What is magnetic resonance imaging (MRI)?
Magnetic resonance imaging, or MRI, is a non-invasive medical imaging test that produces detailed images of almost all internal structures in the human body, including organs, bones, muscles, and blood vessels. Magnetic resonance imaging scanners create images of the body using large magnets and radio waves. Unlike X-rays, magnetic resonance imaging does not produce radiation. These images provide your doctor with important information to diagnose your medical condition and plan treatment.
Why magnetic resonance imaging is done?
Magnetic resonance imaging can detect a variety of conditions of the brain such as cysts, tumors, bleeding, swelling, developmental and structural abnormalities, infections, inflammatory conditions, or problems with the blood vessels. It can determine if a shunt is working and detect damage to the brain caused by an injury or a stroke.
MRI of the brain can be useful in evaluating problems such as persistent headaches, dizziness, weakness, and blurry vision or seizures, and it can help to detect certain chronic diseases of the nervous system, such as multiple sclerosis.
In some cases, magnetic resonance imaging can provide clear images of parts of the brain that can’t be seen as well with an X-ray, CAT scan, or ultrasound, making it particularly valuable for diagnosing problems with the pituitary gland and brain stem.
How does an MRI work?
An MRI works by using a powerful magnet, radio waves, and a computer to create detailed images. Your body is magnetic with millions of hydrogen atoms (80% of the human body is water). When you place your body in a magnetic field, these molecules merge with the field like compass points toward the north pole.
A radio wave “knocks down” the atoms and alters their polarity. The sensor detects the time it takes for the molecules to return to their original arrangement. In essence, magnetic resonance imaging measures the water content (or fluid properties) of different tissues, which is then processed by a computer to create a black and white image. The image is very detailed and shows even the smallest anomaly.
Similar to CT, magnetic resonance imaging allows your doctor to view your body as narrow pieces, each about an inch thick. For example, imagine that you are slicing bread and taking a picture of each slice. You can see parts from the bottom (axillary), front (coronal), or shoulders (sagittal) depending on what your doctor wants to see.
A dye (contrast agent) may be injected into the bloodstream to enhance certain tissues. The color contains gadolinium, which has magnetic properties. It circulates through the bloodstream and fuses with certain tissues, then placed in a scan. Magnetic resonance angiography (MRA). MRI can be used to look at arteries and veins.
A standard magnetic resonance imaging does not see the fluid moving like blood in the artery and creates “flow voids” that appear as black holes in the image. A contrast medium (gadolinium) injected into the bloodstream helps the computer “see” the arteries and veins. The contrast medium is used to look for tumors and arterial abnormalities (AVMs).
How safe is an MRI?
The powerful magnetic field of an MRI system can attract objects made of certain metals (i.e., called ferromagnetism) and cause them to move abruptly and with great force. This can endanger the patient or anyone in the “flight path” of the object. Therefore, care must be taken not to introduce external objects such as ferromagnetic screwdrivers and oxygen tanks into the area of the magnetic resonance imaging system.
As a patient, you may need to remove all metal objects prior to the MRI exam, including external hearing aids, watches, jewelry, cell phones, and clothing that contains metal thread or fasteners. Also, makeup, nail polish, or other cosmetics that contain metallic particles should be removed if magnetic resonance imaging is applied to the affected body area.
The powerful magnetic field of the MRI system attracts the medical implant, some aneurysm clips, or an action pump on any object in the body that contains iron. Each magnetic resonance imaging facility includes a comprehensive screening procedure and protocols. If carefully followed, these steps will allow the magnetic resonance imaging technician and radiologist to be aware of the presence of any metallic and material implants in the patient.
In general, special precautions can be taken. In some rare cases, the test may need to be canceled due to an unacceptable implant or device. For example, an MRI examination cannot be performed if there is a ferromagnetic aneurysm clip because the clip is at risk of moving and can cause serious harm to the patient.
In some cases, some medical implants can heat up significantly during the MRI exam, resulting in the radiofrequency energy used for the procedure. This heating can cause injury to the patient. Therefore, it is important to inform the magnetic resonance imaging technician of any implants or other internal objects you may have before entering the MRI scanner room.
The strong magnetic field of the magnetic resonance imaging system can damage external hearing aids or cause malfunction or injury of the cardiac pacemaker, electrical stimulator, or neurostimulator. If you have a bullet or any other piece of metal in your body, it is likely to change position and cause injury.
Also, a metal implant or other object can cause signal loss or disrupt the magnetic resonance imaging images, making it difficult for the radiologist to view the images correctly. This may be unavoidable, but if the radiologist knows it, compromises can be made when obtaining and interpreting the MR images.
For some MRI tests, a contrast material called gadolinium may be injected intravenously to help enhance the information seen on magnetic resonance imaging images. Unlike contrast materials used in x-ray exams or computed tomography (CT), gadolinium does not contain the contrast agent iodine and therefore rarely causes an allergic reaction or other problems.
However, if you have a history of kidney disease, kidney failure, kidney transplant, liver disease, or other conditions, you should inform the MRI technologist and/or radiologist before taking the gadolinium contrast agent. If you are unaware of these conditions, discuss these issues with a magnetic resonance imaging technologist or radiologic before having an MRI exam.
During an MRI scan
A radiologist or MRI technician will usually perform the scan in a hospital, clinic, or imaging center using specialized equipment.
- You lie on a movable table that slides into the MRI machine. The machine looks like a long metal tube.
- Depending on the part of your body that you want to check, you can place a small coil in that part of the body to send radio waves and receive the MRI signal.
- Your technician will see you from another room. You can talk to him or her through the microphone. In some cases, a friend or family member may be in the room with you.
- The MRI machine creates a strong magnetic field around you and pictures of radio waves are obtained in your body. It does not experience magnetic fields or radio waves.
- During an MRI, the magnet makes pressing sounds or loud knocks and other noises. You can fit earplugs or listen to music with headphones to help prevent noise.
- In some cases, you may have an intravenous (IV) line in your hand or arm to inject the contrast agent into your veins (for MRA). The contrast agent produces better images of your tissues and blood vessels. It does not contain iodine and is less likely to cause an allergic reaction compared to the agents used for computed tomography (CT).
- The MRI lasts between 30 and 90 minutes.
- You should still lie down during the test as movement can blur your body images. If you do not feel comfortable indoors, tell your doctor before the test. You can get narcotics to keep calm. Some clinics have machines with fewer magnets or wider openings to make you feel more comfortable.
After an MRI
- You should move slowly while getting up from the scanner table to avoid dizziness or minor headaches while lying down during the procedure.
- If you take sedatives for this procedure, you will need to rest until they are exhausted. You should also avoid driving.
- Call your doctor right away if you use contrast dye during the procedure and if you experience any side effects or reactions to the dye, such as itching, swelling, a rash, or trouble breathing after your appointment. If you think it is a fatal emergency, call 911.
- If you notice any pain, redness, and/or swelling at the IV site after you return home, you should inform your doctor, as this may indicate an infection or other type of reaction.
- Otherwise, no special care is required after the magnetic resonance imaging. You can resume your normal diet and activities unless your doctor tells you otherwise.
- Your doctor may give you further instructions or alternate instructions after the procedure, depending on your specific situation.