Low-dose megavoltage cone-beam computed tomography using a novel multi-layer imager (MLI).

Area of Science:

• Medical Imaging Physics
• Radiotherapy Technology
• Detector Development

Background:

• Megavoltage cone-beam CT (MVCBCT) is crucial for image-guided radiotherapy.
• Conventional detectors often require higher doses, potentially limiting imaging frequency.
• Improving detective quantum efficiency (DQE) is key to enhancing low-dose imaging.

Purpose of the Study:

• To investigate the feasibility of low-dose MVCBCT using a novel multi-layer imager (MLI).
• To assess Hounsfield Unit (HU) accuracy, noise, and contrast-to-noise ratio (CNR) of MVCBCT reconstructed with the MLI.
• To compare MLI performance against a standard AS1200 single-layer imager.

Main Methods:

• A four-layer MLI with copper filter/converter, GOS scintillator, and a-Si detector was utilized.
• MVCBCT projections were acquired using 2.5 and 6 MV flattening filter-free (FFF) beams on a Varian TrueBeam LINAC.
• Images were reconstructed with varying projection numbers to achieve different dose levels; HU uniformity, accuracy, noise, and CNR were quantified.

Main Results:

• MLI reconstructions demonstrated excellent HU uniformity (95%-99%) and accurate relative electron density estimation.
• The MLI achieved a 2-4x greater CNR compared to the AS1200 imager.
• The 2.5 MV acquisition with MLI showed the lowest noise and optimal CNR-to-dose balance for low-dose imaging.

Conclusions:

• The novel MLI prototype significantly enhances MVCBCT image quality over the standard AS1200 EPID.
• The MLI improves CNR at substantially lower doses, enabling high-quality MVCBCT at clinically acceptable levels.
• Further optimization, especially for 2.5 MV acquisitions, promises even better image metrics.