Proceedings Article | 13 March 2009
KEYWORDS: Modulation transfer functions, Imaging systems, Spatial frequencies, Sensors, X-rays, Chest imaging, Radiography, Quantum efficiency, Scatter measurement, Signal attenuation
In digital radiography, conventional DQE evaluations are performed under idealized conditions
that do not reflect typical clinical operating conditions. For this reason, we have developed
and evaluated an experimental methodology for measuring theeffective detective quantum
efficiency (eDQE) of digital radiographic systems and its utility in chest imaging applications.To
emulate the attenuation and scatter properties of the human thorax across a range of sizes,
the study employed pediatric and adult geometric chest imaging phantoms designed for use in
the FDA/CDRH Nationwide Evaluation of X-Ray Trends (NEXT) program and a third phantom
configuration designed to represent the bariatric population. The MTF for each phantom
configuration was measured using images of an opaque edge device placed at the nominal
surface of each phantom and at a common reference point. For each phantom, the NNPS was
measured in a uniform region within the phantom image acquired at an exposure level
determined from a prior phototimed acquisition. Scatter measurements were made using a
beam-stop technique. These quantities were used along with measures of phantom
attenuation and estimates of x-ray flux, to compute the eDQE at the beam-entrance surface of
the phantoms, reflecting the presence of scatter, grid, magnification, and focal spot blur. The
MTF results showed notable degradation due to focal spot blurring enhanced by geometric
magnification, with increasing phantom size. Measured scatter fractions were 33%, 34% and
46% for the pediatric, adult, and bariatric phantoms, respectively. Correspondingly, the
measured narrow beam transmission fractions were 16%, 9%, and 3%. The eDQE results for the
pediatric and adult phantoms correlate well at low spatial frequencies but show degradation in
the eDQE at increasing spatial frequencies for the adult phantom in comparison to the pediatric
phantom. The results for the bariatric configuration showed a marked decrease in eDQE in
comparison to the adult phantom results, across all spatial frequencies, attributable to the
combined differences in geometric magnification, and scatter. The eDQE metric has been
demonstrated to be sensitive to body habitus suggesting its usefulness in assessing system
response across a range of chest sizes and potentially making it a useful factor in protocol
assessment and optimization.
