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Radiology: CT and MRI Scans

Radiology is undoubtedly a huge part of cancer diagnosis and management. When I started out, it mostly consisted of plain X-rays and variations on them, such as barium enemas, upper GI series, etc. The ensuing years have seen an explosion in the technology of diagnostic radiology that has made plain X-rays almost obsolete in the day-to-day practice of oncology. I refer, of course, to CT scans, MRI scans and PET scans, all of which play major roles in the diagnosis, staging, follow-up and surveillance of patients with cancer. Exactly what each one is, how they differ from each other, and their exact roles is a subject I approach as a non-radiologist.

Computed tomography (CT) scans were developed in the 1970s to get more-detailed images than plain X-rays could provide. In essence, a CT scan takes 3- to 5-mm slices through the organ that is being analyzed, perhaps over 100 slices, and then the computer integrates these images into a three-dimensional whole. In essence, it is taking multiple X-rays and then integrating them to create a composite image. The images tend to be much more detailed and sensitive than a regular X-ray. The 1979 Nobel Prize in Medicine was awarded to the developers of this technology. The detail and clarity of the images can vary depending on how much radiation is used in the scan, with lower-dose techniques being utilized for screening and then supplemented with higher-dose scans if abnormalities are detected.

Magnetic resonance imaging (MRI) scans use a different form of energy rather than ionizing radiation to generate images. They were also developed in the 1970s and 1980s. Radio frequency energy is used to excite the hydrogen atoms in organs—hydrogen atoms are omnipresent in water and fat. When the hydrogen atoms relax, they release energy that is localized with the use of strong magnetic fields. In essence, MRI scans map the locations of water and fat. The Nobel Prize in Medicine in 2003 was awarded to the two scientists responsible for the discoveries that led to the development of MRI imaging.

CT scans are usually used first, versus MRI scans, for diagnostic and staging purposes unless the test is being used for a tumor, which is seen better on MRI. Prostate cancer, uterine cancer and certain brain metastases may show up better on an MRI, for example. CT scans are much faster to perform and have the advantage of avoiding the claustrophobic sensation engendered in some patients by MRI scanners. Furthermore, MRI scanners, because of the very strong magnetic fields they generate, cannot be utilized for patients who have any form of metal within them—pacemakers, older forms of cochlear implants, shrapnel, and the like.

MRI scans tend to produce more detailed images than CT scans and thus are frequently used as follow-up scans for abnormalities found on an initial CT scan for clarification. They also provide more detailed images of soft tissue such as ligaments, joints or certain organs. Examples include herniated discs, torn ligaments, breasts, heart, and soft tissue problems. A problem with a fracture or head trauma, or a general image of an internal organ will generally lead to use of a CT scan. They are less expensive so that may be an issue as well; CT scans are generally half or less the cost of MRI scans.

Both types of scans require the use of contrast dye and so problems with allergies may become an issue. A CT scan without contrast is generally not valuable in the context of oncology. The contrast dye is excreted through the renal system so a patient with significant renal dysfunction generally cannot undergo a CT scan. For CT scans, another issue is the small risk of carcinogenesis from ionizing radiation. Generally, the potential benefit from a medical point of view should far outweigh this minor risk, but MRI scans do not have this problem.

CT scans generally cover a larger anatomic area than MRI scans do, though the trade-off is the level of detail that is obtained. Both types of scan can be good at suggesting whether a particular lesion is malignant or not. But appropriate follow-up is usually necessary with histologic testing and biopsy to be 100% certain.

In our next episode of Thoughts on Cancer, we will address the third major type of scan used in cancer, PET scans.


Alfred I. Neugut, MD, PhD, is a medical oncologist and cancer epidemiologist at Columbia University Irving Medical Center/New York Presbyterian and Mailman School of Public Health in New York.

This article is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment, and does not constitute medical or other professional advice. Always seek the advice of your qualified health provider with any questions you may have regarding a medical condition or treatment.

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