Boyes Richard G, Gunter Jeff L, Frost Chris, Janke Andrew L, Yeatman Thomas, Hill Derek L G, Bernstein Matt A, Thompson Paul M, Weiner Michael W, Schuff Norbert, Alexander Gene E, Killiany Ronald J, DeCarli Charles, Jack Clifford R, Fox Nick C
Dementia Research Centre, Institute of Neurology, Box 16, University College London, Queen Square, London, UK.
Neuroimage. 2008 Feb 15;39(4):1752-62. doi: 10.1016/j.neuroimage.2007.10.026. Epub 2007 Oct 30.
Measures of structural brain change based on longitudinal MR imaging are increasingly important but can be degraded by intensity non-uniformity. This non-uniformity can be more pronounced at higher field strengths, or when using multichannel receiver coils. We assessed the ability of the non-parametric non-uniform intensity normalization (N3) technique to correct non-uniformity in 72 volumetric brain MR scans from the preparatory phase of the Alzheimer's Disease Neuroimaging Initiative (ADNI). Normal elderly subjects (n=18) were scanned on different 3-T scanners with a multichannel phased array receiver coil at baseline, using magnetization prepared rapid gradient echo (MP-RAGE) and spoiled gradient echo (SPGR) pulse sequences, and again 2 weeks later. When applying N3, we used five brain masks of varying accuracy and four spline smoothing distances (d=50, 100, 150 and 200 mm) to ascertain which combination of parameters optimally reduces the non-uniformity. We used the normalized white matter intensity variance (standard deviation/mean) to ascertain quantitatively the correction for a single scan; we used the variance of the normalized difference image to assess quantitatively the consistency of the correction over time from registered scan pairs. Our results showed statistically significant (p<0.01) improvement in uniformity for individual scans and reduction in the normalized difference image variance when using masks that identified distinct brain tissue classes, and when using smaller spline smoothing distances (e.g., 50-100 mm) for both MP-RAGE and SPGR pulse sequences. These optimized settings may assist future large-scale studies where 3-T scanners and phased array receiver coils are used, such as ADNI, so that intensity non-uniformity does not influence the power of MR imaging to detect disease progression and the factors that influence it.
基于纵向磁共振成像的脑结构变化测量方法越来越重要，但可能会因强度不均匀性而降低质量。这种不均匀性在更高的场强下或使用多通道接收线圈时可能会更明显。我们评估了非参数非均匀强度归一化（N3）技术校正阿尔茨海默病神经成像计划（ADNI）准备阶段72例脑部容积磁共振扫描中不均匀性的能力。正常老年受试者（n = 18）在基线时使用磁化准备快速梯度回波（MP-RAGE）和扰相梯度回波（SPGR）脉冲序列，在不同的3-T扫描仪上使用多通道相控阵接收线圈进行扫描，2周后再次扫描。应用N3时，我们使用了五种不同精度的脑掩码和四种样条平滑距离（d = 50、100、150和200毫米），以确定哪种参数组合能最佳地减少不均匀性。我们使用归一化白质强度方差（标准差/均值）来定量确定单次扫描的校正效果；我们使用归一化差异图像的方差来评估来自配准扫描对的校正随时间的一致性。我们的结果显示，当使用识别不同脑组织类别的掩码时，以及当对MP-RAGE和SPGR脉冲序列都使用较小的样条平滑距离（例如50 - 100毫米）时，单次扫描的均匀性有统计学显著改善（p < 0.01），归一化差异图像方差减小。这些优化设置可能有助于未来使用3-T扫描仪和相控阵接收线圈的大规模研究，如ADNI，以便强度不均匀性不会影响磁共振成像检测疾病进展及其影响因素的能力。
