SELF-SCANNING LINEAR DIODE ARRAY DIGITAL RADIOGRAPHY* D. Sashin, E.J. Sternglass, B.S. Slasky, K.M. Bron, J.H. Herron, W.H. Kennedy, L. Shabason, J. Boyer, A.E. Pollitt, R.E. Latchaw, B.R. Girdany, R.W. Simpson Department of Radiology, University of Pittsburgh RC 406 Scaife Hall, Pittsburgh, PA 15261 Abstract The development of high resolution, selfscanning, solid state linear diode arrays and fast computer memories allows for the construc-. tion of an imaging system which overcomes many of the limitations associated with ci.rrent x-ray film-screen and TV-f luoroscopy systems. In our digital radiography method, a fan-shaped x-ray beam illuminates a phosphor screen that is fiber optically coupled to a row of self-scanning, linear diode arrays whose output is then digitized and transmitted to a computer for storage, display and processing. This technique images small density differences which permits the visualization of vessels and lesions with intravenous administration of radiopaque con. trast media rather than the invasive method of directly introducing contrast into the arteries. The fact that images are recorded in a digital memory allows for greater flexibility for processing and display of these images. Introduction The development of high-speed digital computers and self-scanning arrays of light sensitive di.odes has permitted the development of a digital electronic radiography imaging system which has the potential for eventually replacing analogue recording media in many radiological procedures. We report here a suimnary of a number of the advantages and limitations of our electronic radiography utilizing self-scanning, solidstate, linear diode arrays in the light of early clinical experience with a newly developed prototype system at the Department of Radiology 1,2,3 of the University of Pittsburgh -. Technical Background Before describing the details of our elec.- tronic imaging system, we will briefly outline a number of the fundamental limitations of film screen and intensifier-tv systems in order to demonstrate how they may be overcome by the use of solid state self-scanning linear diode arrays. Perhaps the most serious limitations in the use of film-screen is the inability to record a wide latitude of intensities and simultaneously display very small density difference at high contrast. On the other hand, the linear diode 7 arrays can store as many as 8 x 10 individual electrons released by incident light in each 25 lim wide diode prior to saturation, thus providing a dynamic range of the order of 10 5 :1 per picture element together with the ability to display density differences of less than 2 parts in 1000. However, prior to the development of low cost high speed digital circuitry and computer memories, there was no feasible method to utilize the arrays in diagnostic radiology. Thus, analog methods of recording and displaying electronic images are unable to match the dynamic range and high signalto-noise ratios of the solid state detectors and do not allow for the feasibility in image processing and display when compared to digitally stored image data. The reduction in cost of memory per pixel each with 12 to 16 bits which is required to record the full dynamic range, makes it feasible to take diode array digital radiographs, each containing some 10 7 bits. A number of the potential advantages of digital systems are listed below. 1. The presentation of a very wide dynamic range image at selected contrast and brightness under the control of the observer. 2. Precise image subtraction. 3. Enhancement of features of images to be highlighted and suppression of extraneous information. 4. Automatic image retrieval. Although many of the advantages of the system developed in our laboratory are also true of image intensifier-television systems, there are a number of features of our system which overcomes many of the limitations of image intensifiers. A number of these are the elimination of light scatter (veiling glare) and x-ray scatter to limit contrast losses and the superior inherent spatial resolution of the diode arrays. Only in situations where rapid framing is required, such as the evaluation of cardiac wall motion abnormalities do the area detector systems have a clear advantage over a line scanning system. The line scanning system has definite advantages for static planar imaging or contrast studies where very rapid framing is not required. 0195-4210/82/OOO0/0932$OO.75 1982 IEEE 932
Prototype Diode Array Imaging System The basic principles of our digital system, as described in more detail elsewhere3 is to use linear arrays of self-scanning, silicon photodiodes which are fiber-optically coupled to an intensifying phosphor screen illuminated by a fan shaped x-ray beam as shown in Figure 1. The fanshaped beam geometry largely eliminates the detection of most of the scattered radiation. Our earliest system coupled a diode array to the screen viaa lens rather than fiber optics. We have determined that the advantages of a series of six one-inch long diode arrays arranged to give a single continuous strip across the width of the field of view by means of fiber-optics are as follows: a. Avoidance of vignetting and lightscattering. b. Compact, flat geometry and low weight becomes possible. c. High light collection efficiency increasing the signal-to-noise ratios. d. The full theoretical spatial resolution of the system is feasible since imaging can be done on a one-to-one basis. Our prototype unit was fitted into an existing diagnostic x-ray room: a. The x-ray beam is collimated to provide a fan-shaped beam. b. The imaging system is positioned under the table so that the fan-beam is coplanar with the detector arrays. c. The patient is transported through the fan-beam by means of a table drive system in order to take a scan in 0.5 seconds with an exposure time of 8 milliseconds per line. By slowly accelerating the table-top over a distance of 4 inches, a speed of 12 inches per second can be attained with no discomfort to the patient since the acceleration is only 1/20 G. A more practical method which will most likely be adopted for clinical system which will transport the source and detector rather than the patient. It is important to note that the small body or organ motion during a scan does not necessarily result in motion blurring because each line of the image represents a very short exposure time. The associated electronic system is designed for six self-scanning linear photodiode arrays. In the present configuration, the diodes in each linear array are scanned within the array and the output is summed in groups of six in the first preamplification stage. Signals from the preamplifier are then fed to intermediate amplifiers located on the preamplifier board and to video signal processing boards located in a separate chassis through short cables. Each analog signal is digitized in the analog-to-digital converter (A/D) mounted in the diode control unit and the digitized outputs are packed and transmitted using driver circuits to the computer facility located on the floor above. The data is transmitted as 16 bit half words and prior to each group a ready enable pulse is sent to the computer. The final image consisted of 1024 picture elements per row for 256 rows wich each row being transmitted every 2 milliseconds to yield an acquisition time of 0.5 seconds per image. An additional single diode detector system is used to monitor fluctuation in the incident x-ray intensity and is used to correct each picture element for these variations. The monitor system consists of an aluminum filter, a phosphor screen and a single diode. The output of this diode feeds a preamplifier circuit which then leads to the video processor circuitry. The output of the video processor is routed to the same A/D circuitry as all other outputs. This signal is sampled at an interval of 12 microseconds and is digitized to 8 bits. The digitized image data is transmitted via closed circuit cables to the Biomedical Image Processing Unit of the Department of Radiation Health located a few hundred feet away from the clinical examination area. The central processor is a Perkin-Elmer 3230 unit equipped with 1.5 megabytes of memory and extensive peripheral devices. The electronic radiography system is connected to one of two selector channels feeding into the memory. Also attached to the same line is an 80 megabyte disc unit, an 800/1600 characters per inch magnetic tape drive and a logical transform array image processor. The second selector channel provides access to a 300 megabyte disc unit, 800 CPI magnetic tape unit, and a random access memory (RAMTEK) display. The available peripheral equipment includes dual floppy discs, a digitizer, a printer/plotter, a line printer, card reader, paper tape unit, and a number of CRT and graphics terminals, permitting rapid display, storage and image enhancement. As part of the interface of diode array electronics to our computer, a software driver was written to permit INPUT/OUTPUT transfers between the remote electronics and computer memory. The drive is an excessive module of the operating system which permits a user access to peripheral devices, and the ability to perform INPUT/OUTPUT to different devices without requiring device dependent coding. The tasks performed by the driver for the CER electronics include 1) transfer of command signals, 2) initializing the direct memory access channel for high speed image transfer to memory and 3) detecting abnormal status conditions upon termination of data transfer. A high-speed interface between the CER electronics and the Perkin-Elmer computer was constructed. This interface permits transfer of data between a private bus for direct memory access and remote electronics. The interface accepts 16 data lines and three status lines from the CER system, and provides two status lines to 933
the CER system. The status lines include BUSY, END OF TRANSFER, DEVICE UNAVAILABLE, ERROR DETECT, and CHANNEL READY. At present the maximum transfer rate is 2 megabytes per second which is equivalent to the transfer of a 1024 by 256 image in 0.2 seconds. This matrix is being increased to 1024 x 1024. One of the features of the method is its ability to acquire serial images each requiring 0.5 seconds a few seconds apart. Future systems can be built to decrease the interval between images to an even shorter time. A technique for accurately timing the arrival of the bolus of contrast material at the organ of interest has been developed in which the patient is kept stationary and the passage of a small bolus of iodine is recorded to determine the transit time. Preliminary clinical studies have shown that this technique greatly assists in obtaining successful clinical studies on digital intravenous angiography. Using the 6 inch wide fiber-optically coupled system with 1024 pixels across the field, we measured a limiting spatial resolution in the horizontal direction at 3.6 lp/mm while for a 1536 pixels, the limiting resolution of 4.2 lp/mm. The most recent phantom studies of contrast sensitivity showed that the system is able to visualize iodinated contrast diluted to 0.56 mg/cm of iodine with simulated vessels only 1 mm in diameter through 4" of Lucite corresponding to a contrast of only 0.2%. For the conditions used during most of our clinical studies, 80 KVp, 640 ma, the entrance exposure for a 0.5 second scan was measured at 100 mr per image. For some of the images of the chest in adults, we were able to reduce the x-ray technique and exposure. We feel that still further reduction should be possible in the future. Clinical Results One of the major potential applications of digital radiography is in the examination of the chest. Here, the wide dynamic range together with the ability to vary window width and level has proved to be of particular value. A single exposure permitted the demonstration of areas of widely differing density, such as the lung parenchyma, mediastinum, ribs and thoracic spine. (See Figure 2). series is chosen as a mask, and the difference between the mask and other images is digitally calculated and displayed. Figure 3 shofws the results of a study of a 71 year old male following recent femoral-popliteal graft surgery. 45 cc of 76% M.D. was injected at 15 cc/second for 3 seconds into his superior vena cava. Conclusion Initial animal and clinical studies indicated that there exists the potential for the replacement of film by self-scanning linear diode array systems of wide dynamic range and high spatial resolution in a number of radiographic examinations. Widespread utilization of digital electronic imaging techniques should not only improve the diagnostic information produced by radiological procedures, but may increase the efficiency and information management capability of radiology departments. References 1. Sashin, D., Sternglass, E.J., "Radiography Apparatus," U.S. Patent 4,179,100, Filed August 1, 1977, Issued December 18, 1979. 2. Sashin, D., Sternglass, E.J., Spisak, M.J., Boyer, J., Bron, K., Davis, L., Fong, C.B., Herron, J., Hoy, R., Kennedy, W., Li, C.C., Thompson,, J., Preston, K., "Computer Electronic Radiography for Early Detection of Vascular Disease," Application of'optical Instrumentation in Medicine VII, Society of Photo-Optical Instrumentation Engineers, Vol. 173, pp. 88-97, 1979. 3. Sashin, D., Bron, K.M., Slasky, B.S., Sternglass, E.J., Gur, D., Kennedy, W.H., Herron, J.M., Spisak, M.M., "Computerized Electronic Radiography", in Digital Subtraction Arteriography: An'Application of Computerized Fluoroscopy, C.A. Mistretta, A.B. Crummy, C.A. Strothed', J.F. Sackett (eds.), Chicago, Year Book Medical Publishers, Inc., pp. 117-122, 1982. *Supported in part by contract NHLBI NOI HV 02929. Another important application of diode array digital radiography is digital subtraction angiography in which images are acquired via the less invasive intravenous injection of contrast. Our system currently has the capability of acquiring serial images of the passage of contrast through blood vessels at intervals as short as 4 seconds with each image acquired in 0.5 seconds with an effective exposure time of 8 milliseconds, Superficial and deep vessels have been imaged in a number of patients in order to demonstrate a variety of pathologies. For the subtraction, one of the digital images in the 934
. t^text \ ~~~~~~~~~~~~~~~~OISK ~~~~~MOTION FilLTERS X-R X- ROMY PHOSPHOR SCREEN- DIODE XSLOT COLLIMATOR FIBER OPTICDCOPUTE ROO LIPLI O URF S spee m 12Y s n i n _ sste ~~~~~~~~~ELECTRONICS CRT~~~~~~~~~~0 TR TERMINAL CRT5L^? ~~~~x-ray ROOM < X ~COMIPUTERROOM INTE FACE E LECT tonics. _ (AP UPI 00 MGYTE Figure I. Block diagram of the high speed 1/2 second imaging system Figure II-b. Bone windowing confirms the early involvement of the second posterior rib by the peripherally based lung mass (Pancoast tumor). Figure II-a. Digital radiograph of a 59 year old male of right upper lung. Parenchyma windowing demonstrates mass. Figure III-a. I.V. digital subtraction intravenous arteriogram of left knee after injection of 45 cc of M.D. 76 into the superior vena cava of a 71 year old male following recent femoralpopliteal graft surgery. This early i'mage shows prompt filling of graft with faint opacification of popliteal artery. 935
Figure III-b. This subtractton image taken five seconds after Figure III-a, shows early washout of the graft with good retrograde filling of distal superficial femoral artery from proximal popliteal artery. Note the atherosclerotic irregularity of the superficial femoral artery. 936