System Overview of Major CT Scan

Before a detailed analysis and description of the mayor components in CT Scanner, it beneficial to present a system overviews in terms of how different component work together   to produce CT system in this figure.

It can be argued that the actual system architecture for different commercial scanners may deviate somewhat from the diagram; the general functions of these 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 views. The purpose of this scan is to determine the anatomical landmarks of the patients and determine the exact location and range of CT Scans. In the scan mode, both the x-ray tube abd the detector remain stationary while the table travels at a constant  speed. The scan is similar to aconventional x-ray taken at either an anterior –posterior postion (tube at 12 o clock) or lateral position(tube at 3 or 9 oclock) once such as a scan is initiated , an operational control computer instruct the gantry to rotate to the desired orientation as prescribed by operator. The computer then send instructions to patients table, the x-ray generation system , the xx ray detection system aand the images generation system to perform the scan. The table subquently reaches the starting scan location and maintains a constant speed during the entire scan process . The hight –voltage  generator quickly reaches 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 tubes produces x-ray flux and the x-ray photon are detected by an x ray detector to pruduce electrical sugnals. At the same time the data acquitions system samples the dectector outputs at a uniform sampling rate and converts analog signals to digitals signals. The samples data are then to sent to the image generations system for processing. Typically, the system contains high sspeed computers and DSP(digital signal processing) chips. The acquired data are preprocessed and enhanced before they are sent to the display device for operator viewing and to the data storage device archiving.

Once the precise location and the range determined( based on scanogram image), the operator prescribed CT Scans based on either predetermined sets of protocols or newly crated protocols. These protocols determine the collimator aperture, detector aperture, x-ray voltage and current, scan mode, table index speed. Gantry speed, reconstruction field of view(FOV), kernel and may aperture other parameters. With the selected scanning protocol, the operational control computer send a series of commands to gantry, the x-ray generations system, the table, the xray detection system, and the image generation systems in a manner similar to what we outlined for the scanogram operation. The major difference is that the x ray gantry is no longer stationary. It needs to reach and maintain a constant rotational speed during the entire operation. Since a CT Gantry typically weighs over several hundred pounds, it takes time for the gantry to reach stability. There fore the gantry is typically one of the first components to respond to scan command. All other operating  sequences are similar to the ones described for the scanogram operations.

It should be pointed out that in many clinical applications, the operational sequence may deviate from one described above. For example , in intervention procedures, x ray generation may triggered by a foot paddle rather that the keyboard on operator’s console . In contrast enhanced CT Scans, the injection of contrast agent must be synchronized with the scan, this may require the integration of power injector with the CT scan protocols. In other operations, generated x-ray images are sent to filming devices directly to produce hard copies. These deviations, however , should not affect our general understanding of the CT operation mechanism.