Solid-State Detectors for Small-Animal Imaging

Semiconductor detector technology, initially developed for high energy physics applications, has found a distinctive role in high performance systems for X-ray and gamma-ray medical imaging applications, including small animal imaging. Single Photon Emission Computed Tomography (SPECT) small animal imaging requires the development of compact detectors with intrinsically ultrahigh spatial resolution, high energy resolution and good detection efficiency, in addition to suitable radiation collimation strategies. This overall performance can only partly be guaranteed by scintillator based systems with photomultiplier tube readout, the most used technology at present for small animal SPECT scanners. On the other hand, with respect to scintillator based detectors, semiconductor detectors can of-fer a gain by approximately a factor two in energy resolution at typical radionu-clide energies, a factor greater than two in intrinsic spatial resolution, and a com-parable intrinsic detection efficiency, though usually at a reduced field of view. Moreover, their compactness could be crucial in devising animal “personalized” miniature scanners. An additional interesting feature of semiconductor based small animal SPECT scanners is that the detector technology can be used both for gamma-ray imaging and for X-ray imaging, when coupling the SPECT scanner to a low resolution X-ray CT scanner for anatomical registration. The requirement of high spatial resolution, coupled to high sensitivity, becomes also stringent in microPET systems, where semiconductor detectors could be the technology of choice for future high performance PET scanners. This Chapter illustrates the basic technology of pixel and microstrip semicon-ductor detectors for these small animal imaging applications, with reference to the most used technologies, relative to the CdTe, CdZnTe (CZT) and Si semiconduc-tors. Still at its early stage, but of increasing interest, is the technology of semi-conductor detectors – CdTe, CdZnTe, Si room-temperature semiconductors, in addition to liquid nitrogen cooled Ge – specifically for Positron Emission Tomog-raphy (PET) in ultra high resolution small animal scanners. Examples of develop-ment of semiconductor SPECT and SPECT/CT scanners, and of semiconductor PET scanners for small animal imaging, are presented. In illustrating those sys-tems, the emphasis will be on their technical description. In order to illustrate the application of semiconductor detectors to small animal imaging in the area of microSPECT and microPET, a brief outline follows on the physical characteristics of those detectors. Basic terminology, relevant data and basic techniques will be introduced for completeness of illustration.