Mulkern R V, Bowers J L, Peled S, Williamson D S
Department of Radiology, Children's Hospital, Boston, Massachusetts, USA.
J Magn Reson B. 1996 Mar;110(3):255-66. doi: 10.1006/jmrb.1996.0041.
Citrate detection and quantitation with proton spectroscopic methods are of current interest as potential tools in the diagnosis and staging of prostate cancer. The stimulated echo acquisition mode (STEAM) sequence is a commonly used volume-localization method for detecting citrate signal. Since the 1H citrate resonance at clinically available field strengths arises from a strongly coupled two-spin system, the 90 degrees RF pulses and localizing gradients used in STEAM sequences result in a complicated dependence of signal intensity on timing intervals and gradient amplitudes. The density-matrix formalism has been applied to arrive at a general solution to this problem. Citrate-signal properties at 1.5 T for different gradient localization schemes are examined with the solution. Optimal interpulse delays, deleterious gradient balances, zero-quantum oscillations with mixing time, and a low-frequency, large-amplitude oscillation with echo time are identified for signals acquired with the standard disposition of gradients in STEAM. The generality of the solution also allows for an examination of non-standard gradient disposition schemes for enhancing citrate signal and for quantifying the sensitivity of such approaches to both field inhomogeneities and off-resonance effects.
利用质子光谱法检测和定量柠檬酸盐作为前列腺癌诊断和分期的潜在工具，目前受到关注。受激回波采集模式（STEAM）序列是检测柠檬酸盐信号常用的容积定位方法。由于在临床可用场强下的1H柠檬酸盐共振来自强耦合双自旋系统，STEAM序列中使用的90度射频脉冲和定位梯度导致信号强度对时间间隔和梯度幅度存在复杂的依赖性。密度矩阵形式已被应用以得出该问题的通用解决方案。利用该解决方案研究了1.5T下不同梯度定位方案的柠檬酸盐信号特性。对于用STEAM中标准梯度配置采集的信号，确定了最佳脉冲间延迟、有害梯度平衡、混合时间的零量子振荡以及回波时间的低频、大幅度振荡。该解决方案的通用性还允许研究用于增强柠檬酸盐信号的非标准梯度配置方案，并量化此类方法对场不均匀性和失谐效应的敏感性。
