Total Body PET: Why, How, What for?

Abstract

PET instruments are now available with a long axial field-of-view (LAFOV) to enable imaging the total-body, or at least head and torso, simultaneously and without bed translation. This has two major benefits, a dramatic increase in system sensitivity and the ability to measure kinetics with wider axial coverage so as to include multiple organs. This manuscript presents a review of the technology leading up to the introduction of these new instruments, and explains the benefits of a LAFOV PET-CT instrument. To date there are two platforms developed for TB-PET, an outcome of the EXPLORER Consortium of the University of California at Davis (UC Davis) and the University of Pennsylvania (Penn). The uEXPLORER at UC Davis has an AFOV of 194 cm and was developed by United Imaging Healthcare. The PennPET EXPLORER was developed at Penn and is based on the digital detector from Philips Healthcare. This multi-ring system is scalable and has been tested with 3 rings but is now being expanded to 6 rings for 140 cm. Initial human studies with both EXPLORER systems have demonstrated the successful implementation and benefits of LAFOV scanners for both clinical and research applications. Examples of such studies are described in this manuscript.

Keywords: (TB-PET); Long axial-field-of-view (LAFOV); PET-CT; PennPET EXPLORER; Total-Body PET; uEXPLORER.

Fig. 1.

Contour plot of the percent sensitivity of a point source in air as a function of crystal thickness and scanner axial length. The detector ring diameter was fixed at 85 cm and LSO was used as the scintillator material.

Fig. 2.

Figure 1: Illustration of axial coverage of a TB-PET scanner of 70 cm, 140 cm, and 200 cm length for an average 6-year child (115 cm (45”) tall) and an average adult male (177 cm (70”) tall). Images are from the XCAT digital phantoms [26, 27].

Fig. 3.

Percentage of annihilation events reaching the scanner plotted as a function of scanner AFOV. Three different imaging setups were simulated: Setup A is a point source in air, Setup B is a point source in a 200 cm long water-filled cylinder, and Setup C is a uniformly distributed source in a 200 cm long water-filled cylinder. The cylinder diameter included 20 cm, 27 cm, 35 cm, and 40 cm, representing small to large patients.

Fig. 4.

Illustration of detectors integrated in current TB-PET scanners. a) Detector used in uEXPLORER from United Imaging Healthcare with a 7×6 block of LYSO crystals, each 2.76×2.76×18 mm³, coupled to 4 SensL SiPMs, each 6-mm², thus relying on a light-sharing scheme for crystal identification (courtesy of Dr. Hongdi Li, United Imaging Healthcare America). b) Detector used in PennPET EXPLORER with an 8×8 block of LYSO crystals, each 3.76×3.76×19 mm³, coupled to the PDPC 64-channel digital SiPM in a 1:to:1 scheme for crystal identification [28].

Fig. 5.

Illustration of the multi-ring PennPET EXPLORER scanner. The scalable design, shown in a 3-ring and 6-ring configuration, allows flexibility in building a TB-PET scanner of various axial length. The rings are closely stacked with a small gap of < 1 cm, but can also be separated with a larger gap to extend the axial FOV.

Fig. 6.

Axial sensitivity (percentage of events reaching the scanner) profile for a line source in a 35-cm diameter × 200-cm long cylinder, shown for a scanner with 20 cm, 70 cm, 140 cm, and 200 cm AFOV.

Fig. 7.

Images (reconstructed into 2-mm³ voxels) of a 170 cm male acquired on the PennPET EXPLORER in its prototype (3-ring) configuration. A dose of 551 MBq (14.9 mCi) was injected and the scan acquired for 20 min at 1.75 hr post-injection.

Fig. 8.

A 164 cm female was scanned on the PennPET EXPLORER in its prototype (3-ring) configuration. A dose of 577 MBq (15.5 mCi) was injected and the scan acquired for 20 min at 1.5 hr post-injection. (a) Data reconstructed into 1-mm³ voxels show selective images of brain. (b) Data reconstructed into 4-mm³ voxels, but sub-sampled with 1/16 the counts, corresponding to a scan time of 1.25 min or a dose of 1 mCi.

Fig. 9.

Example of dynamic study acquired on PennPET EXPLORER in its prototype (3-ring) configuration, illustrating ability to measure whole-body kinetics. Five representative frames, from 5 s to 5 min duration, are shown over the course of 1 hour, following injection of 15 mCi FDG.