HIGH-SPATIAL-RESOLUTION MEDICAL-IMAGING SYSTEM USING A HARPICON CAMERA COUPLED WITH A FLUORESCENT SCREEN

A high-sensitivity HARPICON(TM) camera was developed for medical X-ray imaging using a fluorescent screen. It is an avalanche-multiplication -type image pick-up tube and is 32 times more sensitive than conventio nal tubes. The camera also has a wider dynamic range than conventional medical-imaging cameras because a maximum output signal current of 2. 3 mu A is obtained and, in high-illumination-intensity regions, photoc urrent is not proportional to illumination intensity. The fluorescent screen is an intensifying screen of the type used for radiographic scr een-film combinations in medical examination. An X-ray image on the sc reen is focused on the photoconductive layer of the pick-up tube using a coupling lens with f/0.65. Experiments were performed using monochr omatized X-rays at the Photon Factory. An image of a spatial resolutio n test chart was taken in a 525 scanning-line mode of the camera. The chart pattern of 5 line-pairs mm(-1) (spatial resolution of 100 mu m) was observed at an X-ray input field of 50 x 50 mm. Real-time digital images of the heart of a 12 kg dog were obtained at a frame rate of 60 images s(-1) after injection of a contrast medium into an artery. The images were stored in digital format at 512 x 480 pixels with 12 bits pixel(-1). High-spatial-resolution and high-contrast images of corona ry arteries were obtained in aortography using X-rays with energy abov e that of the iodine K edge; the image quality was comparable with tha t of conventional selective coronary angiography.