Digital Radiography Using the Individual Photon Counting Technique

Abstract

In the last ten years the field of medical imaging has been revolutionized by the introduction of new methodologies (CAT, Digital Radiography, NMR, etc...) combined with powerful digital techniques. On the other hand, the field of medical radiation detectors has remained more steady. Almost all of the existing devices are still of the integrating type, which are often hampered by linearity, noise or dynamic range problems. In this respect, a detection system counting individual photons should have several advantages: i) low noise, given by definition by the photon statistics; ii) high linearity and dynamic range limited only by dead-time losses (which can be accounted for). A MultiWire Proportional Chamber (MWPC) is a poissonian, electronic, area detector and for this reason since its introduction it has received great attention for its possible application in the biomedical field. However, to be suitable for medical imaging a MWPC should satisfy the following stringent requirements: 1) good detection efficiency; 2) good spatial resolution; 3) high-rate capability. To satisfy the first requirement one has to use a gas with a high atomic number (Xenon), possibly at high pressure. For the second requirement, the use of a monochromatic X-ray source with energy just above the K-edge of Xenon (=34.6 KeV) can greatly help to achieve a submillimeter spatial resolution. Until a few years ago, the speed of the electronic data acquisition system was the most severe bottleneck for the solution of the third problem. The recent introduction of very fast TDC’s and of Histogramming Memories has greatly simplified the problem, allowing a data capability of more than 1 MHz to be reached in a straightforward way. The modern generation of MWPCs seems to be competi tive with the existing imaging devices, especially when quantitative studies on stationary objects must be performed (i.e. bone or lung densitometry)