Introduction: In CT imaging, the Hounsfield unit (HU) is proportional to the degree of X-ray attenuation in tissue. In a radiographic device (3D C-arm), the degree of X-ray attenuation is represented by shades of gray (Gray Scale Value - GSV). Demonstrating a correlation between HU and GSV would enable broader and easier use of radiographic imaging in fields beyond biomedicine and healthcare, as well as the application of quantitative research methods. Objective: The purpose of this study was to investigate the relationship between GSV on a C-arm and HU on a CT scanner. Methods: In this study, a CT phantom was scanned with both a C-arm and a CT scanner. The meta-data from the resulting DICOM images were used to calculate the GSV values on the C-arm and HU values on the CT scanner. The data obtained were analyzed using statistical software to explore the relationship between GSV and HU. Results: The results obtained using the MedCalc Statistical Software show that the measured and displayed correlation coefficient (r) of r = 0.185 between the C-arm at 115 kV and the CT scanner at 110 kV is described as insignificant and positive. A weak positive correlation (r = 0.450) was measured and displayed between the CT scanner and the C-arm with the tube power of both devices set to 80 kV. A weak positive correlation (r = 0.434) was obtained from the measurements between the CT scanner at 110 kV and the C-arm at 80 kV. Conclusion: Based on the results of this study and the review of available literature, it can be concluded that there is a relationship between GSV on the C-arm and HU on the CT scanner. However, the results of this study are insufficient to validate the gray scale on the C-arm in relation to HU on the CT scanner.