# Comparing the Diagnostic Accuracy of CBCT
## 1 Introduction  
Osteoporosis is a common disease that affects over two hundred million people worldwide [1]. It is associated with bone fractures among men and women, especially hip and vertebral column bone fractures [2]. It affects nearly 30% of postmenopausal women in the United States and Europe [2]. Dual X-ray absorptiometry(DXA) (previously known as DEXA) is a simple, safe and precise tool that uses two X-ray beams of different energy levels for the identification of peripheral and total body bone mineral density (BMD) [3]. This technique attained its popularity and is considered the gold standard paradigm for bone quality and quantity assessment, because it allows high resolution images, precise short time scanning and, stability of calibration [3]. The outcomes of a DXA scanare displayed as T or Z score; the T score represents the comparison between a bone mineral density (BMD) of an individual and the optimal peak bone mineral density within the gender/ethnic group according to the World Health Organization norms (WHO, 2007) [3]. Thus, the T score is determined by calculating the ratio of the difference (and the standard deviation) between subjects' BMD and the mean BMD of the young population of the same gender and ethnicity [3; 4; 5]. A T score value below '- 2.5' indicates osteoporosis, between '-1' and '-2.5' denotes osteopenia/low bone mass, and '-1' or above means normal BMD. On the other hand, the Z score is calculated in a similar way to the T score, but the BMD data of an individual is compared to the mean BMD of a normal population with similar age group [6]. T scores are mostly used, as most of the studies are performed on white men and postmenopausal women. However, Z scores are considered helpful in the diagnosis of secondary osteoporosis, especially in children, young adults and premenopausal women [7; 8]. Although DXA is considered as an effective method for BMD analysis, the accuracy and precision of DXA scanning depends on the machine, operator and patient-related factors [9]. Moreover, existing bone pathology, systemic diseases and individuals' ability to comply with the positioning requirements of the system create difficulties in precision and diagnosis [10]. Furthermore, DXA's high cost and high X-ray doses limit its use for large scale screening for osteoporosis [11]. The literature lacks enough research regarding the use of DXA for evaluation of BMD and osteoporosis of jaw bones. The use of DXA for jaw bones is associated with some shortcomings, including high cost, high radiation dose, low image quality, high distortion, difficulties in positioning the system for jaws, and long scanning time [12]. Consequently, further research is required to identify alternative techniques for osteoporosis screening, that especially cost less and are associated with less radiation. Cone beam computed tomography (CBCT) has been a popular diagnostic tool in dental settings since its introduction for dental purposes in 1998 [12; 13; 14]. It is a relatively noninvasive technique for the assessment of bone quality. It allows a dimensional evaluation of jaw anatomic structures as well as measurement of jaw bone mineral density, which is expressed in grayscale (GS) values. CBCT increasingly replaces the multi-slice CT methods, as it is associated with a lower radiation dose and costs less than CT [15]. Moreover, CBCT is more available and cost effective than DXA. Moreover, CBCT is an effective diagnostic tool to determine bone length, width and density in implant dentistry, because it is associated with low scanning time, adequate image quality and lower image artifacts [16]. However, the GS values derived from CBCT to indicate jaw bone mineral density are not identical to those derived from multi-slice CT [17]. Hounsfield units in the multi-slice CT scan are proportional to the attenuation level of X-ray and are assigned to each pixel, whereas voxel value represents the X-ray attenuation in CBCT. Moreover, the bone density obtained from DXA is two dimensional information.