Thick phosphor screens for on-line portal imaging

Area of Science:

• Medical Physics
• Radiological Imaging
• Materials Science

Background:

• Megavoltage imaging systems require efficient phosphor screens for high-quality X-ray detection.
• Gadolinium oxysulfide (Gd2O2S) is a promising material for such applications due to its luminescent properties.
• Optimizing phosphor screen parameters is crucial for balancing light output and spatial resolution.

Purpose of the Study:

• To evaluate Gd2O2S phosphor screens of varying thicknesses (250-1000 mg/cm2) for megavoltage imaging.
• To investigate the impact of phosphor thickness and optical back reflectors on imaging performance.
• To determine optimal parameters for light output and spatial resolution in high-energy X-ray systems.

Main Methods:

• Gd2O2S phosphor screens were prepared on brass plates (1-5 mm thick) with and without optical back reflectors.
• Light output and spatial resolution (modulation transfer function - MTF) were measured at 6-MV and 23-MV X-ray energies.
• Empirical relationships were analyzed to describe the observed variations in MTF with phosphor thickness.

Main Results:

• Light output increased linearly with phosphor thickness up to 500 mg/cm2, plateauing at 1000 mg/cm2.
• Spatial resolution (MTF) decreased exponentially with increasing phosphor thickness, reaching a minimum at 750 mg/cm2.
• The relationship between MTF and phosphor thickness followed a predictable empirical pattern.

Conclusions:

• Gd2O2S phosphor screens offer a viable option for megavoltage imaging, with performance dependent on thickness.
• A phosphor thickness of approximately 500 mg/cm2 appears optimal for balancing light output and spatial resolution.
• The established empirical relation aids in predicting and optimizing MTF for specific megavoltage imaging applications.