DEXA Prototype Using SrI2:Eu2+ Coupled to Silicon Photomultiplier

Methods: Bone mineral density (BMD) evaluation tools are crucial to the proper diagnosis of osteoporosis. The commercially available DEXA systems utilize CZT as the detector of the small dose of ionizing radiation passing through the area of interest. Our paper presents a novel Strontium Iodide doped with Europium (SrI2:Eu) scintillator crystal coupled to a Silicon Photomultiplier (SiPM) array as a less expensive alternative to CZT detector. Dual energy (60 keV and 122 keV) exposure was used for BMD measurements of a phantom containing CaHPO4 to mimic bone and resin to mimic soft tissue. We also determine the mass thickness of copper and aluminum samples.


Introduction
Osteoporosis is a bone disease characterized by low Bone Mineral Density (BMD) leading to a reduction in bone quality. Up to 1 in 2 women and 1 in 4 men over the age of 50 will endure osteoporosis related fractures, making the disease a public health threat [1]. Dual-energy X-ray absorptiometry (DEXA) is the gold standard technique to measure BMD and diagnose osteoporosis.
DEXA uses a small dose of ionizing radiation to create images of bones and to measure BMD. Although DEXA's ability to reproduce BMD measurements is commended, there are some limitations to its technology causing its results to be misleading. Faulty interpretation of DEXA results by physicians leads to an over estimation of BMD in people with large bones and an underestimation of BMD in people with smaller bones [2]. Interpretation leading to misdiagnosis leads to increased medical expenses. In 2017, the National Osteoporosis Foundation reported that osteoporosis related fractures costs are responsible for up to $19 billion annually to patents and the healthcare system [1]. By 2025, experts project the cost to increase to $25 billion annually [1]. With an increased awareness of osteoporosis and the inaccurate assessments of BMD by DEXA, we aim to tackle DEXA's lack of accuracy in determining BMD by improving and refining materials used as a gamma ray detector. We have developed a DEXA prototype that couples a SrI 2 :Eu 2+ scintillator with a SiPM. SrI 2 :Eu 2+ scintillator is a novel material developed at Fisk University.
The SiPM is a low power, high efficiency photo-sensor that can offer the capabilities to develop imagers with better spatial resolution than Photomultiplier Tube (PMT).
The optimization of the DEXA detector will lower the cost of the scanner and enhance its performance for proper BMD reports.
Europium activated strontium iodide was discovered to be a highperformance scintillator at room temperature in 2007. Cherepy et al. [3] reported on its scintillation properties for crystals grown by the Bridgman method showing a light yield of 90,000 photons/ MeV. SrI 2 :Eu 2+ shows promise for high energy resolution, with a reported FWHM energy resolution at 662 keV of 2.5% close to the 2% of the semiconductor CZT [4,5]. The price for CZT detectors is considerably high due to raw material processing and defect-free growth difficulties. The segregation coefficient of Zn along the growth axis in CZT is large (k = 1.35) leading to the high compositional variation [6]. SrI 2 :Eu 2+ scintillation emission spectrum presents a sharp peak around 435 nm which is in the maximum photon detection efficiency range of the SiPM produced by SensL that will be used in the experiments [3,7]. The low power consumption and the small size of SiPM creates the premises to reduce the size of the device and make it portable.  A 3x3x8 mm3 SrI 2 :Eu 2+ crystal b.

Experimental Methods
The encapsulated SrI 2 :Eu 2+ array c.
A SensL 4x4 SiPM array.  Table 1 presents where I is the count rate measured through the attenuating material, I 0 is the count rate measured without the attenuating material, µ/ρ is the mass attenuation coefficient, x = ρ t is the mass- Using Eq 2, experimental value of BMD will be determined.
Aluminum and Copper samples served as reference in terms of densities to validate the performance of the system. Since no soft tissue substituent is present Eq 1 and Eq 2 will be simplified as

Results
The