In the United States, breast cancer is the second leading cause of cancer death after lung cancer. While x-ray mammography has a high success rate in the detection of cancerous tumors, it also has a 20% false-negative detection rate; that is, cancer is present but not detected.
A new imaging technique developed by scientists at Florida International University collaborating with the Sylvester Comprehensive Cancer Center and Advanced Medical Specialties (all in Miami, FL) for clinical breast imaging studies uses a fiber-optic probe to both deliver and channel reflected illumination from a 785-nm-emitting laser diode back to a CCD detector to generate two-dimensional (2D) surface maps of total hemoglobin absorption.1 Higher hemoglobin presence has been shown to be indicative of the increased vasculature and higher blood content because of the presence of a cancerous tumor.
Diffuse optical imaging
Clinical studies have proven that breast cancer is characterized by increased hemoglobin (HbT) concentration, which is detected by its increased optical absorption beyond visible wavelengths.
Two different probes were designed for 2D imaging. In the generation-1 device, the 500 mW output from a 785 nm laser diode is split into six fibers, each with less than 5 mW optical power at the tissue end. In addition, 165 fibers channel light back to the CCD for illumination from a 4 × 9 cm2 contoured probe face. The generation-2 device has six laser diodes with 100 mW output resulting in about 5 mW of output on the tissue from two 4 × 5 cm2 probe faces, each with three illumination and 96 collection fibers.
The second-generation probe was designed to increase illumination uniformity as well as allow reflectance and transmittance imaging of the breast, apart from bilateral simultaneous illumination of both breasts. For the probes, 600-μm-diameter multimode fibers with a numerical aperture of 0.22 were used.
The CCD data consisted of 5 to 10 measurements with a 0.2 s exposure time averaged across the measurement set. A custom-developed MATLAB software routine post-processes the data to generate 2D contour plots of optical intensity vs. probe surface location.
A 785 nm wavelength was chosen as it is close to the "isosbestic point," whereby the absorption spectra of oxyhemoglobin (HbO) and deoxyhemoglobin (HbD) intersect and higher absorption regions indicate increased total hemoglobin concentrations that are not dependent on the oxygen saturation content of hemoglobin. Normal breast tissue scans from the contralateral breast of each patient were subtracted from cancerous tissue to subtract background noise from the collected scans.
In a study of five breast cancer patients aged 51 to 74, the fiber probe was able to detect invasive ductal carcinoma, ductal carcinoma in situ, and lymphatic spread of cancer. Future work centers on coupling the 2D measurements with real-time tracking and computational tools (as demonstrated by previous studies in phantoms and normal human subjects), resulting in a 3D tomographic image of HbT concentration. Furthermore, adding more source wavelengths (690 and 830 nm) to the 785 nm probe will allow the collection of both HbO and HbD concentrations as well as total HbT concentration. HbO and HbD can indicate whether a tumor is benign or malignant.
"An accessible, safe imaging device using light is what the future generations need to image the breast tissues with no radiation, increased affordability, and ease of real-time imaging during periodic visits to the doctor's office," says Anu Godavarty of the Optical Imaging Laboratory at Florida International University.
REFERENCE
1. S. J. Erickson-Bhatt et al., Biomed. Phys. Eng. Express, 1, 045003, 1–11 (Oct. 23, 2015).
