TU-E-217BCD-01: X-Ray Spectra for Breast CT

To study a range of x-ray spectra with regard to their suitability for dedicated breast CT, taking into account realistic tube loading limitations.

A simple theoretical model was used to get a preliminary indication of the best mono-energetic x-ray energy as a function of object size. That model was extended to poly-energetic spectra and used to study a range of object sizes, tube voltages, filter materials, and filter thicknesses. Spectra from IPEM Report 78 were used as input to the model. Initially we have looked at lanthanide filters (Z = 57-60) and tube voltages from 30 to 80 kVp. Outputs from the model included contrast, contrast-to-noise ratio (CNR), dose, dose efficiency (CNR̂2/dose), and tube loading. Dose was estimated as incident minus transmitted energy fluence. We have also started experiments to verify the computational results. Experiments are done using a bench-top cone beam CT and a 14 cm diameter cylindrical PMMA phantom filled with liquid mixtures equivalent to breast tissue of varying glandularity. We use reconstructed data to determine the CNR of a test block representing 100% glandular breast tissue, immersed in the liquid. Air kerma measurements are obtained at the center and periphery of the phantom, and used in the calculation of dose efficiency. Monte Carlo calculations are being done to get a more accurate dose estimate.

For a constant CNR, computational results indicate that as filter thickness increases above 0.4 mm dose efficiency plateaus. However, the power required to maintain a constant CNR becomes prohibitively large for filter thicknesses greater than 0.3 mm.

Spectra generated at tube voltages above 40 kVp are likely to provide the best trade-off between dose efficiency and tube loading. For lanthanide filters, there is little benefit from using thicknesses greater than 0.3 mm.