John M. Sandrik

Modeling the x-ray energy characteristics of DQE for full-field digital mammography

The modulation transfer function and detective quantum efficiency are modeled for a Full Field Digital Mammography detector constructed with a CsI scintillator deposited on an amorphous silicon active matrix array. The model is evaluated against experimental measurements using different exposure levels, x-ray tube voltages, target composition and beam filtrations as well as varying thicknesses and compositions of filtration materials placed in the path between the tube and detector. Available x-ray tube emission spectrum models were evaluated by comparison against the measured transmission through aluminum. The observed variation of DQE at zero spatial frequency among different target/filter conditions, acrylic filtration thicknesses and kVp is well characterized by a x-ray model. This variation is largely accounted for by just two effects -- the attenuation of x-rays through the detector enclosure and the stopping power of x-rays in the CsI layer. Additional considerations such as the Lubberts effect were included in the analysis in order to match the measured DQE(k) as a function of spatial frequency, k. The pixel aperture and light channeling through the scintillator shape the MTF which acts favorably to avoid aliasing due to digital sampling.

Standardization of image quality measurements of medical x-ray image intensifier systems

Medical image quality is ultimately gauged by the image’s contribution to an accurate diagnosis. Meaningful comparison of the key elements of diagnostic image quality requires that consistent methods be used to measure them. A NEMA subcommittee is drafting standards to evaluate an x-ray image intensifier (XRII) system—the XRII; lenses linking the XRII output to a camera; video, photofluorographic, and cine cameras; and a video monitor. Image quality parameters include contrast ratio, detective quantum efficiency (DQE), fixed pattern noise, gain, image signal uniformity, limiting spatial resolution, modulation transfer function (MTF), noise power spectrum, radial distortion, signal-to-noise ratio, and visible entrance field size. An IEC Working Group is dealing with the XRII itself and first addressed MTF and DQE. This paper reviews the work of these two groups developing standards for measuring the image quality parameters of medical XRIIs and systems.