Diffuse optical tomography in the presence of a chest wall

Abstract

Diffuse optical tomography (DOT) has been employed to derive spatial maps of physiologically important chromophores in the human breast, but the fidelity of these images is often compromised by boundary effects such as those due to the chest wall. We explore the image quality in fast, data-intensive analytic and algebraic linear DOT reconstructions of phantoms with subcentimeter target features and large absorptive regions mimicking the chest wall. Experiments demonstrate that the chest wall phantom can introduce severe image artifacts. We then show how these artifacts can be mitigated by exclusion of data affected by the chest wall. We also introduce and demonstrate a linear algebraic reconstruction method well suited for very large data sets in the presence of a chest wall.

Fig. 1

Schematic of the experimental setup. A collimated CW 785 nm laser source at is raster scanned on one side of the imaging tank. The transmitted light on the detection plane is collected by a CCD for each source position.

Fig. 2

Phantoms used in the experiment. (a) 6 mm thick bar target with ${\mu }_a=0.2{\mathrm{cm}}^{-1}$ and ${\mu }_s^{\prime }=7.5{\mathrm{cm}}^{-1}$ has slots 48 mm tall and 9 mm wide. The outer dimensions are $60\times 50{\mathrm{mm}}^2$. (b) The chest wall phantom with ${\mu }_a=0.1{\mathrm{cm}}^{-1}$ and ${\mu }_s^{\prime }=5.0{\mathrm{cm}}^{-1}$.

Fig. 3

Models for data restriction. (a) Photograph of the drained imaging tank illustrating the position the target with respect to the chest wall phantom. (b) Schematic of the imaging tank. (c) Illustration of the various data sets used in the reconstructions. The dark blue square is the CCD FOV while the inner light blue square indicates the reconstruction region. The red dots indicate the source positions. A sample reconstruction is superimposed with the drawing to illustrate the target shape and position. The red lines indicate the three lowest position of the chest wall phantom (other positions are outside of the CCD FOV) while the green lines illustrate the restricted data sets where all the sources and detectors situated above a given green line have been discarded.

Fig. 4

Images of the central slice obtained by analytical (a) and algebraic (b) reconstruction methods. Different columns show data obtained with the chest wall phantoms at different distances $d$ from the bar target. Different rows of images correspond to different data restrictions NR, as indicated. The color bar applies to all other images shown below.

Fig. 5

Slices through the medium drawn at different depths (from the plane of sources) as indicated. Analytical image reconstruction method with $d=5\mathrm{cm}$ and $d=2\mathrm{cm}$.

Fig. 6

Same as in Fig. 5 but obtained by algebraic reconstruction.