Magnetic Resonance Imaging Health Article

Magnetic resonance imaging

MRI produces a map of hydrogen atoms distributed in the body. Hydrogen is the simplest element known, the most abundant in biological tissue, and one that can be magnetically polarized. It will align itself within a strong magnetic field, like the needle of a compass. The earth's magnetic field is not strong enough to polarize a person's hydrogen atoms, but the superconducting magnet of an MRI machine can do this. The strength of the earth's magnetic field is approximately 1 gauss. Typical field strength of an MRI unit, with a superconducting magnet, is 1,500 gauss, expressed as 1.5 kilogauss or 1.5 Tesla units. This comprises the "magnetic" part of MRI. There are also low field units with 0.5 Tesla strength, often with open MRI units.

Once a patient's hydrogen atoms have been aligned in the magnet, pulses of very specific radio wave frequencies jolt them out of alignment. The hydrogen atoms alternately absorb and emit radio wave energy, vibrating back and forth between their resting (polarized) state and their agitated (radio pulse) state. This comprises the "resonance" part of MRI. The patient does not detect this process.

The MRI equipment detects the duration, strength, and source location of the signals emitted by the atoms as they relax. This data is translated into an image on a television monitor. The amount of hydrogen in diseased tissue differs from the amount in healthy tissue of the same type, making MRI particularly effective at identifying tumors and other lesions. In some cases, chemical agents such as gadolinium can be injected to improve the contrast between healthy and diseased tissue.

A single MRI exposure produces a two-dimensional image of a slice through the entire target area. A series of these image slices closely spaced (usually less than half an inch [1.25 cm]) provides a virtual three-dimensional view of the area.

Definition

Magnetic resonance imaging (MRI) is a unique and versatile medical imaging diagnostic tool. Using MRI, physicians obtain highly refined images of the body's interior. Strong magnetic fields and pulses of radio waves manipulate the body's natural magnetic, producing images not possible with other diagnostic imaging methods. MRI is particularly useful for imaging the brain and spine, as well as the soft tissues of joints and the interior structure of bones. The entire body can be imaged using MRI, and the technology poses few known health risks.

Purpose

MRI was developed in the 1980s. The latest additions to MRI technology are magnetic resonance angiography (MRA) and magnetic resonance spectroscopy (MRS). MRA studies blood flow, while MRS identifies the chemical composition of diseased tissue and produces color images of brain function. The many advantages of MRI include:

• Detail. MRI creates precise images of the body based on the varying proportions of magnetically polarizable elements in different tissues. Very minor fluctuations in chemical composition can be determined. MRI images have greater subject contrast than those produced with standard x rays, computed tomography (CT), or ultrasound, all of which depend on the differing physical properties of tissues. This contrast sensitivity lets MRI distinguish fine variations in tissues deep within the body. It is particularly useful for spotting and distinguishing diseased tissues (tumors and other lesions) early in their development. Often, physicians prescribe an MRI scan to more fully investigate earlier findings from other imaging techniques.
• Scope. All body parts can be imaged using MRI. Moreover, MRI scans are not adversely affected by bone, gas, or body waste, which can hinder other imaging techniques. (The scans can, however, be degraded by motion such as breathing, heartbeat, and normal bowel activity.) A close series of two-dimensional images can provide a three-dimensional view of a targeted area. Unlike other techniques, MRI can provide images in multiple planes.
• Safety. MRI does not depend on potentially harmful ionizing radiation, as do standard x rays and CT scans. There are no known risks specific to the procedure, other than for people who have metal objects in their bodies.

Physicians sometimes choose other imaging techniques, such as ultrasound scanning, because the MRI process is complex, time-consuming, and costly. The process requires large, expensive, and complicated equipment; a highly trained operator; and a physician specializing in radiology. Generally, MRI is prescribed only when serious symptoms or negative results from other tests indicate a need. In many cases, an alternative imaging procedure is more appropriate for the type of diagnosis needed. However, some diseases such as multiple sclerosis are best imaged by MRI.

Physicians may prescribe an MRI scan of different areas of the body.

• Brain and head. MRI technology was developed because of the need for brain imaging. It is one of the few imaging tools that can see through bone (the skull) and deliver high quality pictures of the brain's delicate soft tissue structures. MRI may be needed for patients with symptoms of a brain tumor, stroke, or infection (such as meningitis). MRI also may be needed when cognitive or psychological symptoms suggest brain disease (such as Alzheimer's or Huntington's diseases, or multiple sclerosis), or when developmental retardation suggests a
  birth defect. MRI can also provide pictures of the sinuses and other areas of the head beneath the face.
• Spine. Spinal problems can cause neck or back pain, or numbness or weakness in the arm or leg. MRI is particularly useful for identifying and evaluating degenerated or herniated intervertebral discs. It can also be used to determine the condition of nerve tissue within the spinal cord.
• Joints. MRI scanning is often used to diagnose and assess joint problems. MRI can provide clear images of the bone, cartilage, ligaments, and tendons that comprise a joint. MRI can be used to diagnose joint damage due to sports, advancing age, or arthritis. It can also be used to diagnose shoulder problems, such as a torn rotator cuff. MRI can detect the presence of an otherwise hidden tumor or infection in a joint, and can be used to diagnose the nature of developmental joint abnormalities in children.
• Skeleton. The properties of MRI that allow it to see though the skull also allow it to view the interior of bones. It can be used to detect bone cancer, inspect the marrow for leukemia and other diseases, assess bone loss (osteoporosis), and examine complex fractures.
• The rest of the body. While CT and ultrasound satisfy most chest, abdominal, and general body imaging needs, MRI may be required to provide more detailed images in certain circumstances, or when repeated scanning is necessary. MRI is also used in cases when the progress of therapy, such as liver cancer treatment, needs to be monitored, and the effect of repeated x ray exposure is a concern.