In magnetic resonance imaging (MRI), a strong magnetic field and very high-frequency radio waves are used to produce very detailed images. MRI does not use X-rays and is usually very safe.
Overview of Imaging Inspection
Imaging tests provide an image of the entire body or part of the body. Imaging helps doctors diagnose a disorder, determine how severe the disorder is, and monitor a person after the disorder is diagnosed. Most imaging tests are painless, relatively safe, and non-invasive (that is, no incisions in the skin or insertion of instruments into the body are required).
Imaging tests can use:
- Radiation: X-rays, computed tomography (CT), and radionuclide scans
- Sound waves: ultrasound imaging
- Magnetic field: magnetic resonance imaging (MRI)
- Substances taken, injected, or inserted to highlight or outline the tissue or organ to be examined ( called contrast media )
There are some risks of using radiation in medical imaging tests.
For information on general imaging tests for specific diagnosis and screening tests, see:
- Heart and blood vessel disorders
- Lung disorder
- Musculoskeletal disorders
- Brain, spinal cord, and nerve disorders
- Digestive problems
- Liver and gallbladder disorders
- Urinary tract disease
- Eye disorder
- Gynecological diseases
- Treatment during pregnancy: ultrasound
For the MRI scan, the subject lies on a motorized table that goes into the narrow interior of a large tube-shaped scanner that emits a strong magnetic field. In general, the protons (some of the positively charged atoms) in tissues are not specifically arranged. However, when surrounded by a strong magnetic field, like an MRI scanner, the protons align with the magnetic field. And the scanner emits a pulse of radio waves, which causes the protons to be temporarily out of alignment. When protons realign with the magnetic field, they release energy (called a signal). The strength of the signal varies from tissue to tissue. MRI scanners record these signals. Computers are used to analyze signals and generate images.
The inspector can change the way different tissues appear on the scan by changing the radio pulse, the strength and direction of the magnetic field, and other factors. For example, adipose tissue appears dark on one type of scan and bright on another. Each of these different scans provides complementary information, so often more than one is acquired.
A contrast agent that contains gadolinium (a paramagnetic contrast agent ) may be injected into a vein or joint. Gadolinium contrast agents change the magnetic field in a way that makes the image clearer.
Prior to the examination, subjects are provided to remove most or all of their clothing and to wear a gown that does not contain buttons, snaps, zippers, or other metal. All metal objects (keys, jewellery, cell phones, etc.) and other objects that may be affected by magnetic fields (such as credit cards and watches) should be placed outside the MRI laboratory. Subjects should lie still when the video is being taken, and sometimes have to hold their breath. The scanner makes a loud thumping noise, so the subject may be offered to wear headphones or earplugs. Scanning may take 20-60 minutes. After the test, the subject can immediately begin routine activities.
Use of MRI
MRI is preferred over computed tomography (CT) when doctors need more detailed information about soft tissue, for example, when they want to image abnormalities in the brain, spinal cord, muscles, and liver. MRI is particularly useful for identifying tumours in these tissues.
MRI is also used for:
- Measurement of specific molecules in the brain that differentiate between brain tumors and brain abscesses
- Identification of female reproductive organ abnormalities and fractures of the hip and pelvis
- Helps doctors evaluate joint abnormalities (such as a rupture of a knee ligament or cartilage) and sprains
- Help doctors evaluate bleeding and infection
MRI is also used when the risk of CT is high. For example, MRI may be preferred for people who have had a reaction to iodine contrast agents used for CT, or for pregnant women (since radiation can cause problems in the fetus).
MRI, done after a gadolinium contrast agent is injected intravenously, helps doctors evaluate inflammation, tumors, and blood vessels. Injecting these contrast agents into a joint helps doctors get a clearer picture of joint abnormalities, especially in complex cases (such as injuries or degeneration of the knee ligaments and cartilage).
Transformation of MRI
This technique detects metabolic changes that occur when the brain is active. Thus, it can show which parts of the brain are active when the subject performs certain tasks, such as reading, writing, remembering, calculating, and moving limbs.
With this technique, doctors can estimate blood flow to a specific area. This information can be useful in determining whether blood flow to an area of the brain has decreased during a stroke. It can also help identify areas with increased blood flow (such as in a tumour).
This technique detects changes in the movement of water in cells that are not functioning normally. It is primarily used to identify early strokes. It is also used to detect certain brain diseases, to determine if the tumor has spread to the brain, or to distinguish brain abscesses from tumors. The use of this technique to identify areas other than the brain is limited. Diffuse-weighted MRI is usually combined with other techniques to evaluate tumors primarily in areas of the brain.
Magnetic resonance spectroscopy
This technique uses radio waves that are emitted almost continuously rather than pulses in conventional MRI. Magnetic resonance spectroscopy is used to screen for brain diseases such as seizure disorders, Alzheimer’s disease, brain tumours, and brain abscesses. It can differentiate between dead tissue fragments inside the abscess and proliferating cells inside the tumour.
This technique is also used to assess metabolic disorders in the muscle and nervous systems.
Magnetic resonance angiography (MRA)
As with conventional angiography and CT angiography, detailed images of blood vessels can be confirmed with MRA. However, although it is more expensive, the MRA is safer and easier to implement. Often, MRA can be done without injections of contrast agents.
Magnetic resonance angiography shows blood flow through the arteriovenous vein or blood flow in only one direction, so it can only show arteries or veins. As in CT angiography, a computer is used to remove all tissue outside the blood vessels from the image.
Usually, a gadolinium contrast agent is injected intravenously to outline the blood vessels. The examiner carefully sets the scan time to take an image when gadolinium is concentrated in the vessel being evaluated.
The MRA is used to evaluate blood vessels in the brain, heart, abdominal organs, arms, and legs. It is used to screen for:
- Aortic aneurysm
- Aortic dissection
- Narrowing of the arteries in the limb
- Blood clots in the veins of the limbs and pelvis
- Blood flow to the tumour
- Tumours affecting blood vessels
Magnetic resonance venography
Echo plane diagnostic imaging
This ultra-fast technique creates an image sequence in seconds. This technique can be used to image the brain, heart, and abdomen. Because it is fast, the motion of the subject being examined does not blur the image very much. Additionally, this technique can provide information about how an organization is functioning.
However, special equipment is required, and due to the nature of the technique, certain structures are more likely to appear erroneously compared to conventional MRI.