MAGNETIC RESONANCE IMAGING
MRI scanners use strong magnetic fields, radio waves, and field gradients to generate images of the organs in the body.
MRI does not involve x-rays, which distinguishes it from CT or CAT scans.
HOW IT WORKS
An MRI scanner contains two powerful magnets; these are the most important parts of the equipment.
The human body is largely made of water molecules, which are comprised of hydrogen and oxygen atoms. At the center of each atom protons serve as magnets and are sensitive to any magnetic field.
Normally, the water molecules in our bodies are randomly arranged, but upon entering an MRI scanner, the first magnet causes the body's water molecules to align in one direction, either north or south.
The second magnetic field is then turned on and off in a series of quick pulses, causing each hydrogen atom to alter its alignment and then quickly switch back to its original relaxed state when switched off. The magnetic field is created by passing electricity through gradient coils, which also cause the coils to vibrate, resulting in a knocking sound inside the scanner.
Although the patient cannot feel these changes, the scanner can detect them and, in conjunction with a computer, can create a detailed cross-sectional image for the radiologist to interpret.
WHEN IT IS USED
Abnormalities of the brain and spinal cord
Tumors, cysts, and other abnormalities in various parts of the body
Injuries or abnormalities of the joints, such as back pain
Certain types of heart problems
Diseases of the liver and other abdominal organs
Causes of pelvic pain in women (e.g. fibroids, endometriosis)
Suspected uterine abnormalities in women undergoing evaluation for infertility
RISKS AND BENEFITS
MRI is widely used in hospitals and clinics for medical diagnosis, staging of disease and follow-up without exposing the body to ionizing radiation. MRI often may yield different diagnostic information compared with CT. There may be risks and discomfort associated with MRI scans. Compared with CT scans, MRI scans typically take longer and are louder, and they usually require that the subject enter a narrow, confining tube. In addition, people with some medical implants or other non-removable metal inside the body may be unable to undergo an MRI examination safely.
Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel, and metallic foreign bodies in the eyes.
The safety of MRI during the first trimester of pregnancy is uncertain, but it may be preferable to other options.
THE PROCEDURE ITSELF
There is little to no preparation required for patients before an MRI scan. On arrival at the hospital, doctors may ask the patient to change into a gown. As magnets are used, it is critical that no metal objects are in the scanner, so the patient will be asked to remove any metal jewelry or accessories that may interfere with the machine.
Sometimes, patients will be injected with intravenous (IV) contrast liquid to improve the appearance of a certain body tissue.
The radiologist will then talk the individual through the MRI scanning process and answer any questions they may have about the procedure.
Once the patient has entered the scanning room, they will be helped onto the scanner to lie down. Staff will ensure that they are as comfortable as possible by providing blankets or cushions.
Earplugs or headphones will be provided to block out the loud noises of the scanner. The latter is very popular with children as they can listen to music to calm any anxiety.
T1 AND T2
Each tissue returns to its equilibrium state after excitation by the independent processes of T1 (spin-lattice) and T2 (spin-spin) relaxation.
To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time (TR). This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions and in general for obtaining morphological information, as well as for post-contrastimaging.
To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time (TE). This image weighting is useful for detecting edema and inflammation, revealing white matter lesionsand assessing zonal anatomy in the prostate and uterus.
WHEN AND WHY IT IS USED
Neurological cancers (better resolution than CT and offers better visualization of the posterior fossa)
Conditions of the central nervous system, including demyelinating diseases, dementia, cerebrovascular disease, infectious diseases, and epilepsy (contrast provided between grey and white matter)
Since many images are taken milliseconds apart, it shows how the brain responds to different stimuli, enabling researchers to study both the functional and structural brain abnormalities in psychological disorders
Used in guided stereotactic surgery and radiosurgery for treatment of intracranial tumors, arteriovenous malformations, and other surgically treatable conditions using a device known as the N-localizer
Assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases, and congenital heart disease (complementary to other techniques)
Spinal imaging, assessment of joint disease, and soft tissue tumors
LIVER AND GASTROINTESTINAL
To detect and characterize lesions of the liver, pancreas, and bile ducts
Extracellular contrast agents are used widely in liver MRI and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging.
MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors.
MR-colonography may play a role in the detection of large polyps in patients at increased risk of colorectal cancer.
Generates pictures of the arteries to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture).
MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs
MRI for imaging anatomical structures or blood flow do not require contrast agents as the varying properties of the tissues or blood provide natural contrasts. However, for more specific types of imaging, exogenous contrast agents may be given intravenously, orally, or intra-particularly. The most commonly used intravenous contrast agents are based on chelates of gadolinium.