Magnetic Resonance Imaging 1121 Principles of MRI

MRI is a clinical imaging modality that produces three-dimensional anatomic images with high spatial resolution and provides in vivo physiological properties including physiochemical information, flow diffusion, and motion in the tissues. The physics of MRI is complicated and has been detailed in a number of monographs.17,18 The fundamental principles of MRI are briefly reviewed in this section. A normal human adult has approximately 60% of the body weight as water. Water protons have a magnetic moment and the orientations of the magnetic moments are random in the absence of an external magnetic field. When a patient is placed in a MRI scanner, the proton magnetic moments align either along or against the static magnetic field (B0) of the scanner and create a net magnetization pointing in the direction of the main magnetic field of the scanner. The magnitude of the magnetization is proportional to the external magnetic field strength as well as the amount of protons.

Nuclear magnetic resonance (NMR) measures the change of the magnetization by applying radiofrequency (RF) pulses. When an RF pulse is applied to create an oscillating electromagnetic field (Bj) perpendicular to the main field, the longitudinal magnetization is tipped away from the static magnetic field and deviates from the equilibrium state (in longitudinal space). A perfect 90° RF pulse flips the total longitudinal magnetization onto the transverse plane. Immediately after excitation with the RF pulse, the transverse magnetization then undergoes the decay process. The decaying transverse magnetization induces an NMR signal as an electromotive force (emf) on an RF coil. Simultaneously, the nuclear spin system approaches toward the equilibrium state with a characteristic recovery time Tj, which depends upon the physical and chemical environment of the molecules.

Longitudinal Tj relaxation involves the return of protons from the high energy state back to equilibrium by dissipating their excess energy to their surroundings. The process is also called spinlattice relaxation. Transverse (T2) relaxation involves the transfer of the spin angular momentum among the protons via the interactions such as dipole-dipole interaction. The process is called spin-spin relaxation. In biological systems, the T2 relaxation time is typically shorter than T relaxation time. Because the main magnetic field of MRI scanners is not perfectly homogenous, the inhomogeneity of the magnetic field also causes dephasing of individual magnetizations of protons, resulting in more rapid loss of transverse magnetization. The process is called T2* relaxation.

In MR imaging, the position dependent frequency and phase are encoded into the transverse magnetization, and the measured signals are Fourier transformed to construct the spatial map (image). MR images are created based on the proton density, or T and T2 relaxation rates, and flow and diffusion properties in different tissues. These parameters vary between different tissues and create image contrast among the tissues. Tissues with a short T relaxation time give a strong MR signal, resulting in bright images in Tj-weighted MRI. Tissues with a short T2 relaxation time produce a weak MR signal, resulting in dark images in T2-weighted MRI.

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