With the advent of the uEXPLORER scanner, the PET/CT scan has evolved to a total-body imaging in clinics. And its superior performance has made it a powerful tool in oncological applications. [18–19, 27–28] The ultra-high sensitivity provides the feasibility of a fast PET scan in routine practice for an improvement of patient comfort and an increased throughput. This study assessed the image quality from the phantom and clinical perspectives to explore a fast PET scan protocol with consistent image quality to that in a routine digital PET scanner. The PET acquisition using a routine PET/CT scanner in a step-and-shoot mode takes about 10–20 minutes for the body part and an additional ~ 3 minutes for the head to cover the scan range from the skull to the mid-thigh. However, in the total-body PET/CT scanner with an AFOV of 194cm, the PET acquisition can be performed with only one bed position to cover the entire patient body. The study demonstrated a fast PET scan with 30–45 seconds acquisition can provide comparable image quality according to the phantom study and the intra-individual comparison in 30 oncological patients. Furthermore, the study can be regarded as a methodology for an inter-scanner comparison, even for a multi-center study. As known, PET/CT plays an important role in timely monitoring of the therapeutic responses in various diseases, such as lymphoma, where multiple PET/CT scans are needed.  Multiple PET/CT scans for a certain patient in the inter-scanner studies always require a consistent image quality to improve the accuracy of the assessment. Therefore, the standardization and optimization of 18F-FDG protocols are essential in inter-scanner studies as illustrated in the study.
In this study, phantom-based study was performed with a standard NEMA/IEC NU-2 phantom. The phantom was selected since it is a simulation of the patient’s morphology and tracer distribution and commonly used in the image quality assessment of PET studies. However, in whole-body or total-body PET studies, the scan range of the patients, either from the skull to the mid-thigh or from the skull to the feet, were much larger than the phantom height. And for the total-body PET scans with uEXPLORER, the patient’s body were scanned with different parts of the PET detector along the AFOV. During the design of the phantom study, the limited axial coverage of this phantom has been considered. Thanks to the consistent sensitivity of the PET scanners along the AFOV of uEXPLORER , the image quality of the patient body can be regarded uniform. Therefore, the standard NEMA/IEC NU-2 phantom was used to assess the image quality in the total-body PET studies.
In the clinical part, the patients were randomly enrolled in the study without strict exclusion criteria, such as age, preparation during the uptake, diabetes, patient size, and cancer type. The enrolled patients group can be regarded as an epitome of the clinical practice. The enrolled patients in the study included almost all the common cancer types where the lesion uptake SUVmax varying from 1.0 to 40. The body mass index (BMI), known as an impact factor on the image quality [29, 30], of the enrolled patients varied in a large range (from 18.1 to 30.4). In addition, they were found to have accompanying diseases other than cancer, such as liver cirrhosis, ascites or with complication of systemic inflammation. And thus, the results of the study indicated a high compatibility and feasibility in the clinical practice.
Furthermore, the reference protocol in uMI 780 was a typical clinical protocol used in our hospital, with a compromise of the image quality and patient throughput. Although not optimal, the protocols obtained in the study can provide image quality as that in routine oncological studies. Based on the results in the qualitative and quantitative analysis, this study proposed a protocol using a 30s-45s acquisition on uEXPLORER with a consistent image quality to that on uMI 780. The phantom and clinical study showed slightly different results. It is well understood that the phantom study can just simulate the patient morphology and tracer distribution in a simplified way. In the phantom study, the tracer were uniformly distributed in the background and in each hot sphere. However, the tracer distribution in patients was totally different, and with more complexity. The patient related factors, such as body weight, blood glucose level, and liver cirrhosis, can impact the tracer distribution. And thus, a phantom with more anthropomorphic structures and different administered activity will be considered in future studies. Other factors may also impact the results of the clinical study. Due to the different reconstruction slice thickness, it was sometimes difficult to find the same slice between the uEXPLORER and uMI 780 images. The variation between two consecutive slices can induce a bias of the results. In future studies, it can be improved by using an average of several consecutive slices to minimize such bias or drawing a volume of interest (VOI) instead of ROI.
Our study had several limitations. The scans are always performed in a step-and-shoot mode with multiple bed positions with some overlap in a whole-body PET acquisition, whereas the total-body PET acquisition are performed with one bed position to cover the entire patient body. However, in the phantom study, we simplified the acquisition protocol using one bed position for both the scanners. In addition, we only assessed the image quality of the patient body, and assessment on patient head was beyond the scope of the study. Due to the intrinsic limitation of the phantom study, it was difficult to study patients with diversities. Furthermore, the lesions selected in the clinical study were all less than 40mm in diameter according to the sphere size in the phantom study, which may lead a bias on the results.