Simplest form of a CT scanner consists of a radiographic tube that emits a finely collimated, fan-shaped X-ray beam directed to a series of scintillation detectors or ionization chambers.
Depending on the scanners mechanical geometry, both the radiographic tube & detectors may rotate synchronously about the patient, or the detectors may form a continuous ring about the patient & the X-ray tube may move in a circle within the detector ring.
Depending on the type of movement for image acquisition scanners can be:
Incremental scanners: Final image set consists of a series of contiguous or overlapping axial images.
Spiral or helical CT: Gantry containing the x-ray tube and detectors revolves around the patient, the table on which the patient is laying continuously advances through the gantry. This results in the acquisition of a continuous spiral of data as the x-ray beam moves down the patient.
It is reported that, compared with incremental CT scanners, spiral scanners provide:
improved multiplanar image reconstructions,
reduced examination time (12 seconds versus 5 minutes), and a reduced radiation dose (up to 75%).
CT completely eliminates the superimposition of images of structures outside the area of interest.
Because of inherent high contrast resolution, differences between tissues that differ in physical density by less than 1% can be distinguished.
Data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.

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