# Experimental and Geant4 Simulation Study of MURA Mask for Scintimammography
## 1 Introduction  
The detection, localization and mapping of radiation-emitting sources are crucial in nuclear decommissioning, decontamination and maintenance procedures [1; 2; 3], especially for nuclear medicine imaging. Among the existing medical imaging modalities, we can cite scintimammography, which uses radioactive substance (radiopharmaceutical) and a special camera for breast imaging, as breast cancer tissues attract more radioactive material than healthy tissues [4]. The reason for this is that cancer cells grow and divide more rapidly. Such rapid growth causes the radioactive isotope to accumulate in larger amounts in tumors. Moreover, through the use of coded-aperture gamma-ray imaging systems, a radioactive distribution map can be superimposed on an optical image as a reference for the localization of radioactive hot-spots [5]. It has been extensively developed to improve its capabilities in terms of lightness, usability, sensitivity, spectral capability, etc. [6; 7; 8; 9; 10]. Within that context, the modified uniformly redundant array (MURA) mask has been widely used, for which a specific aperture pattern of holes (an open area) represents half of the collimator area and allows us to minimize exposure to that half [11]. The majority of systems are currently designed to produce images under far-field conditions, where images appear nearly perfect. As a result, the distances between imaging systems and radioactive sources may be minimized in some applications to improve system sensitivity and to reduce measurement time. Furthermore, when high resolutions are desired, the mask projection tends to be magnified and lowering the distance between the source and camera system is also necessary (assuming that the distance between the mask andthe detector is fixed). However, when the distributed source image is produced by the cross-correlation method within near-field geometry, such as for scintillammography examinations, there are always obvious artifacts present [12]. In astronomy and nuclear medicine imaging, masks and anti-masks are used to reduce such artifacts [13; 14; 15; 16]. By using maximum-likelihood expectation-maximization (MLEM) deconvolution methods, near-field artifacts and aperture collimation effects can also be reduced [17; 18]. However, relatively few studies have been conducted on the use of the \"blank-phantom\", which entails extracting the projected image of a uniformly distributed source from the phantom projection before using cross-correlations as an alternative decoding method [19]. To address the problems outlined above, we propose an experimental setup and a Geant4-based computational framework to model a specific scintimammography imaging setup. Moreover, the object-oriented (C\({}^{++}\)) toolkit for Monte Carlo simulation of particle transport through matter, Geant4 (GEometry ANd Tracking version 4), was used for different applications including high-energy physics, space dosimetry, medical imaging, medical dosimetry, radiation therapy and radiation protection [20; 21; 22]. Other than the possibility of tailoring realistic detector geometry, a range of functionalities is included in Geant4, such as tracking, geometry, physics models and hits. Additionally, the comprehensive range of physical processes, including electromagnetic, hadronic and opticalprocesses, as well as long-lived particles, materials and elements over a wide energy range made it a versatile simulation opportunity. The framework consists of the full modeling of an anthropomorphic breast phantom, a NaI(Tl) crystal-based detector and a 41 \(\times\) 41 MURA mask. Our three main goals can be summarized as follows: (1) experimental measurement of projected images, (2) verification of Geant4, and (3) computationally searching for tumor detection and localization. Thus, we first conducted an experimental procedure for imaging an in-house anthropomorphic breast phantom including a \({}^{99 mTc}\)source at different locations and an in-house designed and fabricated lead-based MURA mask (with 41 \(\times\) 41 matrix).