Thick slices from tomosynthesis data sets: phantom study for the evaluation of different algorithms

Abstract

Tomosynthesis is a 3-dimensional mammography technique that generates thin slices separated one to the other by typically 1 mm from source data sets. The relatively high image noise in these thin slices raises the value of 1-cm thick slices computed from the set of reconstructed slices for image interpretation. In an initial evaluation, we investigated the potential of different algorithms for generating thick slices from tomosynthesis source data (maximum intensity projection-MIP; average algorithm-AV, and image generation by means of a new algorithm, so-called softMip). The three postprocessing techniques were evaluated using a homogeneous phantom with one textured slab with a total thickness of about 5 cm in which two 0.5-cm-thick slabs contained objects to simulate microcalcifications, spiculated masses, and round masses. The phantom was examined by tomosynthesis (GE Healthcare). Microcalcifications were simulated by inclusion of calcium particles of four different sizes. The slabs containing the inclusions were examined in two different configurations: adjacent to each other and close to the detector and with the two slabs separated by two 1-cm thick breast equivalent material slabs. The reconstructed tomosynthesis slices were postprocessed using MIP, AV, and softMip to generate 1-cm thick slices with a lower noise level. The three postprocessing algorithms were assessed by calculating the resulting contrast versus background for the simulated microcalcifications and contrast-to-noise ratios (CNR) for the other objects. The CNRs of the simulated round and spiculated masses were most favorable for the thick slices generated with the average algorithm, followed by softMip and MIP. Contrast of the simulated microcalcifications was best for MIP, followed by softMip and average projections. Our results suggest that the additional generation of thick slices may improve the visualization of objects in tomosynthesis. This improvement differs from the different algorithms for microcalcifications, speculated objects, and round masses. SoftMip is a new approach combining features of MIP and average showing image properties in between MIP and AV.

Fig 1.

Scheme of the two different configurations of our phantom for tomosynthesis.

Fig 2.

Diagram of the phantom used and the two configurations of the slabs with the included objects (diameters microcalcification: μcal; SM: spiculated masses; RM: round masses).

Fig 3.

MIP images are computed as the maximum value of a voxel stack, whereas average computes the average of all corresponding values of a pixel. Simular to calculating Average softMip calculates the resulting pixel value through a (more complicated) function.

Fig 4.

Examples of original thin slices and 1-cm slices generated by MIP, Average (AV), and softMip. Top: 200-μm microcalcifications, bottom: round mass.

Fig 5.

Results of contrast measurements of microcalcifications in Configuration I—contrast versus diameter in micrometer.

Fig 6.

Results of contrast measurements of microcalcifications in Configuration II—contrast versus diameter.

Fig 7.

Example of contrast-to-noise ratios of round masses.

Fig 8.

Average contrast-to-noise ratios of round masses (RM) and spiculated masses (SM) in Configuration I and Configuration II.