Computed Tomography

Computed Tomography is probably the most common source of 3 dimensional data. CT scanners are relatively inexpensive, and most hospitals have at least one scanner. CT uses an X-ray radiation source to image the patient. The CT scanner consists of a couch upon which the patient is placed and a circular gantry through which the couch with patient is passed. Within the gantry is a rotating ring with an X-ray source opposed to a linear array of detectors. The X-ray source is collimated so that the X-rays form a flat fan beam with a thickness determined by the user. During the acquisition of a "slice" of data, the source-detector ring is rotated around the patient. The raw output from the detector array is back projected to form an image of the slice of the body. The couch is moved and then another slice is obtained.

The output from a CT scanner is a series of transaxial slices of the patient. Each slice represents a slab of the patients' body with a thickness set by the collimation for the slice (typically 1-10mm). For most CT scanners each slab has 512 by 512 pixels. The size of a pixel can be varied within certain limits (generally 0.5 to 2 mm). Generally each slice is spaced such that they are either overlapping or contiguous, though some protocols call for gaps between the slices. Each pixel ideally represents the absorption characteristics of the small volume within its bounds. This is measured in Hounsfield units (HU).

The Hounsfield unit scale is calibrated upon the attenuation coefficient for water and air, with water reading 0 HU and air -1000 HU. Fat tissue will be in the range of -300 to -100 HU, muscle tissue 10-70 HU and bone above 200 HU. Two methods of patient preparation are used to enhance the radiologists ability to interpret the scans. Before the scan, the patient can be fed a radio-opaque solution termed "oral contrast" that causes the stomach and bowels to have increased attenuation (above 100 HU). To enhance the vascular anatomy, an intravenous injection of a radio-opaque solution can be made prior to or during the scan. This causes parts of the vascular system to have increased attenuation (above 100 HU). This is called "contrast enhanced CT".

Modern CT scanners can generally acquire one slice within 1 to 5 seconds. An entire study of a patient generally represents 30-40 slices, with a study time of 3-15 minutes. The radiation dose from a CT scan is comparable with that of a series of traditional X-rays.

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