Magnetic Resonance Imaging - Tutorial I

11994Magnetic Resonance ImagingSão Carlos - SP - BrasilDepartamento de Física e InformáticaInstituto de Física de São Carlos - Universidade de São Paulo - USPH. Panepucci and A. TannúsTutorial Idonupnnn2nunNature of the MR imageThe MR image is actually a map of the very weak magnetization, originated from some ofthe atomic nuclei in the body tissue, in the presence of an external magnetic field. Since thismagnetization is proportional to the density of those nuclei, the MR image shows the distributionof the selected atoms. Because of the large hydrogen concentration, soft tissue is easily seen inproton MR images, that show even the small differences due to their different chemicalcomposition. The resulting soft tissue contrast is thus better that in X- ray techniques and can befurther improved by methods that use the difference in dynamic behavior of the tissueProperties of the atomic nucleiThe atomic nuclei useful for MRI have two properties that are fundamental for the- an intrinsic angular momentum or spin, Ih - a permanent magnetic moment, m = g I hwhere I is the nuclear spin operator, is Planck's constant and g g is called the nuclearhThe above two properties make these nuclei resemble a small magnet spinning around itsom nFigure 1As shown in Figure 1 , the magnetic field B produces a torque, m m x B , on the magneticdipole moment m of the nucleus. This torque, being normal to the angular momentum vector,m to precess ...