Department of Radiation Oncology and Molecular Radiation Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21231, USA.
Med Phys. 2013 Sep;40(9):091712. doi: 10.1118/1.4818656.
Current pretreatment, 4D imaging techniques are suboptimal in that they sample breathing motion over a very limited "snapshot" in time. Heretofore, long-duration, 4D motion characterization for radiotherapy planning, margin optimization, and validation have been impractical for safety reasons, requiring invasive markers imaged under x-ray fluoroscopy. To characterize 3D tumor motion and associated variability over durations more consistent with treatments, the authors have developed a practical dynamic MRI (dMRI) technique employing two orthogonal planes acquired in a continuous, interleaved fashion.
2D balanced steady-state free precession MRI was acquired continuously over 9-14 min at approximately 4 Hz in three healthy volunteers using a commercial 1.5 T system; alternating orthogonal imaging planes (sagittal, coronal, sagittal, etc.) were employed. The 2D in-plane pixel resolution was 2 × 2 mm(2) with a 5 mm slice profile. Simultaneous with image acquisition, the authors monitored a 1D surrogate respiratory signal using a device available with the MRI system. 2D template matching-based anatomic feature registration, or tracking, was performed independently in each orientation. 4D feature tracking at the raw frame rate was derived using spline interpolation.
Tracking vascular features in the lung for two volunteers and pancreatic features in one volunteer, the authors have successfully demonstrated this method. Registration error, defined here as the difference between the sagittal and coronal tracking result in the SI direction, ranged from 0.7 to 1.6 mm (1σ) which was less than the acquired image resolution. Although the healthy volunteers were instructed to relax and breathe normally, significantly variable respiration was observed. To demonstrate potential applications of this technique, the authors subsequently explored the intrafraction stability of hypothetical tumoral internal target volumes and 3D spatial probability distribution functions. The surrogate respiratory information allowed the authors to show how this technique can be used to study correlations between internal and external (surrogate) information over these prolonged durations. However, compared against the gold standard of the time stamps in the dMRI frames, the temporal synchronization of the surrogate 1D respiratory information was shown to be likely unreliable.
The authors have established viability of a novel and practical pretreatment, 4D tumor centroid tracking method employing a commercially available dynamic MRI sequence. Further developments from the vendor are likely needed to provide a reliably synchronized surrogate 1D respiratory signal, which will likely broaden the utility of this method in the pretreatment radiotherapy planning context.
目前的预处理 4D 成像技术在采样呼吸运动方面存在不足，因为它们只在非常有限的“快照”时间内采样。迄今为止，出于安全原因，长时间的 4D 运动特征对于放射治疗计划、边缘优化和验证是不切实际的，这需要在 X 射线透视下成像的侵入性标记物。为了更符合治疗时间的特点，作者开发了一种实用的 3D 肿瘤运动和相关变异性的动态 MRI(dMRI)技术，采用连续、交错的方式获取两个正交平面。
在三个健康志愿者中，使用商业 1.5 T 系统连续采集 9-14 分钟，约 4 Hz 的二维平衡稳态自由进动 MRI；交替使用正交成像平面（矢状位、冠状位、矢状位等）。二维平面内像素分辨率为 2×2mm2，切片厚度为 5mm。在图像采集的同时，作者使用 MRI 系统上可用的设备监测一维替代呼吸信号。在每个方向上独立进行基于二维模板匹配的解剖特征配准或跟踪。使用样条插值从原始帧率导出 4D 特征跟踪。
作者成功地对两名志愿者的肺部血管特征和一名志愿者的胰腺特征进行了跟踪。这里定义的注册误差是 SI 方向矢状面和冠状面跟踪结果之间的差异，范围为 0.7 至 1.6mm（1σ），小于采集的图像分辨率。尽管健康志愿者被指示放松并正常呼吸，但仍观察到明显的呼吸变化。为了展示该技术的潜在应用，作者随后探索了假设的肿瘤内部靶体积和 3D 空间概率分布函数的分次内稳定性。替代呼吸信息使作者能够展示如何使用该技术研究这些长时间内内部和外部（替代）信息之间的相关性。然而，与 dMRI 帧中的时间戳的金标准相比，替代 1D 呼吸信息的时间同步性被证明可能不可靠。
作者已经证实了一种新颖实用的预处理 4D 肿瘤质心跟踪方法的可行性，该方法采用商业上可用的动态 MRI 序列。供应商可能需要进一步开发，以提供一个可靠同步的替代 1D 呼吸信号，这将可能拓宽该方法在预处理放射治疗计划中的应用。
