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About CT
CT FAQ
Preparing for a CT Exam


What is the difference between CT and MRI?

CT and MRI scans are both diagnostic tests that provide us with high-resolution pictures of the structure of any organ or area of the body. Both tests use computers to construct pictures of the inside of the body. There are, however, inherent differences in these tests' mechanisms, as well as sophistication and applications.

CT scans were developed in the early 70's. CT or CAT stands for "computerized axial tomography." Basically, CT scans consist of a highly sensitive X-Ray beam that is focused on a specific plane of the body. As this beam passes through the body, it is picked up by a detector, which feeds the information it receives into a computer. The number of "slices" refers to the number of detectors within the CT unit. The higher the number of "slices" the better the speed and image resolution. The computer then analyzes the information on the basis of tissue density. This analyzed data is then fed into a cathode ray tube (the device responsible for producing pictures on your TV screen) and a picture of the X-Rayed, cross-section of the body is produced. Bone shows up as white; gases and liquids as black; and, tissue as varying shades of gray, depending on its density.

Magnetic Resonance Imaging (MRI) began being used in the late 1980s. An MRI machine uses computer-controlled radio waves and very big magnets, which create a magnetic field roughly 25,000 times stronger than the earth's magnetic field. After the machine creates a magnetic field, it sends radio waves into the body and then measures the response of its cells (how much energy they release) with a computer. From these responses, the computer is able to create a three-dimensional picture of the inside of the body. MRI makes use of the fact that all living cells have a certain magnetic quality to them; because of this, MRI can provide a look at the biochemistry of living cells.

What information do CT and MRI scans provide?

Both tests provide detailed pictures of areas of the body that used to be inaccessible by conventional X-Rays. The development of non-invasive imaging techniques reduces invasive, exploratory surgeries and allows detailed views of the anatomic structures in normal and diseased states. CT scans give us excellent information on anatomical (bony) features and tissue density that allows for the detection of tumors, and sometimes the ability to distinguish between malignant and benign tumors. CT scans are useful in sensitive detection of calcification (calcium deposits), cysts, abscesses, and hemorrhaging. CT is also widely used due to short imaging times, widespread availability, and ease of access.

Are their any health risks assocated with CT or MRI?

CT scanning does carry with it the risks associated with X-Ray exposure. CT procedures are typically estimated at 1 to 10 mSV, far below cancer inducing levels (200 mSV). MRI has no known associated health risks. However, people with pacemakers, aneurysm clips, or other implants that contain magnetic materials may not be candidates for MRI testing. MRI offers superior soft-tissue contrast, excellent visualization of vascular structures, fewer artifacts, and imaging in any plan. MRI is best put to use in examining the central nervous system (the brain and spinal cord). It can also be used to identify tumors, strokes, degenerative diseases, inflammation, infection, and other abnormalities in organs and soft tissue of the body. MRI scans require the patient to be very still for extended periods of time and generally is a more expensive exam as compared to CT scans.

What are other differences between CT and MRI?

Both CT and MRIs exams can be constructed "With or Without Contrast." This refers to the dye or contrast agent that is either injected into or ingested by the patient. When the test is performed, the contrast allows the radiologist to see more clearly the image of a certain area or organ of the body.

There are other more complicated differences than described here, especially in terms of how the tests actually work. If you would like more technical information of how CT and MRI works, please speak with your physician or visit our Links/Resources page. (In particular, the How Stuff Works site has a lot of good information; just enter "MRI" and "CT" at the site search.)

What kinds of CT contrast are used?

University MRI uses both Oral and IV (intravenous) CT contrast.

1. Oral Contrast: Oral contrast consists of two 16 oz. bottles, which are kept refrigerated prior to ingestion. Most commonly we offer a Mixed Berry flavored contrast, however there may be times when we also have Banana, Orange, or plain unflavored contrast available. Oral contrast consists of liquid barium, and must be ingested at least one hour prior to the start of the CT scan; this oral contrast helps make the intestinal tract visible. There are no side effects from the ingestion of oral contrast. Oral contrast is thicker than milk, but thinner than a milkshake; it is creamy, and slightly chalky. Patients receiving oral contrast are scheduled at least 1 hour and 15 minutes prior to the start of their exam to allow them time to drink the contrast, and time for the contrast to be absorbed.

Oral contrast can not be consumed prior to:
  • Ultrasound exams of either the abdominal or pelvic region (the ultrasound technician will be unable to see through the oral barium).
  • Bone Density exams (the oral contrast will actually stop the radiation used in a Bone Density exam that is trying to determine the density of the bone).
2. IV Contrast: IV contrast is a non-ionic contrast, however, there is still a small amount of iodine present. All CT patients' requiring IV contrast must fast (NPO 4 = no food, or water) for a minimum of four hours prior to their exam. This fasting is done so there is no food present in the patient's stomach, in the rare event that they experience any nausea or vomiting. Patient's receiving IV contrast may experience a sensation of warmth as the contrast is given. Although, patients allergic to iodine may not have a reaction to non-ionic contrast the CT scan may be done without contrast to avoid any reactions.

CT Quick Facts:
  • CT uses X-Ray-like technology to obtain "slices" or images of the inside of the body.
  • CT is widely used because it is fast (single breath scans) and has high resolution.
  • CT exposes the patient to low level radiation doses like an X-Ray.
  • CT is not appropriate for pregnant women.
MRI Quick Facts:
  • MRI use a magnetic field and radio waves to generate images of the inside of the body.
  • MRI requires the patient to be extremely still for long periods of time in order to obtain the images.
  • MRI images can be viewed with multiple variations/plains.
  • MRI does not expose the patient to any radiation.
  • Some people are not candidates for MRI, such as those with pacemakers or metal implants, pregnant women, or people unable to remain still for long periods of time.


© 2004 University MRI