基于MRI的合成CT生成方法综述

A Review of MRI-Based Synthetic CT Image Generation

放射治疗是治疗肿瘤的重要手段之一,而CT是目前放射治疗的主要基准影像。与CT相比,MRI具有良好的软组织对比度,且对人体完全无害,近年来也越来越会多地被用于软组织的勾画以及引导放射治疗。由于CT扫描的电离辐射影响患者的健康,同时扫描CT和MRI加重患者的经济负担,CT和MRI的配准融合会引入系统误差等弊端,所以仅使用MRI的放射治疗受到研究者的广泛关注。然而,由于MRI与电子密度无联系,不能直接用于剂量计算以及基于X光的患者摆位验证,因此需要研究相关算法,根据MRI图像得到患者组织的电子密度信息或者HU值,即生成合成CT或伪CT。将生成合成CT或伪CT的方法分为3类:基于体素、基于图谱和混合的方法。根据所应用的方法,对序列图像、应用数量以及解剖部位等进行总结和分析。该研究是放射治疗中一个崭新的方向,既可避免使用传统CT带来的电离辐射,又可以应用高分辨率的MRI影像对肿瘤进行监测,使患者得到更精确的放射治疗。

Radiation therapy is one of the important ways in tumor treatments,and CT is the main reference image of current radiation therapy.Compared with CT,MRI has good soft tissue contrast and is completely harmless to the human body.In recent years,MRI has been increasingly used for soft tissue delineation and radiation therapy guidance.However,the ionizing radiation of CT scan affects the health of patients,at the same time,scanning CT and MRI increase the economic burden of patients,and the registration fusion of CT and MRI introduces systematic errors.Radiotherapy using MRI only has received more and more attention from researchers.Nevertheless,MRI has no connection with electron density and cannot be directly used for dose calculation and X-ray based patient placement verification,therefore it is necessary to study the correlation algorithm to obtain the electron density information or HU value of the patient tissue according to the MRI image,that is to generate synthetic CT or pseudo CT.Currently existing methods for generating synthetic CT or pseudo CT are classified into three categories,including voxel-based,atlas-based and hybrid methods.According to the applied methods,we summarized and analyzed the sequence images,the number of applications and the anatomical parts.This study may represent a new direction in radiation therapy,which can avoid the use of ionizing radiation from traditional CT,and can use high-resolution MRI images to monitor tumors and enable patients to perform more precise radiation therapy.