MAGNETIC RESONANCE IMAGING

MRI can reveal subtle differences between areas of dissimilar anatomy, physiology, and pathology that are not possible with other imaging techniques. Other advantages include the following:

1. Unlike conventional radiographs and CT, MRI can obtain images in any plane (transverse, sagittal, coronal, or oblique).
2. MRI provides excellent soft tissue contrast.
3. It may provide intravascular contrast without the need for intravenous contrast media.
4. Very little patient preparation is required.
5. MRI does not produce ionizing radiation, is noninvasive, and does not in itself produce biologically deleterious effects.

Principles of MRI

A number of detailed descriptions of the technical aspects of MRI have been published.^{[36] [37] [38] [39]} Briefly, atomic nuclei with an odd number of protons or neutrons have the potential to act as magnetic dipoles. This property is possessed by all paramagnetic elements (¹ H,¹³ C, ¹⁹ F, ²³ Na, and ³¹ P). Biologic tissues have a high water content. Thus, ¹ H is present in high concentrations, and its detection is the basis of MRI. The single-proton ¹ H can be understood to behave like a tiny bar magnet with north and south poles. Absent a strong magnetic field, hydrogen protons are oriented randomly and do not exhibit any net magnetic field. If they are placed in a strong external magnetic field, the hydrogen protons will align themselves parallel or antiparallel to the applied magnetic field, and the tissue itself will exhibit a sight net magnetism, with the magnetic vector aligned with the MRI field. This tissue magnetism results from the nuclei that are aligned antiparallel to the external magnetic field; they are in a slightly higher energy state than those aligned parallel to the field. This energy difference increases with the strength of the applied magnetic field. In MRI, tissue magnetized by the powerful static magnetic field is suddenly transiently exposed to a second magnetic field aligned perpendicularly to the static field. The transient second magnetic field is generated by a pulse of RF energy. The RF pulse deflects the magnetic vector of ¹ H by altering proton alignment. When the transient RF pulse is terminated, the nuclei return to their original alignment within the static magnetic field, which causes a weak transient RF signal as the protons return to their previous energy state. The RF signal emitted by the tissues is detected by an RF coil that is placed in close proximity to the patient. The intensity of the received signals is plotted in gray scale, and an image is built up by sophisticated computers. The characteristics of the decay of the emitted signal, which is called "relaxation," provide most of the information for generating the image. Relaxation time is measured in two ways: T1 relaxation is the time taken to return to the resting magnetic vector, and T2 is the time taken to return to resting axial spin. Different tissues demonstrate different relaxation characteristics. Multiple, sequential RF pulses within the static field can thus be used to emphasize particular tissues or abnormalities by taking advantage of the relaxation differences between tissues.

The magnetic field strength used in MRI scanners is measured in tesla units. One tesla equals 10,000 gauss, with the strength of the earth's magnetic field at its surface being between 0.5 and 1.0 gauss. Field strengths used in most clinical MRI examinations vary from 0.05 to 2.0 T, but the use of 3-T units is increasing. The advantage of weaker-field MRI scanners is that more open construction can used, and the advantage of those with stronger fields is that they produce better image quality. Stronger-field MRI scanners generally use cryogenic magnets with superconductivity coils operating in liquid helium (4° K).

Hydrogen proton density and relaxation dynamics as detected by MRI vary in different tissues in accordance with the tissue's physical and chemical properties. Because the differences in water content and proton environment in gray and white matter are greater than the differences in electron densities as measured by CT, MRI can produce better resolution between gray and white matter, and the same is also applicable to soft tissue examinations. MRI permits evaluation of blood flow, cerebrospinal fluid flow, contraction and relaxation of organs, and because calcium does not emit a signal in MRI, images of tissues surrounded by bone. This lack of a calcium signal, however, prevents MRI from detecting pathologic calcification in tumors of soft tissue and pathologic changes in cortical bone.^[40]

The RF signal obtained during MRI scanning is of very low intensity and is subject to interference from stray high-frequency electronic radiation such as FM radio signals and electromagnetic emissions from electronic equipment or monitoring devices.^[36] For this reason, the scanning area is enclosed in an RF shield incorporated into the structure of the MRI suite. Preventing interference from monitoring devices situated within the RF shield can be challenging. Attempts have been made to overcome this problem by using isolated power sources or battery power, filtering, or enclosing the monitoring device in its own small RF shield, or any combination of these measures. The implications of this problem for the anesthesiologist are obvious.