Depth Resolved Quantification of Fluorescence in Skin

Photo of Depth Resolved Quantification of Fluorescence in Skin.

Project #1: Diffuse Correlation Tomography + Spatial Frequency Domain Imaging: Quantitative Metabolic Imaging

The current and primary method of determining burn severity is visual assessment. The subjective nature of this assessment, based on the depth of the burn into tissue, carries significant risk and costs both to the patient and the care giver. We are combining both SFDI and DCT imaging modalities to evaluate whether burn wound severity can be objectively characterized via several sources of optical contrast: 1) the relative fraction of methemoglobin absorption to that of oxygenated and deoxygenated hemoglobin can serve as a direct correlate to burn depth for intact wounds, 2) the zone of coagulation can be represented by the depth resolved region absent of flow via DCT, 3) The reduction of the scattering slope in the visible regime is indicative of collagen denaturation and structural damage to the tissue.

Figures for SFDI: characterization of burns (left) and DCT: detection of blood flow as a function of depth (right)

Graphics showing the results from the research..

Not only do these two optical techniques have the potential to isolate complementary physiological properties of tissue in depth, the quantification of blood flow (DCT), volume (SFDI) and oxygenation (SFDI) can also be synergistically used to estimate cellular metabolism. This metric could be profoundly useful in the context of burn wound assessment by correlating the cellular activity in underlying tissue to site-specific “wound healing capacity.” Non-invasive metabolic imaging could also be useful in the detection of cancers and other diseases.

Project #2: Optical Coherence Tomography + Spatial Frequency Domain Imaging: Nanosensitive Rendering of Tissue Structure and Function of Skin Cancers

Emerging research suggested that the chronic inflammation can be a prominent indicator of nonmelanoma skin cancer. Inflammation can cause accumulation of bloods, water, other inflammatory proteins, and biomolecules in the skin dermis. In addition, inflammation can alter tissue structural properties like mass density, refractive index, size, and shapes of scatterers inside dermis tissue. Therefore, it can be hypothesized that the determination of such broad range of chromophores like hemoglobin, water contents, inflammatory biomolecules and structural morphology like mass density, refractive index, size, and shapes together can indicate the future possibility of nonmelanoma skin cancer.

Graphics showing the results from the research..

In this context, SFDI and OCT are mutually beneficial:

  1. SFDI can provide the optical properties of the tissue. These properties can then be used as a model to describe the light signal loss of the OCT data as it propagates deeper into tissue. This light loss propagation information can then be used to make OCT contrast quantitative and recover weak signals, thereby extending the depths at which OCT can characterize tissue structures
  2. OCT can provide distinct boundaries to tissue structures in depth. This would greatly improve the tomographic reconstruction of depth sensitive SFDI methods and therefore improve the specificity of the depth resolved tissue function parameters that SFDI can detect.

Project #3: Depth Resolved Quantification of Fluorescence in Skin

The ability to quantify fluorescence can be useful for understanding the motility of topically applied fluorescing drugs as well as dosimetry planning for light-based therapies like photodynamic therapy (PDT). Previously we have reported a quantitative imaging approach using a Spatial Frequency Domain Imaging (SFDI) technique that can extract bulk drug concentration from skin based on its fluorescent signal. This method, however, assumes that the drug is homogenously distributed within the tissue volume probed.

In this project, we are developing several modified approaches that apply this SFDI approach to tissue where the fluorophore distributions vary in depth.  We are exploring light transport model based approaches that characterize the spectral absorption and scattering properties in tissue in order to better understand and interpret the fluorescent signal generated within the tissue volumes. To extract depth information, we additionally exploit both spectrally selective depth penetrance of multiple excitation sources as well as spatial frequency dependent depth sensitivity from both excitation and emission wavelengths.

(Left): Photo of healthy skin, within the ink markers is where topical ALA is applied; (middle): wide-field image of topical drug distribution in vivo; (right): quantitative drug penetration and distribution into tissue, 5 hours after topical application  

The primary objectives of these approaches are to extend quantitative optical imaging to include depth resolved fluorescence signals in 3 distinct clinical areas:

  • Wound Healing – Cellular activity (NADH/FAD), tissue restructuring and remodeling (Collagen)
  • Photodynamic Therapy (photosensitizer based – fluorescence)
  • Pharmakinetics – Evaluate topical drug delivery and motility

Project members