Magnetic resonance imaging (MRI) scan requires the use of a very strong magnetic field. Unlike other devices used in radiology, MR imaging uses no radiation. The magnet is contained in the housing of the scanner and this creates a magnetic field oriented down the center of the magnet.
The patient is placed within the magnetic field by lying on a table which is placed through the center of the opening of the magnet, similar to lying on a road running through a tunnel.
The strength of the magnetic field is measured in units called gauss or Tesla: 10,000 gauss equals 1 Tesla. The earth's magnetic field is approximately 0.6 gauss. The strongest magnetic field permitted in MRI scanning of humans is 1.5 Tesla (1.5T).
Three types of magnets are available for use in MRI. Most MRI scanners in use today are superconductng magnets. Resistive magnets are electromagnets, similar to superconducting magnets, but they are air cooled therefore have greater resistance to current and create weaker magnetic fields.
Permanent magnets are made of solid magnetic material, similar to bar magnets, and create the weakest magnetic fields. However, they can be arranged in a configuration that doesn't require the patient to be surrounded by the magnet and are used in Open MR scanners.
The strongest is a superconducting magnet. This is a type of electromagnet in which current flowing in a circular direction in a coil of wire creates a magnetic field oriented down the core of the coil. In superconducting magnets, the wire conducts the current without significant resistance because it is cooled to a temperature close to absolute zero by being bathed in a jacket of liquid helium and/or liquid nitrogen.
The picture shows the actual magnet (the outer container resembles a thermos and contains the superconducting wire surrounded by liquid helium).
The physics of MRI are extremely complex. When a patient is placed within and MR scanner, the protons in the patients tissues (primarily protons contained in water molecules) align themselves along the direction of the magnetic field.
A radiofrequency electromagnetic pulse is then applied, which deflects the protons off their axis along the magnetic field. As the protons realign themselves with the magnetic field, a signal is produced. This signal is detected by an antenna, and with the help of computer analysis, is converted into an image.
The process by which the protons realign themselves with the magnetic field is referred to as relaxation. The protons undergo 2 types of relaxtion: T1 (or longitudinal) relaxation and T2 (or transverse relaxation) relaxation.
Different tissues undergo different rates of relaxation, and these differences create the contrast between different structures, and the contrast between normal and abnormal tissue, seen on MRI scans.
T1 weighted images emphasize the difference in T1 relaxation times between different tissues. In these images, water containing structures are dark. Since most pathologic processes (such as tumors, injuries, CVA's, etc.) involve edema (or water), T1 weighted images do not show good contrast between normal and abnormal tissues. However, pathologic processes do demonstrate excellent anatomic detail.
T2 weighted images emphasize the difference in T2 relaxation times between different tissues. Since water is bright on these images, T2 weighted images provide excellent contrast between normal and abnormal tissues, although the anatomic detail is less then that of T1 weighted images.
Proton density images emphasize neither T1 or T2 relaxation times, and therefore produce contrast based primarily on the amount of protons present in the tissue.
Intravenous contrast is often used to improve the sensitivity of MR imaging, especially in the brain and spine.
MR contrast agents contain gadolinium, which increases T1 relaxation and causes certain abnormalities to "light up" on T1 weighted images. These agents contain no iodine, and allergic reactions are extremely rare.