千伏级CBCT图像CT值校正及在放疗剂量计算中应用

目的 研究千伏级CBCT图像CT值校正方法,提高其用于剂量计算的准确性。方法 以扇形束计划CT作为先验信息,将CBCT与计划CT图像进行刚性配准,通过将CBCT与计划CT图像相减得到CBCT散射背景估计,对散射背景进行低通滤波处理,最后将原始CBCT图像减去滤波后的散射背景得到校正的CBCT图像。对Catphan600模体和4例盆腔恶性肿瘤患者的CBCT图像进行校正,配对t检验校正前后CBCT与计划CT的差异,评估校正后的CBCT图像质量并分析用于剂量计算的准确性。 结果 经CT值校正后CBCT图像伪影明显减少,空气、脂肪、肌肉、股骨头的平均值校正前与计划CT分别相差232、89、29、66 HU,而校正后平均值差别缩小至5 HU内(P=0.39、0.66、0.59、1.00)。校正后CBCT图像用于剂量计算误差在2%内。 结论 校正后的CBCT图像CT值与计划CT的CT值相似,用于剂量计算可得到准确的结果。

Investigation of CT numbers correction of kilo-voltage cone-beam CT images for accurate dose calculation

Objective To study CT numbers correction of kilo-voltage cone-beam CT (KV-CBCT) images for dose calculation. Method Aligning the CBCT images with plan CT images, then obtain the background scatter by subtracting CT images from CBCT images. The background scatter is then processed by low-pass filter. The final CBCT images are acquired by subtracting the background scatter from the raw CBCT. KV-CBCT images of Catphan600 phantom and four patients with pelvic tumors were obtained with the linac-integrated CBCT system. The CBCT images were modified to correct the CT numbers. Finally, compare HU numbers between corrected CBCT and planning CT by paired T test. Evaluate the image quality and accuracy of dose calculation of the modified CBCT images. Results The proposed method reduces the artifacts of CBCT images significantly. The differences of CT numbers were 232 HU, 89 HU, 29 HU and 66 HU for air, fat, muscle and femoral head between CT and CBCT respectively (P=0.39,0.66,0.59,1).The differences of CT numbers between CT and CBCT was reduced to within 5 HU. And the error of dose calculation with corrected CBCT images was within 2%. Conclusions The CT numbers of corrected CBCT are similar with plan CT images and dose calculations based on the modified CBCT show good agreement with plan CT.