The detective quantum efficiency (DQE), expressed as a function of spatial frequency, describes the ability of an
x-ray detector to produce high signal-to-noise ratio (SNR) images. While regulatory and scientific communities
have used the DQE as a primary metric for optimizing detector design, the DQE is rarely used by end users to
ensure high system performance is maintained. Of concern is that image quality varies across different systems
for the same exposures with no current measures available to describe system performance. Therefore, here
we conducted an initial DQE measurement survey of clinical x-ray systems using a DQE-testing instrument to
identify their range of performance. Following laboratory validation, experiments revealed that the DQE of five
different systems under the same exposure level (8.0 μGy) ranged from 0.36 to 0.75 at low spatial frequencies,
and 0.02 to 0.4 at high spatial frequencies (3.5 cycles/mm). Furthermore, the DQE dropped substantially with
decreasing detector exposure by a factor of up to 1.5x in the lowest spatial frequency, and a factor of 10x at
3.5 cycles/mm due to the effect of detector readout noise. It is concluded that DQE specifications in purchasing
decisions, combined with periodic DQE testing, are important factors to ensure patients receive the health
benefits of high-quality images for low x-ray exposures.
The ability to improve visualization of structural information in digital radiography without increasing radiation exposures requires improved image quality across all spatial frequencies, especially at high frequencies. The detective quantum efficiency (DQE) as a function of spatial frequency quantifies image quality given by an x-ray detector. We present a method of increasing DQE at high spatial frequencies by improving the modulation transfer function (MTF) and reducing noise aliasing. The Apodized Aperature Pixel (AAP) design uses a detector with micro-elements to synthesize desired pixels and provide higher DQE than conventional detector designs. A cascaded system analysis (CSA) that incorporates x-ray interactions is used for comparison of the theoretical MTF, noise power spectrum (NPS), and DQE. Signal and noise transfer through the converter material is shown to consist of correlated an uncorrelated terms. The AAP design was shown to improve the DQE of both material types that have predominantly correlated transfer (such as CsI) and predominantly uncorrelated transfer (such as Se). Improvement in the MTF by 50% and the DQE by 100% at the sampling cut-off frequency is obtained when uncorrelated transfer is prevalent through the converter material. Optimizing high-frequency DQE results in improved image contrast and visualization of small structures and fine-detail.
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