Single photon emission mammography with convergent collimators

Tese de doutoramento, Engenharia Biomédica e Biofísica, Universidade de Lisboa, Faculdade de Ciências, 2016

The radionuclide based imaging techniques offer substantial potential as complementary diagnostic tools. Dedicated radionuclide systems for breast cancer imaging have been intensively investigated over the past decade. However, these breast-specific systems cannot image the chest wall region. The aim of this work is to present a solution to solve this imaging limitation and demonstrate its feasibility. The proposed solution is based on the cone beam collimation geometry, whose collimator holes are angled toward the breast and chest regions. It is shown that this approach is simpler and more efficient compared to other state of the art approaches. In order to maximize the collimator sensitivity for a fixed spatial resolution, we developed an optimization algorithm for convergent collimators. This algorithm was used to project optimal collimators for two different systems: a single-photon emission mammography (SPEM) scanner and the positron emission mammography (PEM) scanner developed by the Clear-PEM project. Monte Carlo simulations were performed to study the expected performance of both scanners with the respective collimators. The main simulated parameters include: 2.3 mm detector pixels, tungsten collimators, 120-160 keV energy window, 128 step-and-shoot projections acquired over a 5 cm radius circular orbit, acquisition times of 11-21 minutes, and uniform disc (Defrise) and resolution (Derenzo) phantoms filled with Tc-99m. We also simulated a real clinical exam using a realistic breast (XCAT) phantom. The simulated data were reconstructed using an iterative maximum likelihood expectation maximization algorithm (MLEM) with 1 mm3 voxels and each system response was included in the reconstruction process. Overall, due to technical challenges involved in the adaptation of the Clear-PEM to single-photon acquisitions, the best results were obtained with the SPEM scanner. The proposed collimators offer enhanced diagnostic information by detecting down to 3 mm chest wall lesions and have a great potential for use in small field of view applications.