Feature Article

Why have an MRI after a CT?

Magnetic resonance imaging (MRI) and computed tomography (CT) both have their areas of specialty in the medical imaging world. However, MRI and CT are frequently used to image the same areas of the body.  An MRI scanner uses a strong magnetic field and radiofrequency signals to provide information about the tissue structures and organs in the body.1 It does not use ionizing radiation to produce the images. Depending on the area of the body, MR images may be improved with the use of gadolinium-based contrast. CT, on the other hand, uses X-rays to create images of bone and tissue structures, as well as organs.2 Like MRI, a contrast agent may be used to improve the image quality in certain areas of the body, though it is iodine based. For example, CT and MRI both provide valuable information while imaging a knee injury or cancer in the body. The two imaging methods can provide different types of information about the region of interest.


Medical facilities may see patients with a number of different knee injuries, like fractures or ligament or meniscus injuries.3,4 Doctors most likely will perform a physical exam if the patient is able to tolerate it before ordering an imaging test. This physical exam may help determine what kind of images the doctor would like to see. After the physical examination, a patient may have a standard X-ray performed. If the X-ray is normal, the physician may still decide they want additional imaging exams done, especially MRI or CT.

An MRI produces images of your body from a number of different angles without the ionizing radiation necessary for a CT scan.4 MRI scans may be used by the doctor to help differentiate and diagnose a number of knee injuries. MRI can show torn knee ligaments or cartilage, torn rotator cuffs, herniated disks, osteonecrosis and other issues. This is done through its ability to image the soft tissue structures and bones, though the injury the doctor finds using MR images may not be the cause of the knee pain.3

The images produced during a CT scan produce detailed images of a patient's tissues and bones.3,4 A CT scan of the knee, like a standard X-ray, may show the doctor a fracture. However, it is able to show more detail and smaller structures, enabling the doctor to indentify something they may have missed on an X-Ray. CT scans are faster than MRI and are often preferred for providing skeletal information, where MRI scans are often preferred for providing soft tissue information.

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Physicians have a variety of tests that are used to diagnose cancer, including physical exams, lab tests, biopsies and imaging tests.5 After a physician initially spots a tumor, cancer patients may undergo further testing to help their oncologist stage and monitor their cancer. Both MRI and CT are used to produce images of the area of the body where the tumor is located, which can aid an oncologist may use in determining the best treatment options.5,6,7 These images also show the tumor's size. However, MR and CT each have their own strengths.

MRI details the soft tissue structures within the body from a wide variety of angles.6 MRI can produce single-slice (a thin area of the body, much like a bread slice) imaging and can also produce a multiple-slice (like a loaf of bread) image set. Some of these soft tissue parts of the body are difficult to see with imaging tests other than MRI. The images produced from various MR acquisition techniques can help radiologists find and pinpoint cancer and determine the malignancy of tumors. Doctors also use the images provided from scans to plan treatment and monitor for signs of metastasis (the spreading of a cancer from its onset region).

CT, like MR, can produce both single- and multiple-slice image sets of suspected pathology and its surrounding. The details of CT images are used to determine the size shape and location of tumors.7 Both CT and MR may also be used to image the blood vessels that feed the tumor. These images are typically acquired faster on a CT scan than they would be on an MRI. Physicians may use a CT to help guide a needle to remove a small piece of tissue, a process which is called a CT-guided biopsy. Oncologists use the images created by CT and MR scans to guide treatments and to compare a tumor to how it looked before treatment began, aiding physicians to assess the patient's response to treatment.

Both CT and MRI are effectively used in a wide variety of medical specialties, including orthopedics and oncology. A significant point of differentiation is the use of ionizing radiation and the time required for scanning. CT uses X-rays but provides a much faster scan time. Even though their images help physicians in virtually the same diagnostic and evaluation tasks, the physician's choice of which one (or both) to use for a particular patient is dependent on multiple considerations related to the case at hand. These considerations include if the area of interest is skeletal or soft tissue, the level of image detail required, the time available to obtain the needed information, and the use of ionizing radiation.


1. Todd A. Gould and Molly Edmonds. "How MRI Works." howstuffworks.com. 25 October 2010. Web. 16 January 2019. <https://science.howstuffworks.com/mri.htm>.

2. "Computed Tomography (CT)." NIBIB.NIH.gov. Web. 16 January 2019. <https://www.nibib.nih.gov/science-education/science-topics/computed-tomography-ct>.

3. Monica Koplas, Jean Schils and Murali Sundaram. "The painful knee: Choosing the right imaging test." Cleveland Clinic Journal of Medicine. 2008 May; 75(5): 377-384. Web. 15 January 2019. <https://www.mdedge.com/ccjm/article/94916/imaging/painful-knee-choosing-right-imaging-test/page/0/1>.

4. "X-rays, CT Scans and MRIs." OrthoInfo. June 2017. Web. 15 January 2019. <https://orthoinfo.aaos.org/en/treatment/x-rays-ct-scans-and-mris/>.

5. Mayo Clinic Staff. "Cancer." MayoClinic.org. 12 December 2018. Web. 15 January 2019. <https://www.mayoclinic.org/diseases-conditions/cancer/diagnosis-treatment/drc-20370594>.

6. The American Cancer Society medical and editorial content team. "MRI for Cancer." Cancer.org. 30 November 2015. Web. 15 January 2019. <https://www.cancer.org/treatment/understanding-your-diagnosis/tests/mri-for-cancer.html>.

7. The American Cancer Society medical and editorial content team. "CT Scan for Cancer." Cancer.org. 30 November 2015. Web. 15 January 2019. <https://www.cancer.org/treatment/understanding-your-diagnosis/tests/ct-scan-for-cancer.html>.