Signal, Noise and R. F. Power in Magnetic Resonance Imaging
The influences on magnetic resonance imaging of probe design, relaxation times, chemical shift and power deposition are considered in the light of the controversy surrounding the notion of an optimal imaging field strength. It is shown that the noise in the experiment originates in the patient, and that in consequence, the signal-to-noise ratio (S/N) increases, at the most, linearly with field. The removal of chemical shift artifacts is discussed, and the use of a Carr-Purcell sequence recommended as a way of maintaining signal-to-noise ratio while eliminating artifacts. The concept of desired image S/N is then introduced, and it is shown that there exists a vaguely defined field strength at which that ratio is attained in the minimum time. Finally, the dangers of excessive power deposition in a patient are highlighted, and the use of a field in the range 0.5 to 1 Tesla advocated as giving good signal-to-noise ratio and versatility while not exceeding recommended power limits.
KeywordsChemical Shift Larmor Frequency Chemical Shift Image Power Deposition Nuclear Magnetic Resonance Chemical Shift
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