Body Magnetic Resonance Imaging (i.e., MRI)
MRI (magnetic resonance imaging) is an imaging technique used to get cross-sectional images using strong magnetic field and radio waves (radio frequency pulses) instead of X-rays.
In MRI scanning, patients are placed inside a very large and strong magnet, so that all the protons in the atoms of the patient's body can be aligned to a magnetic field. Then, radio waves (called radio frequency pulses) are directed at the protons -- i.e., the nuclei of hydrogen atoms -- to excite the protons. Once the radio waves are stopped, excited atoms emit radio signals received by an antenna (i.e., a surface coil in the MRI machine), which are then measured and processed to form an image using a computer.
In the human body, protons are most abundant in the hydrogen atoms of water. Thus, MRI images represent the water content in the area of the exam. The more water present, the more radio signals emitted, and the whiter the image.
Body MRI provides highly detailed images of the organs of the chest and abdomen, including the lungs, liver, kidney, spleen, pancreas, uterus, prostate gland, and blood vessels.
Types of body MRI. There are four main applications:
- Abdominal MRI
- Male pelvis MRI
- Female pelvis MRI
- Thorax MRI
Provides cross-sectional images in any anatomical plane (upper to lower, right to left, front to back, oblique).
Provides clear, detailed images of various soft tissues, such as internal organs, tumors, and blood vessels.
Compared to the iodine-based contrast medium in X-ray or CAT scan, MRI dye is relatively safe.
Provides angiographic images without being invasive
MRI can evaluate organ function as well as structure.
Abdominal MRI -- diagnosis and evaluation of tumors in internal organs, such as liver, kidney, spleen, pancreas, abdominal vessels
Male pelvis MRI -- used to detect and evaluate prostate cancer
Female pelvis MRI -- used to detect and evaluate endometrial tumor, cervix cancer
Thorax MRI (except heart, aorta) -- used to diagnose and evaluate tumors of the lungs, chest wall, and mediastinum
- Tumors of the liver -- hemangioma, metastases
- Tumors of adrenal gland -- pheochromocytoma, metastases
Male Pelvis MRI:
- Staging of carcinoma of bladder, prostate, or seminal vesicles
Female Pelvis MRI:
-Staging of carcinoma of the bladder, cervix, endometrium, ovary, or rectum
Thorax MRI (Except heart, aorta):
- Staging of lung cancer
- Evaluation of vascular lesions
You will be asked to lie on the scan table. After proper positioning, the exam table will slide into the center of the magnet.
During the scan you will be alone in the exam room, but you can talk to and listen to the technologist using an intercom, who will watch you through a glass window and video camera.
During actual scanning, you will hear loud tapping noises, but you are required to remain still until it's done (to get clear pictures).
In the event that a contrast medium is needed to make organs and blood vessels stand out, it will be injected into your vein during the exam.
The exam usually takes from 15 minutes to an hour half.
Wear comfortable, loose-fitting clothing.
Take off your clothes in the examining area and put on a hospital gown. Remove any metal objects such as watches, car keys, wallets, beepers, cellular phones, zippers, snaps, hairpins, jewelry, accessories, eyeglasses, hearing aids and any removable dental work, because they can be affected by the huge magnet and degrade quality imaging.
The information on credit cards can be erased by this strong magnetic field.
Let your radiologist or technologist know when you have a metal object in your body, such as a cardiac pacemaker, prosthetic heart valve, prosthetic hip or knee joint, implanted infusion pump, intrauterine device (IUD), cochlear implant, aneurysm clip or vascular clips, hearing aid, metal monitoring device, surgical staples, metal plates, pins, screws, bullets, shrapnel or any metal fragments. This is because the strong magnetic fields can cause these ferromagnetic metal objects to move, dislodge, cause burns, or electrical currents.
Tattoos may degrade image quality.
For contrast-enhanced MRI, you will be asked if you have any drug allergies.
If you are pregnant or suspect pregnancy, you should inform your technologist or radiologist.
If you have any history of claustrophobia, you should inform your radiologist or technologist of it. Sedatives can be given before scanning.
Earplugs can be used to protect your ear from loud repetitive noises during scanning.
A radiologist (a physician specialist trained to interpret MRI images or other radiology exams, such as CAT scans, X-ray, mammography, etc.) reviews the body MRI and reports the results to your personal doctor.
The physician's office informs patients when the results are in, and what the results are. The office uses the results as a reference in evaluating and treating patients.
The strong magnetic field can cause metal implants to dislodge, burn, and cause additional injuries. When you have any metal implant in your body, such as a pacemaker, prosthetic valves, or clips, you should let your radiologist or technologist know its brand name and model. If it is not confirmed to be compatible with the magnet, you should not take the MRI. If you have had bullet injuries or possible metal fragments in your body, X-rays can be taken instead for detection of metal objects.
Metallic implants/fragments that might be affected by magnet:
Women in the first 12 weeks of pregnancy should avoid MRI and the contrast medium. There are no known harmful effects to pregnant women and unborn babies. However, because it is a recently-developed technology, the long term effects of MRI are not known.
Allergic reactions to the contrast medium are possible, but very rare.
Claustrophobia. Lying alone inside the tunnel of huge magnet may provoke intense fear of confinement. If you have any history of claustrophobia, you should inform your radiologist or technologist. Sedatives can be given before scanning.
- Cardiac pacemakers
- Artificial cardiac valves
- Cerebral aneurysm clips, vascular clips
- Epidural electrodes
- Prosthetic hip
- Prosthetic knee
- Implanted port (brand names: Port-o-cath, Infusaport, Lifeport)
- Chemotherapy or insulin pumps
- Intrauterine device (IUD)
- Metallic bone plates, pins, screws, surgical staples, especially within 4-6 weeks after surgery.
- Bullets, shrapnel, possible metal fragments, especially in and around eyes
- Metallic ear implants
Limited ability in imaging bone -- conventional X-ray or CAT scan is better in demonstrating bone details.
MRI is less sensitive in demonstrating acute hemorrhage when compared to CAT scans.
Hard to depict calcifications
MRI does not always distinguish between tumor tissue and edema fluid
Less sensitive in detecting small abnormalities compared to CAT scan (poor spatial resolution)
Inability to scan critically-ill patients requiring life-support systems and monitoring devices that employ ferromagnetic materials
May be dangerous in scanning patients with metal implants and other metal objects
May provoke claustrophobia
Longer exam time compared to CAT scans
Safety in scanning pregnant women is not known.