MRI does not involve radiation. Magnetic fields and radio waves are used to measure and analyze multiple tissue parameters such as hydrogen (proton) density, T1 and T2 relaxation times of tissue, and tissue blood flow. Magnetic resonance (MR) provides soft tissue contrast, which is significantly better than any other imaging technique. The tissues are identified by characteristic differences of the T1 and T2 relaxation times. T1 measures how quickly the proton can be magnetized and T2 measures how quickly the protons lose their magnetisms 13 m a is m
Because of this property, the different tissue types can be measured for distinct characteristic rates of energy release using a strong primary magnetic field to align the protons in one direction. A radiofrequency pulse is then used to knock these protons out of alignment. Once the radiofrequency pulse is terminated, the energy released by the realignment of the protons is monitored, localized, and processed using a computer algorithm for creation of a tomographic anatomic image. The "cut" of the images can be manipulated by adjusting the direction of the orientation of the x-axis, the y-axis, and the z-axis of the magnetic field. Different manufacturers use three major pulse sequence techniques and their variations to form MR images. These techniques now make a single breath-hold image possible, but they decrease the induction and monitoring times for image creation. With early MR techniques, the movement of breathing decreased the quality of the images. Gradient recalled echo pulse sequences are now used to perform fast MR and MR angiography by the application of "flip angles" (i.e., < 90 degrees), which intensify the signals during T2 relaxation by creating magnetic field distortions. This process causes a T2* (T2 star) time, which is shorter than the "true" T2 decay times of spin echo (SE) imaging.13
SE imaging uses pulse sequences of standard Tl-weighted imaging, T2-weighted imaging, and proton density-weighted images. The Tl-weighted imaging is important for obtaining the best anatomic details and for identifying subacute hemorrhages and fat. T2-weighted imaging enhances the detection of pathologic lesions by exploiting the differences in the T2 relaxation times. With the proton density-weighted images, the tissue layers are emphasized, making this scan the most useful for brain imaging.
Inversion recovery pulse sequences take advantage of the differences in T1 relaxation times between tissues. In particular, the tissues with a short T1 time produce a brighter signal. To take advantage of this property, short T1 inversion recovery (STIR) sequences are most frequently used. Stated simply, the short T1 relaxation time tissues, like fat, are suppressed (dark), and the high water content tissues such as muscles are enhanced in STIR images. This yields images that accentuate pathologic lesions against a dark background. Echo planar imaging can be performed in 20 seconds. MRI is the standard for noninvasive soft tissue imaging.
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