ABSTRACT
This chapter discusses the key properties of photoconductive materials for high-performance direct conversion detectors for medical imaging applications. It presents two commercially available direct conversion photoconductors, amorphous selenium (a-Se) for large area x-ray imaging and cadmium zinc telluride (CZT) for high-resolution positron emission tomography (PET). Direct conversion detector technology differs from that of indirect conversion by the conversion of incident radiation to charge, which is collected with the aid of an applied bias that drifts the generated charge toward the electrodes. The ultimate choice of conversion material depends on the application, since different radiation energies are used for different imaging modalities, and a material's probability to absorb photons is dependent on factors such as atomic number. For x-ray imaging, the modulation transfer function (MTF) can be used for characterizing the spatial resolution of the system and the detective quantum efficiency (DQE) can be used to quantify the signal quality in terms of signal-to-noise ratio (SNR).
