Real-time trajectory imaging of alpha particles emitted from actinium-225 and its daughter radionuclides

Abstract

In targeted alpha-particle therapy, actinium-225 (Ac-225) has emerged as a radionuclide of potential, driving extensive efforts to develop innovative radiopharmaceuticals. High-resolution imaging of alpha particles is required for precisely detecting alpha-emitting radionuclides in cellular environments and small organs. Here, we report real-time trajectory imaging of alpha particles emitted by Ac-225 and its daughter radionuclides, utilizing an alpha particle trajectory imaging system. This system incorporates a magnification unit, a cooled electron-multiplying charge-coupled device (EM-CCD) camera, and a Ce-doped Gd₃Al₂Ga₃O₁₂ (GAGG) scintillator. Alpha particles were projected onto the GAGG scintillator, producing magnified images that were captured at 100 ms intervals. We successfully tracked particle trajectories with varying lengths and intensities for 4 different alpha particles emitted from Ac-225 and its daughter radionuclides with a spatial resolution of 1.0 μm. Notably, we achieved the imaging of sequentially emitted trajectories from Fr-221 and its decay product At-217, characterized by short decay intervals, along with the extended trajectories of high-energy alpha particles emitted by Po-213. These results demonstrate that high-resolution trajectory imaging, integrated with temporal and energy information, offers profound insights into the real-time behavior of Ac-225 and its daughter radionuclides within living cells or tissue sections, thereby driving advancements in targeted alpha-particle therapy.

Keywords: Actinium-225; Alpha particles; Imaging, energy spectrum; Trajectory.

Fig. 1

Half-life and alpha particle energies of Ac-225 and daughter radionuclides.

Fig. 2

Schematic drawing of developed high-resolution alpha-particle imaging system (A) and photo during imaging (B).

Fig. 3

Energy spectra measurement of alpha-particles emitted from Ac-225 and its daughter radionuclides measured with silicon charged-particle detector.

Fig. 4

Two composite images each created by summing 10 individual frames captured over 100 ms with a 40× lens.

Fig. 5

Two of measured images containing alpha particles from Po-214 (8.4 MeV) (indicated with arrow) measured with 100 ms and 40 x lens (A) and depth (B) and lateral profiles (C) of alpha particles.

Fig. 6

Alpha particle trajectory images contained alpha particles from Fr-221 (6.4 MeV) and At-217 (7.1 MeV) emitted almost simultaneously with “V” shape (indicated in arrow) measured with 100 ms and 40 x lens.

Fig. 7

Measured image with 20 × lens of alpha-particles emitted from Ac-225 and its daughter radionuclides in GAGG plate (A) and simulated image (B).

Fig. 8

Energy spectra estimated from measured with 20 × lens of alpha-particles emitted from Ac-225 and its daughter radionuclides in GAGG plate (A) and smoothed energy spectra calculated by simulation (B).