Simulation studies to evaluate the design options for a stationary tomographic low-dose molecular breast imaging system

Background: A prototype molecular breast imaging (MBI) system is currently under construction based on the use of dual opposing CZT detector arrays and highly-packed multi-pinhole collimators leading to significant multiplexing (MX) in the acquired projections. The system relies on high intrinsic spatial resolution detectors and knowledge of depth-of-interaction (DOI). De-multiplexing, aided by the DOI, is performed either before or during reconstruction to provide tomographic images of the breast. The system is motivated by the need to develop a practical low-dose system which would have clinical use in patients with dense breast tissue where conventional mammography is limited.

Methods: We have investigated several competing factors that need to be considered in design of the system: a) the choice of pinhole aperture, detector spatial resolution and geometry, b) the angular sampling throughout the breast volume and c) the effectiveness of the de-multiplexing algorithms for various degrees of multiplexing dictated by pinhole geometry (aperture spacing, opening angle). Phantom studies (uniform activity, line sources, multiple lesions) have been performed using both analytical calculations and Monte Carlo based analytical simulation to investigate the overall performance of the system and to select possible design parameters for the prototype system.

Results: Assuming a 3D detector resolution of 1 mm, the contrast-to-noise ratio (CNR) in a phantom with multiple lesions of different size and contrast was maximised using over 200 pinholes / detector. This resulted in a total sensitivity of 1375 cps/MBq. The mean spatial resolution over the FoV was 5x5x12 mm. Using DOI, de-MX is possible even with a high degree of MX, although reduced contrast is obtained, as compared to ideal non-MX data (possible only in simulations), at the same noise-level.

Conclusions: Design parameters have been optimised for a stationary MBI system based on multi-pinhole collimators. Tomographic images with high CNR can be obtained, which results in improved lesion detectability and/or reduced dose or scan time as compared to planar MBI. Acknowledgements: UCL staff receive support from the NIHR UCLH Biomedical Research Centre.