用于千伏图像剂量测量的 Gafchromic XRQA2 薄膜的特性。

Characteristics of Gafchromic XRQA2 films for kV image dose measurement.

机构信息

Department Of Radiation Oncology, Thomas Jefferson University, Philadelphia, PA 19107, USA.

出版信息

Med Phys. 2012 Feb;39(2):842-50. doi: 10.1118/1.3675398.

DOI:10.1118/1.3675398

PMID:22320794

Abstract

PURPOSE

In this study, the relevant characteristics of the new Gafchromic XRQA2 film for its application in measuring kV cone beam computed tomography (CBCT) image doses were thoroughly investigated.

METHODS

The film was calibrated free in air to air kerma levels between 0 and 9 cGy using 120 kVp photon beams produced by the x-ray volume imager. Films were scanned using transmission and reflection scanning modes with the Epson Expression 10000 XL flat-bed document scanner. The impact of film size, region of interest for the analysis, scan uniformity, scan resolution, scan orientation and alternate scanning sides on the analysis process were investigated. Energy dependence, postirradiation growth of reflectance with time and irradiation angular dependence of the film were tested at different air kerma levels.

RESULTS

The net reflectance changed by ∼3% when the size of the film piece changed from 1 cm × 2 cm to 10 cm × 11 cm and changed by ∼1% when ROI changed from 0. 7 cm × 0. 7 cm to 8 cm × 8 cm, suggesting a good uniformity of the film. The film was successfully analyzed using the transmission scanning mode, calibration curves from both transmission and reflection scanning modes showed similar behavior. The calibration uncertainty was somewhat lower when the film was scanned using reflection mode (6% and 8% for reflection and transmission modes, respectively.) Higher scanning resolution came with increasing calibration uncertainty. The calibration uncertainty for reflection and transmission modes increased from ∼3.5% to 7% and from ∼3.5% to 9%, respectively when scanning resolution was changed from 50 to 400 dpi. Scanning the film on alternate sides using transmission mode led to variation of 16%-19% in the net optical density at doses commonly used for CBCT procedures. The film response changed by almost 10% when it was exposed to beams of two different energies (100 and 120 kVp.) Other features of the film such as film orientation, postexposure growth, and irradiation angular dependence were also investigated.

CONCLUSIONS

The size of film piece and analysis ROI used for calibration slightly affected the film response. Both transmission and reflection scanning modes can be used to analyze the Gafchromic XRQA2, with the reflection mode having a somewhat lower calibration uncertainty. Scanning films on alternate sides using transmission mode significantly affects the optical density. The film response was shown to be energy dependent. The films reached stability in about 6 h after exposure. The film response was proven to be independent of irradiation angle except when the beam is parallel to the film surface.

摘要

目的

本研究旨在深入探讨新型 Gafchromic XRQA2 胶片在测量千伏锥形束计算机断层摄影术（CBCT）图像剂量方面的相关特性。

方法

使用 X 射线容积成像仪产生的 120 kVp 光子束，在空气中对胶片的空气比释动能水平在 0 至 9 cGy 之间进行自由校准。使用爱普生 Expression 10000XL 平板文档扫描仪，分别采用透射和反射扫描模式对胶片进行扫描。研究了胶片尺寸、感兴趣区域分析、扫描均匀性、扫描分辨率、扫描方向和交替扫描边对分析过程的影响。在不同的空气比释动能水平下，测试了胶片的能量依赖性、随时间的反射率后增长以及辐照角度依赖性。

结果

当胶片片的尺寸从 1 cm×2 cm 变为 10 cm×11 cm 时，净反射率变化约 3%，当 ROI 从 0.7 cm×0.7 cm 变为 8 cm×8 cm 时，净反射率变化约 1%，表明胶片具有较好的均匀性。采用透射扫描模式成功地对胶片进行了分析，透射和反射扫描模式的校准曲线表现出相似的行为。当使用反射模式扫描胶片时，校准不确定度略低（分别为 6%和 8%，用于反射和透射模式。）较高的扫描分辨率伴随着校准不确定度的增加。当扫描分辨率从 50 增加到 400dpi 时，反射和透射模式的校准不确定度分别从约 3.5%增加到 7%和从约 3.5%增加到 9%。采用透射模式在交替侧扫描胶片会导致在 CBCT 程序中常用剂量下的净光密度变化 16%-19%。当胶片暴露在两种不同能量（100 和 120 kVp）的射束下时，胶片的响应变化近 10%。胶片的其他特性，如胶片方向、曝光后增长和辐照角度依赖性，也进行了研究。

结论

胶片片的尺寸和校准用 ROI 稍影响胶片的响应。透射和反射扫描模式均可用于分析 Gafchromic XRQA2，反射模式的校准不确定度略低。采用透射模式在交替侧扫描胶片会显著影响光密度。胶片的响应表现出对能量的依赖性。胶片在曝光后约 6 小时达到稳定。胶片的响应除了当光束平行于胶片表面时，与辐照角度无关。