Penalty on the detective quantum efficiency from off-axis incident x rays

George Hajdok; Ian A. Cunningham

doi:10.1117/12.535933

6 May 2004 Penalty on the detective quantum efficiency from off-axis incident x rays

George Hajdok, Ian A. Cunningham

Proceedings Volume 5368, Medical Imaging 2004: Physics of Medical Imaging; (2004) https://doi.org/10.1117/12.535933
Event: Medical Imaging 2004, 2004, San Diego, California, United States

Abstract

An often neglected assumption related to detector performance metrics such as the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) is that they only apply to a small region around the centre of an x-ray image. In the periphery of an image, image formation is from obliquely incident x rays. These off-axis x rays will introduce an additional degrading effect on the above detector performance metrics. In our study, we use Monte Carlo simulations to quantify the effects of off-axis radiation on the MTF, NPS, and DQE on common diagnostic x-ray detectors. In our simulations, we vary the incident angle of x rays between 0° and 12°, which is a typical range of divergence in diagnostic x-ray imaging. In the case of amorphous selenium, our results show that off-axis incident x rays degrade the MTF above 5 cycles/mm with increasing severity at higher incident angles and x-ray energy, and more importantly has very little effect on the NPS. Hence, the impact is more severe on the DQE due to the MTF squared dependency. For an incident x-ray angle of 12° (~13 cm from central axis or chest wall in mammography), the DQE falls to 50% of its initial value at 10 and 7 cycles/mm for x-ray energies of 20 and 40 keV, respectively. This loss of signal-to-noise ratio may be most significant near the skin line in mammography studies.

Citation Download Citation

George Hajdok and Ian A. Cunningham "Penalty on the detective quantum efficiency from off-axis incident x rays", Proc. SPIE 5368, Medical Imaging 2004: Physics of Medical Imaging, (6 May 2004); https://doi.org/10.1117/12.535933

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