Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Research progress on double-photon-emission coincidence imaging withcascade gamma-ray-emitting nuclide
M. Uenomachi,∗1 Z. Zhong,∗1 K. Shimazoe,∗1 H. Takahashi,∗1 D. Mori,∗2 T. Yokokita,∗2 Y. Komori,∗2 andH. Haba∗2
Positron emission tomography (PET) and singlephoton-emission computed tomography (SPECT) are
important nuclear medicine modalities that have been
widely used for clinical diagnosis. PET detects the coincidence of annihilation gamma-rays from a positronemitting nuclide. SPECT uses a collimator to restrict theincident direction of gamma-rays. Therefore, nuclides for
PET imaging are limited to positron-emitting nuclides
and there is a theoretical spatial resolution limit derived
from the positron range. The sensitivity of SPECT is relatively low because of its use of a collimator, and it is difﬁcult to image multiple nuclides simultaneously. To realize highly sensitive multi-nuclide imaging, we proposed adouble-photon -emission coincidence imaging method.1)
For SPECT imaging, single-photon-emitting nuclides
are preferable. Among them, there are nuclides thatemit two photons in cascade decay. For example, 111 In,
which is widely used in SPECT imaging, emits twogamma-rays at 171 keV and 245 keV. Using two gammarays, the position of nuclides can be determined withhigher sensitivity. One of the methods using two gammarays is double-photon-emission computed tomography(DPECT).1,2) This method can drastically increase thesignal-to-noise (SN) ratio of Compton imaging3) by coincidence detection for cascade gamma-rays with multiple
Compton cameras, which is based on Compton scattering kinetics. Conventional Compton imaging can obtain
only the angle information of incident gamma-rays, anddrawing many Compton cones results in a low SN ratio.
However, DPECT can limit the nuclide position from aCompton cone to an overlap of two Compton cones. We
previously demonstrated DPECT using 134 Cs and 60 Co
for application to fuel debris imaging2) with GAGGSiPM Compton cameras.4) In the present research, wedeveloped small GAGG-SiPM Compton cameras for application to nuclear medicine and demonstrated doublephoton-emission coincidence methods using 43 K, which
is one of the promising double-photon-emitting nuclides
because of its short decay time and suitable gamma-rayFig. 2. Single-photon Compton imaging at (a) 373 keV and(b) 617 keV. (c) Time diﬀerence of DPECT (camera1camera2).

energies.
43
K was produced in the 43 Ca(d, 2p)43 K reaction atthe RIKEN AVF cyclotron. After chemical separation,2 (6) MBq of 43 K in 6 (18) µL H2 O was placed in avial as a gamma-ray source. Figure 1 shows the experimental setup of DPECT for 43 K. 43 K mainly emitscascade gamma-rays at 617 keV and 373 keV. A Compton camera consists of a scatter layer and an absorberlayer. Each layer consists of an 8 × 8 GAGG array coupled to an 8 × 8 SiPM array. The thicknesses of theGAGG arrays is 4 mm and 9 mm for scatterers and absorbers, respectively. The pixel size of the GAGG arrays
is 2.5 mm × 2.5 mm, and each crystal is separated by
BaSO4 reﬂectors. The signal from SiPM is processed by
the dynamic time over threshold (dToT) method5) to extract the energy. The distance from the 43 K source toa scatterer was 30 mm, and two Compton cameras wereplaced in opposite directions.
Figures 2(a) and 2(b) show images reconstructed
through back projection. Figure 2(a) shows a singlephoton Compton image at 373 keV and (b) shows animage at 617 keV. We succeeded in obtaining images ofsingle-photon Compton imaging with our small Compton
cameras. Figure 2(c) shows a histogram of time diﬀerence of double Compton coincidence events at 373 keVand 617 keV. However, the number of coincidence events
that can be used for DPECT with Compton scattering
energy windows was only 12. After this experiment, weincreased the number of Compton cameras to detect double Compton coincident events eﬃciently and repeatedthe experiment with eight modules. The result is underanalysis.
References

Fig. 1. Experimental setup of DPECT for 43 K.
∗1∗2

Department of Engineering, The University of Tokyo
RIKEN Nishina Center

1) Y. Yoshihara et al., Nucl. Instrum. Methods Phys. Res. A873, 51 (2017).
2) M. Uenomachi et al., Nucl. Instrum. Methods Phys. Res.
A 954, 161682 (2020).
3) R. W. Todd et al., Nature 251, 132 (1974).
4) J. Jiang et al., J. Nucl. Sci. Technol. 53, 1067 (2015).
5) K. Shimazoe et al., IEEE Trans. Nucl. Sci. 59, 3213 (2012).
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