Browsing by Subject "X-ray diffraction"

Coherent scatter spectral imaging has been demonstrated as an effective way to classify healthy and malignant breast tissues. Previously in our group, data acquired using sectioned, lumpectomy specimens obtained from surgical pathology have been used to demonstrate the efficacy of this imaging method. Although effective in its current state, the system has not been optimized for use with these types of specimens (i.e. tissue types and thicknesses). Specimens obtained from lumpectomies often vary in thickness (up to 3 mm). The current X-ray tube operating parameters have been considered excessive for these tissues based on heating of the tube’s anode and the unnecessary, high quality of resulting spectra. The purpose of this work was to optimize our spectral imaging system to maintain accurate and consistent results of sectioned lumpectomy specimens while simultaneously maximizing system throughput by reducing the power requirements of the imaging system.

Teflon, adipose breast tissue, and malignant breast tissue were scanned using different combinations of X-ray source parameters (70-125 kVp, 25-500 mAs) to obtain a coherent-scatter diffraction spectrum for each measurement. Cross correlation was performed on the measured spectra to compare their quality against known, ground truth spectra from literature. In addition, a classification algorithm was developed to classify our measured spectra as one of four tissue types (adipose, normal, fibroglandular, and cancer). The locations of the spectral peaks were used to distinguish cancer from adipose and normal (50/50 fibroglandular/adipose) tissue, followed by a weighted cross correlation method used to distinguish cancer from fibroglandular tissue. Classification performance was assessed across all acquisition protocols to evaluate accuracy.

The optimal setting was identified as the minimal power supplied to the X-ray tube that resulted in the highest correlation to the ground truth spectra. The optimal setting was identified at 115 kVp, 100 mAs when using the raw spectra and 95 kVp, 50 mAs after processing the spectra. These settings result in an increase in system efficiency of at least 400% (at 115kVp, 100 mAs) compared to our current system operating protocol. Finally, the optimized system was tested using a new, unknown tumor specimen obtained from a preserved lumpectomy section.

This study successfully demonstrates the optimization of a new coherent scatter spectral imaging system based on classification using cross correlation and a weighted cross correlation method. The efficiency improvement obtained through the work allows for higher system throughput, thereby allowing enhanced data collection with shorter scans or scanning the specimens at higher resolutions.

GaAs1-xBix is a III-V semiconductor alloy which has generated much fundamental scientific interest. In addition, the alloy possesses numerous device-relevant beneficial characteristics. However, the synthesis of this material is very challenging and its properties are not well understood. The focus of this dissertation is to advance the understanding of its synthesis using molecular beam epitaxy (MBE) and, as a result, improve its key as-grown properties that are of great importance to device applications, such as increasing Bi concentration in the alloy and enhancing its optical emission efficiency.

In chapter 3, the discovery of a trade-off between the structural and optical characteristics of GaAs1-xBix , controlled by the degree to which the growth is kinetically-limited, is described. Chapter 4 discusses the exploitation of a growth method that utilizes the spatial distribution of MBE fluxes to facilitate numerous studies of the critical dependence of GaAs1-xBix characteristics on the V/III flux ratio. Chapter 5 describes the results of experiments utilizing vicinal substrates to modify both Bi incorporation and optical emission efficiency of synthesized GaAs1-xBix and enable new understanding of the Bi incorporation mechanism. Specifically, incorporation primarily at A steps, defined as the steps generated by misorienting the GaAs (001) substrate toward the (111)A surfaces, enhances Bi incorporation but reduces optical emission efficiency. Chapter 6 describes the identification of two new signatures in the Raman spectra of GaAs1-xBix that can be used to determine the Bi content and the hole concentration of nominally undoped GaAs1-xBix. Finally, in Chapter 7 the GaAs1-xBix growth using pulsed Ga fluxes is described. The use of pulsed-growth significantly modifies the incorporation of Bi and suggests it is a promising method for widening the GaAs1-xBix MBE growth window enabling improved synthesis control and materials properties.

Although mammography is the gold standard for early screening for breast cancer, there is a need in improving its specificity. X-ray diffraction (XRD) has shown the ability to detect cancer based on its molecular properties; however, most commercial XRD systems focus on very thin targets using very low x-ray energies. To improve the imaging capabilities for thicker targets, we have developed XRD imaging systems capable of running at diagnostic X-ray energies. In this work, we evaluate the performance of our XRD system at two source configurations: 20 keV (mammography) and 60 keV (radiography), to understand and quantify the trade-off of between higher Rayleigh scatter cross section versus reduced penetration depth.

An XRD system was built using a Bremsstrahlung X-ray source, an energy discriminating X-ray detector, and customizable geometry. XRD scans were performed using adipose, fibroglandular, and carcinoma surrogate targets at two mean energies – 20 keV and 60 keV. The 20 keV configuration used a molybdenum filter and 2-mm collimated beam, whereas the 60 keV configuration used a tungsten filter with 1-mm diameter X-ray beam. Each target was scanned 5-10 times to evaluate measurement uncertainty. XRD spectra, normalized to mAs, were extracted from the detected signal and compared against known diffraction data for each material. System performance was evaluated using signal-to-noise ratio (SNR), average-percent-difference (APD) and uncertainty in each measurement.

An analytical calculation was done to test the effects of attenuation on the 20 keV and 60 keV configurations using elastic scatter coefficients and total attenuation coefficients excluding elastic scatter for each energy for breast tissue. Using the mean-free-path and the calculated exponential attenuation, an estimate of the surviving fraction of x-rays that undergo Rayleigh scatter was calculated.

The 20 keV configuration showed 8.25 SNR, 95.96% accuracy, and 1.46% uncertainty. The 60 keV configuration showed 7.05 SNR, 94.72% accuracy, and 0.96% uncertainty. Overall, the 20 keV configuration showed 13.21% improvement in SNR compared to the 60 keV configuration. The analytical estimation of the surviving fraction of x-rays having undergone Rayleigh scatter showed that at the average compressed breast thickness of 4.4 cm – 4.8 cm, the 20 keV system performed on par with the 60 keV system while the surviving fraction of Compton scatter, which is detrimental for XRD analysis, has decreased with energy. Calculating the ratio of the surviving fraction of Rayleigh scattered x-rays to Compton scattered x-rays to estimate the SNR with depth, 20 keV x-rays showed a consistent advantage. Through this, we demonstrated the viability of low-energy XRD imaging for characterizing breast tissues. The 20 keV configuration presents a viable method to characterize breast tissues at energies relevant to mammography, representing a potential method to improve specificity in mammography.