Feasibility of spatial frequency-domain imaging for monitoring palpable breast lesions

Abstract

In breast cancer diagnosis and therapy monitoring, there is a need for frequent, noninvasive disease progression evaluation. Breast tumors differ from healthy tissue in mechanical stiffness as well as optical properties, which allows optical methods to detect and monitor breast lesions noninvasively. Spatial frequency-domain imaging (SFDI) is a reflectance-based diffuse optical method that can yield two-dimensional images of absolute optical properties of tissue with an inexpensive and portable system, although depth penetration is limited. Since the absorption coefficient of breast tissue is relatively low and the tissue is quite flexible, there is an opportunity for compression of tissue to bring stiff, palpable breast lesions within the detection range of SFDI. Sixteen breast tissue-mimicking phantoms were fabricated containing stiffer, more highly absorbing tumor-mimicking inclusions of varying absorption contrast and depth. These phantoms were imaged with an SFDI system at five levels of compression. An increase in absorption contrast was observed with compression, and reliable detection of each inclusion was achieved when compression was sufficient to bring the inclusion center within ∼12 mm of the phantom surface. At highest compression level, contrasts achieved with this system were comparable to those measured with single source-detector near-infrared spectroscopy.

Keywords: breast cancer; compression; near-infrared light; optical mammography; spatial frequency-domain imaging.

Fig. 1

(a) Schematic and (b) photo of SFDI system with phantom under compression.

Fig. 2

Absorption coefficient maps for four phantoms containing inclusions of ${\mu }_{\mathrm{a},\text{inclusion}}=0.058{\mathrm{cm}}^{-1}$ at (a) different initial depths and four phantoms containing inclusions of ${\mu }_{\mathrm{a},\text{inclusion}}=0.104{\mathrm{cm}}^{-1}$ (b) at partial compression (31%) and full compression (55%). Dotted circle represents the size of the inclusion (1.7 cm).

Fig. 3

Absorption coefficient cross sections for each phantom, at five levels of compression, percent compression defined as change in phantom height divided by initial height. (a) ${\mu }_{\mathrm{a},\text{inclusion}}=0.058{\mathrm{cm}}^{-1}$, (b) ${\mu }_{\mathrm{a},\text{inclusion}}=0.079{\mathrm{cm}}^{-1}$, (c) ${\mu }_{\mathrm{a},\text{inclusion}}=0.091{\mathrm{cm}}^{-1}$, and (d) ${\mu }_{\mathrm{a},\text{inclusion}}=0.104{\mathrm{cm}}^{-1}$. The inclusion is positioned at the center of the cross section. As the absorption coefficient of the inclusion increases, the contrast measured with SFDI increases. Similarly, as the compression level increases, the effective inclusion depth decreases and the contrast increases as well.

Fig. 4

(a)–(d) Reduced scattering coefficient cross sections for each phantom, at five levels of compression, percent compression defined as change in phantom height divided by initial height. No significant scattering contrast is detected, and no significant change with compression. The y-axis is scaled to correspond to the same percentage change as in Fig. 3.

Fig. 5

Measured contrast decreases exponentially with depth for (a) SFDI at all compression levels and (b) single source–detector separation NIRS. Although depth during compression was not known precisely, the estimate used here shows the expected exponential relationship, with dependence on ${\mu }_{\mathrm{a},\text{inclusion}}$. The dashed lines in (a) represent the approximate depth threshold for inclusion detection, 12 mm. At lower depths, contrast is greater than 0.05 for each of the four exponential fits. Open circles represent measurements of uncompressed phantoms. For each of the 16 phantoms, (c) compares optical contrast from SFDI at highest compression level to that measured with single source–detector separation NIRS, demonstrating that a similar level of contrast can be achieved by this method.