Investigating the Potential for Super-Resolution in Digital Breast Tomosynthesis

Digital breast tomosynthesis (DBT) is an emerging 3D x-ray imaging modality in which tomographic sections of the breast are generated from a limited range of tube angles. Because non-normal x-ray incidence causes the image of an object to be translated in sub-pixel increments with increasing projection angle, it is demonstrated in this work that DBT is capable of super-resolution (i.e., sub-pixel resolution). The feasibility of super-resolution is shown with a commercial DBT system using a bar pattern phantom. In addition, a framework for investigating super-resolution analytically is proposed by calculating the reconstruction profile for a sine input whose frequency is greater than the alias frequency of the detector. To study the frequency spectrum of the reconstruction, its continuous Fourier transform is also calculated. It is shown that the central projection cannot properly resolve frequencies higher than the alias frequency of the detector. Instead, the central projection represents a high frequency signal as if it were a lower frequency signal. The Fourier transform of the central projection is maximized at this lower frequency and has considerable spectral leakage as evidence of aliasing. By contrast, simple backprojection can be used to image high frequencies properly. The Fourier transform of simple backprojection is correctly maximized at the input frequency. Adding filters to the simple backprojection reconstruction smoothens pixilation artifacts, and reduces spectral leakage found in the frequency spectrum. In conclusion, this work demonstrates the feasibility of super-resolution in DBT experimentally and provides a framework for characterizing its presence analytically.