Magnetic resonance imaging (MRI) can facilitate accurate organ delineation and optimal dose distributions in high-dose-rate (HDR) MRI-Assisted Radiosurgery (MARS). Its use for this purpose has been limited by the lack of positive-contrast MRI markers that can clearly delineate the lumen of the HDR applicator and precisely show the path of the HDR source on T1- and T2-weighted MRI sequences. We investigated a novel MRI positive-contrast HDR brachytherapy or interventional radiotherapy line marker, C4:S, consisting of C4 (visible on T1-weighted images) complexed with saline. Longitudinal relaxation time (T1) and transverse relaxation time (T2) for C4:S were measured on a 1.5 T MRI scanner. High-density polyethylene (HDPE) tubing filled with C4:S as an HDR brachytherapy line marker was tested for visibility on T1- and T2-weighted MRI sequences in a tissue-equivalent female ultrasound training pelvis phantom. Relaxivity measurements indicated that C4:S solution had good T1-weighted contrast (relative to oil [fat] signal intensity) and good T2-weighted contrast (relative to water signal intensity) at both room temperature (relaxivity ratio > 1; r2/r1 = 1.43) and body temperature (relaxivity ratio > 1; r2/r1 = 1.38). These measurements were verified by the positive visualization of the C4:S (C4/saline 50:50) HDPE tube HDR brachytherapy line marker on both T1- and T2-weighted MRI sequences. Orientation did not affect the relaxivity of the C4:S contrast solution. C4:S encapsulated in HDPE tubing can be visualized as a positive line marker on both T1- and T2-weighted MRI sequences. MRI-guided HDR planning may be possible with these novel line markers for HDR MARS for several types of cancer.
磁共振成像（MRI）在高剂量率（HDR）磁共振辅助放射外科（MARS）中能够实现精确的器官勾画和优化的剂量分布。然而，由于缺乏能够在T1加权和T2加权MRI序列上清晰勾画HDR施源器管腔并精确显示HDR源路径的正对比MRI标记物，其在此方面的应用受到限制。本研究探讨了一种新型MRI正对比HDR近距离放射治疗（或介入放射治疗）线状标记物C4:S，该标记物由C4（在T1加权图像中可见）与生理盐水复合而成。我们在1.5 T MRI扫描仪上测量了C4:S的纵向弛豫时间（T1）和横向弛豫时间（T2）。将填充C4:S作为HDR近距离放射治疗线状标记物的高密度聚乙烯（HDPE）管，在组织等效的女性超声训练骨盆体模中测试了其在T1加权和T2加权MRI序列上的可见性。弛豫率测量表明，C4:S溶液在室温（弛豫率比值 > 1；r2/r1 = 1.43）和体温（弛豫率比值 > 1；r2/r1 = 1.38）下均具有良好的T1加权对比度（相对于油[脂肪]信号强度）和良好的T2加权对比度（相对于水信号强度）。这些测量结果通过C4:S（C4/生理盐水 50:50）HDPE管HDR近距离放射治疗线状标记物在T1加权和T2加权MRI序列上的正性可视化得到了验证。方向性不影响C4:S对比剂溶液的弛豫率。封装在HDPE管中的C4:S在T1加权和T2加权MRI序列上均可作为正性线状标记物被可视化。对于多种癌症的HDR MARS，使用这些新型线状标记物可能实现MRI引导的HDR治疗计划制定。
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