previous studies have shown a close relationship between bone density and success of implant treatment [ 27 ] . bone quality is determined by cortical bone thickness , the amount of trabeculae and mineralization   and can be evaluated by bone density assessment . several methods have been proposed for bone quality assessment ; among which , volumetric imaging techniques are the most feasible . these techniques enable the clinicians to assess bone density without any superimposition , which is a major limitation of plain radiography . computed tomography ( ct ) is a useful method for bone density assessment prior to dental implant surgery   and has been accepted as the gold standard for such evaluations . cone - beam computed tomography ( cbct ) has become a widely used imaging modality for pre - implant assessment in the recent years . compared with ct scan , cbct images provide superior resolution and lower patient radiation dose . despite the wide application of cbct in implant dentistry , some studies advocate it as a reliable substitute for ct in this respect and confirm its validity [ 1,2,6,7,1316 ] while others have brought it into question . hounsfield unit ( hu ) is the standard scheme for scaling the reconstructed attenuation coefficient in medical ct systems . in cbct systems , the gray values are used to represent the reconstructed values , although it has not yet been proposed as a standard system . in the present study , the correlation between the gray values obtained from cbct and the hus in ct images was evaluated and the effect of the type of tissue , tissue thickness , acquisition parameters and location was assessed in this regard . in this in vitro diagnostic study , we compared the tissue density values on cbct and ct images in four different tissue phantoms with two different thicknesses , two different acquisition settings and in three locations in the phantoms . the phantoms used in this study simulated different densities namely hard tissue , soft tissue , water and air . hard tissue equivalent phantom was made of bovine bone powder mixed with epoxy resin with the proportion of 0.55:1 in the laboratory of nuclear engineering department of shahid beheshti university . soft tissue equivalent phantom consisted of a 1 cm - thick plexiglas slab , suggested by the medical engineering and medical physics department of shahid beheshti university . they were fixed over each other and placed in a clear plastic cylindrical container ( biokips , komax industrial co. ltd . , the phantoms were positioned at the center of the field of view ( fov ) . for easy repeatability , the phantoms were scanned three times to increase precision using ge bright speed 16 slice multi detector ct scan system ( general electric , schenectady , new york , usa ) with two different exposure settings . it was also scanned three times with two different acquisition settings using three different cbct units namely scanora 3d ( soredex , helsinki , finland ) , newtom vg ( afp , verona , italy ) and pro - max 3d ( planmeca , helsinki , finland ) . exposure parameters for different units in the first phase , six different series of scans were obtained by each unit , three with high and three with low acquisition settings , all in full thickness . in the second phase , the new series of scans were obtained by the same units and the same acquisition parameters but with half thickness . in order to overcome the effect of various computer programs , all ct and cbct images were imported into a single computer program ( workstation 2.1 merge emed , the ct number of ct images and the voxel value in cbct images were evaluated . three definite locations were selected on each image : at the middle ( m ; 270 mm from the periphery ) , on the periphery ( p ) , and an intermediate point between these two ( int ; 140 mm from the periphery ) on the radius of phantom with a constant region of interest ( roi ; fig . these points were selected on a slice and distributed to all using a software option to evaluate the same location in all slices and scans . the same was done for the half thickness images . an oral and maxillofacial radiologist observed and recorded the values in all images . evaluation of hus in hard tissue phantom with merge e - film software in three times of scanning in full thickness phantom . the relationship between ct numbers on ct scans and the gray values of cbct images was studied by anova ( p<0.05 ) . the effects of the type of material , acquisition parameters , phantoms thickness and location of the assessed point on the phantoms were also studied . the relationship between ct numbers on ct scans and the gray values of cbct images was studied by anova ( p<0.05 ) . the effects of the type of material , acquisition parameters , phantoms thickness and location of the assessed point on the phantoms were also studied . in the first evaluation , the interaction effect of the variables was found to be significant using results were not the same in different conditions and each variable had to be examined separately . the results of anova used to compare the cbct and ct data are shown in tables 2 and 3 . comparison of voxel values in cbct systems with hounsfield units in ct in full thickness phantoms ns : non - significant s : significant comparison of voxel values in cbct systems with hounsfield units in ct in half thickness phantoms ns : non - significant s : significant bone quality and quantity assessment is critical for pre - implant surgeries ; thus , reliable and precise radiographic evaluations are essential . however , despite wide use of cbct in different fields of dentistry , it is still not a valid method for bone quality assessment [ 4,1416,1921 ] . object location in cbct systems , object volume , acquisition parameters and some other factors have been evaluated in previous studies and controversial results have been reported [ 7,8,10,11,1316 ] . in this study , the diagnostic accuracy of three cbct systems was compared with that of ct as the gold standard , and the effect of four factors including different tissue phantoms , object volume size , acquisition parameters and location of the objects was assessed . as the results showed , none of the cbct systems revealed the precise bone density as the gold standard . in this study , different tissue phantoms were used to evaluate diagnostic accuracy of cbct systems in different density ranges . review of the literature revealed that only mah et al ,   used standard phantoms . heterogeneity of the hard tissue and air phantoms was a limitation of our study and was due to the production process . they were handmade in a laboratory and even in the best conditions , porosities were seen . however , in the clinical setting , the tissues are not homogenous ; thus , this factor may not be a real limitation for generalization of results to the clinical setting . however , in this study , the results showed no significant effect of this restriction by comparing the data of the four phantoms . the effect of tissue thickness is one of the main subjects for researchers to asses in density studies . in the first phase , the complete thickness and in the second phase , half - thickness samples were evaluated , which showed no considerable effect on assessment of the tissue density . the results were different in the two phases and there was a significant difference in results compared with ct system . katsumata et al ,   showed that the thicker the tissues , the more precise the values of tissue density . they discussed that beam characteristics may be a more important factor than the tissue thickness , which is because of the 360-degree rotation of the system and changes in the intensity of the x - ray beam while passing through different parts of an object , which causes maximum and minimum x - ray intensities . considering the importance of this factor , we used different acquisition parameters to evaluate the importance of the effect of intensity of x - ray beam on tissue density . in our study , despite the changes in the values , no significant difference was shown between the various image acquisition parameters of cbct systems and all values were different from the ct numbers . parsa et al ,   found a significant difference between ct and cbct values and showed changes in cbct values with changes in acquisition parameters . gray values increased with increase in size of fov in accuitomo system but this increase was obtained with an increase in volume in newtom system . this discrepancy between the behaviors of the two systems could be attributed to the variability in reconstruction and post - processing methods applied by the two manufacturers . however , one theory suggests that the tissue density of an object is more effective than the beam intensity and characteristics in showing the actual tissue density . to evaluate and interpret the cbct and ct images , we used only one software which can show the same level of tissue density in both systems , the point which was confirmed by ghasemi et al . by doing so mah et al ,   who used 11 types of cbct systems also used only one software program ( on demand 3d ) to match the observing conditions and roi in all sections . this fact is important in density assessment studies and can affect the results . because of the variable geometric characteristics of different systems , the density values and the selected roi are variable in different software programs provided by the systems . katsumata et al ,   emphasized that the selected roi should be the same in all sections with at least 3% error . in their study , ct images were also examined by the same software program to achieve the same situation in all conditions . nevertheless , mah et al ,   believed that human error is inevitable in selecting the roi . however , no specific method for precise selection of an area in different conditions has been suggested in studies except in a study by naitoh et al ,   who emphasized on selection of equal points via observation . by choosing three different points in an object , we wanted to evaluate the effect of object location in the fov on tissue density . legravere et al ,   did not report any significant difference in different locations in the tube field in their study , which was in contrast to the results of oliveira et al ,   who noticed that the correlation between the object density and ct number in cbct systems was not uniform through the dental arch . we found no significant difference between different locations in the diversity between the voxel values and hus . hua et al ,   found that some artifacts and scatter radiation are responsible for inefficacy of cbct for density assessment , which are inevitable because of the design of cbct systems and their detectors . considering the inefficacy of cbct for density evaluation , the x - ray beam heterogeneity in cbct can affect hu values , and lead to absence of a clear relationship between voxel value in cbct and bone mineral density provided by dual x - ray absorptiometry . the artifacts such as beam hardening or heel effect can decrease the validity of these values . several studies mentioned the difference between the cbct values and the ct values and in most cases , the reported cbct values were higher than the ct values . parsa et al ,   mentioned higher gray value in their study and believed that it may be due to the increased noise level , scattering and artefacts specific to the scanning technology . scarf and farman   proposed that although a sort of association was seen between the provided hus by ct and voxel value in cbct , the variability in measurements by cbct was higher than that by ct . however , legraver et al ,   found a linear relationship in r ( coefficient of relationship between two variables ) between the values in cbct and ct , and indicated that the cbct values were generally higher . in our study , the provided values by cbct were higher than the ct values in one of the systems ( newtom vg 3d ) , which is similar to the results of legrave et al ,   who used the same system for the same purpose . according to this point of view , the difference between the values in our study can be the result of different levels of energy in the three systems ; because the systems with a lower power result in lower intensity of x - ray beam and subsequently less tissue penetration depth . in order to create parallel conditions between the experimental phases and the clinical setting , the suitable acquisition parameters for each system were determined by an expert operator . thus , the levels of kvp and mas were specific and different in the three systems . haristory et al ,   studied the effect of different exposure parameters on the voxel value of cbct and found a strong relationship between cbct and ct using r value . however , because of different results in cbct , they suggested that the use of calibration phantom is necessary before imaging to ensure accurate bone density values . despite the afore - mentioned results , some studies showed inefficacy of this system in providing accurate values of bone density and found a linear relationship ; thus , a high correlation between the results of cbct and ct was achieved . a study confirming the optimal efficacy of cbct systems to estimate bone density introduced a conversion coefficient . nomura et al ,   reported a relationship between two systems in evaluating bone mineral density and voxel value , but because of the nonlinear regression obtained in their study , further studies were suggested . however , a linear relationship and high correlation coefficient between these values have been shown in several studies . by applying the attenuation coefficient in an equation , mah et al , obtained the hu values of tested materials from the voxel values with only a small difference from the actual hu values . considering all the above , we can say that our methodology , which is one reason for the uniqueness of this study , affected the results , and our results do not completely reflect the efficacy of the systems tested . as recently stated by pauwels et al ,   it is logical to postulate that although attempts have been made to correct the gray level variability , quantitative use of values provided by cbct should be generally avoided at this time . according to the results , the cbct systems were not able to show the accurate value of tissue density and the factors such as type of tissue ( hard , soft , water , air ) , thickness ( full against half ) , image acquisition conditions ( high settings against low settings ) and object location ( middle , peripheral and intermediate ) did not affect density evaluation by cbct systems .
objectives : bone quality and quantity assessment is one of the most important steps in implant treatment planning . 
 different methods such as computed tomography ( ct ) and recently suggested cone beam computed tomography ( cbct ) with lower radiation dose and less time and cost are used for bone density assessment . 
 this in vitro study aimed to compare the tissue density values in hounsfield units ( hus ) in cbct and ct scans of different tissue phantoms with two different thicknesses , two different image acquisition settings and in three locations in the phantoms.materials and methods : four different tissue phantoms namely hard tissue , soft tissue , air and water were scanned by three different cbct and a ct system in two thicknesses ( full and half ) and two image acquisition settings ( high and low kvp and ma ) . 
 the images were analyzed at three sites ( middle , periphery and intermediate ) using efilm software . 
 the difference in density values was analyzed by anova and correction coefficient test ( p<0.05).results : there was a significant difference between density values in cbct and ct scans in most situations , and cbct values were not similar to ct values in any of the phantoms in different thicknesses and acquisition parameters or the three different sites . 
 the correction coefficients confirmed the results.conclusions:cbct is not reliable for tissue density assessment . 
 the results were not affected by changes in thickness , acquisition parameters or locations .