CT SCAN : Historical Development

First-generation scanners

   With the earliest scanners, each line integral was collected using an x-ray tube and a single detector. During the scanning procedure, the x-ray tube detector were translated across the scan field of view, and a series of transmitted intensity measurements were made. The tube and detector combination were rotated through a small angle(1 degree)  and the translation repeated. This process of translation and rotation was repeated until 180 degree of projection data had been acquired. It took 41/2 minutes to complete a scan.

Second-generation scanners

    A significant reduction in scan time was obtained by using multiple detectors. These were placed opposite the tube so that the beas sensed by the detectors differed in angle by a small amount. These scanners still required translation but each translation produced many views, one from each detector position. The rotation angle between translation was increased. With these systems, scan times were of the order of 20 to 80 seconds.

Third-generation scanners

    To decrease scan times further, rotate-only systems have been introduced. In these systems a fan beam x-ray source irradiates a large of detectors. For third-generation systems, both the tube and detector array are rigidly coupled and rotate jointly about the patient. At any instant of time a complete fan or view of data is acquired.

Forth-generation scanners

    These are also called rotate-stationary systems in which only the tube rotates about the patient. A fixed detector array completely surrounds the patient. The fan beam projections are formed by grouping all the mesurements made by a single detector. Scan times of 1 to 10 seconds with these rotate only systems.