Initial study of quasi-monochromatic X-ray beam performance for X-ray computed mammotomography

We evaluate the feasibility, benefits, and operating parameters of a quasimonochromatic beam for a newly developed x-ray cone beam computed mammotomography application. The value of a near monochromatic x-ray source for fully 3D dedicated mammotomography is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose levels at or below that of existing dual view mammography. In previous studies, simulations for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast, with a digital flat-panel CsI(Tl) detector model, indicated that thick, rare earth filter materials may provide optimized image quality. Figures of merit computed included: lesion contrast under different filtering conditions; ratio of measured lesion contrast with and without filtering; and exposure efficiency (SNR/sup 2//exposure). Initial experiments are performed with a custom built x-ray mammotomography system, cerium foil filters, and plastic breast and lesion tissue-equivalent slabs. Simulation results showed that tube potentials of 50-70 kVp with filters of Z=57-63 yielded quasimonochromatic x-ray spectra with improved FOMs. Initial experimental measurements corroborate simulation results in that, relative trends and rank order of contrast ratios and exposure efficiency were in agreement. These studies show that this approach can be implemented practically with simple hardware and yield improved exposure efficiency versus the unfiltered or minimally filtered case.

I. Introduction

Limitations in the ability of current breast transmission imaging techniques have resulted in high false-positive rates for breast lesion malignancy and false negative rates of between 10–20% in patients with dense fibroglandular tissue [1]. Various modalities have been applied in unique ways to 3D breast transmission imaging to attempt to improve diagnosis. Some groups are investigating dedicated x-ray computed mammotomography (CmT) alone [2], [3], and others have proposed CmT and emission imaging [4], [5]. Tomographic techniques promise improved contrast resolution versus dual view mammography through the inherent removal of overlying tissues but with an added challenge of limiting patient dose where large numbers of projection images are required. It has been predicted that CmT can potentially be performed at or below dose levels of dual view mammography [2], [3].