Evaluation of novel detector readout modes for MV scatter reduction on kV images.

Background: There is increased interest in concurrent kilovoltage (kV) imaging during megavoltage (MV) irradiation as a means of monitoring intra-fraction motion. However, scatter introduced by the MV beam degrades kV image quality, potentially making tumor visualization more challenging.

Purpose: To implement novel imager readout modes on the on-board imager (OBI) of a Varian TrueBeam to reduce the effect of MV scatter on the kV detector.

Methods: New readout strategies (vertical 3 × 2 binning, vertical region of interest, kV pulse drop functionality) were incorporated to achieve high frame rates. A series of 120 kV images were acquired on the OBI while simultaneously irradiating a phantom with an MV beam (6xFFF and 10xFFF) with field sizes ranging from 2 × 2 cm² to 6 × 6 cm² and a 10 × 10 cm² field for reference. Two datasets, with and without MV beam delivery, were collected at four frame rates varying from 7 to 45 fps, at four gantry angles. The effect of the frame rates was evaluated by the percent change with respect to the kV-only data within selected regions on the images. Image quality was evaluated using contrast-to-noise ratio (CNR).

Results: The amount of MV scatter was directly proportional to the square field size and the detector integration time. At their respective maximum dose rates, 6xFFF and 10xFFF produced comparable amounts of MV scatter for considered frame rates and field sizes. Increasing the frame rates from 7 to 45 fps decreased MV scatter by up to 87%. Over the same frame rates, the CNR was improved by 20% for the 2 × 2 cm² field up to 85% for the 10 × 10 cm² field. These findings were consistent for both energies considered.

Conclusions: MV scatter onto the kV detector can be significantly reduced with the implementation of the new detector readout modes. These strategies increase the CNR and thus improve the image quality.