Jasmina Ludwig

Experimental validation of a three-dimensional linear system model for breast tomosynthesis

A three-dimensional (3D) linear model for digital breast tomosynthesis (DBT) was developed to investigate the effects of different imaging system parameters on the reconstructed image quality. In the present work, experimental validation of the model was performed on a prototype DBT system equipped with an amorphous selenium (a-Se) digital mammography detector and filtered back-projection (FBP) reconstruction methods. The detector can be operated in either full resolution with 85 microm pixel size or 2 x 1 pixel binning mode to reduce acquisition time. Twenty-five projection images were acquired with a nominal angular range of +/- 20 degrees. The images were reconstructed using a slice thickness of 1 mm with 0.085 x 0.085 mm in-plane pixel dimension. The imaging performance was characterized by spatial frequency-dependent parameters including a 3D noise power spectrum (NPS) and in-plane modulation transfer function (MTF). Scatter-free uniform x-ray images were acquired at four different exposure levels for noise analysis. An aluminum (Al) edge phantom with 0.2 mm thickness was imaged to measure the in-plane presampling MTF. The measured in-plane MTF and 3D NPS were both in good agreement with the model. The dependence of DBT image quality on reconstruction filters was investigated. It was found that the slice thickness (ST) filter, a Hanning window to limit the high-frequency components in the slice thickness direction, reduces noise aliasing and improves 3D DQE. An ACR phantom was imaged to investigate the effects of angular range and detector operational modes on reconstructed image quality. It was found that increasing the angular range improves the MTF at low frequencies, resulting in better detection of large-area, low-contrast mass lesions in the phantom. There is a trade-off between noise and resolution for pixel binning and full resolution modes, and the choice of detector mode will depend on radiation dose and the targeted lesion.

A Novel Approach for Filtered Backprojection in Tomosynthesis Based on Filter Kernels Determined by Iterative Reconstruction Techniques

Breast tomosynthesis is a new 3D imaging technique providing 3D slices of the breast. The computation of 3D slices from a set of 2D projections is performed with a reconstruction algorithm --- either a filtered backprojection (FBP) or an algebraic iterative reconstruction method. Both approaches yield different image characteristics of the reconstructed object. The algebraic method has the main disadvantage of a very long processing time. We experimentally developed a set of filter kernels for FBP, determined by iterative reconstruction providing similar image characteristics and quality as an algebraic reconstruction. Additionally we showed that it is possible to approximate these kernels by a polynomial function of order 4. Using clinical data sets we demonstrate how the image quality and the image impression can be varied by using different reconstruction methods.

Optimization of Tomosynthesis Acquisition Parameters: Angular Range and Number of Projections

In breast tomosynthesis several projection views of the object are acquired over a limited angular range from which slice images are reconstructed. The image quality of the reconstructed slices, however, depends on the geometric acquisitions parameters and on the reconstruction algorithm. We investigated the impact of the angular range of data acquisition and number of projection views on image quality by performing 3-D cascaded linear system modeling and phantom measurements. The results show that a larger angular range not only increases resolution perpendicular to the slices but also improves the visibility of low-frequency objects since the sampling area in Fourier space is mainly limited at low-frequencies. The effect on contrast resolution is demonstrated also with clinical data sets.

Image Artifact in Digital Breast Tomosynthesis and Its Dependence on System and Reconstruction Parameters

Digital breast tomosynthesis (DBT) is a three-dimensional (3D) imaging modality that produces cross-sectional image slices parallel to the detector plane using a limited number of views (<50) and a limited angular range (< 50 degrees). Due to the incomplete sampling of DBT, image artifacts are unavoidable. In this paper we investigated the dependence of out-of-plane image artifact on image acquisition and reconstruction parameters. The image artifact of a microcalcification (MC) was correlated with the 3D point spread function (PSF) of the DBT system, which was calculated using a cascaded linear system model. Our results show that the linear system model can be used to quantitatively predict the image artifact in DBT, and can be used as a unified tool for maximizing lesion detectability and minimizing artifact.