A data-acquisition system designed for x-ray medical imaging utilizes a segmented high-purity germanium (HPGe) detector array with 2-mm wide and 6-mm thick elements. The detectors are contained within a liquid-nitrogen cryostat designed to minimize heat losses. The 50-ns pulse-shaping time of the preamplifier electronics is selected as the shortest time constant compatible with the 50-ns charge collection time of the detector. This provides the detection system with the fastest count-rate capabilities and immunity from microphonics, with moderate energy resolution performance. A theoretical analysis of the preamplifier electronics shows that its noise performance is limited primarily by its input capacitance, and is independent of detector leakage current up to approximately 100 nA. The system experimentally demonstrates count rates exceeding 1 million counts per second per element with an energy resolution of 7 keV for the 60-keV gamma ray photon from 241Am. The results demonstrate the performance of a data acquisition system utilizing HPGe detector systems which would be suitable for dual-energy imaging as well as systems offering simultaneous x-ray transmission and radionuclide emission imaging.
Schlüsselwörter: Quantitative Computertomographie, Knochendensitometrie, Osteoporose Es wird ein Computertomographie-System vorgestellt, welches speziell für Knochenuntersuchungen an den Extremitäten des Menschen entwickelt wurde. Um eine risikoarme Quantifizierung kleinster Knochendichteveränderungen zu erreichen, standen bei der Entwicklung vor allem kleine Strahlenbelastung und hohe Genauigkeit im Vordergrund. Unter diesen Aspekten werden die verschiedenen Komponenten des Systems behandelt. Das System wird von einem Mikrocomputer gesteuert, der einerseits die Interaktivität zwischen Benutzer und System gewährleistet und andererseits eine hohe Flexibilität zur Anpassung der Systemparameter an die gegebenen Meßverhältnisse erlaubt. Die rechenintensive Bildrekonstruktion erfolgt auf schnellen Slave-Prozessoren, so daß die Computertomogramme on-line berechnet werden können und unmittelbar nach der Messung zur Verfügung stehen. Anhand von Beispielen wird eine Übersicht über den Einsatzbereich dieses spezialisierten Computertomographie-Systems gegeben. Die für Verlaufsuntersuchungen entscheidende Reproduzierbarkeit wurde zu l °/oo des linearen Attenuationskoeffizienten bestimmt, bei einer Strahlenbelastung von nur 10 mrem.Key-words: Quantitative computed tomography, bone densitometry, osteoporosis We present a computed tomography System which has been specially developed to determine the bone density of human extremities. High precision and low radiation dose were the primary goals of the development. It is under these aspects that we shall describe the System, including mechanical construction, x-ray generation and detection, data-acquisition and processing, and evaluation of the CT-images. The system is based on a second-generation translation-/rotation Scanner. It is controlled by a microcomputer, which on the one hand ensures interactivity between user and system, and on the other permits high flexibility in the adjustment of system parameters to the given measuring conditions. Fast slave processors permit on-line image reconstruction, and thus the immediate availability of the measurement. The computed CT-images are stored in a digital display memory and visualized on a commercial TV monitor. The quantitative evaluation of trabecular and cortical bone is done off-line after completing the investigation. The Utility of the system for evaluating peripheral bone is documented by means of examples. Most important for longitudinal examinations is the high reproducibility of 0.1 % of the mean linear attenuation coefficient, achieved at a radiation dose of 10 mrem.
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