[Timing calibration comparison research of integrated TOF-PET/MR]

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.