**4. Conclusion**

ASD is a disease induced by both environmental and heredity factors, and its major cause and induction process still remain unclear. Early and clear diagnosis of ASD is conducive to improving ASD symptoms in patients, thereby affecting treatment efficacy. PET can be applied to quantitatively and dynamically evaluate cerebral glucose metabolism, task and resting states, neurotransmitter system biomarkers, and cerebral blood flow perfusion, explore ASD pathophysiology, and promote ASD therapeutic drug development, as well as in ASD diagnosis and treatment. The identification of ASD biomarkers and PET probes is critical in ASD diagnosis, and drug development and evaluation. Numerous ASD associated biomarkers have been identified in cerebral blood perfusion, glucose metabolism, neuroinflammation, and neurotransmitter systems. However, due to the differences in ASD subtypes, experimental design (e.g., imaging conditions, anesthesia/sleep/awake, and/or task-state/ resting-state), and subjects (e.g., gender, IQ and age), contradictory conclusions may be drawn from different studies [160]. There a limitation in the current ASD studies.

#### *Positron Emission Tomography in the Neuroimaging of Autism Spectrum Disorder DOI: http://dx.doi.org/10.5772/intechopen.106825*

The PET imaging studies showed that there might be differences in the pathophysiological mechanisms for different ASD subtypes and therefore different results may be observed and different conclusions may be drawn for different ASD subtypes. Therefore, more attention should be paid on this in future ASD PET studies to explore the possible differences in the pathophysiological mechanisms of different ASD subtypes, and selection of experimental subjects and design of experiments should be determined and performed based on these differences to provide more valuable references for clinical diagnosis and treatment of ASD [161].

In this chapter, several ASD related targets, including cerebral blood perfusion, cerebral glucose metabolism, neuroinflammation, arginine vasopressin receptor, neurotransmitter system, as well as the corresponding probes and their applications in ASD imaging studies and the experimental results are summarized. PET studies using different probes can obtain similar results, while PET studies using the same probe can also produce different results due to the heterogeneity of ASD patients and the influence of TSPO gene polymorphism [162]. Therefore, although identify new biomarkers and develop novel PET probes are very helpful for ASD study, evaluation the application value of each probe in the existing system is difficult. Therefore, it is crucial to seek and investigate the associations between a probe, a biomarker, and a specific ASD subtype at molecular level [163], which is the most valuable exploration direction in future ASD studies.

PET molecular imaging is very helpful for ASD diagnosis and treatment, however, there are still some limitations and difficulties in execution. First, patients are required to lie on a machine for PET/MR and PET/CT scanning, and protection from radiation is required during the imaging process, so high coordination of patients is very important. Second, ASD patients often have a large age span and significant differences in IQ, so professional technicians need to train some patients to make them get familiar with drug injection, task process or even anesthesia operation and adapt to scanning environment. Therefore, the joint efforts of patients, doctors, technicians, nurses, researchers, as well as other specialists in neuropsychiatry, rehabilitation, and nuclear medicine to promote continuous advancements in ASD research, diagnosis and treatment.
