The Basics of Medical Imaging Tests

Medical imaging tests are used to give doctors a better understanding of patient condition

/ Author:  / Reviewed by: Robert Carlson, M.D Beth Bolt, RPh

While the doctors on Grey’s Anatomy may commonly speak of PET scans, CT scans and MRIs, many people may not understand these tests or why they are performed.

To clear up some of the misunderstandings, here are descriptions of each of these tests, as well as when and why they are used.


An X-ray is a painless procedure that allows doctors to take pictures of the inside of your body using radiation. In these pictures, different body parts — like the bones, joints, and tissue — will show up in black and white. Patients will be asked to sit or stand in a certain position so that a clear picture can be taken.

X-rays are commonly performed to determine if a patient has broken a bone or during dental exams. They can also be used to check for things like tumors, breast cancer, arthritis, lung infections and heart abnormalities. Because X-rays depend on radiation to produce the pictures, patients may be asked to wear protective material.

Although radiation exposure is minimal, pregnant woman or those who may be pregnant, are more sensitive to potential risks. Before performing an X-ray on a woman, doctors will ask if the woman is or may be pregnant.

CT Scan

A computed tomography (CT) scan is a series of X-rays taken from multiple angles to get pictures of cross sections of the body.

During a CT scan, a patient will be asked to lie down on a table that will slide into a tunnel-shaped scanner. Once inside the scanner, an X-ray beam will rotate around the patient and collect several images of the specific area. The test usually lasts a few minutes. These images can then be viewed together to give a 3-D view of that area.

CT scans are painless and can be used to identify internal trauma from car accidents, diagnose infections, identify tumors and study blood vessels.

MRI and fMRI

Magnetic resonance imaging (MRI) uses magnets and radio waves to produce pictures of the organs and tissues inside of the body.

Similar to a CT scan, when undergoing an MRI, a patient will be asked to lie on a table which will then slide into a tunnel-shaped scanner. The test is painless and usually lasts between half an hour to an hour.

During the MRI, magnets temporarily realign particles in the body and radio waves produce signals from these particles that allow images of a specific area to be taken.

Since magnets are involved in creating the images, patients are not allowed to bring any metal objects, such as jewelry, into the testing room, as they may cause blurry pictures. Metal or electronic devices in the body such as pacemakers or ear implants can also interfere with the tests, so patients who have these devices inside of them should notify their technician before the test is performed. MRIs may be used in place of some CT scans for those who wish to avoid radiation exposure.

Functional magnetic resonance imaging (fMRI) is a type of MRI that measures brain activity after individuals are asked to perform certain tasks. It works by detecting changes in oxygen levels and blood flow. When an area of the brain is more active, it requires more oxygen, and as a result, more blood flows to the area. The scanner then captures these changes and produces pictures of this activity.

PET Scan

A positron emission tomography (PET) scan uses a small amount of a radioactive material, also known as a tracer, to check for disease in the body. The radioactive material is typically injected into the body through a vein on the inside of the elbow.

It takes about one hour for the tracer to be absorbed by the body. During this time, the tracer will travel through the blood and gather in organs and tissues. Once the tracer is fully absorbed, the patient will be asked to lie on a table that will slide into a scanner. In the scanner, signals from the tracer are detected and the scanner converts these signals into 3-D pictures.

A part from the injection of the tracer, this procedure, like the others, is painless.

Key Similarities and Differences

For individuals who don't like small, enclosed spaces, or have trouble sitting still for long periods of time, a CT scan may be a better option than an MRI. Both tests involve being inside a scanner, though some CT scanners are not fully enclosed. Also, a CT scan typically lasts only a few minutes, while an MRI scan can last longer than an hour. Generally, CT scans are better for imaging bones, while MRI scans are better for viewing soft tissue.

"Modern medical imaging is often best thought of as a series of exams designed to give the treating physician specific data about the patient's condition," Dr. Mark Fulmer, a radiologist on the medical staff of Baylor University Medical Center in Dallas, TX, told dailyRx News.

"[CT] scans, ultrasound and X-rays are the most commonly used tools, and often give great clarity regarding a diagnosis for the patient. In neurological conditions and orthopedic conditions, as well as complex liver illnesses, MRI forms the cornerstone of diagnostic testing," Dr. Fulmer explained.

"When doctors decide to order imaging tests, they think about which exam will give the most amount of specific information most efficiently. This takes into consideration the type of test, the cost of the exam and the likely diagnosis," he said.

Potential Risks

Depending on the area requiring imaging, an X-ray, CT or MRI scan may make use of what's called a contrast dye. In this instance, patients will ingest or be injected with a dye that will illuminate and outline specific parts of the body, allowing doctors to better see it. A contrast dye may be used to view things like blood vessels or the intestines. Although the risk is low, some patients may have an allergic reaction to the contrast dye used, so it's important to tell your physicians about any allergies you have before receiving the dye.

Since some radiation exposure occurs during X-ray and CT scans, patients will want to speak with their physician to weigh the potential risks and benefits involved. A typical chest X-ray exposes patients to a radiation dose equal to what they would naturally be exposed to over a 10-day period, while a CT scan exposes them to a year's worth of radiation.

The risks and benefits of identifying medical problems with a CT scan should be discussed between a doctor and the patient to ensure that the benefits outweigh the risks.

Review Date: 
July 8, 2014