Objective To explore an algorithm to define the threshold value for tumor contouring on 18F-fluorodexyglucose (FDG) PET imaging. Methods A National Electrical Manufacturing Association (NEMA)NU 2 1994 PET phantom with 5 spheres of different diameters were filled with 18F-FDG. Seven different sphere-to-background ratios were obtained and the phantom was scanned by Discovery LS 4. For each sphere-to-background ratio, the maximum standardized uptake value ( SUVmax ) of each sphere, the SUV of the border of each sphere ( SUVborder ), the mean SUV of a 1 cm region of background (SUVbg) and the diameter (D) of each sphere were measured. SPSS 13.0 software was used for curve fitting and regression analysis to obtain the threshold algorithm. The calculated thresholds were applied to delineate 29 pathologically confirmed lung cancer lesions on PET images and the obtained volumes were compared with the volumes contoured on CT images in lung window. Results The algorithm for defining contour threshold is TH% = 33.1% + 46.8% SUVbg/SUVmax + 13.9%/D ( r = 0.994) by phantom studies. For 29 lung cancer lesions, the average gross tumor volumes ( GTV ) delineated on PET and CT are ( 7.36 ± 1.62 ) ml and (8.31 ±2.05) ml, respectively (t = -1.26, P＞0.05). Conclusion The proposed threshold algorithm for tumor contouring on PET image could provide comparable GTV with CT.%目的 通过水模研究,探讨影响18F-脱氧葡萄糖(FDG)PET图像靶区勾画阈值(TH%)的因素,建立计算阈值的公式,为不同生物学特征的肿瘤选取不同阈值.方法 对特制的拥有5个不同大小靶区、7种不同靶区本底放射性比值的水模进行PET/CT图像采集,由图像得到每个靶区的最大标准摄取值(SUVmax)、靶区边界处的SUV(SUVborder)、本底中1 cm×1 cm大小感兴趣区的平均SUV(SUVbg)以及靶区内径(D)等,使用SPSS 13.0的曲线估计和线性回归分析方法,得到计算阈值的公式,并以此对29个经病理检查确诊的肺癌原发灶或转移淋巴结进行肿瘤靶区勾画,求出体积,比较PET和CT图像勾画的体积间的差别.结果 通过分析水模数据,得出阈值与靶区的大小呈负相关,与靶区的SUVmax呈负相关,与SUVbg呈正相关,阈值的计算公式为TH%=33.1%+46.8%×SUVbg/SUVmax+13.9%/D,r=0.994.通过对29个病灶进行比较,发现PET和CT勾画的平均大体肿瘤体积(GTV)分别是(7.36±1.62)ml和(8.31±2.05)ml,两者间差异无统计学意义(t=-1.26,P＞0.05).结论 靶区的大小、SUVmax及SUVbg均会影响靶区勾画阈值;通过公式TH%=33.1%+46.8%×SUVbg/SUVmax+13.9%/D可对不同肿瘤计算合适的阈值.按此公式PET和CT勾画的靶区无明显差异.
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