special subjects
Magnetic Resonance Imaging
Introduction
MRI uses magnetic fields and radio waves to produce images of thin slices of tissues (tomographic images). Normally, protons within tissues spin to produce tiny magnetic fields that are randomly aligned. When surrounded by the strong magnetic field of an MRI device, the magnetic axes align along that field. A radiofrequency pulse is then applied, causing the axes of many protons to momentarily align against the field in a high-energy state. After the pulse, protons relax and resume their baseline alignment within the magnetic field of the MRI device. The magnitude and rate of energy release that occurs as the protons resume this alignment (T1 relaxation) and as they wobble (precess) during the process (T2 relaxation) are recorded as spatially localized signal intensities by a coil (antenna) built within the MRI device. Computer algorithms analyze these signals and produce detailed anatomic images. The relative signal intensity (brightness) of tissues in an MRI image is determined by factors such as The radiofrequency pulse and gradient waveforms used to obtain the image Intrinsic T1 and T2 characteristics of different tissues The proton density of different tissues By controlling the radiofrequency pulse and gradient waveforms, computer programs produce specific pulse sequences that determine how an image is obtained (weighted) and how various tissues appear. Images can be T1-weighted T2-weighted Proton density–weighted For example, fat appears bright (high signal intensity) on T1-weighted images and relatively dark (low signal intensity) on T2-weighted images; water and fluids appear relatively dark on T1-weighted images and bright on T2-weighted images. T1-weighted images optimally show normal soft-tissue anatomy and fat (eg, to confirm a fat-containing mass). T2-weighted images optimally show fluid and abnormalities (eg, tumors, inflammation, trauma). In practice, T1- and T2-weighted images provide complementary information, so both are important for characterizing abnormalities. Recently introduced high-resolution MRI scanners increase image quality and diagnostic accuracy and produce a wide variety of additional pulse sequences to further characterize tissues and tumors. 