Quantification of breast density using dual-energy mammography with liquid phantom calibration

Breast density is a widely recognized potential risk factor for breast cancer. However, accurate quantification of breast density is a challenging task in mammography. The current use of plastic breast-equivalent phantoms for calibration provides limited accuracy in dual-energy mammography due to the chemical composition of the phantom. We implemented a breast-equivalent liquid phantom for dual-energy calibration in order to improve the accuracy of breast density measurement. To design these phantoms, three liquid compounds were chosen: water, isopropyl alcohol, and glycerol. Chemical compositions of glandular and adipose tissues, obtained from NIST database, were used as reference materials. Dual-energy signal of the liquid phantom at different breast densities (0% to 100%) and thicknesses (1 to 8 cm) were simulated. Glandular and adipose tissue thicknesses were estimated from a higher order polynomial of the signals. Our results indicated that the linear attenuation coefficients of the breast-equivalent liquid phantoms match those of the target material. Comparison between measured and known breast density data shows a linear correlation with a slope close to 1 and a non-zero intercept of 7%, while plastic phantoms showed a slope of 0.6 and a non-zero intercept of 8%. Breast density results derived from the liquid calibration phantoms showed higher accuracy than those derived from the plastic phantoms for different breast thicknesses and various tube voltages. We performed experimental phantom studies using liquid phantoms and then compared the computed breast density with those obtained using a bovine tissue model. The experimental data and the known values were in good correlation with a slope close to 1 (∼1.1). In conclusion, our results indicate that liquid phantoms are a reliable alternative for calibration in dual-energy mammography and better reproduce the chemical properties of the target material

Quantitative contrast‐enhanced spectral mammography based on photon‐counting detectors: A feasibility study

PurposeTo investigate the feasibility of accurate quantification of iodine mass thickness in contrast-enhanced spectral mammography.Materials and methodsA computer simulation model was developed to evaluate the performance of a photon-counting spectral mammography system in the application of contrast-enhanced spectral mammography. A figure-of-merit (FOM), which was defined as the decomposed iodine signal-to-noise ratio (SNR) with respect to the square root of the mean glandular dose (MGD), was chosen to optimize the imaging parameters, in terms of beam energy, splitting energy, and prefiltrations for breasts of various thicknesses and densities. Experimental phantom studies were also performed using a beam energy of 40 kVp and a splitting energy of 34 keV with 3 mm Al prefiltration. A two-step calibration method was investigated to quantify the iodine mass thickness, and was validated using phantoms composed of a mixture of glandular and adipose materials, for various breast thicknesses and densities. Finally, the traditional dual-energy log-weighted subtraction method was also studied as a comparison. The measured iodine signal from both methods was compared to the known value to characterize the quantification accuracy and precision.ResultsThe optimal imaging parameters, which lead to the highest FOM, were found at a beam energy between 42 and 46 kVp with a splitting energy at 34 keV. The optimal tube voltage decreased as the breast thickness or the Al prefiltration increased. The proposed quantification method was able to measure iodine mass thickness on phantoms of various thicknesses and densities with high accuracy. The root-mean-square (RMS) error for cm-scale lesion phantoms was estimated to be 0.20 mg/cm2 . The precision of the technique, characterized by the standard deviation of the measurements, was estimated to be 0.18 mg/cm2 . The traditional weighted subtraction method also predicted a linear correlation between the measured signal and the known iodine mass thickness. However, the correlation slope and offset values were strongly dependent on the total breast thickness and density.ConclusionThe results of this study suggest that iodine mass thickness for cm-scale lesions can be accurately quantified with contrast-enhanced spectral mammography. The quantitative information can potentially improve the differential power for malignancy

Surface and interface studies of organic semiconductors

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2010.In recent decades, research and development of organic based semiconductor devices have attracted intense interest. One of the most essential elements is the understanding of the electronic structures at various interfaces involved in these devices, as the interface properties control many of the critical electronic processes. However, the conventional theories developed for inorganic semiconductors are often adopted without further experimental confirmation in the design of innovative organic electronic devices. It is, thus, necessary to study the electronic properties of organic semiconductors with surface analytical tools, in order to improve our understanding of the fundamental mechanism involved in the interface formation. This thesis covers experimental investigations on some of the most interesting topics raised in the recent development of organic electronic devices. The intent of this thesis is to reveal the physical processes at the interface and their contributions to the device performance with photoemission and inverse photoemission investigations on the evolution of the occupied and unoccupied electronic structures. The topics include alkali metal doping, insertion layers, spin injection and organic single crystal studies. The electronic structure modification induced by alkali metal doping in tris-(8-hydroxyquinoline) aluminum (Alq) and copper phthalocyanine (CuPc) will be discussed. Based on the experimental observations, I propose a two-stage model to describe the doping effect in organic materials, which differs significantly from the classical theories used for inorganic semiconductors. I investigate the electronic structure of a number of insertion layers used in organic electronic devices, and the mechanisms of the induced performance improvement are discussed based on the observed interface properties. Next, I examine the spin injection and dynamics for organic thin films. Efficient spin injection for the hot electrons across the interface is demonstrated with spin and time-resolved two photon photoemission (STR-2PPE). Finally, I describe my studies about a particularly interesting organic material -- rubrene. The band structure measurement of rubrene single crystal samples is presented with angle-resolved photoemission spectroscopy (AR-PES). The energy level alignment at the interfaces between the rubrene thin film and various metal substrates and the morphology of the amorphous films prepared under various growth conditions are also discussed