Determination of the modulation transfer function using the edge method: influence of scattered radiation

Area of Science:

• Medical Imaging Physics
• Digital Radiography Systems

Background:

• The edge method is a popular technique for measuring the modulation transfer function (MTF) in digital imaging systems.
• Commonly used edge test devices made of thin metal sheets can be semitransparent to X-rays, leading to scattered radiation.

Purpose of the Study:

• To investigate the effect of scattered radiation from edge test devices on MTF measurements.
• To develop a theoretical model and experimentally validate the impact of scatter on MTF and detective quantum efficiency (DQE).

Main Methods:

• Theoretical analysis assuming an ideal detector.
• Experimental investigation using a CsI-based digital detector.
• Evaluation across various general radiography and chest X-ray beam qualities (RQA3, RQA5, RQA7, RQA9).

Main Results:

• Scattered radiation increases the measured MTF for all spatial frequencies above 0 mm⁻¹.
• A theoretical model predicts the maximum error as the ratio of scattered to attenuated radiation (S/A).
• Experimental and theoretical results show good agreement for general radiography, but discrepancies arise for chest X-ray quality due to detector energy response.

Conclusions:

• The MTF error can reach 18% for a 0.25 mm lead edge under RQA9, potentially causing over 36% error in DQE.
• Fully absorbing edge materials are recommended for accurate MTF determination using the edge method.