PERCEPTION OF DETAIL AND GREYSCALE RANGE IN X-RAY FLUOROSCOPY IMAGES CAPTURED WITH A PERSONAL-COMPUTER AND FRAME-GRABBER

Objective: To assess the dynamic range of radiological images captured with a personal computer frame-grabbing system coupled to an X-ray fl uoroscopy machine. Methods: A 386DX-40 MHz, IBM compatible, computer w ith an SVGA monochrome graphics subsystem and a 387 co-processor, inst alled with a Screen Machine frame-grabber and controlled by a program specially written was used. Various systems were examined and the obse rver's perceptions of the results assessed. Results: The dynamic range available to an ordinary X-ray fluoroscopy system was found to be res tricted to about 750 mV. Similar measurements showed that the dynamic range was always restricted to 3/4-1/2 of the full available signal be cause of a high value of the dark voltage of the TV camera's target on all seven systems measured. The dynamic range of the computer - frame grabber system was found to be significantly wider than the Image Int ensifier - TV camera chain but, surprisingly, it was affected by the t ype of file format used for image storing on disk. Clinical images fro m a barium meal examination as well as CT images captured after optimi sation of the frame-grabber were found to contain large quantities of noise in the first two least significant bit planes making them redund ant and limiting the grey levels needed for image display to less than 64. This number was also less than the 80 grey levels that could be d iscriminated by the human eye on the computer monitor. Conclusions: It was concluded that 6 bit digitisation would have been sufficient for image capture. The advantages of the wider dynamic range of the frame- grabber and the processing capabilities of the computer were tested fo r the possibility of improving the perception of detail. However, the results were negative. The limiting spatial resolution measured with a variable density bar pattern at all magnifications was about 0.4 lp/m m lower from that measured directly on the fluoroscopic screen. A deta il perception test had the same result. The perception success was sig nificantly lower with the digital images at all but the highest of the exposure rates and despite the use of image processing filters.