MRI VS. CT FOR CARDIAC IMAGING

Your heart is a powerful muscle that pumps oxygen-rich blood and nutrients through the body. Over the course of a day the heart beats over 100,000 times and pumps about 2,000 gallons of blood, making the cardiovascular system a complex and important part of your overall health.

Any condition that affects the function of the heart is considered heart disease. Coronary artery disease (CAD), or atherosclerosis of the coronary arteries, is the most prevalent in Canada. Substances that normally circulate in your blood, including calcium, fat and cholesterol, can collect to form a plaque along the wall of the coronary arteries. Plaque buildup can cause these arteries to become narrow, stiff, or blocked, decreasing blood flow to the heart and resulting in injury to the heart muscle. If left untreated, it might lead to heart failure or a severe heart attack.

Early warning signs could include chest pain, fatigue, and dizziness. Depending on the seriousness of your symptoms, your doctor could order a number of tests to investigate their cause. Often this involves medical imaging, such as:

• Echocardiogram – a heart ultrasound which examines the function of the heart’s pumping chambers and of the heart valves.
• Exercise stress test – your heart’s electrical activity and your blood pressure are measured while you exercise on a treadmill.
• Myocardial perfusion imaging – your heart’s function is assessed at rest and during exercise using a radiopharmaceutical and a gamma camera.
• Coronary CT angiography – computed tomography imaging is used to gather comprehensive and detailed information about the coronary arteries.
• Cardiac MRI – magnetic resonance imaging is used to create detailed imaging of heart structures and its function.

WHEN WOULD CT OR MRI BE ORDERED?

CT and MRI are both important diagnostic tools which play critical roles in evaluating heart health and diagnosing CAD.

A coronary computed tomography angiography (CCTA) can be used to non-invasively examine the coronary arteries. Using a combination of X-rays and computer technology, CCTA can detect both calcified (hard) plaques and noncalcified (soft) plaques. CCTA has been shown, through major research studies, to be highly sensitive (>90%) in the detection of CAD.

A cardiac MRI is a safe and non-invasive exam that uses radiofrequency pulses and magnets to examine heart structure (chambers, valves, and muscles) and function. It can determine if any areas of the heart have been injured.

Your doctor will determine which type of imaging is appropriate based on your medical and family history, risk factors, and type and duration of symptoms. CCTA exams are often faster than cardiac MRI, so your doctor may order CT first, then cardiac MRI if more information is needed.

WHAT’S THE DIFFERENCE BETWEEN MRI AND CT SCANS?

MRI creates images by exposing hydrogen atoms within our body to a magnetic field which controls the direction and frequency at which 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 respond differently to these magnetic field alterations, a computer can convert the data into a picture.

CCTA imaging on the other hand uses X-ray technology to produce images, so there is some exposure to radiation. This exposure is higher than that of standard X-rays, but the associated radiation risk is still small. For example, the radiation exposure from one CCTA is similar to the exposure from the earth’s natural background radiation in one year. In most cases, the benefits of a CCTA, such as the early detection of a serious illness, significantly outweigh the small increased risk from radiation exposure.

CCTA exams require the use of an iodinated contrast dye which is injected through a vein in the arm.  The contrast dye travels through the targeted body areas resulting in more detailed CT scan images.

Some cardiac MRIs may require contrast. It can help highlight the blood vessels to show their structure, and information related to tissue blood supply, inflammation, and scarring.

WHAT TO KNOW BEFORE YOU BOOK

An MRI uses a strong magnetic field, which can attract metal objects or may cause metal in your body to move. You will need to wear comfortable clothes free of metal zippers or buttons, including metal-free underwear, and you will be screened to exclude internal metal objects that are not safe in the MRI. This includes some medical devices, such as intrauterine devices or some pacemakers. Your technologist may ask you to remove any external metallic objects, which are unsafe near the MRI magnet.

An MRI can be noisy, so you will be given headphones to block some of the noise. The scanner is well lit and well ventilated, and patients can remain in contact with the technologist during the exam through an emergency call button. Eye masks are available upon request for patients who have difficulty relaxing in a confined space.

For CCTA, there are a number of preparation instructions to be aware of:

• You will need an electrocardiogram (ECG) within the year prior to the exam. Your health care practitioner can order this test as needed.
• You will need a blood test to determine your kidney function within three months of the appointment.
• Avoid taking erectile dysfunction medications (Viagra or Cialis) in the 48 hours prior to the procedure.
• Avoid all caffeine products (e.g., coffee, tea, chocolate, Excedrin, decaffeinated coffee, decaffeinated tea) in the 24 hours before the exam.
• If prescribed, you will need to take one tablet (100 mg) of a Beta Blocker the night before the exam. You may be asked to take an additional Beta Blocker one hour before the exam.
• Avoid strenuous exercise the morning of the exam.
• No food or drink (except water) in the two hours before the exam. If you have diabetes, you can have dry toast and juice.
• Arrive 60 minutes before the appointment to allow enough time for adequate preparation.

HOW DO I GET CARDIAC MRI OR CCTA?

We offer cardiac MRI at our Saskatoon location in Saskatchewan as a publicly funded, community-based service under contract with the Saskatchewan Health Authority. This test is also offered privately, in accordance with and under the legislation of the Province of Saskatchewan.

In Alberta, cardiac MRI is only publicly funded, offered at hospitals through Alberta Health Services.

For CCTA, this is a publicly funded test in both Alberta and Saskatchewan. In Alberta only, we offer CCTA as a private pay examination, not covered by Alberta Health Care, at our Mayfair Place location. It can be purchased on its own or as part of a Health Assessment package, which provides a discount on multiple imaging exams when purchased together.

Your health spending account or group medical insurance plan may cover the cost of a private CT that is prescribed by a qualified health care practitioner. You will need to check with your plan administrator for coverage details.

Whether public or private, medical imaging must be requested by a health care practitioner who will provide a requisition. When we receive your requisition, the staff at Mayfair Diagnostics will review it and contact you to schedule your exam, as well as provide you with detailed preparation instructions. 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.

REFERENCES

Alberta Health Services (2022) “MRI of the Heart: About This Test.” myhealth.alberta.ca. Accessed July 2, 2024.

Cleveland Clinic (2024) “Blood Flow Through the Heart.” clevelandclinic.org. Accessed June 27, 2024.

Cleveland Clinic (2024) “Heart MRI.” clevelandclinic.org. Accessed June 27, 2024.

John Hopkins Medicine “Noninvasive Cardiovascular Imaging.” hopkinsmedicine.org Accessed June 27, 2024.

Newton, C. (2010) “Comparing CTA Scan and MRA Scan.” dicardiology.com. Accessed July 2, 2024.

Weberling, L.D., et al. (2023) “Coronary Computed Tomography vs. Cardiac Magnetic Resonance Imaging in the Evaluation of Coronary Artery Disease.” ncbi.nlm.nih.gov. Accessed July 2, 2024.