Medical Physics Department, Anticancer-Oncology Hospital of Athens "Agios Savvas," 171 Alexandras Avenue, 115 22 Athens, Greece.
Med Phys. 2013 Nov;40(11):111918. doi: 10.1118/1.4826160.
To investigate the utility of the nested polymethylacrylate (PMMA) phantom (which is available in many CT facilities for CTDI measurements), as a tool for the presentation and comparison of the ways that two different CT automatic exposure control (AEC) systems respond to a phantom when various scan parameters and AEC protocols are modified.
By offsetting the two phantom's components (the head phantom and the body ring) half-way along their longitudinal axis, a phantom with three sections of different x-ray attenuation was created. Scan projection radiographs (SPRs) and helical scans of the three-section phantom were performed on a Toshiba Aquilion 64 and a Philips Brilliance 64 CT scanners, with different scan parameter selections [scan direction, pitch factor, slice thickness, and reconstruction interval (ST/RI), AEC protocol, and tube potential used for the SPRs]. The dose length product (DLP) values of each scan were recorded and the tube current (mA) values of the reconstructed CT images were plotted against the respective Z-axis positions on the phantom. Furthermore, measurements of the noise levels at the center of each phantom section were performed to assess the impact of mA modulation on image quality.
The mA modulation patterns of the two CT scanners were very dissimilar. The mA variations were more pronounced for Aquilion 64, where changes in any of the aforementioned scan parameters affected both the mA modulations curves and DLP values. However, the noise levels were affected only by changes in pitch, ST/RI, and AEC protocol selections. For Brilliance 64, changes in pitch affected the mA modulation curves but not the DLP values, whereas only AEC protocol and SPR tube potential selection variations affected both the mA modulation curves and DLP values. The noise levels increased for smaller ST/RI, larger weight category AEC protocol, and larger SPR tube potential selection.
The nested PMMA dosimetry phantom can be effectively utilized for the comprehension of CT AEC systems performance and the way that different scan conditions affect the mA modulation patterns, DLP values, and image noise. However, in depth analysis of the reasons why these two systems exhibited such different behaviors in response to the same phantom requires further investigation which is beyond the scope of this study.
研究嵌套聚甲基丙烯酸甲酯（PMMA）体模（许多 CT 设备都可用于 CTDI 测量）作为工具的效用，以展示和比较两种不同的 CT 自动曝光控制（AEC）系统在修改各种扫描参数和 AEC 协议时对体模的响应方式。
通过将两个体模的组件（头部体模和体环）沿其纵轴偏移一半，创建了一个具有三个不同 X 射线衰减部分的体模。在东芝 Aquilion 64 和飞利浦 Brilliance 64 CT 扫描仪上对三部分体模进行了扫描投影射线照片（SPR）和螺旋扫描，选择了不同的扫描参数[扫描方向、螺距因子、层厚和重建间隔（ST/RI）、AEC 协议以及用于 SPR 的管电压]。记录每个扫描的剂量长度乘积（DLP）值，并将重建 CT 图像的管电流（mA）值绘制在体模的相应 Z 轴位置上。此外，还测量了每个体模部分中心的噪声水平，以评估 mA 调制对图像质量的影响。
两台 CT 扫描仪的 mA 调制模式差异很大。任何上述扫描参数的变化都会影响 Aquilion 64 的 mA 调制曲线和 DLP 值，而 Brilliance 64 的变化只会影响管电流调制曲线和 DLP 值。对于 Aquilion 64，mA 变化更为明显，任何扫描参数的变化都会影响 mA 调制曲线和 DLP 值。但是，噪声水平仅受螺距、ST/RI 和 AEC 协议选择的变化影响。对于 Brilliance 64，螺距变化会影响 mA 调制曲线，但不会影响 DLP 值，而仅 AEC 协议和 SPR 管电压选择变化会影响 mA 调制曲线和 DLP 值。噪声水平随着 ST/RI 的减小、较大的 AEC 协议体重类别和较大的 SPR 管电压选择而增加。
嵌套 PMMA 剂量体模可有效地用于理解 CT AEC 系统的性能以及不同扫描条件对 mA 调制模式、DLP 值和图像噪声的影响。然而，深入分析这两个系统对同一体模表现出如此不同行为的原因需要进一步研究，这超出了本研究的范围。
