MRI Overview - CBIC, Amy Chan and Ralph Lee:
Brief overview of MRI and NMR.
CBIC Web Notes provided by Amy Chan, BS & Ralph E. Lee, EdD, RT, Copyright 2006
Magnetic Resonance Imaging (MRI) uses radiofrequency waves and a strong magnetic field to provide clear and detailed images of internal anatomy. Biologic tissue, when placed within a strong magnet, can be induced to give off a detectable signal immediately following stimulation by a pulse of RF (radio frequency) energy.Different types of tissue (muscle, fat, cerebral spinal fluid, etc) each give off measurably different types of “tissue specific” signal following the administration of the same RF pulse. It is these predictable differences between the signal from one tissue vs. another tissue type that allows MRI to produce meaningful images. For example, the water signal from the white matter of the brain appears quantifiably different in intensity than the water signal emitted from other tissue types in the brain such as grey matter.
One of the chief advantages of MRI as a research tool is the fact that, unlike similar anatomic imaging modalities like CAT (computerized axial tomography), MRI does not use ionizing forms of “x-ray” energy (potentially harmful to living tissue) in the formation of its images. This built-in safety feature makes MRI an ideal non-invasive tool for the imaging of healthy volunteers in order to, among other things, better understand the physical and physiologic norms of brain anatomy and function. And additional benefit of MRI, as opposed to CAT, is MRI’s ability to identify physiologic characteristics of tissues (i.e., diffusion coefficients, magnetization transfer characteristics, and chemical composition spectra) in addition to providing detailed anatomic information.
Historically, today’s term for imaging biologic tissue, MRI, stems from the earlier term nuclear magnetic resonance (NMR). NMR was/is used to describe the MRI phenomena as it is commonly applied to the analysis of chemical and/or geologic specimens. The acronym NMR stands for:
Nuclear(N): refers to the spin of some nucleus.
Magnetic(M): refers to an assortment of magnetic fields (3.0T, 4.7T, 7.0T, 9.4T).
Resonance(R): refers to the need to match the (radio) frequency of an oscillating magnetic fields to the processional frequency of the spin of some nucleus.
Imaging(I): refers to the ability, flexibility and sensitivity to produce images.
Both chemical NMR and biologic MRI exploit the nuclear magnetism exhibited by certain “magnet friendly” nuclei such as 1H, 19F, 13C and 31P. The most common nucleus visualized by MRI is 1H (the atom of hydrogen as it appears in biologic tissue) because it is the predominant element in water. The composition of most biological tissue is dominated by water in some form, and it is the difference in water environment between different tissues that MRI visualizes so effectively (i.e., the difference between hydrogen as it appears in complex molecules like fat, as opposed to hydrogen as it appears in simple molecules such as H2O).
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