Conventional filtered back projection (FBP) reconstruction for digital breast tomosynthesis (DBT) can suffer from a low
signal to noise ratio. Because of the strong amplification by the reconstruction filters (ramp, apodization and slice
thickness), noise at high spatial frequencies can be greatly increased. Image enhancement by Wiener filtering is
investigated as a possible method to improve image quality. A neighborhood wavelet coefficient window technique is
used to estimate the noise content of projection images and a Wiener filter is applied to the projection images. The
neighborhood wavelet coefficient window is a non-linear technique, which may cause the Wiener filters estimated before
and after the application of the reconstruction filters to be different. Image quality of a FBP reconstruction with and
without Wiener filtering is investigated using a Fourier-based observer detectability metric ( d' ) for evaluation.
Simulations of tomosynthesis are performed in both homogeneous and anatomic textured backgrounds containing lowcontrast
masses or small microcalcifications. Initial results suggest that improvements in detectability can be achieved
when the Wiener filter is applied, especially when the Wiener filter is estimated for the reconstruction filtered
Digital breast tomosynthesis (DBT) is a limited-view, limited-angle computed tomography (CT) technique that has the
potential to yield improved lesion conspicuity over that of standard digital mammography. To maintain short acquisition
time, the detector must have a rapid temporal response. Transient effects like lag and ghosting have been noted
previously in digital mammography systems, but for the times between successive views (approx. 1 minute), their impact
on image quality is generally negligible. However, tomosynthesis imaging requires much shorter times between
projection images (< 1 s). Under these conditions, detectors that may have been acceptable for digital mammography
may not be suitable for tomosynthesis. Transient effects will generally cause both a loss of signal and an increase in
image noise. A cascaded systems analysis is used to determine the effect of lag on image quality in a DBT system. It is
shown that in the projection images, lag results in artifacts appearing as a "trail" of prior exposures. The effect of lag on
image quality is also evaluated with a simple Monte Carlo simulation of a cone-beam tomosynthesis image formation
incorporating a filtered back-projection algorithm.
Digital Breast Tomosynthesis (DBT) is a 3D x-ray technique for imaging the breast. The x-ray tube, mounted on a gantry, moves in an arc over a limited angular range around the breast while 7-15 images are acquired over a period of a few seconds. A reconstruction algorithm is used to create a 3D volume dataset from the projection images. This procedure reduces the effects of tissue superposition, often responsible for degrading the quality of projection mammograms. This may help improve sensitivity of cancer detection, while reducing the number of false positive results.
For DBT, images are acquired at a set of gantry rotation angles. The image reconstruction process requires several geometrical factors associated with image acquisition to be known accurately, however, vibration, encoder inaccuracy, the effects of gravity on the gantry arm and manufacturing tolerances can produce deviations from the desired acquisition geometry. Unlike cone-beam CT, in which a complete dataset is acquired (500+ projections over 180°), tomosynthesis reconstruction is challenging in that the angular range is narrow (typically from 20°-45°) and there are fewer projection images (≈7-15). With such a limited dataset, reconstruction is very sensitive to geometric alignment. Uncertainties in factors such as detector tilt, gantry angle, focal spot location, source-detector distance and source-pivot distance can produce several artifacts in the reconstructed volume. To accurately and efficiently calculate the location and angles of orientation of critical components of the system in DBT geometry, a suitable phantom is required.
We have designed a calibration phantom for tomosynthesis and developed software for accurate measurement of the geometric parameters of a DBT system. These have been tested both by simulation and experiment. We will present estimates of the precision available with this technique for a prototype DBT system.