III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Double photon emission nuclides for double photon coincidence imaging
H. Takahashi,∗1 M. Uenomachi,∗1 K. Shimazoe,∗1 and H. Haba∗2
Positron emission tomography (PET) is known as avery high sensitivity imaging method. One reason forthis is that it utilizes a coincidence technique, where
two detectors are used to identify the event; therefore,a high signal-to-noise ratio is realized. However, PET
uses only 511 keV gamma rays. Thus, it cannot be used
to distinguish all positron emitters.
Some nuclides successively emit two gamma-ray photons in a very short time. The use of such gamma
photons can lead a new type of coincidence imaging method.1,2) Recently, we proposed a new conceptof time/position correlation type tomography method
based on a directionality sensitive gamma camera. For
this, we need a nuclide that emits two photons in a relatively short period. Thus far, we have proposed the useof 111 In, which emits a 171 keV photon and after thelife time of 85 ns, it emits a 245 keV photon. When themeasurement system identiﬁes a 171 keV photon, thesystem waits for a 245 keV photon for approximately
300 ns. If the system recognizes the 245 keV photon,
it records two gamma photons and their incident angles, and then, it proceeds to the next measurement.
To record angles, an electron tracking type Compton
imager can be used for 300–600 keV gamma-ray photons; however, multi-pinhole cameras can be used for
low-energy gamma rays. This enables double photonemission computed tomography (DPECT). Although111
In is feasible for measurements, the advantage ofthe double-photon coincidence method lies in its multinuclide capability. Therefore, we are exploring othercandidates for double-photon emission nuclides. Thegamma ray energies should not be too large since thedetection eﬃciency for high energy gamma rays is lim-Fig. 2. Single photon Compton image of a 43 K source. The
color bar shows relative intensity.

ited.
For nuclear medicine applications, the half-life of thenuclide should not be too short or too long. The doublephoton yield should be large. The gamma-ray energies
should not be too large since the detection eﬃciency
for high-energy gamma rays is limited. Further, twophotons should be emitted within a short period. Although there are various possibilities for double-photonemission nuclides. they cannot all be used owing to theabove requirements. Then we successfully found thefollowing nuclides thus far.
43
K is a beta minus nuclide followed by gamma photon emissions. The half-life of the nuclide 43 K is 22.3 h,which is a reasonable value. The decay scheme of thisnuclide is shown in Fig. 1. It emits three pairs of gammarays. The ﬁrst pair is a 617 keV photon (79%) and a372 keV photon (87%). The second pair is a 397 keVphoton (11.9%) and a 593 keV photon (11.3%). Finally,
the third pair is a 221 keV photon (4.8%) and a 372 keVphoton. The longest life time is 81 ps. Therefore, two
gamma-ray photons are observed almost simultaneously
in the measurement system. The time resolution is very
close to that of positron annihilation. The TOF method
can be used to localize the event for this nuclide. Asa ﬁrst step, the single-photon Compton image was obtained as shown in Fig. 2. In this research, 43 K wasproduced at RIKEN for radioisotope imaging.
ReferencesFig. 1. Decay scheme of∗1∗243K.

School of Engineering, University of Tokyo
RIKEN Nishina Center

3)1) M. Uenomachi et al., Nucl. Instrum. Methods Phys. Res.
A, 954, 161682 (2020).
2) K. Shimazoe et al., J. Instrum. 12, C12055 (2017).
3) A. Dhal et al., EPJ Web Conf. 146, 10013 (2017).

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