1. How is the X-ray beam used in CT scanning?
Answer In CT scanning a beam of X-rays is used to image sections of the body. The X-rays fall onto detectors rather than onto film, as is the case with radiographs. The X-ray tube rotates around the patient. This allows multiple data to be collected concerning all of the tissues in the section being imaged. A computer records the data and two- and three-dimensional images can be generated.
2. How is the computer used to form CT images?
Answer The X-rays that penetrate the patient strike iodide crystals within the detectors. These crystals emit photons of light, which are detected by a photomultiplier. This converts the light into electrical pulses. The number of pulses is directly controlled by how much of the X-ray hits the crystals, and thus how radio opaque (dense) the subject is. The electrical pulses are recorded as digital information and converted into images by a computer.
3. What are the advantages of CT scans?
Answer Useful in showing bony lesions. More useful than MRI for investigating cortical bone fractures and calcification of organs. Good contrast between different tissues. Good contrast is seen between tissues which are mainly bone, fat, water, and air. Use of a narrow X-ray beam and windowing can produce detailed images. However CT cannot differentiate well between different parts of the same organ. Useful in imaging metallic foreign body. MRI is unsuitable in metallic foreign body as it can move the object causing dislocation or tissue damage. Rapid imaging. Modern machines can produce images in a matter of a few seconds, depending on the type of scan where as MRI scan takes much longer to complete.
4. What are the main disadvantages of CT scans?
AnswerHigh ionizing dose. Bony artefacts. Brain scans may be distorted by bony artefacts Very small lesions may be missed. The wavelength of the X-ray beam means that lesions under 1cm may be missed Limited contrast. CT cannot differentiate between tissues of very similar density or between areas of inconsistancy within an organ.
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