Positron emission tomography with additional γ-ray detectors for multiple-tracer imaging.
OBJECTIVE
Positron emission tomography (PET) is a useful imaging modality that quantifies the physiological distributions of radiolabeled tracers in vivo in humans and animals. However, this technique is unsuitable for multiple-tracer imaging because the annihilation photons used for PET imaging have a fixed energy regardless of the selection of the radionuclide tracer. This study developed a multi-isotope PET (MI-PET) system and evaluated its imaging performance.


METHODS
Our MI-PET system is composed of a PET system and additional γ-ray detectors. The PET system consists of pixelized gadolinium orthosilicate (GSO) scintillation detectors and has a ring geometry that is 95 mm in diameter with an axial field of view of 37.5 mm. The additional detectors are eight bismuth germanium oxide (BGO) scintillation detectors, each of which is 50 × 50 × 30 mm3, arranged into two rings mounted on each side of the PET ring with a 92-mm-inner diameter. This system can distinguish between different tracers using the additional γ-ray detectors to observe prompt γ-rays, which are emitted after positron emission and have an energy intrinsic to each radionuclide. Our system can simultaneously acquire double- (two annihilation photons) and triple- (two annihilation photons and a prompt γ-ray) coincidence events. The system's efficiency for detecting prompt de-excitation γ-rays was measured using a positron-γ emitter,22Na. Dual-radionuclide (18F and22Na) imaging of a rod phantom and a mouse was performed to demonstrate the performance of the developed system. Our system's basic performance was evaluated by reconstructing two images, one containing both tracers and the other containing just the second tracer, from list-mode data sets that were categorized by the presence or absence of the prompt γ-ray.


RESULTS
The maximum detection efficiency for 1275 keV γ-rays emitted from22Na was approximately 7% at the scanner's center, and the minimum detection efficiency was 5.1% at the edge of the field of view. Dual-radionuclide imaging of the point sources and rod phantom revealed that our system maintained PET's intrinsic spatial resolution and quantitative nature for the second tracer. We also successfully acquired simultaneous double- and triple-coincidence events from a mouse containing18F-fluoro-deoxyglucose and22Na dissolved in water. The dual-tracer distributions in the mouse obtained by our MI-PET were reasonable from the viewpoints of physiology and pharmacokinetics.


CONCLUSIONS
This study demonstrates the feasibility of multiple-tracer imaging using PET with additional γ-ray detectors. This method holds promise for enabling the reconstruction of quantitative multiple-tracer images and could be very useful for analyzing multiple-molecular dynamics.
RIKEN Center for Life Science Technologies, Kobe, 650-0047, Japan.
Tomonori
T
Fukuchi
RIKEN Center for Life Science Technologies, Kobe, 650-0047, Japan.
Takashi
T
Okauchi
RIKEN Center for Life Science Technologies, Kobe, 650-0047, Japan.
Mika
M
Shigeta
Department of Radiological and Medical Laboratory Sciences, Nagoya University Graduate School of Medicine, Nagoya, 461-0047, Japan.
Seiichi
S
Yamamoto
RIKEN Center for Life Science Technologies, Kobe, 650-0047, Japan.
Yasuyoshi
Y
Watanabe
RIKEN Center for Life Science Technologies, Kobe, 650-0047, Japan.
Shuichi
S
Enomoto
D000818:Animals; D006801:Humans; D007091:Image Processing, Computer-Assisted; D051379:Mice; D019047:Phantoms, Imaging; D017785:Photons; D049268:Positron-Emission Tomography; D011868:Radioisotopes
D011868:Radioisotopes
dual-radionuclide imaging; positron emission tomography; positron gamma emitter; small animal imaging