Department of Radiology and Imaging Sciences (NIH-DR&IS), National Institutes of Health, Bethesda, MD 20892, USA.
IEEE Trans Med Imaging. 2010 Nov;29(11):1927-41. doi: 10.1109/TMI.2010.2053552. Epub 2010 Jun 21.
Low noise, high resolution, fast and accurate T₁ maps from MRI images of the brain can be performed using a dual flip angle method. However, B₁ field inhomogeneity, which is particularly problematic at high field strengths (e.g., 3T), limits the ability of the scanner to deliver the prescribed flip angle, introducing errors into the T₁ maps that limit the accuracy of quantitative analyses based on those maps. A dual repetition time method was used for acquiring a B₁ map to correct that inhomogeneity. Additional inaccuracies due to misregistration of the acquired T₁-weighted images were corrected by rigid registration, and the effects of misalignment on the T₁ maps were compared to those of B₁ inhomogeneity in 19 normal subjects. However, since B₁ map acquisition takes up precious scanning time and most retrospective studies do not have B₁ map, we designed a template-based correction strategy. B₁ maps from different subjects were aligned using a twelve-parameter affine registration. Recomputed T₁ maps showed an important improvement with respect to the noncorrected maps: histograms of all corrected maps exhibited two peaks corresponding to white and gray matter tissues, while unimodal histograms were observed in all uncorrected maps because of the inhomogeneity. A method to detect the best nonsubject-specific B₁ correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available B₁ correction was detected in almost all subjects while corrections comparable to the T₁ map corrected using the B₁ map from the same subject were detected in the others.
利用双翻转角方法可以从脑部 MRI 图像中获得低噪声、高分辨率、快速且精确的 T₁ 图。然而，在高场强（例如 3T）下，B₁ 场非均匀性会限制扫描仪提供规定翻转角的能力，从而导致 T₁ 图出现误差，限制了基于这些图的定量分析的准确性。采用双重复时间方法获取 B₁ 图以校正这种非均匀性。通过刚性配准校正了因采集的 T₁ 加权图像配准不良而导致的额外误差，并在 19 名正常受试者中比较了这些误差对 T₁ 图的影响与 B₁ 非均匀性的影响。然而，由于 B₁ 图采集会占用宝贵的扫描时间，并且大多数回顾性研究都没有 B₁ 图，因此我们设计了一种基于模板的校正策略。使用十二参数仿射配准对齐来自不同受试者的 B₁ 图。重新计算的 T₁ 图与未校正的图相比有了重要的改善：所有校正图的直方图都显示出两个峰值，分别对应于白质和灰质组织，而所有未校正的图由于非均匀性而呈现单峰直方图。设计了一种基于一组特征检测最佳非特定于个体的 B₁ 校正的方法。计算了这些特征的最佳权重因子集。在几乎所有受试者中都检测到了最佳的可用 B₁ 校正，而在其他受试者中则检测到了与使用相同受试者的 B₁ 图校正的 T₁ 图相当的校正。
