We want to thank the authors of this letter to the editor for their comments regarding our article. We are fully aware of the limitation of using Hounsfield units (HU) to predict bone mineral density (BMD) from cone-beam computed tomography (CBCT) images; this was the reason that we designed this longitudinal study and only reported the relative HUs or HU changes. We chose HUs because they are commonly associated with BMD.
In general, HUs are linked to gray values through linear transformation. In CBCT images, the absolute values of both gray values and HUs are not very reliable because they are affected by many parameters, as indicated in the authors’ referenced article. The parameters include object difference, beam hardening, scattered radiation, discontinuity-related effect, differences between the CBCT device and field of view (FOV) changes, and size and position changes. Our study design ensured that these parameters remained the same in this longitudinal study, meaning that the changes of HUs or gray values are reliable, although their absolute values may not be reliable. Some of the uncontrollable errors were handled statistically. Thus, the results of the study were reliable, and we acknowledged this.
Gray values and HUs are linearly related. We chose HUs because previous studies linked HUs to BMD, which was the physical parameter we were investigating. We used a custom-designed phantom with standard BMD rods to check whether the zeros of the HUs and BMD agreed. The zeros were very close; this was the basis for calculating the percentages of HU changes and using them to investigate BMD changes. Therefore, we used HUs instead of gray values in this study.