Fundamental x-ray interaction limits in diagnostic imaging detectors: frequency-dependent Swank noise

Area of Science:

• Medical Physics
• Radiological Imaging
• Materials Science

Background:

• Detective quantum efficiency (DQE) is a critical measure of digital x-ray detector performance.
• Optimizing DQE requires understanding signal and noise correlations introduced by x-ray interactions within converter materials.
• Existing benchmarks may not fully account for frequency-dependent factors and complex interactions.

Purpose of the Study:

• To determine a frequency-dependent x-ray Swank factor for common converter materials.
• To establish a theoretical performance benchmark for detective quantum efficiency (DQE) in digital x-ray detectors.
• To provide guidance on material selection for different x-ray imaging applications based on spatial frequency.

Main Methods:

• Monte Carlo analysis to calculate the frequency-dependent x-ray Swank factor.
• Evaluation of four converter materials: amorphous silicon (a-Si), amorphous selenium (a-Se), cesium iodide (CsI), and lead iodide (PbI2).
• Analysis across incident photon energies from 10 to 150 keV and various converter thicknesses.

Main Results:

• The x-ray Swank factor was calculated for a-Si, a-Se, CsI, and PbI2, considering x-ray interaction modulation transfer function and noise power spectrum.
• The factor is largely insensitive to converter thickness for quantum efficiency > 0.5, with values for thick converters (QE=0.99) being broadly applicable.
• A simple expression for x-ray interaction DQE was derived, showing good agreement with published data.

Conclusions:

• The calculated x-ray Swank factor serves as an ideal performance benchmark for digital x-ray detectors.
• Material selection recommendations: a-Si for low-energy (mammography), a-Se promising below 100 keV.
• Increased quantum efficiency near K-edges in CsI and PbI2 is offset by reduced Swank factor at high spatial frequencies, impacting high-resolution imaging.