Bone Densitometric Analysis using Cone Beam Computed Tomography (Cone Beam CT) and Computed Tomography (CT): Establishing the Correlation between Predicted and Actual Values

Abstract

Background:

One of the major drawbacks of cone beam computed tomography (cone beam CT) is that unlike CT, the gray density values obtained from cone beam CT are relative, and not absolute as obtained in the case of CT. The present study was conducted with the intent to perform a comparative bone densitometric analysis using cone beam CT and CT and to determine if there was a mathematical correlation between the two.

Materials and Methods:

The present in-vitro study included CT and cone beam CT scans of 30 dry mandibles wherein the gray density values from well-demarcated, precise anatomical areas were obtained, analyzed, and compared. Also, the dependent t-test and Pearson's correlation coefficient test were used for statistical analysis, while probability values (P values) < 0.05 were contemplated as being statistically significant.

Results:

Pearson's correlation between the gray density values for different combinations of point(s)/group(s) as derived on CT and cone beam CT images revealed the results to be statistically significant for each of the imaging methods used, individually and when compared in between the two methods, in all the cases (P < 0.001) when analyzed at 5% level of significance.

Conclusions:

The mean gray density values obtained with cone beam CT were found to be significantly higher than the ones derived from CT in the present study, though, a linear correlation was observed between the values obtained from cone beam CT and CT, which can be used to convert the relative values obtained with cone beam CT into absolute values derived with CT.

INTRODUCTION

The way the invention of computed tomography (CT) has changed the faces of medicine and dentistry has been magnificent. However, the soaring cost of the technology, restricted access, and high radiation exposure associated with the use of CT have been the main reported drawbacks for the under-utilization of CT.[1,2,3] To overcome these drawbacks, Arai et al.[4] in Japan, and Mozzo et al.[5] in Italy, brought the concept of cone beam computed tomography (cone beam CT) as a refined mode of using CT for various clinical applications in the maxillofacial region, which like CT, offered three-dimensional (3D) imaging. The cost-effective and comparatively lower radiation exposure associated with cone beam CT subsequently led to a speedy ingress of the novel concept of cone beam CT in the field of dentistry.[6,7,8,9] Cone beam CT provides images with sub-millimeter resolution (2 line pairs/mm) further increasing the diagnostic precision while working on a shorter total scan time (~60s).[10,11,12,13,14] Furthermore, the radiation dose from cone beam CT is 10 times less than the conventional CT during similar maxillofacial exposures (68 µSv in cone beam CT as against 600 µSv in CT), while providing greater dimensional accuracy. The other significant advantages of cone beam CT include the technique's capability of allowing for a modified field of view (FOV) as per the requirement, providing high-resolution images with a voxel size range of 0.076-0.4 mm, allowing for the reduction of metal-induced artifacts, a significant concern with CT, and displaying modes which are unique to maxillofacial imaging.[15,16,17,18] Again, the equipment design in the case of cone beam CT is easier to use, while image distortion is minimal, and also, it allows for multiplanar imaging with the images obtained being compatible with other planning and simulation software. The clinical applications of cone beam CT are infinite with the technique serving as a boon in the evaluation of different bone pathologies, developmental anomalies in head and neck regions, and fractures, in determining the size, extent, location, and relative proximity of various vital structures in relation to the actual lesion in cases of cysts and tumors, in object localization, in cases of impacted and supernumerary teeth, in cases of osteomyelitis, soft-tissue calcifications, cleft palate, in pre-surgical planning, as well as in post-operative evaluations.[19,20,21,22,23] Again, there is a great inter-individual variation in the human mandible, while the mandible may undergo various patterns and degrees of resorption, and subsequent re-modeling following loss of tooth in different populations. Similarly, the extent to which generalized osteoporosis in the post-cranial sites is linked to changes in the morphology of the mandible has, also, been debated over a period of time, while the studies conducted in this regard, and the interpretation done, of the results, is complicated by the wide range of techniques used, and the different variables and coordinates assessed.[1,2,3] In this pretext and for all such applications, cone beam CT has largely replaced CT as it provides good spatial resolution while having a significant gray density range and contrast with a good pixel/noise ratio.[24,25,26] One of the major drawbacks with the use of cone beam CT, though, is that unlike CT, the bone density/gray density values obtained from cone beam CT images [voxel value (VV)] are relative, and not absolute as obtained in case of CT [Hounsfield unit (HU)], while, also, CT images can be calibrated using the density values of air (-1000 HU) and pure water (0 HU), and on the contrary, cone beam CT units cannot be calibrated since the values which are based on the differences of the gray scale are already pre-determined by the manufacturers.[27,28,29,30,31] There is a relative dearth of studies based on comparative analysis between CT and cone beam CT considering bone density, while few of the studies conducted in this regard were conducted in a cross-sectional study design by directly taking scans of CT and cone beam CT in patients before and after treatment. The present study was conducted with the intent to perform a comparative bone densitometric analysis using cone beam CT and CT, and to determine if there was a mathematical correlation between the different gray density values measured through CT (HU) and cone beam CT (VV) for an acceptable precision.

MATERIALS AND METHODS

The present study was conducted in an in-vitro study design using CT and cone beam CT scans of 30 partially or, completely edentulous dry mandibles of both male and female adults of all age groups [Figure 1] which were sourced from the Department of Anatomy after taking due permission from the concerned authorities, while damaged and/or, incomplete mandibles, pathological specimens and specimens with images of poor quality, and those which presented with artifacts were excluded. The protocol employed in the present in-vitro study consisted of an integrated sequence that involved using defined anatomical areas including points exactly 1 mm below the mental foramen and 1 mm below the root apex of the second molar on both left and right sides in each dry mandible which were used as landmarks to ensure a perfect overlap, and the execution of CT and cone beam CT scans for all anatomical specimens employing the same spatial coordinates for both types of scans. Cone beam CT scans were acquired with the help of a cone beam CT unit (CS 8100 3D, Carestream Dental LLC, Atlanta, GA, USA) [Figure 2], while spiral CT unit (GE Revolution™, ACTs, Bangalore, India) [Figure 3] was used for procuring the CT images. Furthermore, the cone beam CT and CT units used conformed with the prescribed guidelines on the performance standards for diagnostic x-ray units and their major components as devised by the Food and Drug Administration (FDA) as per the published rule “Electronic Products; Performance Standard for Diagnostic X-Ray Systems and Their Major Components” (70 FR 33997) falling under 21CFR 1020.30 for the cone beam CT unit and under 21CFR 1020.33 for the CT unit except with respect to those characteristics which were authorized by the Variance Number FDA-2014-V0657. The technical specifications used for obtaining cone beam CT scans included a 16384-14-bit gray scale density processing system with a slice thickness of 75 µm, and 90 kVp tube voltage, along with 15 mAs tube current with an exposure time of 14s, while the CT parameters used included 140 kVp tube voltage and 210 mAs tube current with an acquisition time of 1s using multiplanar reformatting by acquiring the data through multiple contiguous scans employing high-resolution bone kernels to produce sharper images with higher spatial resolution using a slice thickness of 0.625 mm and pitch of 0.531:1. Again, the CT unit was calibrated before being put to use, while all the data acquired was saved in Digital Imaging and Communications in Medicine (DICOM) format. The quantified bone density/gray density values obtained were measured and expressed as VV in cone beam CT and HU in CT groups respectively. The values were recorded in a pre-determined proforma, while the data obtained was subsequently evaluated statistically. For each anatomical specimen, the bone density/gray density values were determined at 4 specific points in the CT [Figures 4 and 5] and cone beam CT [Figures 6 and 7] images respectively under the following heads:

Figure 1.

Dry mandible

Figure 2.

Cone beam CT unit (CS 8100 3D, Carestream Dental LLC, Atlanta, GA, USA)

Figure 3.

Spiral CT unit (GE Revolution™, ACTs, Bangalore, India)

Figure 4.

Bone density/gray density value (HU) at Point AL on left side, 1 mm below the mental foramen

Figure 5.

Bone density/gray density value (HU) at Point BR on right side, 1 mm below the root apex of second molar

Figure 6.

Bone density/gray density value (VV) at Point AL* on left side, 1 mm below the mental foramen

Figure 7.

Bone density/gray density value (VV) at Point BR* on right side, 1 mm below the root apex of second molar

For CT:

Point AL: Bone density/gray density values (HU) on the left side, 1 mm below the mental foramen;

Point AR: Bone density/gray density values (HU) on the right side, 1 mm below the mental foramen;

Point BL: Bone density/gray density values (HU) on the left side, 1 mm below the root apex of the second molar;

Point BR: Bone density/gray density values (HU) on the right side, 1 mm below the root apex of the second molar.

For cone beam CT:

Point AL*: Bone density/gray density values (VV) on the left side, 1 mm below the mental foramen;

Point AR*: Bone density/gray density values (VV) on the right side, 1 mm below the mental foramen;

Point BL*: Bone density/gray density values (VV) on the left side, 1 mm below the root apex of the second molar;

Point BR*: Bone density/gray density values (VV) on the right side, 1 mm below the root apex of the second molar.

Sample size estimation:

Sample size calculation for correlation coefficient analysis:

Sample correlation coefficient = 0.60.

Population correlation coefficient = 0.

Power (%) = 95.

Alpha (α) Error (%) = 5.

Sided = 2.

Sample size: n = 30.

Formula used to calculate sample size:

Where

r = correlation coefficient = 0.60.

Z_α = Z-value for α level = 1.682.

Z_b = Z-value for β level = 1.96.

Statistical analysis used

Statistical analysis was performed with the help of Statistical Package for Social Sciences (SPSS) version 20.0 (SPSS Inc., Chicago, IL, USA). Also, dependent t-test and Pearson's correlation coefficient test were applied to compare the data, while probability values (P values) less than 0.05 were contemplated as being statistically significant.

RESULTS

Table 1 shows the descriptive statistics including the minimum-maximum values, mean, and standard deviation of the bone density/gray density values at different point(s)/group(s) in case of data obtained for CT and cone beam CT. Similarly, Table 2 shows the comparison of the different combinations of point(s)/group(s) as derived on CT and cone beam CT images in terms of the bone density values by dependent t-test wherein the results were found to be with statistical significance in all the cases and for all combinations of groups while revealing that the mean bone density values derived from the cone beam CT images were significantly higher than the ones derived from CT images. On analyzing the data, a significant difference was observed between the combinations of groups, Group AL+AR and Group AL*+AR*, and Group BL+BR and Group BL*+BR*, in terms of the bone density/gray density values obtained with the corresponding t-values being −201.0340 and −321.0563, respectively, with the results being statistically highly significant in either case (P < 0.001) at 5% level of significance. The mean scores, in either case, are, also, presented in Table 2 for reference. Likewise, a significant difference was, also, observed when a comparison was drawn between Group AL+AR+BL+BR and Group AL*+AR*+BL*+BR* in terms of the bone density/gray density values obtained with the corresponding t-value being −334.0470, with the results being statistically highly significant (P < 0.001) at 5% level of significance implying that the mean bone density/gray density values were significantly higher in Group AL*+AR*+BL*+BR* than in case of Group AL+AR+BL+BR. Table 3 reveals Pearson's correlation between the different combinations of point(s)/group(s) as derived on CT and cone beam CT images in terms of the bone density/gray density values wherein, again, significant differences were observed between the combinations of groups, Group AL+AR and Group AL*+AR*, and Group BL+BR and Group BL*+BR* with the corresponding r-value being 0.96 and 0.73 respectively, and with the results being statistically highly significant in either case (P < 0.001) at 5% level of significance. On analyzing the data further, a significant difference was, also, observed when comparison was drawn between Group AL+AR+BL+BR and Group AL*+AR*+BL*+BR* in terms of the bone density/gray density values obtained with the corresponding r-value being 0.88 with statistically significant results (P < 0.001). Graph 1a-g represent scatter diagrams showing a correlation between the different combinations of point(s)/group(s) as derived on CT and cone beam CT images in terms of the bone density/gray density values.

Table 1.

Descriptive statistics revealing the minimum-maximum values, mean, and standard deviation of the gray density values at/for different point(s)/group(s)

Point(s)/Group(s)	n	Min.	Max.	Mean	Std. Dev.
AL	30	467.00	518.50	496.55	12.10
AL*	30	1109.00	1237.00	1180.90	35.78
AR	30	472.50	539.50	503.77	23.42
AR*	30	1204.00	1349.00	1271.30	46.93
BL	30	460.00	516.00	487.55	17.44
BL*	30	1103.00	1227.00	1171.77	38.40
BR	30	468.50	523.00	492.27	19.51
BR*	30	1235.00	1327.00	1301.20	22.40
AL+AR	30	948.50	1054.50	1000.32	29.08
AL*+AR*	30	2346.00	2582.00	2452.20	66.99
BL+BR	30	951.00	1028.00	979.82	22.36
BL*+BR*	30	2421.00	2540.00	2472.97	37.10
AL+AR+BL+BR	30	1902.50	2055.50	1980.13	40.74
AL*+AR*+BL*+BR*	30	4785.00	5086.00	4925.17	80.13

Table 2.

Comparison of the different combinations of point(s)/group(s) in terms of the gray density values by dependent t-test

Point(s)/Group(s)	Mean Diff.	Std. Diff.	t	P
AL	−684.35	24.19	−154.9767	<0.001*
AL*
AR	−767.53	24.29	−173.1071	<0.001*
AR*
BL	−684.22	21.80	−171.8782	<0.001*
BL*
BR	−808.93	14.78	−299.7215	<0.001*
BR*
AL+AR	−1451.88	39.56	−201.0340	<0.001*
AL*+AR*
BL+BR	−1493.15	25.47	−321.0563	<0.001*
BL*+BR*
AL+AR+BL+BR	−2945.03	48.29	−334.0470	<0.001*
AL*+AR*+BL*+BR*

*P<0.001: Highly significant

Table 3.

Pearson's correlation between the different combinations of point(s)/group(s) in terms of the gray density values

Point(s)/Group(s)	Summary	AL	AR	BL	BR	AL+AR	BL+BR	AL+AR+BL+BR
AL*	r	0.9719
	P	0.0001*
AR*	r		0.9831
	P		0.0001*
BL*	r			0.9733
	P			0.0001*
BR*	r				0.7594
	P				0.0001*
AL*+AR*	r					0.9672
	P					0.0001*
BL*+BR*	r						0.7397
	P						0.0001*
AL*+AR*+BL*+BR*	r							0.8806
	P							0.0001*

*P<0.001: Highly significant

Graph 1.

(a-g) Scatter diagram showing correlation between the different combinations of point(s)/group(s) in terms of the gray density values. (a) Point AL* vs. Point AL, (b) Point AR* vs. Point AR, (c) Point BL* vs. Point BL, (d) Point BR* vs. Point BR, (e) Points AL*+AR* vs. Points AL+AR, (f) Points BL*+BR* vs. Points BL+BR, (g) Points AL*+AR*+BL*+BR* vs. Points AL+AR+BL+BR

DISCUSSION

Computed tomography (CT) provides cross-sectional radiographic images that facilitate an accurate assessment of hard and soft tissues in a subject. It is a non-invasive technique that enables an accurate assessment of the trabecular and cortical bone densities in the given subject. In line with this, Bassi et al.[32] conducted a study in which mandibular bone density and vertebral density as evaluated by quantitative-CT (Q-CT) were correlated with the resorption of edentulous ridge areas as was assessed on panoramic radiographs in a group of 17 partially edentulous subjects, while the authors observed that the mean mandibular bone density was found to be significantly higher in dentate than in edentulous ridge areas, and that the mean mandibular bone density did not reveal a significant correlation either with the vertebral density or, with the resorption that was seen in edentulous areas. The authors, thus, concluded based on the findings of their study that CT was more of a help in the precise evaluation of trabecular bone as compared to the other imaging methodologies. In line with this and terms of validation of the data obtained with cone beam CT, Taylor et al.[33] conducted a study to evaluate the role of CT in assessing the relative differences of the degree of bone density/gray density values obtained in relation to human mandible and concluded that both cone beam CT and micro-CT provided comparable results in the assessment of the relative differences in the gray level distribution between the alveolar and basal cortical bones in human mandible, while, also, that such differences can be used as reliable markers of the bone density changes when compared in relation to an internal reference such as the basal cortical bone.

Likewise, Kim DG[34] in the year 2014 reviewed the applications of cone beam CT while comparing it with conventional CT and observed that, though, cone beam CT has certain characteristic disadvantages of being associated with an increased noise and reduced contrast than conventional CT, cone beam CT gains edge due to the lower cost and reduced radiation dose apart from offering higher spatial resolution than as compared to the CT. The author, also, provided a brief description of the image artifacts associated with the assessment of gray values in cone beam CT apart from the techniques that can be employed to correct local and conversion errors, and that can be used in converting the relative gray values obtained in cone beam CT into absolute values of CT. Similarly, Liang et al.,[17] also, conducted a similar comparative study to evaluate the accuracy of cone beam CT while comparing it with micro-CT in assessing the osseous changes in the jaw bones and concluded in favor of cone beam CT as an imaging technique that revealed comparable accuracy with high-resolution micro-CT. Similar to the observations made in these studies, the correlation of the micromorphometric data obtained using cone beam CT and micro-CT in augmented sinuses in the study conducted by Kivovics et al.,[35] also, suggested cone beam CT reconstructions provide reliable information on the microarchitecture of the bone.

Contrary to the findings of these studies, though, Campos et al.[36] stated in their review that despite the fact that cone beam CT has often been proposed and used as a substitute for conventional CT in the assessment of bone density and quality of craniofacial bones based on the bone density/gray density values obtained, there is no consensus as of now which has been reached in relation to the accuracy of such values. The authors, also, concluded on the basis of their review that, though, the bone density/gray density values obtained with cone beam CT show a linear relationship with the attenuation coefficients of the structures scanned, the HU values obtained with conventional CT and bone density values obtained with dual-energy x-ray absorptiometry (DEXA), there are high chances of errors when cone beam CT is used because of the inherent inconsistencies in the values obtained, especially, in relation to the abrupt changes in the density of structures, secondary to the beam hardening effect, due to scattered radiation, discontinuity in the projection data, differences between the various cone beam CT units used due to the inherent/inbuilt features, changes in the volume of FOV, and due to changes in the relationships of the size and position of the FOV used and the structures in the scan. Furthermore, the authors, also, highlighted that, though, few of the methods and formulas have been proposed to correct the local and conversion errors, the exactness of the values obtained with the help of cone beam CT is yet to be established by further studies in this regard.

In line with this, Kim et al.,[14] also, conducted a study to determine the efficacy and reliability of dual-energy-CBCT (DE-CBCT) in the assessment of bone mineral density (BMD) as against the conventional multislice-CT (MS-CT) and concluded in favor of DE-CBCT, predicting DE-CBCT may soon replace MS-CT due to the higher costs and increased radiation exposure associated with MS-CT. In another similar study conducted by Nomura et al.,[37] though, the authors observed a significant correlation between the bone density/gray density values obtained with cone beam CT and the CT numbers of MS-CT, the authors predicted that with the in-vitro design used in the study which largely assumes ideal conditions, further studies are warranted including future in-vivo study designs for the validation of the results obtained, while this becomes even more important since the voxel values (VVs) obtained with cone beam CT were not found to be entirely linear with the CT numbers obtained with MS–CT. A systematic review conducted by Guerra et al.[8] in the year 2017, however, indicated a relative scarcity of studies in this regard, though, the authors proposed, based on the limited evidence existing in the literature, the possibility of using radiomorphometric indices and the data obtained using cone beam CT as reliable tools for differentiating individuals with osteoporosis from individuals with normal BMD.

Again, a notable study on similar lines was conducted by Cassetta et al.[38] to assess if there was a statistically significant difference in the bone density/gray density values obtained with two different cone beam CT exposures of 8 mAs and 15 mAs, while the authors created two different datasets using a specially designed software and radiographic template wherein the images were overlapped and each dataset provided respective bone density/gray density values of the same anatomical area with the same spatial coordinates, the authors observed statistically significant differences between the respective gray density values making the authors conclude that the values obtained with the help of cone beam CT are not reliable when taken as the absolute values. In another similar study, however, when the authors tried to compare the data obtained from cone beam CT and conventional MS-CT using a similar methodology, they observed statistically significant differences between the respective cone beam CT (VV) and CT (HU) gray density values with a linear correlation between the VV and HU gray density values when assessed using Pearson's correlation coefficients and Pearson's r-values. The authors, thus, recommended that, though, cone beam CT appears as a useful substitute for CT, a conversion ratio needs to be applied to the relative cone beam CT (VV) gray density values in order to make the values more precise. The authors, also, suggested a conversion ratio of 0.7 when a comparison was drawn between CT and cone beam CT (0.7 × value obtained in cone beam CT = value obtained in CT).[39]

Similar conclusions were, also, drawn in the study conducted by Arisan et al.[40] wherein the authors, though, suggested significantly higher cone beam CT gray density values (229-1042 VV) as compared to the CT values (67-989 HU), it was concluded that similar to the gray density values of CT, the values obtained with the help of cone beam CT could be used to predict the subjective quality of bone while analyzing the results obtained from various correlation tests and regression analyses. The authors, however, recommended further studies for the validation of results obtained using different cone beam CT units, while attributing beam hardening, scattered radiation, projection data discontinuity, and changes in the volume of FOV as the major reasons behind the inconsistencies in the gray density values obtained with cone beam CT and for the decrease in the dynamic contrast obtained with cone beam CT as against CT. Again, the authors, also, mentioned that the effect of beam hardening was more pronounced with increased radiopacity which might be the reason behind the significant differences observed in the case of cortical as compared to the trabecular and low-density maxillary bone. In the present study, as well, the mean bone density/gray density values obtained in the case of cone beam CT were found to be significantly higher than those obtained with CT, though, the dimensional accuracy achieved with cone beam CT was found comparable with CT.

Similarly, significant differences were observed between the gray density values obtained using cone beam CT and MS-CT with higher mean HU values obtained using cone beam CT (418.06 HU) than MS-CT (313.13 HU) in the study conducted by Silva et al.,[41] while the authors considered the values obtained using cone beam CT as being unreliable when taken as sole absolute values. Pauwels et al.,[42] also, based on their review published in the year 2015, mentioned limited field size, relatively high scattered radiation, and restrictions of the currently used conversion ratios and reconstruction algorithms as the major reasons for the high variability seen in the quantitative gray density values obtained using cone beam CT as against multidetector-CT (MD-CT). The authors, however, indicated a shift in paradigm in the assessment of bone quality from density-based methodologies to more of the structural evaluation of the bone while recommending the possibility of applying structural analytical methods which are commonly used in micro-CT and histology.

In line with this, a recently published study conducted by de Castro et al.[24] proposed a cone beam CT-driven composite osteoporosis index (3D-mandibular osteoporosis index- 3D-MOI) based on data obtained using cone beam CT and DEXA in combination with mandibular cortical width (MCW) and cortical quality evaluation performed on cross-sectional and panoramic reconstructed images to assess bone density in the elderly population including the peri- and post-menopausal females. In another similar study conducted by Naitoh et al.,[43] the authors observed a statistically significant correlation between the bone density/gray density values obtained using cone beam CT, and the BMD obtained using MS-CT, while the authors suggested that the gray density values of mandibular cancellous bone as obtained using cone beam CT could be used to precisely estimate bone density. Similar conclusions were, also, drawn in the study conducted by Aranyarachkul et al.[44] wherein bone density was evaluated using quantitative-CBCT (Q-CBCT) and Q-CT, while additionally, a subjective assessment of bone density was performed using Lekholm and Zarb classification, while the findings of the study suggested that, though, the values obtained using Q-CBCT were significantly higher than the values obtained through Q-CT, the values obtained with Q-CBCT and Q-CT revealed significantly high correlations values (0.92-0.98) in spite of the systematic differences between the two methodologies making the authors conclude in favor of Q-CBCT. The findings obtained in the present study, as well, suggested the Pearson's correlation coefficient between the bone density/gray density values obtained in the case of cone beam CT and those obtained with CT being r = 0.88 in close accordance with the values obtained in the studies conducted by Cassetta et al.[39] and Aranyarachkul et al.[44]

Limitations of the present study

One of the major limitations of the present in-vitro study rested in the in-vitro design of the study using CT and cone beam CT scans of 30 partially or, completely edentulous dry mandibles wherein further studies including in-vivo study designs, and data from both the cortical and cancellous bones can provide highly useful information, and, thus, are indeed exceedingly essential to conclude-on and further validate the results obtained in the present study. Also, there is sporadic evidence of studies in this regard which have largely confined to a comparison of two methods while being based on highly variable methodologies, involving methodologies including quantitative-CT (Q-CT) and CBCT (Q-CBCT), and micro-CT, multislice-CT (MS-CT) and multidetector-CT (MD-CT) as against dual-energy-CBCT (DE-CBCT), and dual-energy x-ray absorptiometry (DEXA). Further research in this direction including a comparison of multiple techniques and methodologies in the same set of samples (in-vitro study design) and patients (in-vivo study designs) is, thus, highly recommended to further validate the findings obtained and draw an analogy between the different methodologies available.

CONCLUSIONS

The mean bone density/gray density values obtained with cone beam CT were found to be significantly higher than the ones derived from CT in the present in-vitro study, though, a linear correlation (r = 0.88) was observed between the bone density values obtained from cone beam CT and CT which can be used to convert the relative values obtained with cone beam CT into absolute values derived with CT.

Ethics approval and consent to participate

The ethical approval for the present was duly sought from the Institutional Ethics and Review Board via. Letter approval no. VDCH-VEWT/IERB/01-46-2021 before the start of the study. It can be added as 2nd sentence in the Materials and Methods section of the manuscript as follows: The objectives and need for the study were approved by the Institutional Ethics and Review Board via. Letter approval no. VDCH-VEWT/IERB/01-46-2021 before the start of the study.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.