Purpose/Background  

We have recently developed the Alpha-SPECT-Mini system utilizing CdTe imaging spectrometers [1] and a novel inverted compound-eye collimator design. To ensure an excellent energy resolution over a wide energy range of 80-500 keV, we have developed and utilized several machine learning algorithms [2] to compensate for the degradation in spectral response due to the charge-sharing effect. Furthermore, we have also implemented a spatially-variant dynamic energy windowing technique, triple-window scattering correction, to improve the signal-to-noise ratio in the projection domain. In this presentation, we will present the performance of the Alpha-SPECT-mini system with phantom studies and ex-vivo/in-vivo studies of mice injected with Ac-225 radiopharmaceuticals. 

Methods

The CdTe Detector and Alpha-SPECT-Mini System Design:

The Alpha-SPECT-Mini system [3, 4] is built around uses 24 CdTe detectors that offer an exquisite energy resolution of close to 1 keV at 140 keV with single-pixel events and an average of  <2 keV FWHM with all events and after charge loss correction. The detection system is able to resolve all major gamma-ray lines from the Ac-225 and its daughters across 60 keV to 600 keV. In teh Alpha-SPECT system, each CdTe detector is coupled with four 1-mm diameter pinholes to offer an ultra-high system sensitivity. The system is designed to offer <0.75 mm spatial resolution focusing on 18 mm x 18 mm x 12 mm (in axial) FOV with an open bore of 43 mm in diameter. 

System Performance Optimization:

We have implemented several signal correction techniques to optimize the spectral performance of the Alpha-SPECT-mini system and to ensure an accurate determination of the projections from different gamma-ray lines from Ac-225 and its daughters. These include (a) a charge-sharing reconstruction algorithm based on machine-learning [2] that helps to determine real energy loss with multi-pixel events, (b) a spatially-variant dynamic energy windowing approach to ensure an ultimate uniformity in detector response, (c) several different scattering-correction techniques to achieve an optimized signal-to-noise ratio in the projections and subsequently in reconstructed images.

Results:  Fig. 1 (Left) shows the image of mice injected with unbonded Ac-225 mostly deposited in the bones. Fig. 1 (Right)  shows the distribution of Ac-225 and its daughters (Fr-221, Bi-213, and Tl-209) in the tumor, liver, and kidney of a live mouse with a total of 1.5 uCi Ac-225 injected.

Conclusion: We have developed the Alpha-SPECT-Mini system for pre-clinical quantitative analysis of -RPT. Ac-225 phantom study, ex-vivo and in-vivo mice experiments have been conducted to illustrate the system’s imaging capabilities. While the demonstration is based on Ac-225 alpha-RPT, the Alpha-SPECT-mini system could be an interesting imaging tool for multi-functional molecular theranostics.

Image/Figure:

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Image/Figure Caption:

Fig. 1 (Left) shows the image of mice injected with unbonded Ac-225 mostly deposited in the bones. Fig. 1 (Right)  shows the distribution of Ac-225 and its daughters (Fr-221, Bi-213, and Tl-209) in the tumor, liver, and kidney of a live mouse with a total of 1.5 uCi Ac-225 injected.

Presentation Poster:
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