Hybrid total-body pet scanners-current status and future perspectives

Abstract

Purpose Since the 1990s, PET has been successfully combined with MR or CT systems. In the past years, especially PET systems have seen a trend towards an enlarged axial field of view (FOV), up to a factor of ten. Methods Conducting a thorough literature research, we summarize the status quo of contemporary total-body (TB) PET/CT scanners and give an outlook on possible future developments. Results Currently, three human TB PET/CT systems have been developed: The PennPET Explorer, the uExplorer, and the Biograph Vision Quadra realize aFOVs between 1 and 2 m and show a tremendous increase in system sensitivity related to their longer gantries. Conclusion The increased system sensitivity paves the way for short-term, low-dose, and dynamic TB imaging as well as new examination methods in almost all areas of imaging.

Keywords: CT; Long axial FOV; MRI; PET; Sensitivity; Total-body.

Fig. 1

The evolution of PET system components, involving a transition to novel scintillator materials and new photo-sensor techniques, ultimately resulting in the usage of LYSO and SiPMs as one small detector unit in state-of-the-art PET systems. The readout electronics transitioned from mostly hardware-controlled solutions to highly integrated SPUs. Illustration of the photo-sensor techniques follows [81]

Fig. 2

Schematic drawing of the readout architecture in modern PET systems. Analog signals from single or groups of photo-sensors are digitized by an ASIC employing analog- and time-to-digital converters (ADCs/TDCs) and then routed to an SPU. A digital SiPM already digitizes the trigger itself. The SPU commonly houses an FPGA for first event sorting and processing steps before the digitized event information is processed on a data acquisition and processing server (DAPS). Here, detected single events or already matched coincidences are given in list-mode format

Fig. 3

Evolution of the aFOV of commercial and research multi-modal human PET systems. The timeline uses the dates of the first available reports, performance studies and NEMA (National Electrical Manufacturers Association) characterizations of the respective system. A slight delay to the actual product launch is therefore possible. The presented information has been extracted from [6, 11, 22, 25, 30, 35, 38, 46, 52, 53, 60, 76, 86, 88, 100, 107]

Fig. 4

Siemens Biograph Vision Quadra. a Front view. b Side view. c Top view and comparison of the footprint with the predecessor Biograph Vision. d Optiso UDR detectors. Pictures have been taken from [103] with kind permission of Siemens Healthineers

Fig. 5

Sensitivity and NECR of existing PET/CT systems (see Fig. 3) depending on their aFOV. The NECR was normalized according to the activity concentration used in the FOV. Data have been taken from [, , , , , , –88, 100, 105]. Lines have been added to the data points to guide the eye

Fig. 6

Integration options of TB PET (orange) with an MRI system (blue). a TB PET and MRI share the same table, but are two separate systems. This system shows the least interference due to the low B0 stray field at the PET location; b TB PET and MRI aligned, with medium interference due to strong B0 stray field; c simultaneous acquisition of TB PET and MRI with strong interference of PET with the B0, gradient, and RF fields of MRI