At this point, we have referred to MR in two different ways: nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). NMR is the analytical technique of exciting and obtaining MR signal from an entire sample, breaking the measured signal into its individual frequency components to identify different molecules containing the nucleus of interest, but not separating the signal by location within the sample. In contrast, MRI is the technique of separating a sample or region of tissue into individual volume elements (voxels) and producing images based on the total signal from the nucleus of interest in each voxel. The techniques of dividing tissue into voxels to produce MR images will be discussed in Chapter 3. This chapter describes tissue relaxation times that provide the contrast seen in MR images. Relaxation times are strictly MR-based parameters that describe the re-growth of longitudinal magnetization (T1) and loss of transverse magnetization (T2) after a radiofrequency (RF) pulse flips magnetization out of alignment with the externally applied static magnetic field, B0.
KeywordsTransverse Magnetization Cystic Fluid Breast Magnetic Resonance Imaging Nuclear Magnetic Resonance Spectrometer Magnetic Field Inhomogeneity
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