Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands.
C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands.
Brachytherapy. 2020 Sep-Oct;19(5):618-623. doi: 10.1016/j.brachy.2020.06.010. Epub 2020 Jul 31.
The individual channels in an endorectal applicator for high-dose-rate endorectal brachytherapy are not visible on standard MRI sequences. The aim of this study was to test whether an ultrashort echo time (UTE) MRI sequence could be used to visualize the individual channels to enable MR-only treatment planning for rectal cancer.
We used a radial three-dimensional (3D) UTE pulse sequence and acquired images of phantoms and two patients with rectal cancer. We rigidly registered a UTE image and CT scan of an applicator phantom, based on the outline of the applicator. One observer compared channel positions on the UTE image and CT scan in five slices spaced 25 mm apart. To quantify geometric distortions, we scanned a commercial 3D geometric quality assurance phantom and calculated the difference between detected marker positions on the UTE image and corresponding marker positions on two 3D T-weighted images with opposing readout directions.
On the UTE images, there is sufficient contrast to discern the individual channels. The difference in channel positions on the UTE image compared with the CT was on average -0.1 ± 0.1 mm (left-right) and 0.1 ± 0.3 mm (anteroposterior). After rigid registration to the 3D T-weighted sequences, the residual 95th percentile of the geometric distortion inside a 550-mm-diameter sphere was 1.0 mm (left-right), 0.9 mm (anteroposterior), and 0.9 mm (craniocaudal).
With a UTE sequence, the endorectal applicator and individual channels can be adequately visualized in both phantom and patients. The geometrical fidelity is within an acceptable range.
高剂量率腔内近距离放射治疗用直肠内施源器的各个通道在标准 MRI 序列中不可见。本研究旨在测试超短回波时间（UTE）MRI 序列是否可用于显示各个通道，以便对直肠癌进行仅基于 MRI 的治疗计划。
我们使用径向三维（3D）UTE 脉冲序列对直肠内施源器模型和两名直肠癌患者进行了成像。我们基于施源器的轮廓，刚性地将 UTE 图像与 CT 扫描配准。一位观察者在五个间隔 25mm 的切片上比较 UTE 图像和 CT 扫描中的通道位置。为了量化几何变形，我们扫描了一个商用 3D 几何质量保证体模，并计算了 UTE 图像上检测到的标记位置与两个具有相反读出方向的 3D T 加权图像上相应标记位置之间的差异。
在 UTE 图像上，可以有足够的对比度来分辨各个通道。UTE 图像与 CT 图像上的通道位置差异平均为-0.1±0.1mm（左右）和 0.1±0.3mm（前后）。在刚性配准到 3D T 加权序列后，在 550mm 直径球体内部的几何变形的 95%分位数残余值为 1.0mm（左右）、0.9mm（前后）和 0.9mm（头脚）。
使用 UTE 序列，可以在模型和患者中充分显示直肠内施源器及其各个通道。几何保真度在可接受的范围内。
