Optical breast imaging

Abstract

Optical breast imaging uses near-infrared light to assess the optical properties of breast tissue. It can be performed relying on intrinsic breast tissue contrast alone or with the use of exogenous imaging agents that accumulate at the tumor site. Different tissue components have unique scattering and absorption characteristics for each wavelength. Using multiple wavelengths allows for the calculation of relative concentrations of oxy- and deoxyhemoglobin, lipid, and water in the tissue (spectroscopic imaging), which may enable discrimination between malignant and benign tissue. With the use of exogenous imaging agents, such discrimination may be improved even further, especially with imaging agents specifically targeted to cancer-associated molecular changes (molecular imaging). Currently used breast imaging modalities (X-ray mammography, ultrasound, and magnetic resonance imaging) have limitations in clinical utility, including early detection, diagnosis, and treatment monitoring of breast cancer. Optical breast imaging is being pursued as an adjunct to the current modalities for its potential to provide biophysical and molecular information on tissue. Moreover, important strengths of optical imaging are that the technique uses no ionizing radiation, is relatively cheap, has a very high sensitivity for probe detection, and its imaging agents are easy to generate. First, a literature overview is given of the various clinical optical breast imaging studies and the preclinical optical imaging studies in the evaluation and development of novel target-specific imaging agents for breast cancer. Then, the early evaluation of a new diffuse optical tomography prototype dedicated for breast imaging is described. We showed that based on intrinsic breast tissue contrast alone, our new prototype has the potential to discriminate malignant from benign breast tissue by assessing the optical properties of breast tissue in a reproducible quantitative and qualitative way. We demonstrated the ability of the technique to visualize cysts and elucidate their high water and low total hemoglobin content by spectroscopic analysis. In addition, we showed ability to safely detect malignant breast tumors in patients using a low dose of a novel fluorescent imaging agent. Main limitations of the prototype included geometry issues hindering the visualization of lesions close to the patient’s chest wall, and the poor spatial resolution limiting the detection of very small breast lesions. Finally, our preclinical optical imaging studies are reported. In phantom studies, we showed the potential of a commercially available optical breast scanner to detect newly developed molecular imaging agents based on modulating light transmission. Nanomolar concentrations of nanotubes and micromolar concentrations of a small molecule dye could be detected. In a mouse model, we showed that non-invasive in vivo monitoring of changes in protein expression as a response to treatment is feasible using optical imaging. Although optical breast imaging is still in its infancy, the technique is certainly promising in breast cancer detection and treatment management. Further technical developments and the availability of relevant molecular imaging agents for human use in the near future could eventually allow for this technique to be used in routine clinical practice