CREATIS, Université de Lyon, Lyon, France.
Invest Radiol. 2012 Nov;47(11):662-70. doi: 10.1097/RLI.0b013e31826ef901.
Diffusion-weighted imaging (DWI) and the introduction of the intravoxel incoherent motion (IVIM) model have provided a unique method for evaluating perfusion and diffusion within a tissue without the need for a contrast agent. Despite its relevance, cardiac DWI has thus far been limited by low b values because of signal loss induced by physiological motion. The goal of this study was to develop a methodology for estimating IVIM parameters of in vivo cardiac magnetic resonance imaging using an efficient DWI acquisition framework. This was achieved by investigating various acquisition strategies (principal component analysis [PCA] filtering and temporal maximum intensity projection [PCATMIP] and single trigger delay [TD]) and fitting methods.
Simulations were performed on a synthetic dataset of diffusion-weighted signal intensity (SI) to determine the fitting method that would yield IVIM parameters with the greatest accuracy. The required number of b values to correctly estimate IVIM parameters was also investigated. Breath-hold DWI scans were performed for 12 volunteers to collect several TD values during diastole. Thirteen b values ranging from 0 to 550 s/mm were used. The IVIM parameters derived using the data from all the acquired TDs (PCATMIP technique) were compared with those derived using a single acquisition performed at an optimized diastolic time point (1TD).
The main result of this study was that PCATMIP, when combined with a fitting model that accounted for T1 and T2 relaxation, provided IVIM parameters with less variability. However, an acquisition performed with 1 optimized diastolic TD provided results that were as good as those provided using PCATMIP if the R-R variability during the acquisition was sufficiently low (± 5%). Furthermore, the use of only 9 b values (that could be acquired in 2 breath-holds), instead of 13 b values (requiring 3 breath-holds), was sufficient to determine the IVIM parameters.
This study demonstrates that IVIM is technically feasible in vivo and reports for the first time the perfusion fraction, f, and the diffusion coefficients, D and D*, for the cardiac DWI of healthy volunteers. Motion-induced signal loss, which is the main problem associated with cardiac DWI, could be avoided with the combined use of sliding acquisition during the cardiac cycle and image postprocessing with the PCATMIP algorithm. This study provides new perspectives for perfusion imaging without a contrast agent and demonstrates that IVIM parameters can act as promising tools to further characterize microvascular abnormalities or dysfunction.
扩散加权成像（DWI）和体素内不相干运动（IVIM）模型的引入为评估组织内的灌注和扩散提供了一种独特的方法，而无需使用造影剂。尽管具有相关性，但由于生理运动引起的信号丢失，心脏 DWI 的 b 值一直较低，从而受到限制。本研究的目的是开发一种使用高效 DWI 采集框架估算活体心脏磁共振成像 IVIM 参数的方法。通过研究各种采集策略（主成分分析[PCA]滤波和时间最大强度投影[PCATMIP]和单触发延迟[TD]）和拟合方法来实现这一目标。
在扩散加权信号强度（SI）的合成数据集上进行模拟，以确定能够产生最准确 IVIM 参数的拟合方法。还研究了正确估计 IVIM 参数所需的 b 值数量。对 12 名志愿者进行屏气 DWI 扫描，在舒张期采集多个 TD 值。使用 0 至 550 s/mm 的 13 个 b 值。使用从所有采集的 TD 获得的数据（PCATMIP 技术）得出的 IVIM 参数与使用在优化的舒张时间点（1TD）执行的单次采集得出的 IVIM 参数进行比较。
本研究的主要结果是，当与考虑 T1 和 T2 弛豫的拟合模型结合使用时，PCATMIP 提供的 IVIM 参数变化较小。但是，如果采集过程中的 R-R 变异性足够低（±5%），则使用单个优化的舒张 TD 进行采集可以提供与使用 PCATMIP 相同的结果。此外，仅使用 9 个 b 值（可以在 2 次屏气中采集）而不是 13 个 b 值（需要 3 次屏气）就足以确定 IVIM 参数。
本研究表明 IVIM 在体内是可行的技术，并首次报告了健康志愿者心脏 DWI 的灌注分数 f、扩散系数 D 和 D*。运动引起的信号丢失是心脏 DWI 主要问题，可以通过在心动周期中使用滑动采集和 PCATMIP 算法进行图像后处理相结合来避免。本研究为无造影剂的灌注成像提供了新的视角，并表明 IVIM 参数可以作为有前途的工具，进一步表征微血管异常或功能障碍。
