Author: Qihui Lyu, Javier Caravaca Rodriguez, Youngho Seo, Ke Sheng, Jingjie Yu 👨🔬
Affiliation: University of California San Francisco, University of California, San Francisco, Department of Radiation Oncology, University of California, San Francisco 🌍
Purpose:
Simultaneous broad-energy imaging is critical for many theragnostic applications, but the current Single-Photon Emission Computed Tomography (SPECT) can only image low energy photons with limited energy band width. At higher energy (>300keV), SPECT results in severe artifacts due to photon penetration or low signal-to-noise ratio (SNR) due to thick collimation. Compton imaging using electronic collimation is desirable for high energy applications, but its angular resolution is inadequate for low-energy applications. To overcome this challenge, we propose a dual-modality imaging system combining SPECT-collimators with Compton imaging for simultaneous multi-energy imaging.
Methods:
The system comprises 12 CZT-based detector heads (64×176×64 mm), each with 1.6×1.6×0.5 mm pixels. A 24-mm collimator with parallel square holes (0.9 mm width, 1.6 mm spacing) is mounted in front. The detector to isocenter distance is 245.25 mm. Each head rotates ±42.5° about its axis. The detector energy resolution is 2%. We performed Monte Carlo simulation using Geant4 to acquire detector data, followed by image reconstruction using the maximum likelihood expectation maximization (MLEM) algorithm, for both the SPECT and Compton imaging. The imaging system was tested using a Derenzo phantom for image quality benchmark, and another phantom with 6 different photon energies (140 keV, 218 keV, 269 keV, 364 keV, 440 keV, and 1000 keV) for simultaneous broad-energy imaging evaluation.
Results:
The combined imaging reconstruction achieved simultaneous imaging of the 6 photon energies. The angular resolution was 3.3°, 3.6°, and 4.0° for 140 keV, 218 keV, and 269 keV using SPECT, and 3.9°, 3.8°, and 3.0° for 364 keV, 440 keV, and 1000 keV using Compton imaging. The practical resolution was 12 mm for <300 keV photons and 20 mm for >300 keV photons, estimated using two Derenzo phantoms.
Conclusion:
This combined SPECT-Compton imaging system demonstrates the feasibility of simultaneous multi-energy photon imaging for radiopharmaceutical applications.