Pappas E P, Seimenis I, Moutsatsos A, Georgiou E, Nomikos P, Karaiskos P
Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece.
Phys Med Biol. 2016 Oct 7;61(19):6993-7011. doi: 10.1088/0031-9155/61/19/6993. Epub 2016 Sep 20.
This work provides characterization of system-related geometric distortions present in MRIs used in Gamma Knife (GK) stereotactic radiosurgery (SRS) treatment planning. A custom-made phantom, compatible with the Leksell stereotactic frame model G and encompassing 947 control points (CPs), was utilized. MR images were obtained with and without the frame, thus allowing discrimination of frame-induced distortions. In the absence of the frame and following compensation for field inhomogeneities, measured average CP disposition owing to gradient nonlinearities was 0.53 mm. In presence of the frame, contrarily, detected distortion was greatly increased (up to about 5 mm) in the vicinity of the frame base due to eddy currents induced in the closed loop of its aluminum material. Frame-related distortion was obliterated at approximately 90 mm from the frame base. Although the region with the maximum observed distortion may not lie within the GK treatable volume, the presence of the frame results in distortion of the order of 1.5 mm at a 7 cm distance from the center of the Leksell space. Additionally, severe distortions observed outside the treatable volume could possibly impinge on the delivery accuracy mainly by adversely affecting the registration process (e.g. the position of the lower part of the N-shaped fiducials used to define the stereotactic space may be miss-registered). Images acquired with a modified version of the frame developed by replacing its front side with an acrylic bar, thus interrupting the closed aluminum loop and reducing the induced eddy currents, were shown to benefit from relatively reduced distortion. System-related distortion was also identified in patient MR images. Using corresponding CT angiography images as a reference, an offset of 1.1 mm was detected for two vessels lying in close proximity to the frame base, while excellent spatial agreement was observed for a vessel far apart from the frame base.
这项工作对用于伽玛刀（GK）立体定向放射外科（SRS）治疗计划的磁共振成像（MRI）中存在的与系统相关的几何畸变进行了表征。使用了一个与Leksell立体定向框架模型G兼容且包含947个控制点（CP）的定制体模。在有框架和无框架的情况下获取了MR图像，从而能够区分框架引起的畸变。在没有框架且对场不均匀性进行补偿后，由于梯度非线性导致的测量平均CP位移为0.53毫米。相反，在有框架的情况下，由于其铝材料闭环中感应的涡流，在框架底部附近检测到的畸变大大增加（高达约5毫米）。与框架相关的畸变在距框架底部约90毫米处消失。尽管观察到最大畸变的区域可能不在GK可治疗体积内，但框架的存在会在距Leksell空间中心7厘米处导致约1.5毫米的畸变。此外，在可治疗体积之外观察到的严重畸变可能主要通过对配准过程产生不利影响而影响递送精度（例如，用于定义立体定向空间的N形基准点下部的位置可能配准错误）。通过用丙烯酸棒替换框架的前侧开发的框架修改版本获取的图像，从而中断了铝闭环并减少了感应涡流，显示出畸变相对减小的优势。在患者MR图像中也识别出了与系统相关的畸变。以相应的CT血管造影图像作为参考，对于靠近框架底部的两条血管检测到1.1毫米的偏移，而对于远离框架底部的一条血管观察到了极好的空间一致性。
