DENSITY-MATRIX CALCULATIONS OF THE 1.5 T CITRATE SIGNAL ACQUIRED WITHVOLUME-LOCALIZED STEAM SEQUENCES

Citrate detection and quantitation with proton spectroscopic methods a re of current interest as potential tools in the diagnosis and staging of prostate cancer, The stimulated echo acquisition mode (STEAM) sequ ence is a commonly used volume-localization method for detecting citra te signal. Since the H-1 citrate resonance at clinically available hel d strengths arises from a strongly coupled two-spin system, the 90 deg rees RF pulses and localizing gradients used in STEAM sequences result in a complicated dependence of signal intensity on timing intervals a nd gradient amplitudes, The density-matrix formalism has been applied to arrive at a general solution to this problem, Citrate-signal proper ties at 1.5 T for different gradient localization schemes are examined with the solution, Optimal interpulse delays, deleterious gradient ba lances, zero-quantum oscillations with mixing time, and a low-frequenc y, large-amplitude oscillation with echo time are identified for signa ls acquired with the standard disposition of gradients in STEAM, The g enerality of the solution also allows for an examination of nonstandar d gradient disposition schemes for enhancing citrate signal and for qu antifying the sensitivity of such approaches to both field inhomogenei ties and off-resonance effects. (C) 1996 Academic Press, Inc.