Reyhan Meral L, Chen Ting, Zhang Miao
Rutgers Cancer Institute of New Jersey.
J Appl Clin Med Phys. 2015 Nov 8;16(6):325–332. doi: 10.1120/jacmp.v16i6.5743.
Magnetic resonance (MR) imaging of Gafchromic film causes perturbation to absolute dosimetry measurements; the purpose of this work was to characterize the perturbation and develop a correction method for it. Three sets of Gafchromic EBT2 film were compared: radiation (control), radiation followed by MR imaging (RAD + B), and MR imaging followed by radiation (B + RAD). The T1-weighted and T2-weighted MR imaging was performed using a 1.5T scanner with the films wedged between two chicken legs. Doses from 0 to 800 cGy were delivered with a 6MV linac. The time interval between radiation and MR imaging was less than 10 min. Film calibration was generated from the red channel. Microscopic imaging was performed on two pieces of film. The effect of specific absorption rate (SAR) was determined by exposing another three sets of films to low, medium, and high levels of SAR through a series of pulse sequences. No discernible preferential alignment was detected on the microscopic images of the irradiated film exposed to MRI. No imaging artifacts were introduced by Gafchromic film on any MR images. On average, 4% dose difference was observed between B + RAD or RAD + B and the control, using the same calibration curve. The pixel values between the B + RAD or RAD + B and the control films were found to follow a linear relationship pixel(Control) = 1.02 × pixel(B + RAD or RAD + B). By applying this correction, the average dose error was reduced to approximately 2%. The SAR experiment revealed a dose overestimation with increasing SAR even when the correction was applied. It was concluded that MR imaging introduces perturbation on Gafchromic film dose measurements by 4% on average, compared to calibrating the film without the presence of MRI. This perturbation can be corrected by applying a linear correction to the pixel values. Additionally, Gafchromic film did not introduce any imaging artifacts in any of the MR images acquired.
磁共振（MR）成像会对Gafchromic胶片的绝对剂量测量产生干扰；本研究的目的是对这种干扰进行表征并开发一种校正方法。比较了三组Gafchromic EBT2胶片：辐射组（对照组）、先进行辐射再进行MR成像组（RAD + B）以及先进行MR成像再进行辐射组（B + RAD）。使用1.5T扫描仪进行T1加权和T2加权MR成像，胶片夹在两条鸡腿之间。用6MV直线加速器给予0至800 cGy的剂量。辐射与MR成像之间的时间间隔小于10分钟。通过红色通道进行胶片校准。对两张胶片进行微观成像。通过一系列脉冲序列，将另外三组胶片暴露于低、中、高比吸收率（SAR）水平，以确定SAR的影响。在接受MRI检查的辐照胶片的微观图像上未检测到明显的优先排列。Gafchromic胶片在任何MR图像上均未引入成像伪影。使用相同的校准曲线时，B + RAD或RAD + B组与对照组之间平均观察到4%的剂量差异。发现B + RAD或RAD + B组与对照胶片之间的像素值呈线性关系：像素（对照组） = 1.02×像素（B + RAD或RAD + B）。通过应用这种校正，平均剂量误差降低到约2%。SAR实验表明，即使应用了校正，随着SAR增加仍存在剂量高估现象。得出的结论是，与在不存在MRI的情况下校准胶片相比，MR成像平均会对Gafchromic胶片剂量测量产生4%的干扰。通过对像素值应用线性校正可以校正这种干扰。此外，Gafchromic胶片在采集的任何MR图像中均未引入任何成像伪影。
