HU variation with various positions in field of view (FOV) and bit-depth in CT simulation

This study aims to investigate the impact of position-dependent CT number variations and image bit-depth, which determines the range of encodable CT numbers, on relative electron density (RED) conversion, a process essential for accurate radiation dose calculation, for patients with high-Z materials or metal implants. A commercially available phantom containing tissue-equivalent rods and metal implant inserts was scanned using a clinical CT simulator at nine positional offsets (isocenter, ±6.5 cm, and ±10.5 cm in lateral and anterior-posterior directions). CT images were acquired with both 12-bit and 16-bit scanning modes under identical scanning conditions. HU values for each insert were measured across positions and converted to RED using the clinical HU-to-RED calibration curve. HU deviations increased with positional displacement, particularly in high-density materials such as titanium and stainless steel. In 12-bit datasets, HU saturation at 3071 HU led to errors in the RED underestimation, while 16-bit imaging preserved accurate HU values even at off-center positions. Off-center positioning and bit-depth limitations significantly affect HU representation and RED conversion, especially with high-Z materials. Adoption of 16-bit acquisition or manual assignment of electron density values is recommended to prevent HU saturation and further ensure robust dose calculation. These findings highlight the importance of imaging bit depth and patient centering as part of routine radiotherapy quality assurance, particularly for cases involving metallic implants or off-isocenter targets.