Combined optical imaging and mammography of the healthy breast: optical contrast derived from breast structure and compression

Abstract

In this paper, we report new progress in developing the instrument and software platform of a combined X-ray mammography/diffuse optical breast imaging system. Particularly, we focus on system validation using a series of balloon phantom experiments and the optical image analysis of 49 healthy patients. Using the finite-element method for forward modeling and a regularized Gauss-Newton method for parameter reconstruction, we recovered the inclusions inside the phantom and the hemoglobin images of the human breasts. An enhanced coupling coefficient estimation scheme was also incorporated to improve the accuracy and robustness of the reconstructions. The recovered average total hemoglobin concentration (HbT) and oxygen saturation (SO2) from 68 breast measurements are 16.2 microm and 71%, respectively, where the HbT presents a linear trend with breast density. The low HbT value compared to literature is likely due to the associated mammographic compression. From the spatially co-registered optical/X-ray images, we can identify the chest-wall muscle, fatty tissue, and fibroglandular regions with an average HbT of 20.1+/-6.1 microm for fibroglandular tissue, 15.4+/-5.0 microm for adipose, and 22.2+/-7.3 microm for muscle tissue. The differences between fibroglandular tissue and the corresponding adipose tissue are significant (p < 0.0001). At the same time, we recognize that the optical images are influenced, to a certain extent, by mammographical compression. The optical images from a subset of patients show composite features from both tissue structure and pressure distribution. We present mechanical simulations which further confirm this hypothesis.

Fig. 1

Block diagram of TOBI breast imaging system. The physical distributions of source and detector fiber positions can be found in Fig. 5.

Fig. 2

Picture of the combined TOBI/DBT system in clinical environment. The TOBI system include both RF and CW source/detector modules and the fiber optics interface attaching to the tomosynthesis system on the right.

Fig. 3

Close-up view of the optical probes.

Fig. 4

Data analysis flow chart.

Fig. 5

Illustrations of sources/detector configurations: (a) sources and (b) detectors for phantom measurements; (c) sources and (d) detectors for patient measurements.

Fig. 6

Balloon phantom experiment illustration: (a) top-view photo of the compressed balloon, (b) forward FEM mesh, and (c) inclusion.

Fig. 7

Reconstructed image slices for a balloon phantom filled with intralipid solution: (a)μ_a (cm^-1) image and (b) ${\mu }_s^{\prime }\left({\mathrm{cm}}^{-1}\right)$ image. Both images were extracted at the horizontal plane 3 cm from the bottom of the phantom. Maximum perturbations for the absorption is ±2.1% and that for the scattering image is ±1.9%.

Fig. 8

Reconstructed μ_a (cm^-1 images of a balloon with a 2.5 cm diameter spherical inclusion: (a) configuration 1 and (b) configuration 2. Image slices were extracted from a horizontal plane 1.5 cm above the bottom plane. Contour of the inclusion is overlayed on the images.

Fig. 9

Reconstructed HbT and DBT images for various subjects. The images in the first row [(a)-(c)] are examples of fatty breast reconstructions; the images (d)-(f) scattered density breasts; images (g)-(i), heterogeneously dense breasts; and (j)-(l), extremely dense cases. All images are slices extracted along a horizontal plane from the reconstructed 3-D HbT maps. The thick dashed line denotes the edge of optical source/detector coverage. The region between the dashed line and the nipple has virtually no sensitivity to the measurements. The white dashed line on the DBT slice and the black solid thick line on the HbT images denote the boundaries of the chest-wall muscle regions which do not show up for all the patients. In all figures, the color scales were set between 80%-120% of the respective bulk HbT value, except for (g) where 70%-130% was used.

Fig. 10

Image slices for (a) ${\mu }_s^{\prime }\left({\mathrm{cm}}^{-1}\right)$ at 830 nm, (b) oxygen-saturation (SO₂) corresponding to case (i) in Fig. 10, and (c) ${\mu }_s^{\prime }\left({\mathrm{cm}}^{-1}\right)$ at 830 nm, (d) SO₂ corresponding to case (d) in Fig. 10. The markings on the images are defined similarly as in Fig. 9.

Fig. 11

Breast compression simulation and von Mise's stress crosscuts: (a) breast model, (b) (x, y) cross section, (c) (y, z) cross section. The darker the color scale, the higher the stress.