Evaluation of a photon-counting breast tomosynthesis imaging system

Digital breast tomosynthesis promises solutions to many of the problems associated with projection mammography, including elimination of artifactual densities due to the superposition of normal tissues and increasing the conspicuity of true lesions that would otherwise be masked by superimposed normal tissue. We have investigated tomosynthesis using a digital camera containing 48 photon counting, orientation sensitive, linear detectors which are precisely aligned with the focal spot of the x-ray source. The x-ray source and the digital detectors are scanned in a continuous motion across the object (patient), each linear detector collecting an image at a distinct angle. A preliminary assessment of tomosynthesis image quality has been performed with both qualitative and quantitative methods. Measured values of MTF and NPS appear concordant with theoretical values. The MTF in the scanning direction is dominated by scanning unsharpness and geometric factors, while the NPS is white. The MTF and NPS in the strip direction are somewhat lower than in the scan direction. The NPS of tomographic images show a slight decrease with increasing spatial frequency, related to the sampling and interpolation in the reconstruction process. A phase I clinical trial is ongoing; 9 women have been recruited. Breast positioning is comparable to other imaging systems. The visualization of breast anatomy appears to be superior to screen-film mammography, at the same average glandular dose. Examination of images reconstructed with a sub-sampled set of projection images appears to support the hypothesis that image quality is superior when more projection images are used in the reconstruction.