Walker Amy, Liney Gary, Holloway Lois, Dowling Jason, Rivest-Henault David, Metcalfe Peter
Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia and Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia; Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia; and South West Clinical School, University of New South Wales, Sydney, NSW 2170, Australia.
Med Phys. 2015 Apr;42(4):1982-91. doi: 10.1118/1.4915920.
Accurate geometry is required for radiotherapy treatment planning (RTP). When considering the use of magnetic resonance imaging (MRI) for RTP, geometric distortions observed in the acquired images should be considered. While scanner technology and vendor supplied correction algorithms provide some correction, large distortions are still present in images, even when considering considerably smaller scan lengths than those typically acquired with CT in conventional RTP. This study investigates MRI acquisition with a moving table compared with static scans for potential geometric benefits for RTP.
A full field of view (FOV) phantom (diameter 500 mm; length 513 mm) was developed for measuring geometric distortions in MR images over volumes pertinent to RTP. The phantom consisted of layers of refined plastic within which vitamin E capsules were inserted. The phantom was scanned on CT to provide the geometric gold standard and on MRI, with differences in capsule location determining the distortion. MRI images were acquired with two techniques. For the first method, standard static table acquisitions were considered. Both 2D and 3D acquisition techniques were investigated. With the second technique, images were acquired with a moving table. The same sequence was acquired with a static table and then with table speeds of 1.1 mm/s and 2 mm/s. All of the MR images acquired were registered to the CT dataset using a deformable B-spline registration with the resulting deformation fields providing the distortion information for each acquisition.
MR images acquired with the moving table enabled imaging of the whole phantom length while images acquired with a static table were only able to image 50%-70% of the phantom length of 513 mm. Maximum distortion values were reduced across a larger volume when imaging with a moving table. Increased table speed resulted in a larger contribution of distortion from gradient nonlinearities in the through-plane direction and an increased blurring of capsule images, resulting in an apparent capsule volume increase by up to 170% in extreme axial FOV regions. Blurring increased with table speed and in the central regions of the phantom, geometric distortion was less for static table acquisitions compared to a table speed of 2 mm/s over the same volume. Overall, the best geometric accuracy was achieved with a table speed of 1.1 mm/s.
The phantom designed enables full FOV imaging for distortion assessment for the purposes of RTP. MRI acquisition with a moving table extends the imaging volume in the z direction with reduced distortions which could be useful particularly if considering MR-only planning. If utilizing MR images to provide additional soft tissue information to the planning CT, standard acquisition sequences over a smaller volume would avoid introducing additional blurring or distortions from the through-plane table movement.
放射治疗计划（RTP）需要精确的几何形状。在考虑将磁共振成像（MRI）用于RTP时，应考虑在采集图像中观察到的几何失真。虽然扫描仪技术和供应商提供的校正算法提供了一些校正，但即使考虑比传统RTP中使用CT通常采集的扫描长度小得多的扫描长度，图像中仍然存在较大的失真。本研究调查了与静态扫描相比，使用移动检查床进行MRI采集对RTP潜在的几何益处。
开发了一个全视野（FOV）体模（直径500mm；长度513mm），用于测量与RTP相关体积的MR图像中的几何失真。体模由几层精制塑料组成，其中插入了维生素E胶囊。在CT上对体模进行扫描以提供几何金标准，并在MRI上进行扫描，胶囊位置的差异决定了失真情况。使用两种技术采集MRI图像。对于第一种方法，考虑标准的静态检查床采集。研究了二维和三维采集技术。对于第二种技术，使用移动检查床采集图像。先在静态检查床上采集相同序列，然后以1.1mm/s和2mm/s的检查床速度采集。使用可变形B样条配准将所有采集的MR图像与CT数据集配准，得到的变形场为每次采集提供失真信息。
使用移动检查床采集的MR图像能够对整个体模长度进行成像，而使用静态检查床采集的图像只能对513mm体模长度的50%-70%进行成像。使用移动检查床成像时，在更大的体积范围内最大失真值降低。检查床速度增加导致层面方向上梯度非线性引起的失真贡献更大，胶囊图像的模糊增加，在极端轴向FOV区域导致胶囊表观体积增加高达170%。模糊随着检查床速度增加而增加，并且在体模的中心区域，与相同体积下2mm/s的检查床速度相比，静态检查床采集的几何失真更小。总体而言，检查床速度为1.1mm/s时获得了最佳的几何精度。
设计的体模能够进行全视野成像，用于RTP目的的失真评估。使用移动检查床进行MRI采集可在z方向上扩展成像体积，同时减少失真，这在仅考虑基于MR的计划时可能特别有用。如果利用MR图像为计划CT提供额外的软组织信息，在较小体积上的标准采集序列将避免因层面方向的检查床移动引入额外的模糊或失真。
