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6.1 System Overview Before giving a detailed analysis and description of major components in a CT scanner, this chapter will present a system overview to explain how the different components work together to produce CT images. Figure 6.1 presents a generic block diagram of a CT system. The actual system architecture for different commercial scanners may deviate from this diagram, but the general functionalities of all CT scanners are more or less the same. For a typical CT operation, an operator positions a patient on the CT table and prescribes a scanogram or “scout view.” The purpose of this scan is to determine the patient's anatomical landmarks and the exact location and range of CT scans. In this scan mode, both the x-ray tube and the detector remain stationary while the patient table travels at a constant speed. The scan is similar to a conventional x ray taken either at an A-P position (with the tube located in the 6 or 12 o'clock position) or a lateral position (with the tube located in the 3 or 9 o'clock position). Once such a scan is initiated, an operational control computer instructs the gantry to rotate to the desired orientation as prescribed by the operator. The computer then sends instructions to the patient table, the x-ray generation system, the x-ray detection system, and the image generation system to perform a scan. The table subsequently reaches the starting scan location and maintains a constant speed during the entire scanning process. The high-voltage generator quickly reaches the desired voltage and keeps both the voltage and the current to the x-ray tube at the prescribed level during the scan. The x-ray tube produces x-ray flux, and the x-ray photons are detected by an x-ray detector to produce electrical signals. At the same time, the data acquisition system samples the detector outputs at a uniform sampling rate and converts analog signals to digital signals. The sampled data are then sent to the image generation system for processing. Typically, the system contains high-speed computers and digital signal processing (DSP) chips. The acquired data are preprocessed and enhanced before being sent to the display device for operator viewing and to the data storage device for archiving.
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