How MRI and CT Work Together

Mayfair • Jun 10, 2020

Computed tomography (CT) and magnetic resonance imaging (MRI) are two types of medical imaging that have a reputation for providing a thorough look inside the body. They are often ordered when more detail is needed or the cause of symptoms is unclear during a physical exam or on other types of imaging.

But sometimes it can be confusing to understand why one exam is requested and not the other, or why a patient might be sent for both types of scans. A patient’s medical and family history, risk factors, and type and duration of symptoms, all affect the physician’s decision on which type of imaging is appropriate.

“Although both CT and MRI are excellent tools for diagnostic imaging, each has its own strengths. This may influence which modality is best for a specific diagnosis,” says Rick Myszkowski, lead MRI technologist and Mayfair Place Clinic Manager.

An MRI scan creates images by exposing hydrogen atoms within our body to a magnetic field which controls the direction and frequency at which these hydrogen protons spin. A radio frequency pulse is then directed at a specific area of the body, while smaller magnets are used to alter the magnetic field on a small, but localized level.

As tissues responds differently to these magnetic field alterations, a computer can convert the data into a picture. MRI images can be taken of most body parts.

A CT scan is made from X-rays.  While a standard X-ray machine sends only one radiation beam, a CT scanner emits a series of beams as it moves in an arc around the body. CT can demonstrate different levels of tissue density. This data is transmitted to a computer, which builds a 3D cross-sectional picture of area the body being scanned.

WHAT CAN THEY DIAGNOSE?

Whether a doctor requests a CT scan or an MRI scan often depends on the presumed diagnosis. For example, MRI is very good at examining soft tissues such as tendons and ligaments, evaluating the spinal cord, and identifying strokes in the brain. CT scans, in comparison, are best for imaging bone, soft tissues in the chest or abdomen, and blood vessels. So a CT may be preferred to evaluate fractures or look for cancers or blood clots.

“Whether it’s identifying a targeted area for treatment with surgery or an injection, finding an illness early, or excluding serious pathology, a clear diagnosis can provide peace of mind for patients. Ultimately it’s about taking care of our patients,” says Dr. Sarah Koles, radiologist at Mayfair Diagnostics.

An important difference between CT and MRI is that a CT scan involves radiation. This exposure to radiation in a CT scan is higher than that of standard X-rays, although the associated risk is still small. For example, the radiation exposure from one low-dose CT scan of the chest is the same amount of radiation as every person receives from the earth's natural background radiation over six months.

MRI images, on the other hand, use a strong magnetic field, which can attract metal objects or may cause metal in your body to move. This means that before an MRI can be performed all patients will need to be screened to exclude internal metal object that are not safe in the MRI. Compared to CT, the inside of the MRI scanner is slightly smaller and there are noises caused by changes in the magnetic field, which require ear protection.

ARE THESE EXAMS SAFE?

To determine which imaging type is appropriate for a patient’s circumstances, a health care practitioner will review the risks and benefits associated with each type of imaging, and determine whether CT or MRI will provide the most valuable information. Often, the benefit of detecting serious illness early will outweigh any discomfort or the small increased risk from radiation exposure.

CT and MRI exams are both available in hospitals and covered under the Alberta Health Care Insurance Plan, but they can also be performed as private pay exams which complement the public health care system.

Whether public or private, medical imaging must be requested by a health care practitioner who will provide a requisition. Mayfair Diagnostics will schedule your exam and provide you with detailed information to prepare for it. Once your exam is completed, your images will be reviewed by a specialized radiologist who will compile a report that is sent to your doctor.

Mayfair Diagnostics is owned and operated by over 60 radiologists who are subspecialty trained which guarantees an expert opinion of your imaging.

Mayfair Diagnostics offers CT and MRI imaging as private pay services at our Mayfair Place location. For more information, visit radiology.ca.


REFERENCES

National Institute of Biomedical Imaging and Bioengineering (2020) "Computed Tomography (CT)." National Institutes of Health, www.nibib.nih.gov. Accessed June 10, 2020. 

Sawyers, T. (2019) "CT Scans vs. MRIs." www.healthline.com. Accessed June 10, 2020.