CT stands for “computerized tomography” — it consists of many x-rays taken from various angles that are assembled by computer to be able to visualize internal organs. It used to be called a CAT Scan “computerized axial tomography,” with “axial” meaning up & down a main axis. Sometimes dye is injected as well, in order to make a variety of diagnoses.
There are different ways of doing CT scans. They can be done plain, or with contrast (injecting dye). Contrast is usually needed to find infections, and sometimes certain cancers, but requires two scans, a plain one before the dye and then the next. CT-angiograms also inject dye, but they’re timed to capture blood flow through a specific organ, depending on the disease being sought. There are also “PET-CT Scans” which adds nuclear medicine tracers to find cancer metastases, and many more sophisticated CT examinations.
The advantage of CT scanning is that it can identify much more than other imaging like simple x-rays and ultrasounds. It’s cheaper than the MRI, and can be done much more quickly; so quick, in fact, that the scan itself takes less time than clerical tasks like entering patient information (some serious delays have occurred due to the latter). But the MRI is often preferable, depending on what we’re looking for, & to avoid risks of CT scans.
The main risk of CT’s is radiation. One CT scan equals 2-3 times the amount of natural radiation we receive in a year from the sun. As exposure to radiation increases, so does the risk of cancer; risks are greatest for younger persons, since cancer takes many years to develop. See Radiation Risks of X-rays.
Another risk is from the dye that’s injected. Serious allergic reactions are possible. Today, most CT scans use “low-osmolal contrast material” for injection; this is safer for patients with asthma and those who take drugs like beta-blockers and NSAIDs pain medicines, so such patients should inquire to be sure that’s what’s being injected. Dye can also cause kidney damage, which is almost always mild & gets better quickly, but can be serious & permanent if the kidneys weren’t working well to begin with.
Finally, as for all tests, there’s a risk of happening to find abnormalities we weren’t looking for, and which may not mean anything (medical term is “incidentaloma”). But since it’s hard to know for sure, we wind up doing more tests just for those, like biopsies (with their own risks), or more CTs (with more radiation). The results usually turn out to be negative.