Center for Image Sciences, Imaging Division, University Medical Center Utrecht, Heidelberglaan 100, Room Q.03.4.21, 3508 GA Utrecht, The Netherlands. Author to whom any correspondence should be addressed.
Phys Med Biol. 2019 Sep 11;64(18):185001. doi: 10.1088/1361-6560/ab35c3.
In this work, we present a new method for visualization of fiducial markers (FMs) in the prostate for MRI-only radiotherapy with a positive contrast directly at the MR console. The method is based on high bandwidth phase-cycled balanced steady-state free precession (bSSFP) sequence, which is available on many clinical scanners, does not require any additional post-processing or software, and has a higher signal-to-noise (SNR) compared to conventional gradient-echo (GE) imaging. Complex phase-cycled bSSFP data is acquired with different RF phase increment settings such that the manifestation of the artifacts around FMs in the acquired complex images is different for each dynamic acquisition and depends on the RF phase increment used. First, we performed numerical simulations to investigate the complex-valued phase-cycled bSSFP signal in the presence of a gold FM, and to investigate the relation of the true physical location of the FM with the geometrical manifestation of the artifacts. Next, to validate the simulations, we performed phantoms and in vivo studies and compared the experimentally obtained artifacts with those predicted in simulations. The accuracy of the method was assessed by comparing the distances between the FM's centers and the center of mass of FMs system measured using phase-cycled bSSFP MR images and using reference CT (or MRI-only) images. The results show accurate (within 1 mm) matching of FMs localization between CT and MR images on five patients, proving the feasibility of in vivo FMs detection on MR images only. The FMs show a positive contrast with respect to the prostate background on real/imaginary phase-cycled bSSFP images, which was confirmed by simulations. The proposed method facilitates robust FMs visualization with positive contrast directly at the MR console, allowing RT technicians to obtain immediate feedback on the anticipated feasibility of accurate FMs localization while the patient is being scanned.
在这项工作中，我们提出了一种新的方法，用于在仅使用 MRI 的前列腺放射治疗中可视化基准标记（FM），并在磁共振控制台直接呈现正对比度。该方法基于高带宽相控平衡稳态自由进动（bSSFP）序列，该序列在许多临床扫描仪上都可用，不需要任何额外的后处理或软件，并且与传统的梯度回波（GE）成像相比具有更高的信噪比（SNR）。使用不同的射频相位增量设置获取复杂的相控 bSSFP 数据，使得在采集的复杂图像中，FM 周围伪影的表现对于每个动态采集都是不同的，并且取决于所使用的射频相位增量。首先，我们进行了数值模拟，以研究存在金 FM 时的复数相控 bSSFP 信号，并研究 FM 的真实物理位置与伪影的几何表现之间的关系。接下来，为了验证模拟，我们进行了体模和体内研究，并将实验获得的伪影与模拟预测的伪影进行了比较。通过比较使用相控 bSSFP MR 图像和参考 CT（或仅 MRI）图像测量的 FM 中心与 FM 系统质心之间的距离，评估了该方法的准确性。结果表明，在五名患者中，FM 的定位在 CT 和 MR 图像之间具有准确的匹配（在 1mm 以内），证明了仅在 MR 图像上进行体内 FM 检测的可行性。在真实/虚部相控 bSSFP 图像上，FM 相对于前列腺背景呈现正对比度，这一点通过模拟得到了证实。该方法便于在磁共振控制台直接进行稳健的 FM 可视化，使放射治疗技术员在扫描患者时能够立即获得有关准确 FM 定位预期可行性的反馈。
