Comprehensive characterization of geometric distortions for MRI simulators and MRI‐guided treatment delivery systems is typically performed with large phantoms that are costly and unwieldy to handle. Here we propose an easily implementable methodology for MR distortion determination of the entire imaging space of the scanner through the use of a compact commercially available distortion phantom. The MagphanRT phantom was scanned at several locations within a MR scanner. From each scan, an approximate location of the phantom was determined from a subset of the fiducial spheres. The fiducial displacements were determined, and a displacement field was fitted to the displacement data using the entire multi‐scan data set. An orthogonal polynomial expansion fitting function was used that had been augmented to include independent rigid‐body transformations for each scan. The rigid‐body portions of the displacement field were thereafter discarded, and the resultant fit then represented the distortion field. Multi‐positional scans of the phantom were used successfully to determine the distortion field with extended coverage. A single scan of the phantom covered 20 cm in its smallest dimension. By stitching together overlapping scans we extended the distortion measurements to 30 cm. No information about the absolute location or orientation of each scan was required. The method, termed the Multi‐Scan Expansion (MSE) method, can be easily applied for larger field‐of‐views (FOVs) by using a combination of larger phantom displacements and more scans. The implementation of the MSE method allows for distortion determination beyond the physical limitations of the phantom. The method is scalable to the user’s needs and does not require any specialized equipment. This approach could open up for easier determination of the distortion magnitude at distances further from the scanner’s isocenter. This is especially important in the newly proposed methodologies of MR‐only simulation in RT and in adaptive replanning in MR linac systems.

中文翻译：

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技术说明：通过多位置体模成像确定扩展视野 (FOV) MRI 失真

MRI 模拟器和 MRI 引导的治疗实施系统的几何变形的综合表征通常使用昂贵且难以处理的大型模型进行。在这里，我们提出了一种易于实施的方法，用于通过使用紧凑的商用失真模型来确定扫描仪的整个成像空间的 MR 失真。MagphanRT 体模在 MR 扫描仪内的多个位置进行扫描。从每次扫描中，从基准球的一个子集确定了体模的大致位置。确定基准位移，并使用整个多扫描数据集将位移场拟合到位移数据。使用了正交多项式展开拟合函数，该函数已被增强为包括每次扫描的独立刚体变换。位移场的刚体部分随后被丢弃，然后得到的拟合代表畸变场。体模的多位置扫描成功地用于确定具有扩展覆盖范围的畸变场。幻影的单次扫描在其最小尺寸中覆盖了 20 厘米。通过将重叠扫描拼接在一起，我们将失真测量扩展到 30 厘米。不需要关于每次扫描的绝对位置或方向的信息。该方法称为多扫描扩展 (MSE) 方法，通过结合使用更大的模型位移和更多扫描，可以轻松地将其应用于更大的视野 (FOV)。MSE 方法的实施允许超出模型的物理限制的失真确定。该方法可以根据用户的需要进行扩展，并且不需要任何专门的设备。这种方法可以更容易地确定距离扫描仪等中心较远的失真幅度。这在 RT 中新提出的仅 MR 仿真方法和 MR 直线加速器系统中的自适应重新规划中尤为重要。