Exploration of Coincidence Detection of Cascade Photons to Improve Multi-Nuclide SPECT Imaging

Radiopharmaceutical Therapy (RPT) has garnered increasing interest in recent years, especially in SPECT imaging involving the simultaneous use of multiple tracers. Traditional imaging methods are prone to scattering and crosstalk from different energy γ-rays, leading to significantly reduced image quality. To address this issue, the authors Yifei Jin and Ling-Jian Meng propose a method called Coincidence Detection of Cascade Photons (CDCP), aimed at significantly reducing scattering and crosstalk contamination in low-activity therapeutic radionuclide imaging based on cascade photon detection.

Research Background

The proposal of CDCP technology stems from the limitations of traditional methods in low-activity radiopharmaceutical therapy imaging. Therapeutic radionuclides such as Ac-225, In-111, Ra-223, and Lu-177 emit cascade photons with energy ranges from 60 to 700 keV. In the presence of multiple-energy γ-rays, especially in low-activity situations such as in-vivo imaging of Ac-225 in targeted alpha therapy, traditional scatter correction methods are inefficient. Therefore, CDCP technology fundamentally reduces downscatter and crosstalk contamination through coincidence detection of cascade photons.

Paper Source

This paper is written by Yifei Jin and Ling-Jian Meng from the Department of Nuclear, Plasma, and Radiological Engineering, Department of Bioengineering, and Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign. The paper is published in the IEEE Transactions on Medical Imaging, May 2024 issue.

Research Details

Research Process

1. Selection of Candidate Therapeutic Radionuclides: The study selected four radionuclides, including Ac-225, Ra-223, Lu-177, and In-111, which emit cascade photons with relatively short intermediate level half-lives, resulting in a higher probability of detecting coincident photons. The characteristics of these radionuclides were described in detail at the beginning of the study, and their decay processes were illustrated (as shown in Figure 2).

2. Principle of the CDCP-SPECT System: CDCP technology uses cascade photons for imaging. The primary γ photons are used for SPECT imaging, while the secondary γ photons serve as a coincidence gate, fundamentally reducing downscatter and crosstalk contamination. The authors mentioned that this technique matches traditional methods in terms of sensitivity but significantly improves signal detection efficiency.

3. Construction of Prototype CDCP-SPECT System: The study constructed a prototype CDCP-SPECT system, including a large-volume CdZnTe (CZT) imaging spectrometer and a pinhole collimator. The CZT imaging spectrometer boasts excellent energy resolution (3 keV FWHM at 200 keV) and spatial resolution (FWHM below 0.5 mm in three dimensions). Additionally, an adaptive rotating stage and plastic holder were used to secure the radioactive source for sufficient angular sampling.

4. Phantom Study: Using Ac-225 and In-111 radioactive sources, the effectiveness of CDCP technology was evaluated through phantom experiments. Quantitative analysis was conducted using the Maximum-Likelihood Expectation-Maximization (MLEM) algorithm for reconstruction.

5. Experimental and Simulation Evaluation: The authors evaluated the impact of detector energy resolution and temporal resolution on CDCP technology through Monte Carlo simulations and artificial energy blur experiments. The results indicated that in high temporal resolution systems, shorter time windows significantly improve the signal-to-contamination ratio.

Main Results

1. Spectral and Projection Analysis: In the study of In-111 radioactive source without background, the spectral signal-to-contamination ratio (Spectral-SCR) significantly improved after using CDCP technology. In the presence of Ac-225 background, CDCP technology increased the signal-to-contamination ratio of the 171 keV energy window to 12.7, compared to only 0.59 without CDCP. Moreover, the projection signal-to-contamination ratio at 171 keV improved from 0.31 to 15.9.

2. Hot Pepper Imaging: Before adopting CDCP technology, the imaging of the In-111 radioactive source was nearly unrecognizable due to downscatter contamination caused by the high-energy γ photons of Ac-225. However, after applying CDCP, the image quality significantly improved, allowing clear identification of the In-111 radioactive source’s position.

3. Comparison with Traditional Scatter Correction Methods: Compared to the traditional Triple-Energy Window (TEW) scatter correction method, CDCP technology significantly reduced contamination levels in projections under the same conditions. Although the TEW correction method requires a longer acquisition time to achieve a reasonable projection signal-to-contamination ratio, CDCP technology provided a higher projection signal-to-contamination ratio in a shorter time.

Research Significance and Value

This study demonstrates the significant potential of CDCP technology in ultra-low-activity therapeutic radionuclide imaging. By substantially reducing downscatter and crosstalk contamination, this technology enhances image quality in low-activity scenarios, providing clear and accurate imaging results. Meanwhile, the research findings provide important references for future construction of high temporal resolution, high sensitivity CDCP-SPECT systems.

Research Highlights

1. Significant Improvement in Image Quality: Experimental results indicate significant improvement in imaging quality with CDCP technology, especially in low-activity imaging scenarios.

2. Innovative Technical Method: Utilizing cascade photons for detection, CDCP technology fundamentally reduces downscatter and crosstalk contamination, representing a novel imaging method.

3. Broad Application Prospects: This technology shows great potential not only in low-activity therapeutic radionuclide imaging but also provides new ideas for the development of future high-precision imaging technology.

Conclusion

In radiopharmaceutical therapy imaging, traditional scatter correction methods perform poorly in low-activity situations, while CDCP technology significantly reduces scatter and crosstalk contamination by utilizing coincidence detection of cascade photons. Through experimental research and simulation evaluations, this paper demonstrates the superiority of CDCP technology in improving imaging quality, particularly for important therapeutic radionuclides such as Ac-225, Ra-223, Lu-177, and In-111. The research results indicate that imaging systems based on CDCP technology have broad application prospects in future preclinical imaging.