Dual-energy contrast-enhanced breast tomosynthesis: optimization of beam quality for dose and image quality

Abstract

Dual-energy contrast-enhanced breast tomosynthesis is a promising technique to obtain three-dimensional functional information from the breast with high resolution and speed. To optimize this new method, this study searched for the beam quality that maximized image quality in terms of mass detection performance. A digital tomosynthesis system was modeled using a fast ray-tracing algorithm, which created simulated projection images by tracking photons through a voxelized anatomical breast phantom containing iodinated lesions. The single-energy images were combined into dual-energy images through a weighted log subtraction process. The weighting factor was optimized to minimize anatomical noise, while the dose distribution was chosen to minimize quantum noise. The dual-energy images were analyzed for the signal difference to noise ratio (SdNR) of iodinated masses. The fast ray-tracing explored 523 776 dual-energy combinations to identify which yields optimum mass SdNR. The ray-tracing results were verified using a Monte Carlo model for a breast tomosynthesis system with a selenium-based flat-panel detector. The projection images from our voxelized breast phantom were obtained at a constant total glandular dose. The projections were combined using weighted log subtraction and reconstructed using commercial reconstruction software. The lesion SdNR was measured in the central reconstructed slice. The SdNR performance varied markedly across the kVp and filtration space. Ray-tracing results indicated that the mass SdNR was maximized with a high-energy tungsten beam at 49 kVp with 92.5 µm of copper filtration and a low-energy tungsten beam at 49 kVp with 95 µm of tin filtration. This result was consistent with Monte Carlo findings. This mammographic technique led to a mass SdNR of 0.92 ± 0.03 in the projections and 3.68 ± 0.19 in the reconstructed slices. These values were markedly higher than those for non-optimized techniques. Our findings indicate that dual-energy breast tomosynthesis can be performed optimally at 49 kVp with alternative copper and tin filters, with reconstruction following weighted subtraction. The optimum technique provides best visibility of iodine against structured breast background in dual-energy contrast-enhanced breast tomosynthesis.

Figure 1

Major components of the three dimensional breast phantom model used in the study (a). A single cross sectional depiction of the phantom at the plane of the breast lesions (b).

Figure 2

Schematic of simulated imaging system.

Figure 3

The image is the maximum intensity projection display of SdNR over all filter materials and filter thicknesses as a function of beam energy at tomosynthesis projection dose.

Figure 4

The image is the maximum intensity projection display of SdNR over all filter thicknesses for different filter materials at 49 kVp and tomosynthesis projection dose. The diagonal is not zero because each cell reflects the maximum intensity projection through the filter space such that two different thicknesses of a filter material might have been used.

Figure 5

SdNR as a function of filter thickness at 49 kVp with two different filter materials. The filter thicknesses are indicated in value layers with half-value layer (hvl), quarter-value layer (qvl), twentieth-value layer (tvl), and fiftieth-value (fvl) at tomosynthesis projection dose (a). The spectra corresponding to the optimal (fvl) thicknesses of Sn and Cu filtration, the lower-right corner of (a) diagram (b), and those corresponding to the sub-optimal (fvl) thicknesses of Sn and Cu filtration, the upper-left corner of (a) diagram (c).

Figure 5

SdNR as a function of filter thickness at 49 kVp with two different filter materials. The filter thicknesses are indicated in value layers with half-value layer (hvl), quarter-value layer (qvl), twentieth-value layer (tvl), and fiftieth-value (fvl) at tomosynthesis projection dose (a). The spectra corresponding to the optimal (fvl) thicknesses of Sn and Cu filtration, the lower-right corner of (a) diagram (b), and those corresponding to the sub-optimal (fvl) thicknesses of Sn and Cu filtration, the upper-left corner of (a) diagram (c).

Figure 6

Projection SdNR calculated with ray-tracing technique for four conditions corresponding to corners of Figure 5. The filter thicknesses are indicated in value layers with half-value layer (hvl) and fiftieth-value layer (fvl). The total glandular dose for each dual-energy projection pair was equal to one tomosynthesis projection.

Figure 7

Projection SdNR calculated with Monte Carlo for the four conditions of Figure 5. The filter thicknesses are indicated in value layers with half-value layer (hvl) and fiftieth-value layer (fvl). The total glandular dose for each dual-energy projection pair was equal to one tomosynthesis projection.

Figure 8

Reconstruction SdNR calculated with Monte Carlo for the four conditions of Figure 5. The filter thicknesses are indicated in value layers with half-value layer (hvl) and fiftieth-value layer (fvl). The total glandular dose for each dual-energy acquisition was equal to one tomosynthesis scan (25 projections).

Figure 9

Tomosynthesis projections obtained from Monte Carlo model for the four conditions of Figure 5. The filter thicknesses are indicated in value layers with half-value layer (hvl) and fiftieth-value layer (fvl). The breast compressed thickness was 4 cm and the lesions iodine density 1%.

Figure 10

Tomosynthesis reconstructions obtained from Monte Carlo model for the four conditions of Figure 5. The filter thicknesses are indicated in value layers with half-value layer (hvl) and fiftieth-value layer (fvl). The breast compressed thickness was 4 cm and the lesions iodine density 1%.

Figure 11

Projection and reconstructed slices for the optimal beam (high-energy tungsten beam at 49 kVp with 92.5 μm of copper filtration and low-energy tungsten beam at 49 kVp with 95 μm of tin filtration). The breast compressed thickness was 4 cm and the lesions iodine density 1%.

Figure 12

Effect of the order in which reconstruction and dual-energy subtraction were performed. Either the image could be first subtracted and then reconstructed (left) or first reconstructed and then subtracted (right). The breast compressed thickness was 4 cm and the lesions iodine density 1%.