Magnetic Resonance Imaging (MRI)

Introduction
Magnetic Resonance Imaging (MRI) has evolved into one of the most powerful non-invasive techniques in diagnostic clinical medicine and biomedical research. The technique is an application of nuclear magnetic resonance (NMR), a well known analytical method of chemistry, physics and molecular structural biology. MRI is primarily used as a technique for producing anatomical images, but as described below, MRI also gives information on the physical-chemical state of tissues, flow diffusion and motion information. Magnetic Resonance Spectroscopy (MRS) gives chemical/composition information.

Theory and Instrumentation
Most elements have at least one reasonably abundant isotope whose nucleus is magnetic. In biological materials, the magnetic nuclei of 1H, 13C, 23Na, 31P, and 39K are all abundant. The hydrogen nucleus (a single proton) is abundant in the body due to the high water content of non-bony tissues. When the body is immersed in a static magnetic field, slightly more protons become aligned with the magnetic field than against the static field. At 0.25 T (2500 gauss) and 25°C the difference between these aligned populations of about one proton in a million produces a net magnetization. A rapidly alternating magnetic field at an appropriate resonant frequency in the radio frequency (RF) range, applied by a coil near the subject or specimen in the static magnetic field, changes the orientation of the nuclear spins relative to the direction of the static magnetic field (Figure 1).

(a)	(b)
Figure 1. The concept of NMR. The effect of a static magnetic field on tissue nuclei: (a) shows the natural state, with randomly spinning nuclear moments (b) shows alignment of nuclei after application of a magnetic field

These changes are accompanied by the absorbtion of energy (from the alternating magnetic field) by nuclei which undergo the transition from a lower energy state to a higher one. When the alternating field is turned off, the nuclei return to the equlibrium state, emitting energy at the same frequency as was previously absorbed. The nuclei of different elements, and even of different isotopes of the same element, have very different resonance frequencies. For a field of 0.1 T (1000 gauss), the resonance frequency of protons is 4.2 MHz and that of phosphorus is 1.7 MHz. Thus, the magnetic nuclei in the body, when placed in a static magnetic field, can be thought of as tuned receivers and transmitters of RF energy.

The principal components of the MRI machine are the magnet, radiofrequency (rf) coils and the gradient coils. Magnet types in current use are of the superconducting, resistive and permanent magnet designs ranging in strength from 0.08 to 4 T (T = 10,000 gauss). The majority of MR systems use superconducting magnets which provide fields of high strength and stability. Most currently produced magnets are based on niobium-titanium (NbTi) alloys, which are remarkably reliable, but require a liquid helium cryogenic system to keep the conductors at approximately 4.2 Kelvin (-268.8 Celcius). The rf coils used to excite the nuclei usually are quadrature coils which surround the head or body, but small (e.g. 6-10 cm) flat coils placed on the surface of the head or body are also used. Besides being the essential element for spatial encoding, the gradient-coil sub-system of the MRI scanner is responsible for the encoding of specialized contrast such as flow information, diffusion information, and modulation of magnetization for spatial tagging.

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© 2003 UC Regents. Last updated June 17, 2003 7:31 PM