Investigation of Computed Tomography Numbers on Multiple Imaging Systems using Single and Multislice Methods

Area of Science:

• Medical Physics
• Radiological Imaging

Background:

• Accurate Hounsfield Unit (HU) values are crucial for CT-based electron density calculations in radiotherapy.
• Variations in HU can arise from different imaging techniques and acquisition methods, potentially affecting treatment planning.

Purpose of the Study:

• To evaluate Hounsfield value variations using single and multi-slice methods across fan-beam CT (FCT), linac-based cone-beam CT (CBCT), and Icon-CBCT.
• To assess the impact of these variations on dose calculations.

Main Methods:

• Scanned Gammex and Advanced Electron Density (AED) phantoms using Toshiba CT, five linac-based CBCT systems, and Leksell Gamma Knife Icon.
• Developed in-house MATLAB software to analyze HU statistics and longitudinal trends.
• Assessed dosimetric changes using a CIRS Model 605 Radiosurgery Head Phantom.

Main Results:

• FCT showed minimal HU variation (± 3 HU) along the long axis, consistent across clinical protocols.
• Linac-based CBCTs exhibited insignificant variation among themselves, but notable HU deviations were observed in specific cases (e.g., water insert).
• Gamma Knife CBCT demonstrated the most significant HU variation along the long axis, with dose differences up to 1 Gy compared to CT.

Conclusions:

• Single-slice HU calibration is sufficient for FCT due to minimal variation.
• CBCT systems, particularly Gamma Knife, show significant HU variations that can impact dose calculations.
• Multiple slice assessment of Hounsfield values is recommended for CBCT-based dose calculations.