Zhou Yifang, Nute Jessica, Scott Alexander, Lee Christina
Departments of Radiation Safety & Imaging, Cedars-Sinai Medical Center, Los Angeles, CA, 90048, USA.
Department of Biomedical Sciences, Cedars-Sinai Medical Center, Los Angeles, CA, 90048, USA.
Med Phys. 2017 Mar;44(3):861-872. doi: 10.1002/mp.12085. Epub 2017 Feb 21.
For CT dose optimization, one needs to address two important questions. The first is how various lesion-specific detection tasks demand different patient doses for the same patient. The second is how the variation of the patient size requires different patient doses for the same lesion detection task. In this study, we attempted to find quantitative solutions to these questions by utilizing a wide range of abdomen phantoms.
A simplified model with a monochromatic fan beam passing through a bowtie-filter and an elliptical object was proposed. The model relates the minimum detectable contrast (MDC) to the size-specific dose by power index of -1/2 and to the lesion size by power index of -1 with a patient size dependence function (PSDF) as the proportionality factor. The experimental validation was performed using seven abdomen phantoms (lateral ranges: 10 cm-39 cm) scanned with helical modes at various dose levels on two 64-slice scanners (Siemens mCT and GE HD 750). Noise images were obtained using subtractions among adjacent slices in the images reconstructed with filtered backprojection. It was verified that the mean pixel value distributions from various small regions (1.8 mm-10 mm) are Gaussian, thus the concept of the statistically defined minimum detectable contrast (SD-MDC), defined as distribution's standard deviation multiplied by 3.29, can be applied. The impact of the helical pitch and the high-definition (HD) acquisition was also studied.
The experimental data from all phantoms were found to fit the power law well (R ≥ 0.983). The PSDF was found to be scanner dependent - modeled with a Gaussian amplifier (R = 0.983) for one manufacturer and with an exponential function for the other (R = 0.990). The MDC relationship was not found to be impacted by different pitches or by HD acquisition. The results were used to find the size-specific doses and corresponding acquisition techniques required by consistent low-contrast detectability for variable patient sizes. Visual comparisons on the low-contrast insert images demonstrated that the derived techniques delivered consistent low-contrast detectability.
We have modeled and verified the relationship of the minimum detectable contrast to the patient size, the patient dose, and the lesion size from the images reconstructed with filtered backprojection. The findings can be useful for task-specific dose modulation on abdomen CT studies.
为了优化CT剂量，需要解决两个重要问题。第一个问题是，对于同一患者，各种特定病变检测任务所需的患者剂量有何不同。第二个问题是，对于同一病变检测任务，患者体型的变化如何需要不同的患者剂量。在本研究中，我们试图通过使用多种腹部体模来找到这些问题的定量解决方案。
提出了一个简化模型，其中单色扇形束穿过蝴蝶结滤波器和椭圆形物体。该模型通过幂指数为-1/2将最小可检测对比度（MDC）与特定尺寸剂量相关联，并通过幂指数为-1将其与病变大小相关联，以患者尺寸依赖函数（PSDF）作为比例因子。使用七个腹部体模（横向范围：10 cm - 39 cm）在两台64层扫描仪（西门子mCT和GE HD 750）上以不同剂量水平进行螺旋扫描来进行实验验证。通过滤波反投影重建图像中相邻切片相减获得噪声图像。验证了各个小区域（1.8 mm - 10 mm）的平均像素值分布呈高斯分布，因此可以应用统计定义的最小可检测对比度（SD - MDC）概念，其定义为分布的标准差乘以3.29。还研究了螺距和高清（HD）采集的影响。
发现所有体模的实验数据都很好地符合幂律（R ≥ 0.983）。发现PSDF与扫描仪相关 - 对于一个制造商用高斯放大器建模（R = 0.983），对于另一个制造商用指数函数建模（R = 0.990）。未发现MDC关系受不同螺距或HD采集的影响。结果用于确定不同患者体型下一致的低对比度可检测性所需的特定尺寸剂量和相应的采集技术。对低对比度插入图像的视觉比较表明，所推导的技术提供了一致的低对比度可检测性。
我们已经对滤波反投影重建图像中最小可检测对比度与患者体型、患者剂量和病变大小之间的关系进行了建模和验证。这些发现可用于腹部CT研究中特定任务的剂量调制。
