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Magnetic resonance imaging (MRI) has proven to be a valuable tool to radiologists since its introduction in the early 1 970’s (Esser and Johnson, 1984). The advent of stable high-field, superconducting magnets has made it possible to carry out high resolution MRI microscopy experiments in the laboratory environment (Callaghan, 1991). When nuclei, such as hydrogen are placed in a static magnet field, the spins of the nuclei will align with the external field. These nuclei will then reach a state of thermal equilibrium with respect to their environment. A radiofrequency (RF) signal, which is dependent on the magnetic field strength, is applied to the sample that is in thermal equilibrium. The nuclei absorb this energy and flip out of alignment with the magnetic field. After the RF pulse, the system is allowed to return to thermal equilibrium and the energy given off is detected. The frequency of the energy that is emitted is known as the Nuclear Magnetic Resonance (NMR)) frequency. In MRI this signal is spatially encoded by the selective application of three magnetic field gradients along the X, Y, and Z axis. The spatially encoded NMR signal is Fourier transformed into a peak, it is then converted into a gray scale image based on the intensity of the resultant peak. The brighter the area of the image, the more intense the signal which is related to the quantity of nuclei present in the sample and to their environment. |
