Understanding How MRI Works

MRI, or Magnetic Resonance Imaging, doesn't use X rays like CT scan, but uses a strong static magnetic field and radio-frequency energy to generate images. MRI and CT scans offer tomographic imaging studies that produce images of discrete slices of tissue and eliminate confusing shadows from adjacent overlying structures as in a traditional X ray. This allows the doctor to see detailed images of soft tissues, bone, fat, muscles and internal organs in our body.

In an MRI, the patient lies in a motorized chamber surrounded by huge powerful magnets produced by passing an electric current through coils of wire. Other coils placed around the part of the body being imaged send and receive pulses of radio waves, producing signals that are detected by the coils. Hydrogen atoms in water in the tissues get magnetized and aligned by the powerful magnetic field, and the Radio-frequency fields systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce signals that are detected by the scanner.

Different tissues of the body produce different signals, depending on their physical and biochemical properties, and it is possible to evaluate several physiological factors such as blood flow or cerebrospinal fluid flow. MRI scanners convert data based on the location and strength of the incoming signals to determine relative tissue density and display them as three-dimensional images on a monitor or transfer to x-ray film. The collected data is reconstructed into a two dimentional image through any axis of the body. Each of the images show a thin slice of the body, which can be studied from different angles by the doctor.

MRI scanners are best suited for imaging soft tissue. Bone is virtually void of water and therefore does not generate any image data. This leaves a black area in the images. Bones and cartilage undergo change as a result of aging and injury, and MRI records such information well. 

Patients with spine fractures and acute head trauma are more often advised MRI than CT scan, yet the two technologies are complementary. MR is currently being used for the diagnosis of aneurysms and stenosis of blood vessels. For patients who may be at increased risk for conventional radiographic methods, MRI offers a safer and better alternative.  

Magnetic resonance imaging produces complex images in 256 levels of gray in any of planes for viewing — sagittal (left/right), coronal (front/back), axial (head/toe), and oblique (slanted) —  without the need to move the patient. Abdominal organs such as the pancreas and adrenal glands are virtually invisible to standard X rays, but are visualized using CT and MRI, and it offers contrast between bones, tissues and fluids.

• The patient is sometimes asked to ingest a contrasting substance (iodine, barium or gadolinium) containing elements of a higher atomic number than the surrounding flesh. Iodine can cause  allergic reactions in certain people. Gadolinium-based contrast agents are contra-indicated for patients with kidney failure, as this may lead to Nephrogenic Systemic Fibrosis (NSF) or Nephrogenic Fibrosing Dermopathy (NFD). This causes muscles to weaken and the joints to stiffen. Patients with chronic liver disease are warned against the use of gadolinium too.

• You should tell the technologist if you have electronic devices in your body, because they may interfere with the examination or potentially pose a risk. People with pacemakers, aneurysm clips, or other implants that contain magnetic materials are generally advised not to undergo MRI testing. Metal and electronic objects are not allowed in the exam room. The patient must remove jewelry, body piercings, hearing aids, hairpins, metal zippers, dentures, metal dental fillings, pens, pocketknives and eyeglasses, as they can interfere with the magnetic field of the MRI unit. People with the pacemaker, cochlear implants and surgical clips, stencils, plates or screws should inform their physicians.

•    CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized.
•    MRI can differentiate visually any abnormal tissue from normal tissues better than a CT scan. Whereas MRI provides better detail about soft tissues, CT scan provides better detail about bony structures. That makes CT scans better at detecting osteporosis or plaque-forming calcium deposits in the arteries.
•    Also, an MRI scan can take pictures from almost every angle, whereas a CAT scan shows only axial (horizontal) pictures.
•    CT scanning uses x-rays. It is believed that prolonged exposure to ionizing radiation can destroy cells and lead to cancer. Unless underlying symptoms warrant it, the diagnostic testing by CT scan should be used sparingly. Pregnant women are advised to avoid abdominal CT scans because of potential harm to the fetus.
•    During an MRI scan you may have to remain motionless for up to an hour, which can be difficult. Some people, especially those who are obese, can feel claustrophobic, or have difficulty lying still. They are usually given a mild sedative or advised other diagnostic methods.