Optimization of exposure parameters in digital mammography: experimental studies and Monte Carlo simulation

Author(s)

Rodrigo Trevisan Massera

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin
• Defense date: 2018-04-12

Examining board members

Alessandra Tomal; Mario Antonio Bernal Rodriguez; Martin Eduardo Poletti

• Advisor: Alessandra Tomal

Abstract

In this work, optimal exposure parameters were determined in digital mammography by Monte Carlo (MC) simulation and experimental methods, using the figure of merit (FOM) and detective quantum efficiency (DQE). For this purpose, the image quality was quantified by the contrast-to-noise ratio (CNR), modulation transfer function (MTF) and noise power spectrum (NPS). The dosimetric evaluation was based on the incident air kerma and mean glandular dose (DGM). The PENELOPE 2014 MC code was used, with validated modifications, to study the DGM and the CNR for different breast models, types of breast lesions (calcification and tumor), detector technologies and antiscatter grids. The FOM was calculated using MC simulation for the X-ray spectra used in contemporary equipment, and new spectra were also proposed. Two clinical mammography units were evaluated experimentally, with different detector technologies and anode/filter combinations. The CNR was obtained using three different phantoms, varying the anode/filter combinations and tube potentials. The MTF and NPS were determined for different anode/filter combinations, tube potentials and current-time products (mAs). The simulated FOM values showed that the detector and grid types affect the optimal tube potential but do not alter the optimum anode/filter combination. Moreover, the FOM increasing using an optimal spectrum, if compared with the reference spectrum, is more noticeable for denser and thicker breasts. The skin models only affect the FOM values, increasing up to 16(2)% without changing the optimum spectra for each breast characteristic. Among the contemporary spectra, the W/Rh and W/Ag provided the best performance for thinner and thicker breasts, respectively. The W/(1,2 mm Al) and W/(0,06 mm Ag) combinations proposed in this work provided an improvement up to 20%, when compared with the optimal contemporary spectra. The optimum spectra obtained experimentally based on FOM were consistent with the simulated ones. In this case, a dose reduction of 50(5)%, compared with the used reference spectrum, can be achieved for the same image quality. The optimal exposure parameters based on the DQE for the mammography units were: W/Rh 28 kV or W/Rh 25 kV with the tube current-time product of 40 or 20 mAs. These results indicate that the FOM and DQE can be used as complementary metrics for a complete optimization of the system. Finally, it can be concluded that the optimum spectra can be implemented in clinical environment, aiming to reduce the dose to produce images with the same quality (AU)

• FAPESP's process: 16/15366-9 - Optimization of the exposure parameters in digital mammography: experimental and simulation studies
• Grantee: Rodrigo Trevisan Massera
• Support Opportunities: Scholarships in Brazil - Master