CREATIS, CNRS UMR5220, Inserm U630, INSA-Lyon, University of Lyon, F-69621 Villeurbanne, France.
Med Phys. 2011 Jan;38(1):166-78. doi: 10.1118/1.3523619.
Four-dimensional computed tomography (4D CT) can provide patient-specific motion information for radiotherapy planning and delivery. Motion estimation in 4D CT is challenging due to the reduced image quality and the presence of artifacts. We aim to improve the robustness of deformable registration applied to respiratory-correlated imaging of the lungs, by using a global problem formulation and pursuing a restrictive parametrization for the spatiotemporal deformation model.
A spatial transformation based on free-form deformations was extended to the temporal domain, by explicitly modeling the trajectory using a cyclic temporal model based on B-splines. A global registration criterion allowed to consider the entire image sequence simultaneously and enforce the temporal coherence of the deformation throughout the respiratory cycle. To ensure a parametrization capable of capturing the dynamics of respiratory motion, a prestudy was performed on the temporal dimension separately. The temporal parameters were tuned by fitting them to diaphragm motion data acquired for a large patient group. Suitable properties were retained and applied to spatiotemporal registration of 4D CT data. Registration results were validated using large sets of landmarks and compared to consecutive spatial registrations. To illustrate the benefit of the spatiotemporal approach, we also assessed the performance in the presence of motion-induced artifacts.
Cubic B-splines gave better or similar fitting results as lower orders and were selected because of their inherently stronger regularization. The fitting and registration errors increased gradually with the temporal control point spacing, representing a trade-off between achievable accuracy and sensitivity to noise and artifacts. A piecewise smooth trajectory model, allowing for a discontinuous change of speed at end-inhale, was found most suitable to account for the sudden changes of motion at this breathing phase. The spatiotemporal modeling allowed a reduction of the number of parameters of 45%, while maintaining registration accuracy within 0.1 mm. The approach reduced the sensitivity to artifacts.
Spatiotemporal registration can provide accurate motion estimation for 4D CT and improves the robustness to artifacts.
四维计算机断层扫描（4D CT）可以为放射治疗计划和治疗提供患者特定的运动信息。由于图像质量降低和伪影的存在，4D CT 中的运动估计具有挑战性。我们旨在通过使用全局问题公式并为时空变形模型采用限制参数化，来提高应用于肺部呼吸相关成像的可变形配准的稳健性。
基于自由形态变形的空间变换被扩展到时间域，通过使用基于 B 样条的循环时间模型来明确地对轨迹建模。全局配准准则允许同时考虑整个图像序列，并在整个呼吸周期内强制保持变形的时间一致性。为了确保具有捕获呼吸运动动力学的参数化，在时间维度上进行了预研究。通过将时间参数拟合到大患者组获得的膈肌运动数据来调整时间参数。保留合适的特性并将其应用于 4D CT 数据的时空配准。使用大量的地标验证配准结果，并将其与连续的空间配准进行比较。为了说明时空方法的优势，我们还评估了在存在运动伪影时的性能。
三次 B 样条比低阶的 B 样条具有更好或相似的拟合结果，并且由于其固有更强的正则化，因此被选择。拟合和配准误差随着时间控制点数的增加而逐渐增加，这代表了在可实现的准确性和对噪声和伪影的敏感性之间的折衷。发现分段平滑轨迹模型最适合在吸气末时能够允许速度的不连续变化，以解释在该呼吸阶段运动的突然变化。时空建模允许减少 45%的参数数量，同时将配准精度保持在 0.1 毫米以内。该方法降低了对伪影的敏感性。
时空配准可以为 4D CT 提供准确的运动估计，并提高对伪影的稳健性。
