How does the Computed Tomography scan work?

The CT scan procedure uses a computer and a CT scanner containing a rotating x-ray device to create cross-sectional images of the body.

The CT scanner is a large, square machine with a doughnut-like hole in the center. The CT scanner contains a gantry: a rotating frame that contains the x-ray tubes, x-ray detectors, and a large opening into which the patient can be inserted. The patient lies still on a table that can be moved up or down, and can be placed into and out of the center of the hole.

During a CT scan, the patient lies still on a table. The table is rolled into or through the center of the hole in the scanner. As the CT scan progresses, the gantry rotates, and the x-ray tube moves around the patient's body to produce the required images. Meanwhile, the x-ray detectors also rotate around the patient, at all times opposite to the x-ray tubes.

X-rays are rays of electromagnetic radiation. In a CT scan, several x-ray beams pass through the body part being examined at different angles. As the beams pass through, different tissues absorb different amounts of radiation. Special sensors measure the amount of radiation absorbed by different tissues.

The scanner parts rotate around the patient and emit and record x-ray beams. One rotation takes about one second. The x-ray tube emits a fan-like beam of x-rays onto the patient. These beams can be anywhere from 1 milimetre to 10 milimetres thick.

Meanwhile, detectors on one side of the patient record the x-rays from the section of the patient's body being examined. Each x-ray leaves the patients body as an x-ray "snapshot", showing one position (angle). Up to 1,000 different "snapshots" (angles) are collected during one complete rotation.

The data is then sent to a special computer, which uses the “snapshots” to form three-dimensional cross-sectional images of the scanned areas. These cross-sections are called tomograms.

An x-ray technologist will monitor the CT scan from behind a screen or through video equipment from another room, though he or she will not actually be in the room with the patient. An intercom within the scanner allows the patient and technologist to communicate throughout the procedure.

Patients should expect to hear whirring sounds throughout the CT scan; this is the sound of the gantry and x-ray tube rotating. At times, the technologist may ask the patient to hold his or her breath to prevent the CT images from blurring.

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