Signal, noise, and contrast in nuclear magnetic resonance (NMR) imaging

Calculations of the sensitivity of the saturation recovery and inver-sion recovery pulse sequences used in nuclear magnetic resonance imaging show the former to be superior in discriminating between tissues with the same proton density but different T₁'s. Two other pulse sequences, which are combinations of the above, have also been analyzed. These have lower T₁ discrimination sensitivity, but other considerations, such as self-normalization, may still make them attractive. The calculations are only valid for selective excitation pulse sequences in which the selected slice profiles are approximately rectangular, and thus a sin(bt)/t radiofrequency excitation is desirable. In order to ensure that the saturation recovery sequence gives valid results for pulse repetition times comparable to or shorter than T₂, it is necessary to destroy the coherence between pulse applications. For this purpose we use a series of “spoiler” gradient pulses between pulse trains. The saturation recovery pulse sequence also has the advantage that, by the correct choice of interpulse spacing, sensitivity close to the optimum T₁, discrimination can be achieved over a wide range of T₁ values. This has the potential advantage to the clinician of simplifying his choice of parameters for imaging.