UC Davis

This dissertation is focused on the development and characterization of a novel x-ray tube prototype for digital breast tomosynthesis (DBT) and the application of this multisource approach to cone beam CT (CBCT) and dedicated breast CT (bCT). The design and testing of a prototype Multi-X-ray-source Array (MXA) for DBT is reported. The MXA is comprised of an array of tungsten filament cathodes with focus cup grid-controlled modulation and a common rotating anode housed in a single vacuum envelope. Prototypes consisting of arrays of three-source elements and eleven-source-elements were fabricated and evaluated. The prototype sources demonstrated focal spot sizes of 0.3 mm at 45 kV with 50 mA. Measured X-ray spectra were consistent with the molybdenum anode employed, and the tube output (air kerma) was between 0.6 mGy/100 mAs at 20 kV and 17 mGy/100 mAs at 45 kV with a distance of 100 cm. HVL measurements ranged from 0.5 mm Al at 30 kV to 0.8 mm Al at 45 kV, and X-ray pulse widths were varied from 20 ms to 110 ms at operating frequencies ultimately to be limited by source turn-on/off times of ~1 ms. Initial results of reconstructed tomographic data were presented.

Multisource configurations were applied to CBCT using phantom imaging and Monte Carlo simulations. Image quality, scatter, and dose were evaluated in both overlapping (large cone angle) and collimated (small cone angle) configurations for CBCT. Four x-ray tube configurations were considered: traditional one source, three source overlapping, six source overlapping, and six sources collimated. Image quality was evaluated on a prototype breast CT system using the following five phantoms: a Defrise phantom, a previously reported CBCT QA phantom (Corgi), a polyethylene cylinder, and two anthropomorphic phantoms (hand and knee). Scatter contamination and radiation dose were evaluated using Monte Carlo simulations of a voxelized polyethylene cylinder.

The modulation of the Defrise phantom disks on average was 2.7X greater for the six-source collimated configuration than the six-source overlapping configuration. The data lost from cone beam artifact (spatial domain) and the null cone (frequency domain) in the overlapping configuration were completely recovered using the collimated configuration. The maximum scatter to primary ratio (SPR) for the overlapping configuration was 0.81 and the maximum SPR for the collimated configuration was 0.26. The average dose and maximum dose was 4X less in the collimated six source configuration when compared with the overlapping configurations. The maximum dose for the overlapping configurations (one, three & six) remained constant, but the average dose for the multisource (three & six source) overlapping configurations increased 25% when compared to the one source configuration. Use of a collimated multisource x-ray tube configuration was shown to provide significant improvements in image quality throughout the cone-beam geometry field-of-view, reduction in scatter contamination, and more efficient use of dose in comparison to both traditional cone-beam CT geometry with a single source and the overlapping multisource configurations.

A phantom was developed to simulate microcalcification lesions and a non-prewhitening-matched filter (NPW) model was applied to evaluate detectability for various technique parameters and cone angle regions in bCT. The detectability of microcalcifications was improved with smaller pixel size (75 µm) and higher dose (6 mGy) when compared to larger pixel size (150 µm) and lower dose (3 mGy). The number of projections (N = 250 or N = 500) did not affect the detectability significantly. The detectability decreased from small to medium cone angle regions and from small to large cone angle regions but was not significantly affected from medium to large cone angle regions. The differences in detectability across cone angle regions was more prominent in the coronal plane than the sagittal and axial planes.