Magnetic resonance imaging (MRI) scanning requires the use of a very strong magnetic field but does not use ionizing radiation. The magnet is contained in the housing of the scanner and creates a magnetic field oriented down the bore 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. 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. Most MRI scanners contain superconducting magnets with a field strength of approximately 1.5 Tesla. In superconducting magnets, the wire conducts the current without resistance because it is cooled to a temperature close to absolute zero (4 K) by being bathed in a jacket of liquid helium. The electrical current flows continuously around many loops, creating the required magnetic field.
The physics of MRI are extremely complex, and only an extremely simplified explanation is given. When a patient is placed within an MR scanner, the protons in the patients tissues (primarily protons contained in water molecules) will tend to align themselves along the direction of the magnetic field. A radio-frequency 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 two types of relaxation: 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 tissues. T1-weighted images emphasize the difference in T1 relaxation times between different structures. In these images, watercontaining
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, they 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. Figure bellow shows a typical T1-weighted image. Figure 12 shows the difference between T1 and T2-weighted images.
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. MRI images can be obtained in any imaging plane without moving the patient. The three standard views usually used
1. Transverse (axial): viewed as if standing at the patient's feet (similar to standard CT).
2. Coronal: Viewed from the front.
3. Sagittal: Viewed from the side.
MR angiography (MRA) permits imaging of the blood vessels in several parts of the body. The most frequent vessels studied include the Circle of Willis (and its main branches) in the brain and the carotid bifurcation in the neck.
The basic principle is that special pulse sequences are used by the MR scanner which causes flowing blood to appear very bright and all stationary tissue to appear very dark. If arterial structures are being studied, additional pulses are applied to erase the signal in veins. Multiple slices are obtained at adjacent levels through the region of interest. A computer then stacks these images on top of each other and creates a 3-D
image similar in appearance to a contrast angiogram, although no contrast agent is used. The constructed images can be rotated 360 degrees so that the vessels can be studied in all projections. Figure below is an example of the raw images created by the scanner. Note that the vessels are bright and all of the surrounding tissues are dark.
Although multiple studies have been performed, no significant permanent biological hazards have been demonstrated as a result of exposure to patients from the magnetic fields or radio-frequency electromagnetic pulses used in magnetic resonance imaging. However, there can be adverse effects on various medical devices implanted into patients and therefore all patients must be carefully screened to determine if MR
scanning can be safely performed. Potential risks include the following:
1. Cardiac pacemakers: Absolute contraindication. These patients cannot be scanned.
2. Cerebral aneurysm clip: Unless there is documented proof that a non-ferromagnetic clip was used, these patients cannot be scanned.
3. Metal fragments in body (bullet, BB, shrapnel, etc.): Safe, unless in contact with vital organ, such as heart, spinal cord, eye.
4. Surgical clips: Safe. Flow and Atherosclerosis (Research Topic) There appears to be a relationship between the hemodynamics of blood flow (details of the fluid flow such as
shear) and the progression of vascular disease. Unfortunately, only simple flow factors such as velocity can be measured in vivo (e.g. with MRA) while the important hemodynamic factors cannot. One way around this problem is to use MRI to determine the size and shape of an artery (or, for example, the carotid bifurcation which is a common location for vascular disease), MRA to determine the pulsatile velocity at the input and output of the vessel, and to use a computer to simulate the actual flow pattern through the vessel so as to match the input and output conditions.
calculated flow pattern through the carotid artery of a healthy
volunteer. The regions of high shear can be determined in this
calculation. In the future, it may be possible to perform this
calculation and identify patients with an elevated risk of
atherosclerotic plaque development.
the appearance of the MRA. The same data is used to create sagittal, transverse and coronal images