CTDI 中点剂量比的研究：对 CT 剂量评估的意义

A study of the midpoint dose to CTDI ratio: Implications for CT dose evaluation.

作者信息

Li Xinhua, Yang Kai, Liu Bob

机构信息

Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114.

出版信息

Med Phys. 2016 Nov;43(11):5878. doi: 10.1118/1.4963811.

DOI:10.1118/1.4963811

PMID:27806599

Abstract

PURPOSE

In multidetector CT, the volume CT dose index (CTDI) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scanner-independent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length.

METHODS

A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDI(water)/CTDI (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose D(0) (water) to CTDI ratio in a CT scan series with a scan length equal to L.

RESULTS

For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of D(0)/CTDI from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDI to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, D(0)/CTDI decreased considerably as L decreased. D(0)/CTDI was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, D(0)/CTDI decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm.

CONCLUSIONS

The midpoint dose to CTDI ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT.

摘要

目的

在多排螺旋CT中，每个扫描序列都会报告容积CT剂量指数（CTDI），并使用剂量转换因子进行特定尺寸剂量估算（SSDE）和独立于扫描仪的器官剂量评估。本研究旨在探讨转换因子对扫描长度、管电压和受检者体型的依赖性。研究结果对于评估患者体型和扫描长度差异较大的CT检查剂量可能具有参考价值。

方法

使用先前开发的蒙特卡洛模拟程序，对两个标准CTDI体模[材料为聚甲基丙烯酸甲酯（PMMA），直径分别为16 cm和32 cm]以及多个不同尺寸的水模在五种管电压（70、80、100、120和140 kV）下进行单圈轴向扫描模拟。所得纵向剂量分布用于计算CTDI(水)/CTDI (PMMA)，其中L为剂量积分长度。该比值等于扫描长度等于L的CT扫描序列中中点剂量D(0)（水）与CTDI的比值。

结果

对于直径从11 cm到50 cm的水模以及扫描长度从15 cm到30 cm的情况，与120 kV和20 cm扫描长度相比，D(0)/CTDI的变化在-18.4%至11.7%之间。这与美国医学物理师协会第204号报告中的CTDI到SSDE转换因子一致。对于扫描长度小于15 cm的情况，D(0)/CTDI随着L的减小而显著降低。当管电压为120 kV且体模直径范围为11 cm至50 cm时，L = 5 cm时的D(0)/CTDI比L = 15 cm时低17.3% - 31.4%。随着管电压从80 kV增加到140 kV，小直径时D(0)/CTDI降低，而大直径时则升高。直径为18 cm且扫描长度为20 cm时变化为9.4%，直径为40 cm且扫描长度为30 cm时变化为17.6%。