CT Scan (“Cat Scan”)

J. Taylor Whaley, MD
Last Modified: June 5, 2016

What is a CT scan?

  • A CT scan is a non-invasive radiology test that creates pictures of internal organs using x-ray technology. CT scans were first used in the 1970s. With the revolution in computer technology, CT scans have evolved dramatically. The most critical changes include improved resolution, allowing radiologists to see very small differences (on the order of millimeters), as well as quicker exams, dramatically improving patient comfort.
  • The implementation of CT scans in medicine has revolutionized health care, allowing healthcare professionals to better evaluate patients quickly, using CT scans to support their physical exam findings and labwork results. It is very important to understand, however, that without talking to the patient and knowing the medical history, CT scans can be very difficult to interpret.
  • As CT scans have become critical in the proper evaluation of many health concerns, CT scanners are now widely available throughout the U.S. and many countries worldwide.
  • A CT scan takes pictures in a cross sectional manner, meaning the pictures are "slices" of the body from the top of the area of interest to the bottom - as if you were slicing the body like a loaf of bread. These slices allow the radiologist to look at organs, tissues and bones in small sections or combine them to create 3D images.

What is this test used for?

  • Because any part of the body can be evaluated with a CT scan quickly, it can be used to evaluate emergent conditions, such as a CT of the head to check for bleeding on the brain, or as part of non-emergent conditions, such as a staging evaluation to determine the extent of cancer.
  • CT scans are often used as an initial evaluation for cancer to see how large the primary tumor may be and if the cancer has spread beyond the primary location. Following cancer treatments, CT scans are often used to evaluate how the cancer has responded to treatments.
  • Some cancers are better evaluated by MRI (magnetic resonance imaging), and which test is preferred will be determined by your team.

How is this test performed?

  • CT scans can be performed as an inpatient or outpatient procedure. The test generally involves lying flat for a few minutes on a flat table. The scan will focus on the specific part of the body that the provider wants to assess.
  • A CT scan involves the use of X-rays that are sent through the patient, painlessly, and hit a detector behind the patient. As the X-rays pass through the patient, some are deflected by bones and soft tissue. The X-rays create an image based on how many x-rays reach the detector. The computer then processes this to create a slice-by-slice figure of the patient, similar to a loaf of bread that has been sliced. A radiologist, which is a doctor who specializes in looking at different types of images of patients, evaluates the scan and generates a report.
  • CT scans have only a large, thin donut that the patient quickly passes through while lying on the table. Many people worry about the possibility of being in a tube during the exam; however, this does not generally pose a problem with a CT scan, as the "donut" area is very thin. Additionally, as technology has improved, CT scans have become very quick to obtain. Scans can be performed in as little as 15-45 minutes, making the exam much more comfortable for patients.
  • It is very critical to not move during the actual exam, as it can make the results blurry (just like taking a picture with a camera). You may be asked to hold your breath at certain points in the exam to prevent movement.
  • Finally, because the CT scan involves a very small amount of radiation, no one else can be in the room. During the exam, a radiation technologist will be in the adjoining room with a window and microphone to see and hear you should you need help.
  • Sometimes, if the doctor wants the blood vessels and lymph nodes to be evaluated, IV contrast is given. IV contrast is a fluid that is injected in the vein during the exam. Although it does not hurt, some patients do feel warm or flushed following the injection, and this is normal. Some patients have allergic reactions to contrast. If you have ever had a reaction, you should tell your doctor prior to the exam, and a short course of steroids and Benadryl can be given to prevent any reaction.
  • There is a small amount of radiation given during the CT scan. This radiation is very small and generally insignificant; however, because of this, CT scans should only be obtained when your healthcare provider deems it necessary.

Picture of a CT Scanner
Picture of a CT Scanner

How do I prepare for a CT scan?

  • Preparing for a CT scan is easy, but it can vary based on what is being imaged, as well as based on whether or not contrast is being used.
  • If IV contrast is being used during the exam, you will be asked to fast for 4 hours prior to the test. This is because the IV contrast can cause a reaction or upset stomach.
  • If the scan will involve looking at the stomach or intestines, oral contrast is frequently given to differentiate the bowels from other important structures. In rare cases, the contrast may be administered through an enema.

How do I interpret the results of a CT scan report?

  • The results of the test depend on which part of the body was imaged. Following the scan, the images are processed by a computer and read by a radiologist. The radiologist then generates a report for the medical professional responsible for ordering the CT scan.
  • The report generally states the patient's name, date of birth, and indication (reason for the CT scan) at the top of the report. Radiology reports follow a standard outline, regardless where they are obtained. Radiologists report both normal and abnormal findings in a very systematic approach. For this reason, it is very important to discuss the results with your doctor.
    • The first paragraph typically includes the specific technical information involved in obtaining the scan (i.e. whether contrast was given, what the exam involves, etc).
    • The middle paragraphs generally begin the description of findings, both normal and abnormal. Because reports are generated for other medical professionals, the terminology is often medically oriented and can be difficult to interpret.
    • Following the detailed interpretation above, an impression generally follows. This is a summary of the findings, often generated to answer the question posed by the ordering physician.
    • The radiologist may compare these findings to any previous scans of that body site, if the images are accessible.


Click on any of these terms for more related articles