The geometric calibration of cone-beam imaging and delivery systems in radiation therapy

TL;DR

This paper introduces a geometric calibration method for cone-beam imaging systems in radiation therapy, using a simple phantom and a model with ten parameters to achieve precise device geometry description.

Contribution

It presents a novel calibration approach employing a light needle phantom and a comprehensive model to accurately determine system geometry in radiation therapy devices.

Findings

Consistent and reproducible calibration results from multiple scans

Detected differences between clockwise and counterclockwise scan modes

Achieved sub-pixel accuracy in geometry description

Abstract

We propose a method to achieve the geometric calibration of cone-beam imaging and delivery systems in radiation therapy; our approach applies to devices where an X-ray source and a flat-panel detector, facing each other, move in circular orbits around the irradiated object. In order to extract the parameters of the geometry from the data, we use a light needle phantom which is easy to manufacture. A model with ten free parameters (spatial lengths and distortion angles) has been put forth to describe the geometry and the mechanical imperfections of the units being calibrated; a few additional parameters are introduced to account for residual effects (small effects which lie beyond our model). The values of the model parameters are determined from one complete scan of the needle phantom via a robust optimisation scheme. The application of this method to two sets of five counterclockwise (ccw) and five clockwise (cw) scans yielded consistent and reproducible results. A number of differences have been observed between the ccw and cw scans, suggesting a dissimilar behaviour of the devices calibrated in the two rotation modes. The description of the geometry of the devices was achieved at the sub-pixel level.