Improving image quality and reducing dose with 2.5 MV diamond target volume-of-interest cone beam CT imaging

PURPOSE

Over the past decades, continuous efforts have been made to improve megavoltage (MV) image quality versus dose characteristics, including the implementation of low atomic number (Z) targets in MV beamlines and the development of more efficient detectors. Recently, a diamond target beam within a commercial radiotherapy treatment platform demonstrated improved planar contrast-to-noise-ratio (CNR) per unit dose using a novel 2.5 MV sintered diamond target beam, which enabled image acquisition on the order of mGy. The present work assesses cone beam CT (CBCT) image quality characteristics for the novel 2.5 MV diamond target beam and the effects of volume-of-interest (VOI) collimation on the image quality and imaging dose distribution.

METHODS

A sintered diamond target was incorporated into the target arm of the linear accelerator, replacing the 2.5 MV commercial copper imaging target. CBCT image quality was evaluated against the commercial imaging beam with regard to spatial resolution and CNR versus dose. In addition to full-field acquisitions, we investigated VOI techniques that collimate the imaging beam to preselected anatomy, to determine potential image quality improvements and dose sparing capacity. Using an anthropomorphic phantom, VOI regions were defined to encompass the maxillary and ethmoid sinuses and ranged in dimension from 3 cm to 4.85 cm equivalent radius. The MLC was fit to each VOI structure throughout a full CBCT arc and the corresponding MLC sequences were produced as XML scripts for acquisition. Calibrated radiochromic film was used in phantom to measure cumulative axial dose distributions during each CBCT acquisition.

RESULTS

In full-field CBCT, the 2.5 MV diamond target beam demonstrated improved CNR versus dose compared to the commercial imaging beam, by factors of up to 1.7. The calculated modulation transfer function (MTF) displayed an increase of nearly 30% in f₅₀ for the 2.5 MV diamond target beam compared to the commercial beam. Using VOI techniques, CNR increased monotonically as a function of equivalent radius at the bone-tissue interface. At the bone-sinus interface, the CNR for the full-field case was slightly decreased compared to the largest VOI case. Imaging dose in the anteroposterior direction increased with increasing VOI equivalent radius.

CONCLUSION

The novel 2.5 MV sintered diamond target beam presents a simple modification to the commercial imaging beam which provides improved image quality in full-field CBCT and the potential for simultaneous dose sparing and CNR improvement at high-contrast interfaces using VOI acquisition techniques.

Region-of-interest intra-arc MV imaging to facilitate sub-mm positional accuracy with minimal imaging dose during treatment deliveries of small cranial lesions

Purpose

To automate the generation of region‐of‐interest (ROI) apertures for use with megavoltage imaging for online positional corrections during cranial stereotactic radiosurgery.

Materials and methods

Digitally reconstructed radiographs (DRRs) were created for a 3D‐printed skull phantom at 5 degree gantry angle increments for a three‐arc beam arrangement. At each angle, 3000 random rectangular apertures were generated, and 100 shifts on a grid were applied to the anatomy within the frame. For all shifts, the mutual information (MI) between the shifted and unshifted DRR was calculated to derive an average MI gradient. The top 10% of apertures that minimized registration errors were overlaid and discretely thresholded to generate imaging plans. Imaging was acquired with the skull while implementing simulated patient motion on a linac. Control point‐specific couch motions were derived to align the skull to its planned positioning.

Results

Apertures with a range of repositioning errors less than 0.1 mm possessed a 42% larger average MI gradient when compared with apertures with a range greater than 1 mm. Dose calculations with Monte Carlo exhibited an 84% reduction in the dose received by 50% of the skull with the 50% thresholded plan when compared to a constant 22 × 22 cm² imaging plan. For all different imaging plans (with and without motion), the calculated median 3D‐errors with respect to the tracking of a metal‐BB fiducial positioned at isocenter in the skull were sub‐mm except for the 80% thresholded plan.

Conclusions

Sub‐mm positional errors are achievable with couch motions derived from control point–specific ROI imaging. Smaller apertures that conform to an anatomical ROI can be utilized to minimize the imaging dose incurred at the expense of larger errors.