SNR and noise measurements for medical imaging. II. Application to fluoroscopic X-ray equipment

Abstract

This paper reports on signal-to-noise ratio (SNR) and noise-power-spectrum (NPS) measurements on a fluoroscopic X-ray unit by the methods of an earlier publication. It is shown that image noise in fluoroscopy can be considered to be multivariate normal and stationary for small image areas. Therefore, the ideal observer of low-contrast details is the prewhitening matched filter. This observer is well approximated by the DC-suppressing observer for signals whose spatial frequencies are in a range where the NPS is reasonably constant. In this paper fluoroscopic image noise is characterized by the spatio-temporal NPS, and the noise lag factor, a descriptor of the temporal blurring of noise, is introduced. In the fluoroscopic system measured, lag is seen to vary depending on the light input to the video camera and results in noise lag factors in the range 4.1-9.9 s-1. The detectability of an iodine contrast material detail in 20 cm and 30 cm thick acrylic phantoms is studied and SNR measurements are used for assessing the dose-to-information conversion efficiency, SNR2/D, of various fluoroscopic techniques. When the X-ray field covers the whole image intensifier input field, the use of an anti-scatter grid results to an improvement of dose efficiency by a factor of 2-3 as compared to fluoroscopy without the grid. The dose-to-information conversion efficiency for small X-ray fields is seen to be high, and the presence of the anti-scatter grid is not important in this case. When aluminum filtration is employed the optimum X-ray tube voltage is 55-60 kV for the 20 cm phantom, and 60-70 kV for the 30 cm phantom. Primary-beam filtration by gadolinium (140 mg cm-2 Gd2O2S) in addition to aluminium (2.5 mm Al) results in a dose efficiency improvement of up to 40%, depending on other imaging factors.