Determining whether an MRI or CT (or CAT) scan is more appropriate for your imaging needs may seem challenging, but there are several factors that can make the decision easier. Some of these considerations include whether the need for imaging is emergent; the conditions or body parts being examined; your age, size, and ability to calmly remain still in an enclosed space; potential adverse reactions to contrast agents; and cost of the procedure. Below are four factors of an MRI vs CT brain scan to consider regarding the procedure that’s right for you.
What’s the Difference Between MRI and CT?
When your doctor orders an imaging test, you may well wonder why they select one type of imaging over the other. MRI and CT (or CAT) scans can be used for similar purposes, and several factors play into the decision to use one over the other.
What is a CT scan?
A computed tomography (CT) scanner uses X-rays to generate detailed images for close examination. X-ray beams and detectors rotate around the patient to create cross-sectional images of the body that can be reformatted to produce three-dimensional pictures.
How are CT scans used?
CT scanning is ideal for getting detailed images of bony structures as well as imaging bone, soft tissue, and blood vessels all at once. CT is capable of producing good soft tissue differentiation, as well, particularly when IV contrast is used. CT is commonly used for diagnosing lung and chest problems and detecting cancers, and emergency rooms rely heavily on CT scanning because it produces fast, detailed results.
What are the drawbacks of CT?
Precision of Soft Tissue Images
CT offers less precise visualization of soft tissue detail and subtle differences in soft tissues than MRI. However, CT is still often the preferred choice for some soft tissue examinations because of other benefits, such as speed, cost, and availability.
CT uses ionizing radiation, which is associated with an increased risk of cancer. There is insufficient evidence, however, to support the notion that relatively low levels of radiation contribute to increased cancer risk. Depending on the exam, a CT scan might deliver an effective radiation dose of 1.5 to 20 mSv. US. and international radiation safety organizations, however, have stated repeatedly that the long-term effects of radiation doses below 100 mSv are too small to be observed and may be nonexistent. Still, imaging professionals adhere to the ALARA (as low as reasonably achievable) standard, using as little radiation as possible to obtain the medical information needed for patient care.
Finally, the contrast solution that is used with CT typically contains iodine, which can cause allergic reactions in some patients. Such reactions are rare, however, and imaging professionals are prepared to deal with them when they occur.
What is MRI?
Magnetic resonance imaging (MRI) uses a powerful magnetic field to produce detailed pictures of the body’s interior structures. The patient lies on the exam table, which slides into a tube in the center of the magnet. Different types of MRI machines are available, with different types of bores—traditional, short, open, or wide—to accommodate different patient needs. For example, a wide-bore MRI will be more comfortable for claustrophobic or larger-bodied patients than a traditional bore.
How is MRI used?
MRI is the star when it comes to close examination of soft tissue detail. MRI technology allows radiologists to adjust the contrast in images, enabling them to more clearly visualize the differences between various types of soft tissue. As a result, MRI is a procedure of choice for examining ligament and tendon injuries, spinal cord injuries, and brain tumors, and it can produce superior images of other types of tumors as well. Because MRI doesn’t use ionizing radiation, it can provide the benefit of detailed imaging without increasing a patient’s cancer risk.
What are the drawbacks of MRI?
While a CT scan often takes less than five minutes from start to finish, MRI requires the patient to remain motionless for prolonged periods in order to obtain usable images. A typical MRI exam takes 30–60 minutes, but some can take longer. Not only can this result in blurring due to small movements, but it also makes MRI unsuitable for time-critical emergency evaluations. MRI can also cause anxious or claustrophobic patients to find the procedure difficult to endure. When necessary, doctors can prescribe sedatives to enable these patients to complete an MRI exam.
Not all patients are appropriate candidates for MRI. The powerful magnet used makes MRI unsuitable for patients with implanted ferromagnetic elements such as aneurism clips, pacemakers, cochlear or ocular implants, and imbedded shrapnel. Because the magnetic field is incredibly strong, it is vitally important to understand all warnings that a clinic provides prior to the exam about what can and cannot be brought into the exam room.
MRI is more costly and less widely available than CT. Because CT provides many similar benefits to MRI at around half the cost, it is often a more practical choice for patients’ imaging needs. Additionally, CT is generally able to accommodate larger individuals, with weight limits around 450 pounds (compared to 350 for MRI).
Like CT scans, MRI scans sometimes involve the use of contrast dye. Instead of the iodine-based contrast used in CT, however, MRI relies on gadolinium-based contrast agents. While these are less prone to triggering allergic reactions than iodine contrast agents, they can pose serious problems for some patients, particularly those with impaired kidney function. For more information about gadolinium contrast agents, see our free ebook.
There are many reasons your doctor might choose to order an MRI or CT exam. If you have questions about why a specific imaging test has been ordered for you, be sure to ask. At Iowa Radiology, we want you to have all the information you need to make wise health care choices. For regular updates about imaging and other health topics, subscribe to our blog.
Colwell J. Obesity complicates diagnosis. ACP Hospitalist. https://acphospitalist.org/archives/2011/10/obesity.htm. Published October 2011. Accessed December 11, 2020.
Computed Tomography (CT)—Body. Radiologyinfo.org. https://www.radiologyinfo.org/en/info.cfm?pg=bodyct. Updated April 10, 2018. Accessed December 11, 2020.
Magnetic Resonance Imaging (MRI)—Body. Radiologyinfo.org. https://www.radiologyinfo.org/en/info.cfm?pg=bodymr. Updated June 18, 2018. Accessed December 11, 2020.
McCollough CH. Radiation Risk from Medical Imaging. Mayo.edu. https://www.mayo.edu/research/documents/radiation-risk-from-medical-imaging/doc-20156687. Published May 2014. Accessed December 11, 2020.
Radiation Dose in X-Ray and CT Exams. Radiologyinfo.org. https://www.radiologyinfo.org/en/info.cfm?pg=safety-xray. Updated March 20, 2019. Accessed December 11, 2020.