State of the art in total body PET

Abstract

The idea of a very sensitive positron emission tomography (PET) system covering a large portion of the body of a patient already dates back to the early 1990s. In the period 2000-2010, only some prototypes with long axial field of view (FOV) have been built, which never resulted in systems used for clinical research. One of the reasons was the limitations in the available detector technology, which did not yet have sufficient energy resolution, timing resolution or countrate capabilities for fully exploiting the benefits of a long axial FOV design. PET was also not yet as widespread as it is today: the growth in oncology, which has become the major application of PET, appeared only after the introduction of PET-CT (early 2000).The detector technology used in most clinical PET systems today has a combination of good energy and timing resolution with higher countrate capabilities and has now been used since more than a decade to build time-of-flight (TOF) PET systems with fully 3D acquisitions. Based on this technology, one can construct total body PET systems and the remaining challenges (data handling, fast image reconstruction, detector cooling) are mostly related to engineering. The direct benefits of long axial FOV systems are mostly related to the higher sensitivity. For single organ imaging, the gain is close to the point source sensitivity which increases linearly with the axial length until it is limited by solid angle and attenuation of the body. The gains for single organ (compared to a fully 3D PET 20-cm axial FOV) are limited to a factor 3-4. But for long objects (like body scans), it increases quadratically with scanner length and factors of 10-40 × higher sensitivity are predicted for the long axial FOV scanner. This application of PET has seen a major increase (mostly in oncology) during the last 2 decades and is now the main type of study in a PET centre. As the technology is available and the full body concept also seems to match with existing applications, the old concept of a total body PET scanner is seeing a clear revival. Several research groups are working on this concept and after showing the potential via extensive simulations; construction of these systems has started about 2 years ago. In the first phase, two PET systems with long axial FOV suitable for large animal imaging were constructed to explore the potential in more experimental settings. Recently, the first completed total body PET systems for human use, a 70-cm-long system, called PennPET Explorer, and a 2-m-long system, called uExplorer, have become reality and first clinical studies have been shown. These results illustrate the large potential of this concept with regard to low-dose imaging, faster scanning, whole-body dynamic imaging and follow-up of tracers over longer periods. This large range of possible technical improvements seems to have the potential to change the current clinical routine and to expand the number of clinical applications of molecular imaging. The J-PET prototype is a prototype system with a long axial FOV built from axially arranged plastic scintillator strips.This paper gives an overview of the recent technical developments with regard to PET scanners with a long axial FOV covering at least the majority of the body (so called total body PET systems). After explaining the benefits and challenges of total body PET systems, the different total body PET system designs proposed for large animal and clinical imaging are described in detail. The axial length is one of the major factors determining the total cost of the system, but there are also other options in detector technology, design and processing for reducing the cost these systems. The limitations and advantages of different designs for research and clinical use are discussed taking into account potential applications and the increased cost of these systems.

Keywords: PET; PET-CT; Sensitivity.

Fig. 1

The three major improvements in PET technology during the last three decades

Fig. 2

The difference between a current PET-CT (top figure) and a total body PET-CT (bottom figure). Different bed positions to complete a body scan are not required anymore. Inside the total body PET FOV, higher sensitivity is obtained for each point in the FOV by the larger solid angle coverage (indicated by the shading). For the same activity injected in the patient, the total acquisition time can be reduced by a large factor due to the higher sensitivity

Fig. 3

The geometrical acceptance for a point-like source and line sources of 10 cm, 100 cm and 200 cm length in the transverse centre of a PET scanner with a diameter of 80 cm. The y-axis shows the fraction of the solid angle

Fig. 4

The influence of the oblique angle on attenuation and detection efficiency in a long axial FOV scanner

Fig. 5

The fraction of detected two-photon events for a central point source taking into account the detector acceptance (Acc), detection with 20-mm-thick LYSO crystals (Acc & Det), attenuation caused by a 20-cm phantom (Acc & Att) as well as (Acc & Att & Det) and selection of event forming events (Acc & Att & Det & Sel). The y-axis displays fraction with maximum value equal to 1

Fig. 6

Sensitivity for detection and selection of image forming events for a point-like, 10-cm-long, 100-cm-long and 200-cm-long central line source taking into account the attenuation caused by a 20-cm phantom and the detection efficiency of a 20-mm-thick LYSO detector. The y-axis displays fraction with maximum value equal to 1

Fig. 7

The sensitivity gain versus a 20-cm axial FOV PET system for a central point source, and for a 200-cm-long line source, the curves for pet and det and sel overlap as the gains remain the same. The gain on the y-axis is the ratio of sensitivity versus the sensitivity of a 20-cm axial FOV PET system

Fig. 8

Total body PET systems with an axial length of 70 cm, 100 cm, 140 cm and 200 cm

Fig. 9

Increase in component costs for a 70-cm, 100-cm, 140-cm and 200-cm system versus a system with 20 cm axial length; the y-axis is the system cost in relative units

Fig. 10

The increase in sensitivity as a function of axial length for a point source, short object 10 cm, a 100-cm-long phantom and a 200-cm–long phantom all filled with activity. The y-axis displays the relative gain versus a system with 2-cm axial FOV

Fig. 11

The increase in NEC as a function of axial length for a 70-cm-long and a 140-long phantom for systems of 20, 70, 100, 140 and 200 cm axial length. Simulations are based on a Paralyzable 300-ns dead time per detector block of 5 × 5 cm

Fig. 12

Different options to reduce the cost of the detectors (scintillator, sensor, electronics) in total body PET systems

Fig. 13

Proposed cost-effective designs for total body PET systems

Fig. 14

The axial arrangement allows for concentric layers of scintillation material

Fig. 15

Sensitivity gain, with respect to 20-cm length LYSO PET, as a function of the axial length for LYSO (2 cm thick) and plastic (two 3-cm-thick layers) detectors. Results for a point-like, single organ (10 cm), as well as 100-cm and 200-cm sources are shown

Fig. 16

Two possible improvements in future total body PET systems: introduction of DOI and better TOF