将多层成像技术应用于X射线计算机断层扫描聚合物凝胶剂量测定法。

Johnston H, Hilts M, Jirasek A

Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada.

Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada and Medical Physics, BC Cancer Agency, Vancouver Island Centre, Victoria, British Columbia V8R 6V5, Canada.

Med Phys. 2015 Apr;42(4):1666-77. doi: 10.1118/1.4914419.

PURPOSE

To evaluate multislice computed tomography (CT) scanning for fast and reliable readout of radiation therapy (RT) dose distributions using CT polymer gel dosimetry (PGD) and to establish a baseline assessment of image noise and uniformity in an unirradiated gel dosimeter.

METHODS

A 16-slice CT scanner was used to acquire images through a 1 L cylinder filled with water. Additional images were collected using a single slice machine. The variability in CT number (NCT) associated with the anode heel effect was evaluated and used to define a new slice-by-slice background subtraction artifact removal technique for CT PGD. Image quality was assessed for the multislice system by evaluating image noise and uniformity. The agreement in NCT for slices acquired simultaneously using the multislice detector array was also examined. Further study was performed to assess the effects of increasing x-ray tube load on the constancy of measured NCT and overall scan time. In all cases, results were compared to the single slice machine. Finally, images were collected throughout the volume of an unirradiated gel dosimeter to quantify image noise and uniformity before radiation is delivered.

RESULTS

Slice-by-slice background subtraction effectively removes the variability in NCT observed across images acquired simultaneously using the multislice scanner and is the recommended background subtraction method when using a multislice CT system. Image noise was higher for the multislice system compared to the single slice scanner, but overall image quality was comparable between the two systems. Further study showed NCT was consistent across image slices acquired simultaneously using the multislice detector array for each detector configuration of the slice thicknesses examined. In addition, the multislice system was found to eliminate variations in NCT due to increasing x-ray tube load and reduce scanning time by a factor of 4 when compared to imaging a large volume using a single slice scanner. Images acquired through an unirradiated, active gel revealed NCT varies between the top and bottom of the 1 L cylinder as well as across the diameter of the cylinder by up to 7 HU.

CONCLUSIONS

Multislice CT imaging has been evaluated for CT PGD and found to be the superior technique compared to single slice imaging in terms of the time required to complete a scan and the tube load characteristics associated with each scanning method. The implementation of multislice scanning is straightforward and expected to facilitate routine gel dosimetry measurements for complex dose distributions in modern RT centers.

目的

使用CT聚合物凝胶剂量测定法（PGD）评估多层计算机断层扫描（CT）对放射治疗（RT）剂量分布进行快速可靠读出的能力，并在未受辐照的凝胶剂量计中建立图像噪声和均匀性的基线评估。

方法

使用16层CT扫描仪通过一个装有水的1升圆柱体获取图像。还使用单层机器收集了其他图像。评估了与阳极足跟效应相关的CT值（NCT）的变异性，并用于定义一种新的逐片背景减法伪影去除技术用于CT PGD。通过评估图像噪声和均匀性来评估多层系统的图像质量。还检查了使用多层探测器阵列同时采集的切片在NCT方面的一致性。进行了进一步的研究以评估增加X射线管负荷对测量的NCT稳定性和总扫描时间的影响。在所有情况下，将结果与单层机器进行比较。最后，在未受辐照的凝胶剂量计的整个体积中采集图像，以在进行辐射之前量化图像噪声和均匀性。

结果

逐片背景减法有效地消除了使用多层扫描仪同时采集的图像中观察到的NCT变异性，并且是使用多层CT系统时推荐的背景减法方法。与单层扫描仪相比，多层系统的图像噪声更高，但两个系统的整体图像质量相当。进一步的研究表明，对于所检查的每种切片厚度探测器配置，使用多层探测器阵列同时采集的图像切片之间的NCT是一致的。此外，发现多层系统消除了由于X射线管负荷增加而导致的NCT变化，并且与使用单层扫描仪对大体积进行成像相比，扫描时间减少了4倍。通过未受辐照的活性凝胶采集的图像显示，在1升圆柱体的顶部和底部之间以及圆柱体直径上，NCT变化高达7 HU。