III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of image reconstruction method for a multiple-isotope
PET using 44m Sc†
T. Fukuchi,∗1 M. Shigeta,∗1 H. Haba,∗2 D. Mori,∗2 T. Yokokita,∗2 Y. Komori,∗2 S. Yamamoto,∗3 andY. Watanabe∗1
Positron emission tomography (PET) is a powerfultool for radio-tracer imaging in a living biological object. However, conventional PET is useful only for singletracer imaging because of the energy constancy of annihilation photons, which are utilized for PET imaging. In
order to improve PET imaging, we have developed a newsmall-animal PET system that can be used for multipletracer simultaneous imaging. Our PET system, named
multiple-isotope PET (MI-PET), detects not only annihilation photons but also prompt γ-rays, which are emitted successively after positrons, using additional γ-ray
detectors. Previously, we succeeded in proving the basic
principle of MI-PET using a prototype system.1,2)
Because of the imperfectness of the prompt γ-ray detection in MI-PET imaging with a pure positron emitterand positron-γ emitter, an image for the pure positron
emitter taken by MI-PET is superposed by the positronγ emitter. Therefore, to create an isolated image ofthe pure positron emitter, we developed an image reconstruction method based on subtraction between datawith the absence (data-D) and presence (data-T) of
the prompt γ-ray detection. For this subtraction, thespatial normalization of the prompt γ-ray sensitivity isneeded. Therefore, long-period normalization scans of apositron-γ emitter, 44m Sc, was performed using a cylindrical phantom 180 mm in length and 78 mm in diameter. These normalization data were also used for theanalysis of the counting-rate dependence of the sensitivity based on its proper decay half-life of 58.6 h. The
initial activity of the 44m Sc phantom was 2.45 MBq, andthe measurement time was 235 h, which corresponds toapproximately 4 half-lives.
To evaluate the practical performance of the developed image reconstruction method, dual-isotope mouse
imaging was performed using 18 F-FDG and a simple substance, 44m Sc. In this experiment, 1.13-MBq of 18 FFDG and 1.23-MBq of 44m Sc were administered to an
8-week-old normal male mouse by a tail vein injection.
After 38 min from administration, a 30-min scan withbed motion was performed under anaesthesia. This animal experiment was performed in accordance with thePrinciples of Laboratory Animal Care (NIH Publication
No. 85-23, revised 1985) and approved by the Institutional Animal Care and Use Committee (IACUC) ofRIKEN, Kobe Branch.
Scandium-44m was produced at RIBF via the45Sc(d, p2n)44m Sc (for normalization scan) and†∗1∗2∗3

Condensed from the article in Jour. of Inst., (in press) (2021)
RIKEN Center for Biosystems Dynamics Research
RIKEN Nishina Center
Department of Radiological and Medical Laboratory Science,Nagoya University

Fig. 1. Photograph (left) and reconstructed images of amouse administrated with 18 F-FDG and 44m Sc. (i) imageD (44m Sc and 18 F-FDG), (ii) image-T (44m Sc), and (iii)
image-A (18 F-FDG).
Table 1. VOI analysis of the mouse. Values are shown in kBq.
HeartLiverBladder

Image-D
31.7
421.3
69.0

Image-T
10.2
427.6
9.2

D-T
21.5
−6.3
59.8

Image-A
23.9
0.9
77.3

Ca(d, 2n)44m Sc (for mouse imaging) reactions with a24-MeV deuterium beam from the AVF cyclotron, purified by chemical processes at the hot lab, and transported to the RIKEN Center for Biosystems Dynamics
Research in Kobe.
Reconstructed mouse images from data-D (image-D:44m
Sc and 18 F-FDG) and data-T (image-T: 44m Sc) as
well as an isolated image of the pure-positron emitter(image-A: 18 F-FDG) are shown in Fig. 1. From imageT, we can clearly observe 44m Sc accumulation in theliver, whereas from image-A, 18 F-FDG accumulated in
the heart and urinary bladder. These distributions arereasonable from a physiological viewpoint.
To make a quantitative analysis, 3D volumes of interest (VOIs) were set on the heart, liver, and bladder for image-D, image-T, and image-A. The values ofthese VOIs and direct subtraction between image-D andimage-T (D-T) are listed in Table 1. The D-T activity inthe liver was negative. However, activity in the liver ofimage-A, which has no biological specific accumulationof 18 F-FDG, was nearly zero and was comparable withother non-accumulated sections. From this result, weconcluded that our newly developed image-A isolation
method is useful for practical multiple-isotope imaging
using MI-PET.
44References

1) T. Fukuchi et al., Med. Phys. 40(6), 2257 (2017).
2) T. Fukuchi et al., RIKEN Accel. Prog. Rep. 53, 21 (2020).

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