III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Simultaneous imaging of Na+ /K+ by semiconductor Compton camera
GREI
S. Motomura,∗1 I. Kii,∗1,∗2 H. Haba,∗3 H. Yakushiji,∗1 Y. Watanabe,∗1 and S. Enomoto∗4
We have been studying the simultaneous imaging ofsodium (Na) and potassium (K) ions that are dynamically controlled in a living body. Na+ and K+ arevery common cations in body ﬂuids; however, interestingly, they are controlled very diﬀerently by somebiomolecules. Owing to the diﬀerent behavior anddistribution of Na+ and K+ , membrane potential canbe formed in neurons, and Na+ can be reabsorbed in
the kidneys. Therefore, the imaging of Na+ /K+ dynamics may be beneﬁcial in inspecting the function ofbiomolecules.
As summarized in a previous report, we were able tosimultaneously take the images of 24 NaCl and 43 KClsolutions using our imaging apparatus GREI.1,2) However, we determined some challenges to be addressedin the imaging of 24 Na, in addition to the low detection
eﬃciency for the 1369-keV gamma ray. In the gammaray energy spectrum of 24 Na + 43 K (Fig. 1), there isa gamma-ray peak at 1732 keV that corresponds tothe double escape peak for the 2754-keV gamma rayof 24 Na. This originates from the escape of two 511keV gamma rays created by the annihilation of thepositron that was created by the pair-creation interaction of gamma rays in the gamma-ray detectors. Inaddition, the E1+E2 = 1732 keV events were observedin the E1-E2 histogram (Fig. 2), where E1 and E2
represent the detected energies by the front and reardetectors, respectively. Furthermore, the E1 + E2 =2754 keV events were also observed, although the measurable energy range for each detector was less than2000 keV in the current GREI system. We inferredthat these 1732- and 2754-keV gamma events doubled
the dead time of the GREI system. Hence, to realizeeﬃcient imaging experiment, we need to further develop a proper method to process the gamma events.
To circumvent the current problem of 24 Na, we decided to use 22 Na for now. Although 22 Na is notsuitable for clinical use, owing to its long half-life of
2.6 years, we can use it for preliminary or preclinical experiments. 22 Na is a positron emitter and emits
two 511-keV gamma-rays and one 1275-keV intrinsic
gamma ray.
We performed GREI imaging experiment of 22 Na
and 43 K simultaneously injected in a live mouse. We
can use a 511-keV gamma-ray peak to identify 22 Na,
which is in the optimal imaging eﬃciency region for
∗1
∗2
∗3
∗4

GREI (Fig. 3). The E1-E2 histogram for 22 Na + 43 Kis shown in Fig. 4. Clear lines can be observed for eachgamma ray that were all covered by the energy rangeof GREI. This study is still ongoing, and the results
are being prepared to be considered for publication.

Fig. 1. Gamma-ray energy
spectrum of 24 Na and 43 K
measured by GREI.

Fig. 2. E1-E2 histogram of24
Na and 43 K.

Fig. 3. Gamma-ray energy
spectrum of 22 Na and 43 K
measured by GREI.

Fig. 4. E1-E2 histogram of22
Na and 43 K.

References

1) S. Motomura et al., J. Anal. At. Spectrom. 28, 934(2013).
2) T. Ida et al., Jpn. J. Appl. Phys. 58, 016002 (2019).

RIKEN Center for Biosystems Dynamics ResearchCompass to Healthy Life Research Complex Program,
RIKEN
RIKEN Nishina Center for Accelerator-Based Science
RIKEN Center for Life Science Technologies- 159 -1