Principles of MR Imaging

Abstract

In an external magnetic field, the magnetic moments of spin of hydrogen nuclei align themselves and create macroscopic magnetization in the direction of the field. This magnetization is rotated by 90 degrees into the transverse plane when appropriate electromagnetic radiation is applied. The magnetization then precesses and shrinks in the transverse plane with a time constant T2, because the magnetic moments of spin spread out with time. This process is known as transverse relaxation. After transverse relaxation, the magnetization builds up in the direction of the external magnetic field with a time constant T1, because the magnetic moments of spin begin to realign in the direction of the external magnetic field. This second process is referred to as longitudinal relaxation. In fact, the magnetization decays much faster in the transverse plane due to inhomogeneities of the external magnetic field than it would on the basis of transverse relaxation alone. When another appropriate form of electromagnetic radiation is delivered, fanned-out magnetic moments of spin will refocus and reestablish the magnetization. This technique, which can cancel the influence of inhomogeneities, is called the spin-echo sequence. This magnetization precesses around the axis of the external magnetic field and induces an echo signal in a coil. TE is the time between a 90-degree rotation of the magnetization and echo-signal production, and TR is the time between consecutive 90-degree rotations of the magnetization. T2-weighted images are generated using a long TE and a long TR, and T1-weighted images using a short TE and a short TR.