct readout to the computer, while CR does not. The direct detector consists of layers containing semi conductors, storage capacitors, thin film transistor arrays (TFT) and a glass substrate. 2,11,14 The detector is enclosed by a protective coating and is connected to a computer by a cable. The thick top layer (nearest entry of the X-ray beam) is called the photoconductive layer because it directly converts X-rays to electrical charges. It is composed of amorphous selenium, a semi conductor with excellent photoconductor properties. X-rays exiting the patient interact with selenium and release electrons. Because of a bias charge placed across the photoconductive layer before exposure, electrons released in the top selenium layer rapidly migrate perpendicularly to the nearest charge collection electrode. These electrodes are located at the bottom of the photoconductive layer and are linked the next layer of the detector, the TFT. Thin film transistor arrays are placed beneath the selenium photoconductive layer and are responsible for readout of charges collected by the photoconductive layer. 2,3 The TFT array covers the entire detector surface in a matrix that corresponds to pixels viewed on the computer screen. Thin film transistor arrays are complex, consisting of multiple layers of amorphous selenium deposited on a Indirect Converting Detectors Indirect converting detectors indirectly convert X-ray energy into electric charges. A scintillator in the top layer of the flat panel converts X-rays into visible light and a diode layer immediately beneath the scintillator transforms light energy into electric charges. These layers replace the single photoconductive layer of direct converting detectors. A TFT array sends a digitized signal to the computer. Cesium iodide crystals in either structured or nonstructured form are commonly used for the scintillator layer. To eliminate lateral light spread, crystals are grown on the detector as thin parallel needles perpendicular to the flat panel. 2,3 Discrete 5 10 mm wide needles act as light pipes and channel visible light to the photodiode layer, enhancing spatial resolution. Because of the minimal light spread with structured crystals, scintillator material can be relatively thick, increasing the probability of X-ray interaction and number of light photons produced. Nonstructured (amorphous) cesium iodide crystals are also available, but manufacturers are moving toward structured scintillator material with needle grown crystals to improve quantum efficiency. 2,3 The photodiode arrays of indirect detectors are usually composed of amorphous silicon. Photodiodes are charged before exposure of the detector and this charge is depleted in proportion to the light it receives. The charge is temporarily stored by the capacitor of the TFT array for readout. Each photodiode and TFT array represents a pixel, providing true digital output from the detector. 11 CCD Detectors Charged coupled technology was invented by Bell Laboratories in 1969 for memory hardware of computers. 2 Because of their sensitivity to light, CCDs became popular photodetectors in the imaging field. Today CCD technology is commonly used for many indirect conversion applications in diagnostic imaging, including fluoroscopy. Charge coupled detectors were the first direct readout detectors offered for digital radiography systems. They are popular in digital radiography because of their small size and cost effectiveness. 2 These two factors have allowed CCD detectors to compete with flat panel detectors in the veterinary imaging market. A typical CCD digital imaging system consists of a scintillator, optics for minification, a CCD optical detector

5 S6 WIDMER 2008 and, in some instances, an image intensifier (Fig. 4). X-rays exiting the patient interact with a scintillator, as with flat panel detectors, producing visible light. Because the area of the CCD chip is small compared with the scintillator, an optical coupling is used to minify the field of light that strikes the CCD chip. Some CCDs utilize a fiber optic coupling to taper the light image to the dimensions of the CCD chip, eliminating the need for a de-magnifying lens. An in-depth discussion on CCD technology is beyond the scope of this review and only a basic description follows. A detailed description of CCD technology is available. 16 The CCD detector chip contains a pixel matrix that converts light photons delivered by the optical system to electrical charges. Pixel arrays are formed by polysilicongate and silicon photosensitive-silicon layers. The gate layer shapes incoming light photons to a pixel dimension while the photosensitive layer coverts light to electrical charges by means of the semiconductor properties of silicon. The pixels are square and range from 6.8 to 26 mm. 17 Electrical charges in the pixel array are transformed from analog to digital format as they are readout. The above is a full frame CCD that utilizes the entire pixel in each array during light photon detection. This 100% fill factor maximizes signal to noise, but increases acquisition time. 17 Other types of CCDs can multi-task pixels to obtain rapid readout at the expense of image resolution. These are employed in the digital camera industry but are less useful for digital radiography. Using CCDs, some light photons are lost during optical reduction and do not reach the CCD chip. This produces increased image noise compared with flat panel systems that do not have image minification. There is also loss of radiation dose efficiency compared with flat screen systems. Therefore, exposure reduction may not be a feature of CCD systems. Finally, the optical system of CCD devices necessitates more space for housing than flat panel detectors. Purchasing Decisions Limitations of film-screen (analog) radiography include narrow exposure latitude of silver halide systems, constant need for dark room quality control and processor maintenance, cost of X-ray film and image storage and lack of image postprocessing capability. 16 These factors often motivate veterinarians to replace conventional analog radiographic systems. Expected benefits include of a digital purchase include (1) convenience of digital image format, (2) elimination of X-ray film, processor and chemicals, (3) elimination of filing and storage of X-ray films, (4) improvement in image quality (primarily image contrast), (5) wide latitude for exposures, (6) fewer repeat exposures and (7) rapid image acquisition leading to increased throughput. Perhaps the advantage of a digital image format is the best overall reason for purchasing a digital system. One benefit of a digital format is utilization of remote imaging interpretation via the Internet (teleradiology). Depending on the situation in a particular imaging department or practice, not all of the above will come to fruition with a purchase of digital equipment. Patient positioning errors, inadequate views and breaches in radiation safety will not be overcome with the purchase of digital equipment. Potential buyers are encouraged to consult with boarded veterinary radiologists for aid in decision making. The digital imaging market has grown rapidly over the past 10 years. Most digital systems aimed at the veterinary market are human based-imaging systems adapted for animal use. These are largely promoted by vendors and not directly by the manufacturer. When searching for digital imaging systems, it is prudent to identify the manufacturer and exact specifications of the detector, and how it has been used for human imaging. For instance, a system marketed for chiropractors may provide excellent musculoskeletal image quality in animals, but may not produce adequate abdominal and thoracic radiographs. Buyers should be aware that, like computer hardware, a complete digital system might have major components that are outsourced to separate manufacturers. In addition, while the company assembling the components may market the system, a separate vendor may be used for sales to veterinarians. Thus, set-up questions and troubleshooting may involve more than contacting the vendor. CR vs. DDR Options The two main categories of digital equipment CR and DDR are frequently compared when making purchasing decisions. The main disadvantage of CR is the lack of direct readout and can be significant in a busy small animal imaging environment or an equine center. However, CR is considered a mature imaging modality that has been continuously improved over the past several years. For computed radiographic systems, the quantum efficiency and spatial resolution of the detectors are directly comparable to the best analog systems. 3 This concept is supported by the fact that CR is now routinely used for mammography in people. CR systems are generally less expensive than DDR systems and can work well in veterinary imaging centers where throughput is not a major concern. Therefore, CR affords veterinary practitioners an easy entry into digital radiography. A more detailed comparison of CR vs. DDR is given in Table 1. If a decision to purchase a DDR system is made, one must consider the options among direct vs. indirect flat panel detectors and CCD. 2,3,11,13 19 Flat panel detectors may have superior imaging characteristics, relating to spatial resolution, dose reduction, and

6 Vol. 49, No. 1, Supplement 1 ACQUISITION HARDWARE FORDIGITAL IMAGING S7 Table 1. Comparison of Purchase Factors for Radiographic Systems 17,19 Conventional Radiography (analog) Computed Radiography Direct Digital Radiography Start-up cost Low Moderate High Throughput Low Low Moderate Exposure latitude Low Moderate Moderate to high Image resolution High Moderate Moderate Image contrast Low High High Post processing No Yes Yes DQE Low Moderate Moderate to high Portability High High Low Life time 7 10 years Unknown Unknown DQE. The efficiency of a detector for identifying incident X-ray photons is measured by detective quantum efficiency or DQE. 3 Because detectors do not identify all incident radiation, the value of DQE is always less than unity. Theoretically systems with a high DQE will have a relatively low exposure dose. dynamic range. 2,3 Comparing direct and indirect converting flat panel detectors at this time is difficult because of continual technologic developments by manufacturers. In addition many of the details regarding composition and performance of flat panel detectors remain proprietary. Most clinical trials on digital detectors have compared DDR flat panels to CR systems and not to each other, thus it is difficult to recommend direct vs. indirect flat panel detectors. Table 2 provides a comparison of characteristics of several commercially available flat panel detectors. A more thorough comparison of CR and DDR detectors is available. 20 Table 2. Specifications for Some Commercially Available Flat Panel Digital X-Ray Detectors 2 DirectRay (Hologic, Kodak, Rochester, NY) Direct conversion of X-rays Photoconductive layer ¼ amorphous selenium (500 mm) TFT þ capacitor ¼ amorphous silicon Matrix ¼ Pixel size ¼ 139 mm Cannon CXDI-11 Indirect conversion of X-rays Scintillator ¼ terbium-doped gadolinium dioxide sulfide Photodiode¼ hydrogenated amorphous silicon (a-si:h) TFT ¼ hydrogenated amorphous silicon Matrix ¼ Pixel size ¼ 160 mm 4046 gray scale (14 bit resolution) General Electric Medical Systems (Milwaukee, WI) Indirect conversion of X-rays Scintillator ¼ cesium iodide Photodiode ¼ amorphous silicon TFT ¼ amorphous silicon Matrix ¼ Pixel size ¼ 200 mm Trixell (Phillips, Siemens, Thompson) Indirect conversion of X-rays Scintillator ¼ structured thallium doped cesium iodide Photodiode ¼ amorphous silicon Matrix ¼ Pixel size ¼ 143 mm 4096 gray scale (14 bit resolution) Direct converting detectors have the potential for better spatial resolution than indirect converting detectors because of the higher resolution qualities of amorphous selenium and lack a light generating step. 2 However, many other factors influence spatial resolution, including pixel size, spacing of pixel rows (pitch) and X-ray scattering. Presently resolution and other measured of performance is similar for most flat panel detectors. The major disadvantage of flat panel detectors is cost they are more expensive than CCD detectors. Specifications of some commercially available detectors are given in Table 2. 3 Detectors based on CCD technology may be less expensive than flat panel detectors and are a reasonable alternative to expensive flat panel detectors. CCD-based detectors were the first direct readout detector and are presently offered by many vendors of veterinary imaging equipment. Image quality is good, but may less than with obtained by some flat panel detectors. As mentioned these detectors have a lower signal to noise ratio than most other detectors. A major disadvantage of CCD detectors is their nonportability. The detector is usually permanently affixed beneath the X-ray table. Because of the optical system, the tube head must be kept perpendicular to the X-ray table, eliminating horizontal beam or cross-table projections or any other projection made with nonperpendicular beam angulation. Charge coupled device technology is not presently applicable to large animal imaging. Digital imaging is growing rapidly in the veterinary imaging marketplace. CR systems are cost-effective and provide similar or superior image quality compared with conventional screen film systems used by most veterinarians. Direct readout modalities include flat panel and CCD detectors and also provide high-quality digital images. Flat panel direct digital detectors are likely to be the mainstay of future digital imaging solutions. Disclosure of Conflicts of Interest: The authors have declared no conflicts of interest.

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