July 14, 2022
MRI is a non-invasive imaging technique, that uses a powerful magnetic field and radio waves to create images of thin slices of body tissue (tomographic images) to help diagnose or monitor the treatment of different diseases and conditions.
How do MRI scanners work?
MRI scanners are large, hi-tech machines. There are different types of MRI scanners but they all work on the same principle of physics. Normally within tissues protons or Hydrogen atoms spin to create tiny magnetic fields that are randomly aligned. A superconducting magnet in an MRI machine aligns the magnetic axes of these protons. A radio wave pulse causes protons to flip around in a high-energy state. When the pulse is switched off, the protons come back to their previous alignments in a magnetic field and release energy. The magnitude and rate of release of energy as they resume their position is picked up by the scanner and the data is sent to a computer to make useful images. The gradient coils are placed near specifically targeted tissue. They switch on and off rapidly and send and receive radio signals. As they switch on and off, they contract and expand creating a loud banging noise.
What does the MRI machine look like?
The traditional MRI machine is a large cylinder-shaped tube, encircling a powerful magnet. Patients are positioned on a movable exam table that slides inside the tunnel. The machine is well lit inside. Some patients do not fit inside properly. Wide bore MRI machines and Open MRI scanners are better for larger patients and patients with claustrophobia (fear of closed space). The open MRI machine has no side walls. The patient can look around and even can hold a friend or parent’s hand for comfort while doing the test.
Uses of MRI
MRI is preferred to CT scans when detailed images of specific body areas are required. Such as intracranial or spinal cord abnormalities, Tumors, trauma, inflammation, etc. Other uses of MRI are the following:
- Magnetic Resonance angiography (MRA). Magnetic Resonance Angiography is less invasive than conventional angiography and has good diagnostic accuracy.
- Magnetic Resonance cholangiopancreatography- (MRCP) is a highly accurate and non-invasive technique to see the pancreatic and biliary duct system.
- Masses in the female reproductive system
- Infiltration and metastasis of bone marrow
- Disorders of joints and certain fractures
Disadvantage of MRI
- Relatively more expensive than a CT scan. May not be available in certain areas. It takes a longer time than a CT scan.
- The strong magnetic field is not harmful but it can affect some implanted medical devices such as pacemakers, defibrillators, vascular clips, metallic wires, cochlear implants, etc. The implant may be displaced or burnt by the strong magnetic field and can injure nearby tissue or organ sometimes resulting in hemorrhage. Displacement is more likely if the implant is in place less than 6 weeks that is before the scar tissue formation. They can also create imaging artifacts. Different MRI scanners have different magnetic strengths so you have to ask the doctor about the safety of the implanted material.
- Claustrophobia- The imaging tube of MRI is an enclosed space that can create claustrophobia in some patients. A sedative is given to anxious patients. Patients should be warned that the machine makes loud banging noises during scanning. Patients are given earplugs or headphones to reduce the noise.
- Contrast agents are often used while doing MRI. The most commonly used contrast agents are Gadolinium derivatives. Gadolinium-based contrast agent injected IV causes headache, nausea, distortion of taste as well as the sensation of cold at the injection site. Serious contrast reactions are rare and are much less common than iodinated contrast agents. However, Nephrogenic systemic fibrosis is a risk in patients with renal impairment. Nephrogenic systemic fibrosis is a rare but life-threatening disorder, that involves fibrosis of the skin, blood vessels, and internal organs resulting in severe disability or death. Renal function tests are done in patients with diabetes, Hypertension, Heart failure, or the elderly before doing a contrast MRI. In patients with renal impairments test is done with precautionary measures or an alternative imaging method is used.
Preparation before doing MRI
Wear loose, comfortable clothing, remove any jewelry or accessories at home, or take them off before MRI. A hospital gown is given to wear before doing the test. Guidelines for eating and drinking before MRI varies depending on the specific exam. Take your food and medicines as usual unless your doctor tells you otherwise. You may be given a mild sedative 15-30 minutes before MRI to relieve anxiety.
MRI does not cause any harm to the fetus, however, your doctor may delay it until after delivery. Gadolinium contrast agents are generally avoided in pregnant women except in very specific circumstances. The doctor may advise contrast MRI after the third trimester if needed after discussing the risks and benefits of doing an MRI. According to contrast manufacturers, mothers should not breastfeed babies 24-48 hours after contrast MRI. Newer studies according to the American college of radiology show that the amount of contrast media absorbed by the baby during breastfeeding is very low and does not cause any harm.
How MRI is performed
MRI is done on an outpatient basis. It takes 15 minutes to an hour usually. The patient is asked to lie down on a movable exam table. The technician place devices which contains coils, close to the area of examination. The coil sends and receives radio signals. The exam table is slid inside the MRI machine. The technician can see, hear and communicate on a two-way intercom. They will also give you a ball to squeeze if you need immediate help. It is important that you remain perfectly still while MRI is done. After every few minutes machine makes a sequence of images with a loud tapping noise. The patient is given earplugs or headphones to lie inside comfortably. This takes a few seconds at a time. You can relax in between, but you should lie completely still until the scanning is over. Breathing movement during chest, abdominal or pelvic MRI creates artifacts and images get blurred. With newer techniques, images are clearer. You may be asked to withhold your breath when the machine is taking the images. A radiologist interprets the images and sends a signed report to referring physician who then shares the report with you.