Reproducibility and Accuracy of HU Assignments In CT Imaging
Abstract
Purpose
In Radiation Oncology, a crucial step for accurate patient treatment is converting Hounsfield Units (HU) from Computed Tomography (CT) scans into mass or electron density for dose calculations. However, several factors – like scan energy, metal artifact reduction (MAR) algorithm, and reconstruction kernel – can influence these values.
Methods
We compared HU values for 16 different density rods within a Gammex 467 tissue characterization phantom across multiple institutions in the Pacific Northwest. Our comparative analysis included both CT simulators and Cone-Beam CT (CBCT) systems housed on linear accelerators. CT and CBCT scans were examined with five parameters: MAR presence, scan energy, institution, machine, and scan protocol.
Results
Using a Python program, we generated a parallel coordinate plot to compare HU values for each rod as a function of the scan parameters. We also plotted relative electron density against HU values to investigate deviations from literature values, which were most pronounced for high-density (HD) structures with HU values far from 0. The magnitude of these deviations varied based on the specific scan parameters.
Conclusion
Our preliminary findings suggest that while HU assignments are repeatable across CT and CBCT scans, deviations from literature values are also observed, particularly at higher densities. The relative balance between reproducibility and accuracy appears parameter dependent. These findings highlight the importance of local HU-to-density calibration and cautious use of HU values for high-density tissues in treatment planning. Further investigation with a larger dataset is recommended to confirm these trends.