What is MRI?
MRI is a diagnostic procedure that uses a combination of a large magnet, radio frequencies, and a computer to produce detailed images of organs and structures within the body. MRI does not use ionizing radiation, as do X-rays and computed tomography (CT scans).
Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field temporarily realigns hydrogen atoms in your body. Radio waves cause these aligned atoms to produce very faint signals, which are used to create cross-sectional MRI images.
The MRI machine can also be used to produce 3-D images that may be viewed from many different angles.
MRI is a noninvasive way for your doctor to examine your organs, tissues and skeletal system. It produces high-resolution images that help diagnose a variety of problems.
MRI of the brain and spinal cord:
MRI is the most frequently used imaging test of the brain and spinal cord.
It’s often performed to help diagnose:
- Disorders of the eye and inner ear
- Multiple sclerosis
- Spinal cord injuries
Functional MRI of the brain (fMRI) can be used to identify important language and movement control areas in the brain in people who are being considered for brain surgery.
How does an MRI scan work?
The MRI machine is a large, cylindrical (tube-shaped) machine that creates a strong magnetic field around the patient and sends pulses of radio waves from a scanner. The strong magnetic field causes the hydrogen atoms in your body to align along the same axis. The radio waves knock the nuclei of the atoms in your body out of this aligned position. As the nuclei realign back into proper position, they send out radio signals. These signals are received by a computer that analyzes and converts them into an image of the part of the body being examined. This image appears on a viewing monitor. Cross-sectional views can be obtained to reveal further details. Some MRI machines look like narrow tunnels, while others are more open.
Magnetic resonance imaging (MRI) may be used instead of computed tomography (CT) in situations where organs or soft tissue are being studied, because MRI is better at telling the difference between different types of soft tissues, as well as the difference between normal and abnormal soft tissues..
Because ionizing radiation is not used, there is no risk of exposure to ionizing radiation during an MRI procedure.
Due to the use of the strong magnet, MRI cannot be performed on most patients with implanted pacemakers, older intracranial aneurysm clips, cochlear implants, certain prosthetic devices, implanted drug infusion pumps, neurostimulators, bone-growth stimulators, certain intrauterine contraceptive devices, or any other type of iron-based metal implants. MRI is also not recommended for people who have internal metallic objects such as bullets or shrapnel, as well as most surgical clips, pins, plates, screws, metal sutures, or wire mesh in their bodies. Dyes used in tattoos may contain iron and potentially could heat up during an MRI, but this is a rare occurrence.
Newer uses and indications for MRI have contributed to the development of additional magnetic resonance technology. Magnetic resonance angiography (MRA) is a procedure used to evaluate blood flow through arteries in a noninvasive (the skin is not pierced) manner. MRA can also be used to detect aneurysms within the brain and vascular malformations (abnormalities of blood vessels within the brain, spinal cord, or other parts of the body).
Magnetic resonance spectroscopy (MRS) is another noninvasive procedure used to assess chemical abnormalities in body tissues such as the brain. MRS may be used to assess disorders, such as HIV infection of the brain, stroke, head injury, coma, Alzheimer’s disease, tumors, and multiple sclerosis.
Functional magnetic resonance imaging of the brain (fMRI) is used to determine the specific location within the brain where a certain function, such as speech or memory, occurs. The general areas of the brain in which such functions occur are known, but the exact location may vary from person to person. During functional resonance imaging of the brain, you will be asked to perform a specific task, such as recite the Pledge of Allegiance, while the scan is being done. By pinpointing the exact location of the functional center in the brain, doctors can plan surgery or other treatments for a particular disorder of the brain.
Another advance in MRI technology is the “open” MRI. Standard MRI units have a closed cylinder-shaped tunnel into which the patient is placed for the procedure. Open MRI units do not completely surround the patient, and some units may be open on all sides. Open MRI units are particularly useful for procedures involving:
- Children. Parents or other caregivers may stay with a child during the procedure to provide comfort and security.
- Claustrophobia. Before the development of open MRI units, persons with severe claustrophobia often required a sedative medication prior to the procedure.
- Very large or obese persons. Almost anyone can be accommodated in most open MRI units.
How is an MRI performed?
An MRI may be performed on an outpatient basis, or as part of inpatient care. Although each facility may have specific protocols in place, generally, an MRI procedure follows this process:
- Because of the strong magnetic field, the patient must remove all jewelry and metal objects, such as hairpins or barrettes, hearing aids, eyeglasses, and dental pieces.
- If a contrast medication and/or sedative are to be given by an intravenous line (IV), an IV line will be started in the hand or arm. If the contrast is to be taken by mouth, the patient will be given the contrast to swallow.
- The patient will lie on a table that slides into a tunnel in the scanner.
- The MRI staff will be in another room where the scanner controls are located. However, the patient will be in constant sight of the staff through a window. Speakers inside the scanner will enable the staff to communicate with and hear the patient. The patient will have a call bell so that he or she can let the staff know if he or she has any problems during the procedure.
- During the scanning process, a clicking noise will sound as the magnetic field is created and pulses of radio waves are sent from the scanner. The patient may be given headphones to wear to help block out the noises from the MRI scanner and hear any messages or instructions from the technologist.
- It is important that the patient remain very still during the examination.
- At intervals, the patient may be instructed to hold his or her breath, or to not breathe, for a few seconds, depending on the body part being examined. The patient will then be told when he or she can breathe. The patient should not have to hold his or her breath for longer than a few seconds, so this should not be uncomfortable.
- The technologist will be watching the patient at all times and will be in constant communication.
The Role of MRI in Multiple Sclerosis:
Because MRI is particularly useful in detecting central nervous system demyelination, it is a powerful tool in helping to establish the diagnosis of MS. However, approximately 5 percent of people with clinically-definite MS do not initially show lesions on MRI at the time of diagnosis. If repeat MRIs continue to show no lesions, the diagnosis of MS should be questioned.
- Since many lesions seen on MRI may be in so-called “silent” areas of the brain that don’t produce symptoms, it is not always possible to make a specific correlation between what is seen on the MRI scan and the person’s clinical signs and symptoms.
- In addition, with advancing age (probably over age 50), there are often small areas seen on MRI in healthy people that resemble MS but are actually related to the aging process.
Clinically Isolated Syndrome (CIS)
MRI is particularly helpful in patients who have had a single demyelinating attack that is suggestive of MS, also called a clinically isolated syndrome (CIS).
The number of lesions on an initial MRI of the brain (or spinal cord) can help the physician assess the person’s risk of developing a second attack (and therefore “clinically-definite MS”) in the future. Some of the treatments for MS have been shown to delay the occurrence of a second episode of symptomatic demyelination in people who have had only one.
The MRI can also be used to identify a second neurological event in a person who has no additional symptoms — thereby helping to confirm a diagnosis of MS as early as possible.
Tracking disease progress
Once a diagnosis of MS has been clearly established, no additional MRI scans are needed for diagnostic purposes. However, subsequent scans are important for tracking the progress of the disease and making treatment decisions. For example, a neurologist may consider disease activity on MRI as well as a person’s clinical symptoms and relapses in order to determine whether the current treatment is effective or a change in treatment needs to be considered.
Healthcare professionals differ in their opinion about how often an MRI should be done for MS, but most now recommend follow-up MRIs on a yearly basis. When possible, follow-up MRIs should be obtained on the same scanner as this will help the radiologist and your healthcare provider make a comparison from one MRI to the next.
Different scan types provide different information
A T1-weighted brain MRI scan, enhanced with gadolinium (injected intravenously to further enhance scan sensitivity), supplies information about current disease activity by highlighting areas of active inflammation. Because gadolinium is a large molecule, it normally cannot pass through the blood-brain barrier (a cell layer around blood vessels in the brain and spinal cord that prevents substances from passing from the blood stream into the central nervous system). However, when there is active inflammation, the blood brain barrier is disrupted and gadolinium can enter and highlight the inflamed areas.
- These areas of inflammation appear as active lesions, meaning that they are new or getting bigger.
- T1-weighted images also show dark areas (hypointensities) that are thought to indicate areas of permanent nerve damage.
- T2-weighted images provide information about disease burden or lesion load (meaning the total amount of lesion area, both old and new).
FLAIR (fluid attenuated inversion recovery) images are used to better identify brain lesions associated with MS.
Spinal cord imaging can identify pathology in the cord. It can also help establish the diagnosis of MS by demonstrating that damage has occurred in different parts of the central nervous system (dissemination in space) at different points in time (dissemination in time).
Although other types of scans are used for research purposes, these are the ones most commonly used in clinical care.
Different magnets provide different information
The strength of the magnet used in the MRI machine is important to the quality of the images. Magnet strength is measured in Tesla (T).
Most conventional MRI machines are 1.5T or 2.0T.
Open MRIs are usually less than 1.5T and do not provide the best images for detecting MS activity, although they may be used when someone has difficulty tolerating a closed MRI machine.
MRI machines used for research purposes have much higher T.
MRI Other Medical Conditions:
MRI of the heart and blood vessels:
An MRI that focuses on the heart or blood vessels can assess:
- The size and function of the heart’s chambers
- Thickness and movement of the walls of the heart
- The extent of damage caused by heart attack or heart disease
- Structural problems in the aorta, such as aneurysms or dissections
- Inflammation or blockages in the blood vessels
MRI of other internal organs:
An MRI may be used to check for tumors or other abnormalities of many organs in the body, including the:
MRI of bones and joints:
MRI may be used to help evaluate:
- Joint disorders, such as arthritis
- Joint abnormalities caused by traumatic or repetitive injuries
- Disk abnormalities in the spine
- Bone infections
- Tumors of the bones and soft tissues
MRI of the breasts:
MRI may be used in addition to mammography to detect breast cancer, particularly in women who have dense breast tissue or who may be at high risk of the disease.
The presence of metal in your body may be a safety hazard or affect a portion of the MRI image. Before receiving an MRI, tell the technologist if you have any metal or electronic devices in your body, such as:
- Metallic joint prostheses
- Artificial heart valves
- An implantable heart defibrillator
- A pacemaker
- Metal clips
- Cochlear implants
- A bullet, shrapnel or any other type of metal fragment
Before you schedule an MRI, tell your doctor if you think you’re pregnant. The effects of magnetic fields on fetuses aren’t well understood. Your doctor may recommend choosing an alternative exam or postponing the MRI.
It’s also important to discuss any kidney or liver problems with your doctor and the technologist, because problems with these organs may limit the use of injected contrast agents during your scan.
After the test:
If you haven’t been sedated, you may resume your usual activities immediately after the scan.