一体化TOF-PET/MR时间校正方法的对比研究

Timing calibration comparison research of integrated TOF-PET/MR

一体化TOF-PET/MR是获得高精度正电子断层扫描(PET)图像同时一并获得磁共振(MR)图像的多模态成像设备,具备飞行时间(TOF)功能。其中PET系统利用了正电子湮灭产生的两个方向180°的光子被探测器接收的时间差,来缩小响应线上湮灭发生位置的范围。由于每个探测晶体自身硬件条件不同和外部噪声影响,不同晶体的时间偏移不同,所以需要对一体化TOF-PET/MR进行精确的时间校正来使得系统正常工作。我们采取三种不同原理的方法对系统进行时间校正,第一种为利用几何方法构建扇形束模型,来拟合时间偏移的高斯分布,从而求晶体时间偏差的迭代方法,简称扇形束法;第二种为构建超定方程组的解,并利用L1范数最小化求解的方法,简称L1-norm法;第三种利用大量数据构建直方图,拟合寻峰后的结果构成超定方程组,利用L2范数最小化进行求解,简称L2-norm法。本文对这三种方法所需数据量和计算时间进行了比较,对时间校正后系统重建图像进行了分析。为了减小采集数据时放射源位置偏移对校正结果的干扰,我们设计了位置校正算法对采集数据进行预处理,该算法能直接计算出筒源摆放位置,并减少因为偏心产生的图像伪影。实验结果证明,L2-norm法噪声比较小,但是计算速度慢;L1-norm法拥有最快的计算速度,但是图像质量较差;扇形束法较其他两种方法拥有更高的图像质量,尤其是对于微小病灶的探查能力更强,因此在一体化TOF-PET/MR中使用扇形束法进行时间校正最佳。

Integrated TOF-PET/MR is a multimodal imaging system which can acquire high-quality magnetic resonance(MR)and positron emission tomography(PET)images at the same time,and it has time of flight(TOF)function.The TOF-PET system usually features better image quality compared to traditional PET because it is capable of localizing the lesion on the line of response where annihilation takes place.TOF technology measures the time difference between the detectors on which the two 180-degrees-seperated photons generated from positron annihilation are received.Since every individual crystal might be prone to its timing bias,timing calibration is needed for a TOF-PET system to work properly.Three approaches of timing calibration are introduced in this article.The first one named as fan-beam method is an iterative method that measures the bias of the Gaussian distribution of timing offset created from a fan-beam area constructed using geometric techniques.The second one is to find solutions of the overdetermination equations set using L1 norm minimization and is called L1-norm method.The last one called L2-norm method is to build histogram of the TOF and find the peak,and uses L2 norm minimization to get the result.This article focuses on the comparison of the amount of the data and the calculation time needed by each of the three methods.To avoid location error of the cylinder radioactive source during data collection,we developed a location calibration algorithm which could calculate accurate position of the source and reduce image artifacts.The experiment results indicate that the three approaches introduced in this article could enhance the qualities of PET images and standardized uptake values of cancer regions,so the timing calibration of integrated TOF-PET/MR system was realized.The fan-beam method has the best image quality,especially in small lesions.In integrated TOF-PET/MR timing calibration,we recommend using fan-beam method.