(MR) Resonance phenomenon resulting in the absorption and/or emission of electromagnetic energy by nuclei (for that reason also nuclear magnetic resonance) or electrons in a static magnetic field, after excitation by a suitable RF magnetic field.
The peakresonance frequency is proportional to the magnetic field, and is given by the Larmor equation. Only unpaired electrons or nuclei with a spin exhibit magnetic resonance. The absorption or emission of energy by atomic nuclei in an external magnetic field after the application of RF excitation pulses using frequencies, which satisfy the conditions of the Larmor equation.
The magnetic resonance phenomenon may be used in one of these ways:
By manipulation of the external field (application of gradient fields), the resonance frequency can become dependent on spatial location, and hence images may be generated (MRI).
The effect of the electron cloud in any atom or molecule is to slightly shield the nucleus from the external field, thus giving any chemical species a characteristic frequency. This gives rise to 'spectra' where nuclei in a molecule give rise to specific signals, thus facilitating the detection of individual chemicals by means of their frequencyspectra (MRS)
(RFOV) A different field of view (the scanned region) in the frequency and phase encoding directions that means the data acquisition with fewer measurement lines. Because there are fewer rows than columns, a rectangular image is obtained. To reduce the FOV in phase encoding direction (foldover direction) saves scan time by decreasing signal but invariable spatial resolution.
Also called HFI or undersampling.
Image Guidance
If the scanned object is oval, e.g. head or abdomen, a rectangular FOV is an easy to use scan parameter to reduce the scan time without loss of resolution.
(SPGR) The SPGR pulse sequence is similar to the spoiled GRASS sequence. The spoiled gradient recalled (SPGR)
acquisition in steady state uses semi-random changes in
the phase of the radio frequency (RF) pulses to produce a spatially independent phase shift.