Rashid Ivan A, Szczepankiewicz Filip, Gunnlaugsson Adalsteinn, Olsson Lars E, Brynolfsson Patrik
Department of Translational Medicine, Medical Radiation Physics, Lund University, Malmö, Sweden.
Department of Medical Radiation Physics, Clinical Sciences Lund, Lund University, Lund, Sweden.
Magn Reson Med. 2025 Oct;94(4):1514-1528. doi: 10.1002/mrm.30579. Epub 2025 Jun 2.
Neglecting imaging gradients in b-value calculations has been a documented issue for decades and remains unaccounted for in the current postprocessing pipelines. This omission may introduce inaccuracies that propagate into diffusion parameter estimates, such as in ADC and DTI analysis. Because intravoxel incoherent motion (IVIM) makes use of low b-values, these inaccuracies may be of greater importance. This study examines the impact of biased b-values on IVIM analysis in simulations and in vivo.
In simulations, b-values were calculated for two pulsed gradient spin-echo sequence designs: one with large cross-terms between imaging and diffusion gradients, and one with minimal cross-terms. Biased and unbiased b-values were calculated from sequences with 200 diffusion directions. These b-values were used to generate IVIM signal curves for parameter estimation. Simulations were repeated with varying in-plane resolutions (1-4 mm) and slice thicknesses (2-10 mm). Additionally, 15 prostate exams were analyzed with scanner-provided b-values and actual b-values derived from the gradient waveforms of the full pulse sequence.
The magnitude and direction of errors in IVIM parameters depended on pulse sequence design. Errors persisted until the full contribution of imaging gradients was considered. Errors in the in vivo data were coherent with the simulations, showing errors of -0.7% in f, 0.8 μm/ms in D*, and 0.07 μm/ms in D.
Ignoring imaging gradients in b-value calculations introduces unnecessary inaccuracies, making IVIM results spurious and highly dependent on specific pulse sequence design and imaging parameters. These inaccuracies can be corrected by adjusting the b-value calculations, without additional measurements.
几十年来，在b值计算中忽略成像梯度一直是一个有记录的问题，并且在当前的后处理流程中仍然没有得到考虑。这种疏忽可能会引入不准确因素，并传播到扩散参数估计中，例如在ADC和DTI分析中。由于体素内不相干运动（IVIM）使用低b值，这些不准确因素可能更为重要。本研究在模拟和体内实验中检验了有偏差的b值对IVIM分析的影响。
在模拟中，针对两种脉冲梯度自旋回波序列设计计算b值：一种在成像和扩散梯度之间有较大的交叉项，另一种交叉项最小。从具有200个扩散方向的序列中计算有偏差和无偏差的b值。这些b值用于生成IVIM信号曲线以进行参数估计。以不同的面内分辨率（1 - 4毫米）和切片厚度（2 - 10毫米）重复模拟。此外，对15例前列腺检查进行了分析，使用扫描仪提供的b值和从全脉冲序列的梯度波形导出的实际b值。
IVIM参数误差的大小和方向取决于脉冲序列设计。在考虑成像梯度的全部贡献之前，误差一直存在。体内数据中的误差与模拟结果一致，显示f值误差为 - 0.7%，D*值误差为0.8μm/ms，D值误差为0.07μm/ms。
在b值计算中忽略成像梯度会引入不必要的不准确因素，使IVIM结果不可靠且高度依赖于特定的脉冲序列设计和成像参数。通过调整b值计算可以纠正这些不准确因素，而无需额外测量。
