Calamante Fernando, Vonken Evert-jan P A, van Osch Matthias J P
Brain Research Institute, Melbourne, Australia.
Magn Reson Med. 2007 Sep;58(3):544-53. doi: 10.1002/mrm.21362.
Measurement of the concentration of the contrast agent using dynamic susceptibility contrast MRI relies on field inhomogeneities caused by the presence of the paramagnetic agent. The usual method for calculation of the concentration from dynamic T2*-weighted images is based on two key assumptions: 1) a linear relation between the change in R2* and the contrast agent concentration, and 2) a negligible effect on the MR signal due to concurrent T1 changes. In this study the effect of inaccuracies in these two assumptions on perfusion measurements was investigated using simulations and in vivo data. The results of the simulations provide a quantitative characterization of the magnitude of these effects for various experimental conditions (e.g., when a 1-sec TR is used with TE=20 ms, the T1 effects can introduce up to 40% cerebral blood flow underestimation depending on the flip angle). These findings can be used as a guide to estimate the errors in specific practical implementations, as well as to optimize the sequence parameters to minimize their effect. In summary, this study shows that the arterial input function measurement should be corrected for nonlinear R2* effects and that care should be taken in the study design to avoid introducing significant T1 effects in perfusion quantification.
使用动态磁敏感对比磁共振成像测量造影剂浓度依赖于顺磁性造影剂的存在所导致的磁场不均匀性。从动态T2加权图像计算浓度的常用方法基于两个关键假设：1）R2的变化与造影剂浓度之间存在线性关系；2）同时发生的T1变化对磁共振信号的影响可忽略不计。在本研究中，使用模拟和体内数据研究了这两个假设中的不准确因素对灌注测量的影响。模拟结果对各种实验条件下这些影响的大小进行了定量表征（例如，当使用1秒的重复时间且回波时间为20毫秒时，根据翻转角的不同，T1效应可导致高达40%的脑血流量低估）。这些发现可作为估计特定实际应用中误差的指南，也可用于优化序列参数以尽量减少其影响。总之，本研究表明，动脉输入函数测量应针对非线性R2*效应进行校正，并且在研究设计中应注意避免在灌注定量中引入显著的T1效应。
