What is MRI?
Magnetic Resonance Imaging (MRI) (figure 1 and figure 2) is a non-invasive test that uses large and powerful magnetic fields and radio frequency waves to produce detailed computerised images of the different organs of the body and blood vessels. Cardiac MRI can show the heart’s structures (muscles, valves, chambers and major blood vessels) and can visualise blood flows through the heart chambers and major vessels as the heart beats, creating images of heart throughout its pumping cycle.
Why and When Do You Need a Cardiac MRI?
A cardiologist uses this technique to evaluate the anatomy and function of the structures of the heart . It images blood vessel, pericardium (the skin covering around the surface of the heart) and even the lungs to determine the presence of heart disease –
How does MRI work?
The MRI is a highly sophisticated imaging system that creates a strong magnetic field around the patient. Together with radiowaves, this magnetic field alters the hydrogen atoms natural alignment in the body. Using advanced computer softwares – 2D (Dimensional)and cross-sectional images of the heart structure can be obtained based on the activity of the hydrogen atoms as they realign themselves after being knocked out of position, sending out radio signals that are converted into images and viewed on a monitor. Because radiation is not used, there is no risk of exposure to ionizing radiation during an MRI procedure.
Other uses of cardiac MRI
When MRI is used to study the flow of blood in the blood vessels, it is called a magnetic resonance angiography (MRA). Unlike traditional x-ray angiogram, this procedure does not require inserting a catheter into your arteries.
A Stress Perfusion MRI requires the injection of a special MRI dye (contrast medium/agent) called gadolinium during the scan. The contrast enters the heart muscles via the blood vessels that supply the heart and can be visualised if the blood supply is normal. Areas with less or loss of blood supply will not show contrast highlight on the scan images and therefore can be used as an indicator of coronary heart disease (under supply of blood and oxygen in the heart).
The blood flow (perfusion) to the heart is assessed both at rest and under stress. The heart can be stressed using an injection medication called adenosine which stimulates the heart muscle to pump harder and faster and therefore simulate an exercise stress test. This technique can therefore reveal certain parts of the heart muscle that is not receiving adequate blood supply. The rest stage of imaging is usually done after the stress stage to use as a comparison.
The cardiac stress perfusion MRI therefore provides very useful information to what happens to the blood supply to the heart at rest and on demand (stress). It will be able to identify the presence of ischaemia (lack of blood supply to the heart muscle) and can identify the precise blood vessel causing the problem.
What are the risks of a Cardiac MRI?
Due to the use of a strong magnet, special precaution must be taken in patients with implanted metallic device unless the device is certified MRI-safe. Such devices include
Claustrophobia (having a fear for closed space)
If you are claustrophobic, you will have to tell your doctor before the MRI procedure. A sedative can be given to help you relax before the MRI. Special precautions and instructions will be given before prescribing sedatives.