Biomedical Engineering, University of Calgary, 2500 University Dr NW, Calgary, AB, T2N 1N4, Canada.
Hotchkiss Brain Institute, University of Calgary, 2500 University Dr NW, Calgary, AB, T2N 1N4, Canada.
MAGMA. 2019 Dec;32(6):669-677. doi: 10.1007/s10334-019-00769-9. Epub 2019 Jul 23.
Purely exponential decay is rarely observed in conventional mono-exponential T mapping due to transmit field inhomogeneity and calibration errors, which collectively introduce stimulated and indirect echo pathways. Stimulated echo correction (SEC) requires an additional fit parameter for the transmit field, resulting in greater uncertainty in T relative to mono-exponential fitting. The aim of this study was to develop an accurate and precise method for T mapping using SEC.
The proposed method, called two-step SEC (tSEC), leverages spatial correlations in the transmit field to reduce the number of fully independent fitting parameters from three to two. The method involves a two-pass fit: the first pass involves a fast but standard SEC fit. The initially estimated transmit field is smoothed and provided as a fixed input to the second pass.
Simulations and in vivo experiments demonstrated up to 38% and 27% decreases in relative T variance with tSEC relative to SEC. Average T values were unchanged between tSEC and SEC fits. The proposed method uses the same input data as SEC and exponential fits, so it is applicable to existing data.
The proposed method generates reliable and reproducible quantitative T maps and should be considered for future relaxometry studies.
由于发射场不均匀性和校准误差共同引入了激发和间接回波途径，常规单指数 T 映射中很少观察到纯指数衰减。激发回波校正（SEC）需要为发射场添加一个额外的拟合参数，这导致 T 相对于单指数拟合的不确定性更大。本研究旨在开发一种使用 SEC 进行准确、精确 T 映射的方法。
该方法称为两步 SEC（tSEC），利用发射场的空间相关性将完全独立的拟合参数数量从三个减少到两个。该方法涉及两次拟合：第一次拟合是快速但标准的 SEC 拟合。最初估计的发射场进行平滑处理，并作为固定输入提供给第二次拟合。
模拟和体内实验表明，与 SEC 相比，tSEC 可将相对 T 方差降低 38%和 27%。tSEC 和 SEC 拟合之间的平均 T 值保持不变。该方法与 SEC 和指数拟合使用相同的输入数据，因此适用于现有数据。
该方法生成可靠且可重复的定量 T 图，应考虑用于未来的弛豫率研究。
