Quantifying the confounding effect of pigmentation on measured skin tissue optical properties: a comparison of colorimetry with spatial frequency domain imaging

SIGNIFICANCE

Spatial frequency domain imaging (SFDI) is a wide-field diffuse optical imaging technique for separately quantifying tissue reduced scattering (μs  '  ) and absorption (μa) coefficients at multiple wavelengths, providing wide potential utility for clinical applications such as burn wound characterization and cancer detection. However, measured μs  '   and μa can be confounded by absorption from melanin in patients with highly pigmented skin. This issue arises because epidermal melanin is highly absorbing for visible wavelengths and standard homogeneous light-tissue interaction models do not properly account for this complexity. Tristimulus colorimetry (which quantifies pigmentation using the L  *   "lightness" parameter) can provide a point of comparison between μa, μs  '  , and skin pigmentation.

AIM

We systematically compare SFDI and colorimetry parameters to quantify confounding effects of pigmentation on measured skin μs  '   and μa. We assess the correlation between SFDI and colorimetry parameters as a function of wavelength.

APPROACH

μs  '   and μa from the palm and ventral forearm were measured for 15 healthy subjects with a wide range of skin pigmentation levels (Fitzpatrick types I to VI) using a Reflect RS® (Modulim, Inc., Irvine, California) SFDI instrument (eight wavelengths, 471 to 851 nm). L  *   was measured using a Chroma Meter CR-400 (Konica Minolta Sensing, Inc., Tokyo). Linear correlation coefficients were calculated between L  *   and μs  '   and between L  *   and μa at all wavelengths.

RESULTS

For the ventral forearm, strong linear correlations between measured L  *   and μs  '   values were observed at shorter wavelengths (R  >  0.92 at ≤659  nm), where absorption from melanin confounded the measured μs  '  . These correlations were weaker for the palm (R  <  0.59 at ≤659  nm), which has less melanin than the forearm. Similar relationships were observed between L  *   and μa.

CONCLUSIONS

We quantified the effects of epidermal melanin on skin μs  '   and μa measured with SFDI. This information may help characterize and correct pigmentation-related inaccuracies in SFDI skin measurements.

Characterizing reduced scattering coefficient of normal human skin across different anatomic locations and Fitzpatrick skin types using spatial frequency domain imaging

SIGNIFICANCE

Spatial frequency domain imaging (SFDI), a noncontact wide-field imaging technique using patterned illumination with multiple wavelengths, has been used to quantitatively measure structural and functional parameters of in vivo tissue. Using SFDI in a porcine model, we previously found that scattering changes in skin could potentially be used to noninvasively assess burn severity and monitor wound healing. Translating these findings to human subjects necessitates a better understanding of the variation in "baseline" human skin scattering properties across skin types and anatomical locations.

AIM

Using SFDI, we aim to characterize the variation in the reduced scattering coefficient (μs') for skin across a range of pigmentation and anatomic sites (including common burn locations) for normal human subjects. These measurements are expected to characterize baseline human skin properties to inform our use of SFDI for clinical burn severity and wound healing assessments.

APPROACH

SFDI was used to measure μs' in the visible- and near-infrared regime (471 to 851 nm) in 15 subjects at 10 anatomical locations. Subjects varied in age, gender, and Fitzpatrick skin type.

RESULTS

For all anatomical locations, the coefficient of variation in measured μs' decreased with increasing wavelength. High intersubject variation in μs' at visible wavelengths coincided with large values of the melanin extinction coefficient at those wavelengths. At 851 nm, where intersubject variation in μs' was smallest for all anatomical locations and absorption from melanin is minimal, significant intrasubject differences in μs' were observed at the different anatomical locations.

CONCLUSIONS

Our study is the first report of wide-field mapping of human skin scattering properties across multiple skin types and anatomical locations using SFDI. Measured μs' values varied notably between skin types at wavelengths where absorption from melanin was prominent. Additionally, μs' varied considerably across different anatomical locations at 851 nm, where the confounding effects from melanin absorption are minimized.

Noninvasive clinical assessment of port-wine stain birthmarks using current and future optical imaging technology: a review

Port-wine stain (PWS) birthmarks are one class of benign congenital vascular malformation. Laser therapy is the most successful treatment modality of PWS. Unfortunately, this approach has limited efficacy, with only 10% of patients experiencing complete blanching of the PWS. To address this problem, several research groups have developed technologies and methods designed to study treatment outcome and improve treatment efficacy. This article reviews seven optical imaging techniques currently in use or under development to assess treatment efficacy, focusing on: reflectance spectrophotometers/tristimulus colorimeters; laser Doppler flowmetry and laser Doppler imaging; cross-polarized diffuse reflectance colour imaging system; reflectance confocal microscopy; optical coherence tomography; spatial frequency domain imaging; and laser speckle imaging.

Spatial frequency domain imaging of port wine stain biochemical composition in response to laser therapy: a pilot study

BACKGROUND AND OBJECTIVE

Objective methods to assess port wine stain (PWS) response to laser treatment have been the subject of various research efforts for several years. Herein, we present a pilot study using a newly developed, light emitting diode (LED) based spatial frequency domain imaging (SFDI) device to record quantitatively biochemical compositional changes in PWS after laser therapy.

STUDY DESIGN/PATIENTS AND METHODS

A SFDI system was used to image before, and after, five PWS treatment sessions [n = 4 subjects (one subject was imaged before and after two consecutive laser treatments)]. SFDI derived wide-field optical properties (absorption and scattering) and tissue chromophore concentrations including oxy-hemoglobin (ctO(2) Hb), deoxy-hemoglobin (ctHHb), total hemoglobin (ctTHb), and tissue oxygen saturation (stO(2) ) are presented for skin imaged prior to and immediately after laser treatment. The SFDI derived images were analyzed by comparing the above measurements in PWS to those of normal skin and tracking changes immediately after laser exposure.

RESULTS

Elevated oxy-hemoglobin (>20%) and tissue oxygen saturation (>5%) were measured in all PWS lesions and compared to values for normal skin prior to treatment. Laser treatment resulted in an increase in deoxy-hemoglobin (>100%), decrease in tissue oxygen saturation (>10%), and reduced scattering (>15%) in all PWS lesions. One subject was followed before and after two consecutive laser treatments and the overall improvement in PWS lesion blanching was quantitatively assessed by measuring a 45% decrease in dermal blood volume.

CONCLUSION

SFDI is a rapid non-contact wide-field optical technique that shows potential as an imaging device that can be used to quantify biochemical compositional changes in PWS after laser therapy. Future work will investigate the potential of SFDI to provide intra-operative guidance for laser therapy of PWS lesions on an individual patient basis.

Disease impact and patient insight--a study on a local population of asthmatics

The cornerstone of asthma management is achieving adequate symptom control and patient education. We studied in our local population of asthmatic patients how well their symptoms were controlled with currently prescribed treatment and their insight into the disease and its management. Over a 6-month period, 93 asthmatics recruited from two local government health clinics and a state hospital were interviewed using a standard questionnaire. Patients were classified into 4 groups based on the treatment they were on according to Global Initiative for Asthma (GINA) treatment guidelines. The number of patients in Step 1 (rescue medication alone), Step 2 (1 controller medication), Step 3 (2 controller medications) and Step 4 (at least 3 controller medications) were 8, 39, 34 and 12, respectively. Except for day symptoms in Step 1 group, fewer than 50% achieved minimum day or night symptoms and no restriction of daily activities. Questions on patient insight were only available for 50 patients. Weather change (74%), air pollution (66%) and physical stress (46%) were the three highest ranked common asthma triggers. More than half correctly recognized the important symptoms of a serious asthma attack but fewer than 15% were familiar with the peak flow meter and its use or with the asthma self-management plan. Most patients perceived that their treatment had helped reduce disease severity and exacerbations. We conclude that symptom control and some aspect of patient education are still lacking in our local asthmatics.