New absorbed dose measurement with cylindrical water phantoms for multidetector CT
New absorbed dose measurement with cylindrical water phantoms for multidetector CT
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DOI：
10.1088/0031-9155/60/11/4517
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发表时间：
2015-06-07
影响因子：
3.5
通讯作者：
Yamashita, Yusuke
中科院分区：
文献类型：
--
作者：
Ohno, Takeshi;Araki, Fujio;Yamashita, Yusuke
本研究的目的是开发用于多探测器计算机断层扫描 (MDCT) 的圆柱形水体模的新剂量测定。电离测量是在体型和头型圆柱形水模型的中心和四个外围点上使用农夫电离室进行的。使用 Co-60 吸收剂量与水的校准因子和蒙特卡罗 (MC) 计算的校正因子将电离转换为吸收剂量。校正因子是通过 MDCT（Brilliance iCT，64 切片，Philips Electronics）根据 EGSnrc MC 代码使用 GMctdospp（IMPS，德国）软件建模计算得出的。确定了入射 X 射线束的光谱和 MDCT 领结滤波器的配置，以便计算出的铝 (Al) 光子强度衰减曲线和计算出的空气中偏心比 (OCR) 曲线与测量值一致。 MC 计算的剂量通过在两个圆柱形水模型中心测量的吸收剂量进行校准。将计算的剂量与模型中四个外围点和中心的不同射束间距和射束准直的测量剂量进行比较。使用该方法测定的吸收剂量的校准因子和不确定度也与CTDIair（空气中CT剂量指数）获得的结果进行了比较。计算出的 Al 半值层和空气中的 OCR 与测量值的一致性分别在 0.3% 和 3% 以内。不同射束间距和射束准直的四个外围点和中心的计算剂量分别与测量值的一致性在 5% 和 2% 以内。由于过度光束效应，我们方法的 MC 校准因子比 CTDIair 的值低 44-50%。然而，在校正过梁效应后，CTDIair 的校准因子与我们的方法的校准因子一致在 5% 以内。我们的方法可以直接测量 MDCT 的吸收剂量，并且比 CTDIair 测量更稳健、更准确。
The aim of this study was to develop new dosimetry with cylindrical water phantoms for multidetector computed tomography (MDCT). The ionization measurement was performed with a Farmer ionization chamber at the center and four peripheral points in the body-type and head-type cylindrical water phantoms. The ionization was converted to the absorbed dose using a Co-60 absorbed-dose-to-water calibration factor and Monte Carlo (MC) -calculated correction factors. The correction factors were calculated from MDCT (Brilliance iCT, 64-slice, Philips Electronics) modeled with GMctdospp (IMPS, Germany) software based on the EGSnrc MC code. The spectrum of incident x-ray beams and the configuration of a bowtie filter for MDCT were determined so that calculated photon intensity attenuation curves for aluminum (Al) and calculated off-center ratio (OCR) profiles in air coincided with those measured. The MC-calculated doses were calibrated by the absorbed dose measured at the center in both cylindrical water phantoms. Calculated doses were compared with measured doses at four peripheral points and the center in the phantom for various beam pitches and beam collimations. The calibration factors and the uncertainty of the absorbed dose determined using this method were also compared with those obtained by CTDIair (CT dose index in air). Calculated Al half-value layers and OCRs in air were within 0.3% and 3% agreement with the measured values, respectively. Calculated doses at four peripheral points and the centers for various beam pitches and beam collimations were within 5% and 2% agreement with measured values, respectively. The MC-calibration factors by our method were 44-50% lower than values by CTDIair due to the overbeaming effect. However, the calibration factors for CTDIair agreed within 5% with those of our method after correction for the overbeaming effect. Our method makes it possible to directly measure the absorbed dose for MDCT and is more robust and accurate than the CTDIair measurement.