A probability‐based multi‐cycle sorting method for 4D‐MRI: A simulation study
Authors: Liang Xiao , Yin Fang‐Fang , Liu Yilin , Cai Jing 
Published: November 2016

Purpose:To develop a novel probability‐based sorting method capable of generating multiple breathing cycles of 4D‐MRI images and to evaluate performance of this new method by comparing with conventional phase‐based methods in terms of image quality and tumor motion measurement.Methods:Based on previous findings that breathing motion probability density function (PDF) of a single breathing cycle is dramatically different from true stabilized PDF that resulted from many breathing cycles, it is expected that a probability‐based sorting method capable of generating multiple breathing cycles of 4D images may capture breathing variation information missing from conventional single‐cycle sorting methods. The overall idea is to identify a few main breathing cycles (and their corresponding weightings) that can best represent the main breathing patterns of the patient and then reconstruct a set of 4D images for each of the identified main breathing cycles. This method is implemented in three steps: (1) The breathing signal is decomposed into individual breathing cycles, characterized by amplitude, and period; (2) individual breathing cycles are grouped based on amplitude and period to determine the main breathing cycles. If a group contains more than 10% of all breathing cycles in a breathing signal, it is determined as a main breathing pattern group and is represented by the average of individual breathing cycles in the group; (3) for each main breathing cycle, a set of 4D images is reconstructed using a result‐driven sorting method adapted from our previous study. The probability‐based sorting method was first tested on 26 patients’ breathing signals to evaluate its feasibility of improving target motion PDF. The new method was subsequently tested for a sequential image acquisition scheme on the 4D digital extended cardiac torso (XCAT) phantom. Performance of the probability‐based and conventional sorting methods was evaluated in terms of target volume precision and accuracy as measured by the 4D images, and also the accuracy of average intensity projection (AIP) of 4D images.Results:Probability‐based sorting showed improved similarity of breathing motion PDF from 4D images to reference PDF compared to single cycle sorting, indicated by the significant increase in Dice similarity coefficient (DSC) (probability‐based sorting, DSC = 0.89 ± 0.03, and single cycle sorting, DSC = 0.83 ± 0.05, p‐value <0.001). Based on the simulation study on XCAT, the probability‐based method outperforms the conventional phase‐based methods in qualitative evaluation on motion artifacts and quantitative evaluation on tumor volume precision and accuracy and accuracy of AIP of the 4D images.Conclusions:In this paper the authors demonstrated the feasibility of a novel probability‐based multicycle 4D image sorting method. The authors’ preliminary results showed that the new method can improve the accuracy of tumor motion PDF and the AIP of 4D images, presenting potential advantages over the conventional phase‐based sorting method for radiation therapy motion management.