# Experimental and Geant4 Simulation Study of MURA Mask for Scintimammography
## 2 Materials and Methods
### Simulation Procedure  
In this work, we used a breast phantom, a MURA mask and a special detector. The breast phantom was modeled with the material composition taken from the NIST website (G4_ADIPOSE_TISSUE_ICRP) [23], and the geometry consists of the superposition of both a cylinder (50 cm radius and 35 cm height) and a semi-ellipsoid (\(5\times 5\times 7\) cm\({}^{3}\)).  
Additionally, as seen in Figure 1, the phantom includes 22 possible locations (with the same thicknesses of 0.1 cm) for tumor positioning. Locations from 17 to 22 have the same radius of 1.2 cm; however, the other 16 options have different radii from 0.1 to 0.25 cm. All locations were cylindrical and filled with air as the medium. On the other hand, the MURA mask consists of a \(41\times 41\) array pattern. Designed at the center of a thin sheet holder of lead with the dimension of \(60\times 60\times 0.15\) cm\({}^{3}\), the mask was formed with a given distribution of box-shaped holes (\(0.2\times 0.2\times 0.15\) cm\({}^{3}\)), resulting in a global dedicated volume of \(8.2\times 8.2\times 0.15\) cm\({}^{3}\). Similar to the desired mask, as described in the literature, it contains 50% open/close area, i.e., the same surface area of gamma ray-opaque and gamma-ray-transparent zones. A more detailed description was given by [PERSON] et al. [11]. Moreover, the detection system consists of the NaI(Tl) crystal detector with a surface of \(0.597\times 0.597\) cm\({}^{2}\) and a thickness of 0.95 cm. The detector was pixelated onto \(512\times 512\) scoring pixels.  
Figure 1: Experimental setup including equivalent tissue anthropomorphic breast phantom, MURA mask and imager. Twenty-two possible locations for the inserted \({}^{99m}Tc\) were also given.  
On the other hand, the generated energy spectrum follows a Gaussian distribution with 140.5 keV and 11.85 keV as the mean value and standard deviation, describing the \({}^{99m}Tc\) emission spectrum. With each simulation, we activated the Bremsstrahlung, ionization and multiple scattering models for electrons with an energy threshold of 990 eV. This allowed particles to be tracked almost entirely. A cutoff energy of 990 eV was used to activate the Compton effect, the photoelectric effect, and the Rayleigh scattering of the photon particles. A specific physics builder called 'G4 EmStandardPhysics' was used to implement our electromagnetic physics options. As a result of Geant4's multithreading feature [24; 25], all of the simulations ran in a reasonable amount of time, despite the complex experimental setup.  
This near-field and medium phantom size application required a magnification factor of 3.0, so we fixed the mask-to-detector distance to be three times the source-to-mask distance. In order to create coded images, the detector crystal was illuminated with particles that deposited energy in it. All simulations were conducted on laptops (for testing) and Dell Precision T7610 workstations, each with a 40-core Intel Xeon E5-2680v2 CPU running at 2.80 GHz and 256 GB RAM. Ubuntu 14.04 was used to manage the entire workflow. Since we focused the generation toward the MURA mask, we generated more than \(2\times 10^{9}\) primary particles. Due to the open area (holes) constituting 50% the field of view, the number of counts detected (primary particles) through the MURA mask was half that of the generated primaries. The run time ranged from 20 min to more than 7 h, resulting in 1% statistical uncertainty for all studied cases.