Department of Medical Physics and Biomedical Engineering, University College London, London, United Kingdom.
CD Laboratory for Clinical Molecular MR Imaging, Vienna.
Invest Radiol. 2019 Jun;54(6):340-348. doi: 10.1097/RLI.0000000000000546.
Gradients in the static magnetic field caused by tissues with differing magnetic susceptibilities lead to regional variations in the effective echo time, which modifies both image signal and BOLD sensitivity. Local echo time changes are not considered in the most commonly used metric for BOLD sensitivity, temporal signal-to-noise ratio (tSNR), but may be significant, particularly at ultrahigh field close to air cavities (such as the sinuses and ear canals) and near gross brain pathologies and postoperative sites.
We have studied the effect of local variations in echo time and tSNR on BOLD sensitivity in 3 healthy volunteers and 11 patients with tumors, postoperative cavities, and venous malformations at 7 T. Temporal signal-to-noise ratio was estimated from a 5-minute run of resting state echo planar imaging with a nominal echo time of 22 milliseconds. Maps of local echo time were derived from the phase of a multiecho GE scan. One healthy volunteer performed 10 runs of a breath-hold task. The t-map from this experiment served as a criterion standard BOLD sensitivity measure. Two runs of a less demanding breath-hold paradigm were used for patients.
In all subjects, a strong reduction in the echo time (from 22 milliseconds to around 11 milliseconds) was found close to the ear canals and sinuses. These regions were characterized by high tSNR but low t-values in breath-hold t-maps. In some patients, regions of particular interest in presurgical planning were affected by reductions in the echo time to approximately 13-15 milliseconds. These included the primary motor cortex, Broca's area, and auditory cortex. These regions were characterized by high tSNR values (70 and above). Breath-hold results were corrupted by strong motion artifacts in all patients.
Criterion standard BOLD sensitivity estimation using hypercapnic experiments is challenging, especially in patient populations. Taking into consideration the tSNR, commonly used for BOLD sensitivity estimation, but ignoring local reductions in the echo time (eg, from 22 to 11 milliseconds), would erroneously suggest functional sensitivity sufficient to map BOLD signal changes. It is therefore important to consider both local variations in the echo time and temporal variations in signal, using the product metric of these two indices for instance. This should ensure a reliable estimation of BOLD sensitivity and to facilitate the identification of potential false-negative results. This is particularly true at high fields, such as 7 T and in patients with large pathologies and postoperative cavities.
由于具有不同磁敏感性的组织引起的静态磁场梯度会导致有效回波时间的局部变化，从而改变图像信号和 BOLD 敏感度。在最常用的 BOLD 敏感度度量（即时间信号到噪声比（tSNR））中没有考虑局部回波时间变化，但这些变化可能非常重要，尤其是在接近空气腔（如鼻窦和耳道）以及靠近大体脑病变和术后部位的超高磁场。
我们在 7T 下对 3 名健康志愿者和 11 名患有肿瘤、术后腔和静脉畸形的患者进行了研究，研究了局部回波时间和 tSNR 对 BOLD 敏感度的影响。从具有 22 毫秒标称回波时间的静息状态 echo 平面成像的 5 分钟运行中估计时间信号到噪声比。局部回波时间图是从多回波 GE 扫描的相位得出的。一名健康志愿者进行了 10 次屏气任务的运行。来自该实验的 t 图用作 BOLD 敏感度的标准测量标准。对于患者，使用了两个较不费力的屏气范式的运行。
在所有受试者中，在接近耳道和鼻窦的地方发现回波时间（从 22 毫秒降至约 11 毫秒）明显缩短。这些区域的特点是 tSNR 高，但在屏气 t 图中的 t 值低。在一些患者中，术前计划中特别关注的区域受到回波时间降低至约 13-15 毫秒的影响。这些区域包括初级运动皮层、布罗卡区和听觉皮层。这些区域的 tSNR 值较高（70 及以上）。所有患者的屏气结果都受到强烈运动伪影的干扰。
使用高碳酸血症实验进行标准 BOLD 敏感度估计具有挑战性，尤其是在患者人群中。仅考虑常用的用于 BOLD 敏感度估计的 tSNR，但忽略局部回波时间的降低（例如从 22 毫秒降低至 11 毫秒），将错误地表明足以映射 BOLD 信号变化的功能敏感度。因此，重要的是考虑回波时间的局部变化和信号的时间变化，例如使用这两个指数的乘积度量。这应确保可靠地估计 BOLD 敏感度，并有助于识别潜在的假阴性结果。在高磁场（例如 7T）和具有大病变和术后腔的患者中，这一点尤其正确。
