Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Live mouse imaging with 44m Sc by a multiple-isotope PET
T. Fukuchi,∗1 M. Shigeta,∗1 D. Mori,∗2 T. Yokokita,∗2 Y. Komori,∗2 H. Haba,∗2 and Y. Watanabe∗1
We have been developing a multiple-isotope positronemission tomography (MI-PET) system that can analyze the dynamics of multiple tracers. Using a positronγ emitter, which emits de-excitation γ-ray as a tracer
after the positron emission in β-decay, the MI-PET system identiﬁes the tracer by detecting the prompt γ-ray
emitted after the positron emission. Figure 1 shows aschematic illustration of the developed MI-PET prototype system. This system is composed of a PET scannerand additional γ-ray detectors.1) In this system, in addition to conventional PET imaging, coincidence among
the additional detectors and the PET scanner can beperformed. We expect that MI-PET will be used for
drug discovery research by direct comparison between
old and new drugs.
For multiple-isotope imaging using MI-PET, atleast one positron-γ emitter is necessary as a tracer.
Scandium-44 is one of the promising radioactive-tracer
candidates for MI-PET because of its large positronand γ-ray emission ratio and moderate half-life (44 Sc:3.97 h, 44m Sc: 58.61 h). In our previous work, we
performed dual-isotope phantom imaging using 44m Sc
and 18 F (pure positron emitter) and evaluated the basic
imaging performance of MI-PET for 44m Sc.2)
Therefore, before the future development of a 44m Sclabeled drug for MI-PET applications, in order to test
the practical imaging ability of the MI-PET system for44m
Sc, we conducted dual-isotope live mouse imagingusing 44m Sc, which is a simple substance.
Scandium-44m was produced at the RIKEN AVF cyclotron via the reaction 44 Ca(d, 2n)44m Sc with a 24MeV deuterium beam. As the irradiation target, 44 CaO

Fig. 2. Images of a live mouse with 44m Sc and 18 F-FDG injections (maximum intensity projection images). The reconstructed images were acquired with the absence (A)or presence (B) of γ-ray coincidence.

(97.0% enriched 44 Ca isotope) powder was pressed intoa disk of 10-mm diameter. The irradiated target wasdissolved in 6 M HCl and puriﬁed by the chemical processes.2) Finally, approximately 2 MBq of pure 44m Sc
was produced and transported to RIKEN Kobe campus
for the imaging experiment. Most of the short-lived byproduct, 44 Sc, decayed out during transportation.
In the mouse imaging experiment, 197 kBq 44m Sc(simple substance) and 198 kBq 18 F-FDG were administered to an 8-week-old normal male mouse by tail-vein
injection. Five minutes after administration, a 30-min
whole-body scan was performed under anesthesia.
The result of dual-isotope mouse imaging is shown inFig. 2. From the reconstructed images with the absence(A) or presence (B) of the prompt γ-ray detection, image(A) reﬂects the distribution of both 18 F-FDG and the44m
Sc tracer, whereas image (B) reﬂects the isolated image of the 44m Sc tracer. In these images, we can clearlyobserve the diﬀerence between 18 F-FDG and 44m Sc distributions, i.e., 18 F-FDG is distributed in the heart andurinary bladder, whereas 44m Sc distributed in the liver.
From this experiment, we successfully demonstrated
the practical feasibility of dual-isotope imaging using
MI-PET with 44m Sc as the second tracer. In the future, we will synthesize a useful MI-PET drug labeledby 44m Sc and perform multiple-drug imaging on diseasemodel animals.
ReferencesFig. 1. Schematic illustration of the developed MI-PET prototype system.
∗1∗2

RIKEN Center for Biosystems Dynamics Research
RIKEN Nishina Center- 21 -1) T. Fukuchi et al., Med. Phys. 44, 2257 (2017).
2) T. Fukuchi et al., RIKEN Accel. Prog. Rep. 52, 207(2019).