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Cerebral blood flow; Cerebral blood volume; Arterial spin labelling; Echo planar image; Larmor frequency
Abstract:
In magnetic resonance imaging (MRI), the signal that is measured usually arises from the nuclei of the tissue’s hydrogen atoms (i.e. protons). A proton possesses a physical property, its spin, which behaves roughly speaking like a compass needle: each spin has a small magnetic dipole moment and aligns in an external magnetic field. If tissue is brought into the strong magnetic field inside the magnetic resonance (MR) scanner bore, spins will align either antiparallel or parallel to the magnetic field B. At the field strengths relevant here, a tiny majority of the spins assume the latter alignment and their magnetic moments add up, giving rise to a net macroscopic magnetisation M which is parallel to B, representing a state of equilibrium (Fig. 1, left). Thus, the existence of this magnetisation inside the magnetic field is an indicator of the presence of protons, and the measurement of M with a certain spatial resolution can be used to construct a proton image.
