What Is the Difference Between CT and MRI?

MRI brain
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While plain X-rays are useful tests for evaluating a wide variety of health problems, physicians often need more sophisticated medical imaging exams to help them determine the cause of a patient's symptoms. Computed tomography (CT) and magnetic resonance imaging (MRI) can be used for diagnostic and screening purposes.

In both tests, the patient lies down on a table which is moved through a doughnut-shaped structure as images are acquired.

But there are significant differences between CT and MRI.

Computed Tomography (CT) 

In a CT scan, the x-ray beam rotates around the patient's body. A computer captures the images and reconstructs cross-sectional slices of the body. CT scans can be completed in as little as 5 minutes, making them ideal for use in emergency departments.

CT scan is commonly used for the following body structures and abnormalities:

  • Acute brain hemorrhage from stroke or trauma
  • Bony structures
  • Pulmonary embolism - blood clot in the lungs
  • Lungs, abdomen, and pelvis
  • Kidney stones

A CT exam is also used to guide the placement of the needle during a biopsy of the lungs, liver, or other organs.

In certain cases, a contrast dye is administered to the patient to improve visualization of certain structures during the CT scan. The contrast can be given intravenously, orally, or via an enema. Intravenous contrast would not be used in patients with significant kidney disease or an allergy to the contrast.

CT scans use ionizing radiation to capture images. This type of radiation causes a small increase in an individual's lifetime risk of developing cancer. The response to ionizing radiation varies between individuals. The radiation is riskier in children. For example, a study led by Professor Mark Pierce of Newcastle University, UK, showed an association between radiation from CT scans and leukemia and brain tumors in children However, the authors note that the cumulative absolute risks are small and usually, clinical benefits outweigh the risks.

Also, as technology improves, the dose of radiation needed for a CT scan has been reduced. Radiation safety and radiation awareness have been widely discussed. Two organizations that work on raising awareness are the Image Gently Alliance and Image Wisely. Image Gently is concerned with adjusting radiation doses for children, while Image Wisely campaigns for better education about radiation exposure and addresses different concerns related to radiation doses of different imaging tests. Studies also show the importance of discussing radiation risks with patients; as a patient, you should be involved in a shared decision-making process.

Magnetic Resonance Imaging (MRI)

Unlike CT, an MRI does not use ionizing radiation. Therefore, it is a preferred method for the evaluation of children and for parts of the body that should not be radiated if possible, for example, the breast and pelvis in women.

Instead, MRI uses magnetic fields and radio waves to obtain images. The MRI generates cross-sectional images in multiple dimensions--that is, across the width, length, and height of your body. 

MRI is well suited for visualizing the following body structures and abnormalities:

  • Injuries of the tendons and ligaments surrounding joints like the knee or shoulder. (A tendon connects muscle to bone in order to move the bone. A ligament connects bone to bone in order to stabilize a joint.) For example, a physician may order MRI if someone has signs or symptoms of a torn ligament in the knee.
  • Spinal cord problems, such as a herniated disc or spinal stenosis
  • Brain problems, such as tumor, infection, old strokes, and multiple sclerosis
  • Osteomyelitis - chronic infection of the bones

MRI machines are not as commonplace as CT machines, so there is usually a longer wait time before getting an MRI. An MRI exam is also more expensive. While a CT can be completed in less than 5 minutes, MRI exams may take 30 minutes or longer.

The MRI machines are noisy and some patients feel claustrophobic during the exams. An oral sedative medication or use of an "open" MRI machine can help patients feel more comfortable.

Because MRI uses magnets, the procedure cannot be done for patients with certain types of implanted metal devices, such as pacemakers, artificial heart valves, vascular stents, or aneurysm clips.

Some MRIs require the use of gadolinium as an intravenous contrast dye. Gadolinium is generally safer than the contrast material used for CT scans but can be harmful for patients who are on dialysis for kidney failure.

Sources:

Foray N, Bourguignon M, Hamada N. Individual response to ionizing radiation. Mutation Research-Reviews in Mutation Research. 2016;770(Part B):369-386.

Hill B, Johnson S, Owens E, Gerber J, Senagore A. CT Scan for Suspected Acute Abdominal Process: Impact of Combinations of IV, Oral, and Rectal Contrast. World Journal of Surgery. 2010;34(4):699

Hinzpeter R, Sprengel K, Wanner G, Mildenberger P, Alkadhi H. Repeated CT scans in trauma transfers: An analysis of indications, radiation dose exposure, and costs. European Journal of Radiology. 2017:135-140.

Pearce M, Salotti J, de González A, et al. Articles: Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. The Lancet. 2012;380:499-505.

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