5.8　Challenges and Perspectives

With the development of medicine and life science，health has been paid much attention.Advanced medical imaging technique is one of the core techniques，which serves as an effectively medical diagnostic technique，including fluorescence imaging，photoacoustic imaging，photothermal imaging，ultrasound imaging，magnetic resonance imaging，Raman imaging，etc.Compared with traditional disease diagnosis methods，medical imaging technique is visualized，more accurate and sensitive.In view of the features of each imaging technique，imaging objects，spatial resolution and depth，sensitivity，are different.Thus，a single imaging technique exhibits unavoidable drawbacks：①The spatial resolution of fluorescence imaging is too low to image deep tissue，due to the limitation of penetration depth.②Penetration depth of photoacoustic imaging is still insufficient，and image contrast and detection sensitivity should be further improved.③The molecular contrast agents，which are low cytotoxicity and provide sufficient contrast，are lack.④Ultrasound imaging has low resolution and is dependent on operating experience.⑤MRI is time-consuming，high cost，and relatively low sensitivity，etc.

The development of multimodal imaging in recent years has attracted much attention in the field of medical imaging，which combines two or more imaging techniques to avoid the defects of a single imaging technique.The medical information obtained by different imaging techniques is complementary.Therefore，it is beneficial for the accurate diagnosis of patients to examine them with multiple imaging techniques.However，it will increase the patients'financial burden and the risk of toxic and side effects of angiography，which promotes the presence of multimodal imaging technique.By incorporating anatomical and functional imaging in a common hybrid imaging platform，a synergism in the imaging capabilities can be achieved，thus making it possible to precisely visualize and delineate structural and functional information.Multimodal imaging thereby ensures better elucidation of physiological mechanisms at molecular and cellular levels.Therefore，multimodal imaging has an immensely beneficial role for improved diagnosis and therapeutic planning of a disease.The field of multimodal imaging has seen rapid progress in the last decade，with its value having been demonstrated in numerous studies.(https://www.daowen.com)