Fig. 1. Schematic diagram of a common setup of a shielded high-field superconducting magnet. RF, radiofrequency.

Two independent but simultaneous spin magnetization processes have been described: Tl, or so-called longitudinal/spin-lattice, and T2, or transverse/ spin-spin relaxations. After an RF pulse stimulation, the proton nuclei absorbs energy to move to an excited state which, in order to return to the ground state, energy needs to be transferred to neighboring nuclei or paramagnetic ions and molecules that are fluctuating or precessing at the same Larmor frequency. This energy transfer can occur between the nuclei of the same (intramolecular) or different molecules (intermolecular). A short Tl (300-500 ms) suggests an efficient energy transfer whereby there is approx 67% signal recovery in that period (Fig. 2). Lipid is a medium-sized molecule that tumbles at a rate closest to the Larmor frequency of bound water, thus possessing a short Tl, and appears as bright signal on Tl-weighted images. Free water tumbles too fast and large molecules too slow, both thereby exhibiting long Tl relaxation, leading to low signal on Tl images.

In contrast to spin-lattice relaxation, transverse magnetization describes the rate of loss of magnetic phase coherence secondary to magnetic field (local environment) imperfections in which not all nuclei are precessing at the same frequency. Larger molecules are more efficient at T2 relaxation (~67% signal loss in that period) with faster signal loss, whereas free water (like cerebrospinal fluid [CSF]) has longer T2 (ms to s) and therefore exhibits bright signal on T2-weighted images (Fig. 3). Unlike Tl relaxation, transverse magnetization is independent of external field strength. The term proton density is merely a reflection of the number of proton spins in a particular region where the imaging parameters reflect neither Tl nor T2 weighting, i.e., long TR (repetition time)

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