Segmented phosphors: MEMS-based high quantum efficiency detectors for megavoltage x-ray imaging

Area of Science:

• Medical Physics
• Radiological Imaging Technology
• Detector Physics

Background:

• Current active matrix flat panel imager (AMFPI) electronic portal imaging devices (EPIDs) face a trade-off between X-ray quantum efficiency (QE) and spatial resolution.
• This limitation significantly impacts the imaging performance of existing EPID technologies.

Purpose of the Study:

• To present a novel detector design for indirect detection-based AMFPI EPIDs that overcomes the QE-spatial resolution trade-off.
• To investigate the performance of detectors fabricated using micro-electro-mechanical system (MEMS) techniques with SU-8 photoresist and scintillating phosphors.

Main Methods:

• Fabrication of optically isolated SU-8 cells using MEMS techniques, dimensionally matched to AMFPI pixels and packed with scintillating phosphor.
• Empirical evaluation of prototype detectors measuring X-ray sensitivity, modulation transfer function (MTF), and noise power spectrum (NPS) under 6 MV radiotherapy conditions.
• Monte Carlo simulations for theoretical calculation of quantum efficiency (QE) and inherent spatial resolution.

Main Results:

• SU-8 detectors demonstrated up to 3 times higher QE compared to Lanex Fast-B phosphor screens, with comparable or superior spatial resolution.
• Early prototypes exhibited significantly lower detective quantum efficiency (DQE) than expected due to high optical Swank noise.
• DQE performance was comparable to Lanex Fast-B at zero spatial frequency but lower at higher frequencies.

Conclusions:

• The novel SU-8 detectors show potential for improved QE in EPIDs, addressing a critical limitation of current technologies.
• Strategies for reducing Swank noise are crucial for realizing the full DQE potential of these MEMS-based detectors.
• Further development focusing on noise reduction is necessary to enhance the clinical applicability of these advanced imaging detectors.