The underlying mechanism in projection radiography as well as in computed tomography (CT) is
the accumulative attenuation of a pencil x-ray beam along a straight line. However, when a
portion of photons is deviated from their original path by scattering, it is not valid to assume that
these photons are the survival photons along the lines connecting the x-ray source and the
individual locations where they are detected. Since these photons do not carry the correct spatial
information, the final image is contaminated. Researchers are seeking techniques to reduce
scattering, and hence, improve image quality, by scatter compensation. Previously, we presented
a post-acquisition scatter compensation technique based on an underlying statistical model. We
used the Poisson noise model, which assumed that the signals in the detector individually
followed the Poisson process. Since most x-ray detectors are energy integrating rather than
photon counting, the Poisson noise model can be improved by taking this property into account.
In this study, we developed a Gaussian noise model by the matching-of-the-first-two-moments
method. The Maximum Likelihood Estimator of the scatter-free image was derived via the
expectation maximization (EM) technique. The maximum a posteriori estimate was also
calculated. The Gaussian noise model was preliminarily evaluated on a full-field digital
This study evaluated the physical performance of a selenium-based direct full-field digital mammography prototype detector (Siemens Mammomat NovationDR), including the pixel value vs. exposure linearity, the modulation transfer function (MTF), the normalized noise power spectrum (NNPS), and the detective quantum efficiency (DQE). The current detector is the same model which received an approvable letter from FDA for release to the US market. The results of the current prototype are compared to those of an earlier prototype. Two IEC standard beam qualities (RQA-M2: Mo/Mo, 28 kVp, 2 mm Al; RQA-M4: Mo/Mo, 35 kVp, 2 mm Al) and two additional beam qualities (MW2: W/Rh, 28 kVp, 2 mm Al; MW4: W/Rh, 35 kVp, 2 mm Al) were investigated. To calculate the modulation transfer function (MTF), a 0.1 mm Pt-Ir edge was imaged at each beam quality. Detector pixel values responded linearly against exposure values (R2 0.999). As before, above 6 cycles/mm Mo/Mo MTF was slightly higher along the chest-nipple axis compared to the left-right axis. MTF was comparable to the previously reported prototype, with slightly reduced resolution. The DQE peaks ranged from 0.71 for 3.31 μC/kg (12.83 mR) to 0.4 for 0.48 μC/kg (1.86 mR) at 1.75 cycles/mm for Mo/Mo at 28 kVp. The DQE range for W/Rh at 28 kVP was 0.81 at 2.03 μC/kg (7.87 mR) to 0.50 at 0.50 μC/kg (1.94 mR) at 1 cycle/mm. NNPS tended to increase with greater exposures, while all exposures had a significant low-frequency component. Bloom and detector edge artifacts observed previously were no longer present in this prototype. The new detector shows marked noise improvement, with slightly reduced resolution. There remain artifacts due to imperfect gain calibration, but at a reduced magnitude compared to a prototype detector.
In this study, the beam stop technique was applied to obtain the scatter fraction values for an anthropomorphic breast phantom on a flat-panel full field mammography system. The phantom was equivalent to a compressed breast of 5 cm thickness with 50% glandular tissue content. The images were acquired at 28kVp with Mo/Mo target/filter combination and multiple mAs values with or without an anti-scatter grid. The one-dimensional and two-dimensional scatter fraction profiles of the breast phantom without a grid were plotted. The effect of mAs on scatter fraction calculation was investigated. It was found that in order to get a reliable measurement of scatter fraction, the mAs had to be above about 40mAs without a grid and 160mAs with a grid. In addition, the SNR values at 28kVp and 80mAs (AEC level for the phantom using the same imaging technique on a screen/film system) with and without a grid were compared with each other. The SNR with a grid was slightly smaller than that without a grid. The SNR improvement factor (KSNR ) defined as the ratio of SNR without the grid to SNR with the grid was 0.976. The grid had the primary and scatter transmissions of 73% and 13% respectively.
Here we report on the development of a new molecular imaging technique using inelastic scattering of fast neutrons. Earlier studies demonstrated a significant difference in trace element concentrations between benign and malignant tissue for several cancers including breast, lung, and colon. Unfortunately, the measurement techniques were not compatible with living organisms and this discovery did not translate into diagnostic techniques. Recently we have developed a tomographic approach to measuring the trace element concentrations using neutrons to stimulate characteristic gamma emission from atomic nuclei in the body. Spatial projections of the emitted energy spectra allow tomographic image reconstruction of the elemental concentrations. In preliminary experiments, spectra have been acquired using a 7.5MeV neutron beam incident on several multi-element phantoms. These experiments demonstrate our ability to determine the presence of Oxygen, Carbon, Copper, Iron, and Calcium. We describe the experimental technique and present acquired spectra.