Sensitivity Enhancement in Magnetic Resonance. II. Investigation of Intermediate Passage Conditions

Abstract

Intermediate passage conditions in magnetic resonance experiments are investigated to determine the optimum performance conditions for maximum sensitivity, particularly in connection with time averaging methods. The possible increase in signal height by increasing the sweep rate can be utilized in time averaging methods to enhance the sensitivity. The optimization of the conditions described here is based on numerical solutions of the Bloch equations. The results are given in graphical form and show the optimum conditions for many magnetic resonance experiments where the absorption mode signal is recorded. The improvement of the sensitivity, which is possible if instead of a slow passage, single scan experiment a fast passage, multiple scan experiment is made, is higher for a high ratio of the relaxation times T₁/T₂. The possible gain is limited by the maximum allowed line broadening and line shift. The conditions are nearly optimum if the sweep rate a (cps/sec) and the rf field strength γH₁ (rad/sec) are related by a/(γH₁)²=0.332. Additionally, it is shown that the shift of the resonance maximum due to the intermediate passage conditions can have a negative sign and that an increase of the sweep rate can cause a narrowing of the resonance line under certain conditions.