New Formula for Calculating the Hounsfield Unit in CT Scan of Hard Tissue from Grey Scales in CBCT

HU is a standard numbering system in CT, which is proportional to x-ray attenuation and an indicator of relative tissue density. In CBCT, the x-ray  attenuation degree is demonstrated with the grayscale. The present study aimed to  determine the correlation between the grayscale in CBCT and HU (Hounsfield Unit) in MDCT.

This descriptive-analytical study was approved under the ethics code of IR.GUMS.REC.1398.451. A human dry mandible was immersed in a transparent cylindrical container to simulate soft tissue attenuation. The sample was scanned at three separate imaging centers using three CBCT units with the same brand. The scans were carried out once with standard irradiation conditions and once with high-resolution conditions. The mandible was scanned with a CT scan unit. The grayscale and HU of the enamel, dentin, cortical, and spongy bone were evaluated and compared with each other. Pearson’s correlation coefficient was used for data analysis at a significance level of 5% (P<0.05).

In two standard and high-resolution conditions, there was a significant correlation (P<0.001) between the grayscale and HU. Given the positive values of the mean differences, it could be concluded that the gray level in CBCT has a positive correlation with HU in MDCT.

The grayscale in CBCT and HU in MDCT had a positive correlation, and according to this study, it is possible to calculate the HU from the gray level.