Magnetic resonance imaging

Magnetic resonance imaging (MRI) - also called magnetic resonance tomography (MRT) - is a method of creating images of the inside of opaque organs in living organisms as well as detecting the amount of bound water in geological structures. It is primarily used to demonstrate pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging. MRI has also found many niche applications outside of the medical and biological fields such as rock permeability to hydrocarbons and certain non-destructive testing methods such as produce and timber quality characterization http://www.mri.cl/index.pl/industrial_stud#355.

Background

Nomenclature

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. The original name for the medical technology is nuclear magnetic resonance imaging (NMRI), but the word nuclear is almost universally dropped. This is done to avoid the negative connotations of the word nuclear, and to prevent patients from associating the examination with radiation exposure. Scientists still use NMR when discussing non-medical devices operating on the same principles.

Related Topics:
Nuclear magnetic resonance - Radiation

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Technique

Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water. When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of the atomic nuclei with non-zero spin numbers, within the tissue all align in one of two opposite directions: parallel to the magnetic field or antiparallel.

Related Topics:
Hydrogen - Magnetic field - Spins - Atomic nuclei - Parallel - Antiparallel

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The difference in the number of parallel and antiparallel nuclei is only about one in a million. However, the vast quantity of nuclei in a small volume sum to produce a detectable change in field. Most basic explanations of NMR and MRI will say that the nuclei align parallel or anti-parallel with the static magnetic field, however, because of quantum mechanical reasons beyond the scope of this article, the nuclei are actually set off at an angle from the direction of the static magnetic field.

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The magnetic dipole moment of the nuclei then precess around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency. The tissue is then briefly exposed to pulses of electromagnetic energy (RF pulse) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state. The frequency of the pulses is governed by the Larmor Equation.

Related Topics:
Electromagnetic - Larmor Equation

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In order to selectively image the different voxels (3-D pixels) of the material in question, three orthogonal magnetic gradients are applied. The first is the slice selection, which is applied during the RF pulse. Next comes the Phase encoding gradient, and finally the frequency encoding gradient, during which the tissue is imaged. Most of the time, the three gradients are applied in the X, Y, and Z directions of the machine so that the patient is sliced from head to toe, however, MRI is especially useful because various combinations of the gradients can be combined during the process so that slices can be taken in any orientation.

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As the high-energy nuclei relax and realign, they emit energy which is recorded to provide information about their environment. The realignment with the magnetic field is termed longitudinal relaxation and the time in milliseconds required for a certain percentage of the tissue nuclei to realign is termed "Time 1" or T1. This is the basis of T1-weighted imaging.

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T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time is termed "Time 2" or T2. Both T1- and T2-weighted images are acquired for most medical examinations. Often, a paramagnetic contrast agent, a gadolinium compound, is administered, and both pre-contrast T1-weighted images and post-contrast T1-weighted images are obtained.

Related Topics:
Paramagnetic - Gadolinium

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In order to create the image, spatial information must be recorded along with the received tissue relaxation information. For this reason, magnetic fields with an intensity gradient are applied in addition to the strong alignment field to allow encoding of the position of the nuclei. A field with the gradient increasing in each of the three dimensional planes is applied in sequence. When received, the signals are recorded in a temporary memory termed K-space; this is the spatial frequency weighting in two or three dimensions of a real space object as sampled by MRI. The information is subsequently inverse Fourier transformed by a computer into real space to obtain the desired image. Detailed anatomical information results. Typical medical resolution is about 1 mm3, while research models can exceed 1 µm3.

Related Topics:
Inverse Fourier transformed - Computer

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