224 · BIOMEDICAL SENSORS is required. In order to reduce the image contrast degradation effects of scattered radiation, an energy resolution of 10% is preferred. Detector selection. An energy resolution of 10% opens up the detector choice to most semicon- ductor detectors, some high-light-yield scintillators, and even some gas detectors. The need to image dynamically imposes a strict demand on high detection efficiency, which at 140 keV effectively elimi- nates gas detectors and low-Z materials such as silicon. The imaging requirement of 4 mm × 4 mm is not overly demanding, but will cause some degradation of energy resolution and restrict scintillator choice to those with the highest light yields, such as NaI(Tl) or CsI(Tl). Thus the choices are the high-Z, high-light-yield scintillators and high-Z semiconductors, all of which are capable of meeting the required specifications. In this situation it would be wise to consider the practical issues, which are usually dominated by cost. The most cost-effective solution would be a scintillator backed by photomultiplier tubes in an Anger gamma camera arrangement. However, if a key practical consideration is compactness, then a room-temperature semiconductor such as CdZnTe may be a good solution. ACKNOWLEDGEMENT The authors are grateful to Dr. Beate Planskoy for her valuable comments and fruitful discussions on the theoretical and technical aspects of dosimetry.