Medical CT

What is a CAT scanner and what does CAT stand for?

"CAT" stands for "computed axial tomography" and often the "axial" is dropped, leaving CT to define this particular imaging modality. This technique produces a series of images at various depths in the body (slices). This is in contrast to conventional x rays in which all the structures of the body throughout the depth are superimposed on each other in a single film.

To make the image, the patient is placed in a donut-shaped gantry. An x-ray tube is rotated around the patient and the patient is exposed to a narrow beam of x rays. A ring of detectors measures the radiation transmitted through the patient. (Radiation is transmitted differently through different structures, such as the various organs and bones, in the body.) This transmitted data is then used to construct images of body "slices." CT is better for imaging certain structures since, as mentioned, it eliminates the superimposition of organs and bones in the body. For example, if a patient has lung nodules, you can see them in a plain film (like a chest x ray), but you can't tell how deep they are or what their exact structure is. By looking at a CT "slice" of the lung and processing the data to enhance the nodules, their location and size can better be defines.

Many of your answers state that the effective dose of an abdominal CT scan is 10 mSv. However, yesterday morning I spoke with the radiation safety officer at a local university who claims that the range for the same procedure with contrast is only 1-3 mSv and that the abdominal-pelvic CT, again with contrast, is only 4-6 mSv. Can you explain the discrepancy?

Doses received by patients for any diagnostic exam will vary depending on the type of equipment, settings of equipment (for example, kVp, mAs, etc.) number of pictures, with or without contrast, patient size, and some other miscellaneous items. According to Wall and Hart (1997), abdominal or pelvic CT results in an effective dose of about 10 mSv and can vary from 3 to 14 mSv. The numbers used on the Health Physics Society Ask the Experts section of the website are those printed in the literature and not based on a single organization.

Wall BF, Hart D. Revised radiation doses for typical x-ray examinations. Report on a recent review of doses to patients from medical x-ray examinations in the UK by NRPB. National Radiological Protection Board. British Journal of Radiology 70(833): 437-9; May 1997.

I recently had a cervical CT scan done. I have been feeling sick since then. I am concerned about the dose of radiation I received so have tried calling around here locally but have no idea how to investigate this or resolve this. The facility where the CT was taken indicates that patients do not get exposed to very much radiation during a CT scan. What can I do to determine my dose?
If you want actual radiation dose numbers from the facility where the CT scan was performed, you can request a dose estimate from the facility's radiologist or medical physicist. Even if the facility does not have a radiologist or medical physicist on-site, it must have qualified staff available to read the x-ray films and calibrate the x-ray equipment. There is nothing in the literature, however, to suggest that the amount of radiation from a standard diagnostic CT scan would cause an increase in your body temperature or make you feel sick. Radiation doses to patients from standard diagnostic exams are well below the doses necessary to cause biological effects.
Is there a way to find out how many CT scan slices were used after the scan is done? Is the dose information stored in a computer?
Information about x-ray procedures is recorded on an x-ray request form and included in the radiologist's dictation of the study results. This would include the number of slices and the scanning parameters (from which dose can be calculated).
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