Physics of magnetic resonance imaging

Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body and the physics of the technique involves the interaction of matter with electromagnetic fields. The human body is largely composed of water molecules, each containing two hydrogen nuclei, or protons. When inside the magnetic field (B₀) of the scanner, the magnetic moments of these protons align with the direction of the field.

A radio frequency pulse is then applied, causing the protons to alter their magnetization alignment relative to the field. In response to the force bringing them back to their equilibrium orientation, the protons undergo a rotating motion (precession), much like a spin wheel under the effect of gravity. These changes in magnetization alignment cause a changing magnetic flux, which yields a changing voltage in receiver coils to give the signal. The frequency at which a proton or group of protons in a voxel resonates depends on the strength of the local magnetic field around the proton or group of protons. By applying additional magnetic fields (gradients) that vary linearly over space, specific slices to be imaged can be selected, and an image is obtained by taking the 2-D Fourier transform of the spatial frequencies of the signal (a.k.a., k-space). Due to the magnetic Lorentz force from B₀ on the current flowing in the gradient coils, the gradient coils will try to move. The knocking sounds heard during an MRI scan are the result of the gradient coils trying to move against the constraint of the concrete or epoxy in which they are secured.

Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates (i.e., they have different relaxation times). By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.

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