Wide-field spatial mapping of in vivo tattoo skin optical properties using modulated imaging

Abstract

Background and objectives: Modulated imaging is a new modality capable of wide-field, spatially resolved measurement of in vivo optical properties. Based on spatial light modulation, the method is inexpensive, non-contact, and allows spatial mapping of tissue absorption and reduced scattering coefficients at any wavelength between 450 and 1,100 nm. Currently, clinicians rely on qualitative visual inspection to guide parameter selection for laser-based tattoo removal. MI provides quantitative measurements of multi-colored tattooed skin which may help guide treatment and objectively assess response.

Study design/materials and methods: We have measured the spatially varying optical properties of multi-colored tattooed skin over a 50 mm x 50 mm field of view at wavelengths ranging from 650 to 970 nm using MI. These measurements were compared to a similar field of view of non-tattooed skin from an adjacent area.

Results: We have determined the differentiated optical properties in vivo of multi-colored tattooed skin versus non-tattooed skin.

Conclusions: MI provides spatially resolved quantitative information with potential for quantitative assessment of response to treatment and may provide guidance for laser tattoo removal in the future.

Fig. 1

The MI instrument field of view. a: A tattoo is illuminated with planar light at Frequency 0. Frequency 1 shows the tattoo illuminated with a spatial sine wave pattern that is phase shifted 120° as you move from the leftmost image to the rightmost image. Frequency 2 shows a similar image set with a higher frequency sine wave pattern. b: The three phase shifted patterns are demodulated to form a map of frequency dependent, non-modulated diffuse reflectance similar to the map of diffuse reflectance generated with planar illumination.

Fig. 2

Diagram of the modulated imaging system. Light generated by a Tungsten lamp is directed onto the digital micro-mirror device (DMD) chip which generates the spatial frequency patterns. These patterns are projected directly onto the region of interest on the subject and the diffuse reflectance is captured at discrete wavelengths by the CCD camera and incorporated liquid crystal tunable filter (LCTF).

Fig. 3

A graphical representation of the Monte–Carlo generated look-up table. The center grid is made up of horizontal lines marking absorption values (μ_a) decreasing from left to right, and vertical lines marking reduced scattering values $\left({\mu }_{\text{s}}^{\prime }\right)$ decreasing from top to bottom. Each intersection represents a unique combination of absorption and reduced scattering and has a particular diffuse reflectance value for the non-modulated light on the x-axis and the modulated light on the y-axis.

Fig. 4

Three tattoos investigated with the modulated imaging system: (a) A sunflower tattoo featuring green, orange, and burnt-red colors, (b) a green leaf tattoo feature green and blue colors, (c) a sun tattoo featuring yellow and orange colors. The field-of-view used for each tattoo is outlined by the dashed line.

Fig. 5

Mean optical properties for non-tattooed skin of each subject. From left to right: The calibrated unmodulated diffuse reflectance, the mean recovered absorption coefficient, and the mean reduced scattering coefficient.

Fig. 6

Mean optical properties for selected regions of interest, Subject #1. a: Color photo of tattoo with selected regions of interest highlighted. b: Unmodulated diffuse reflectance. c: Mean recovered absorption coefficients. d: Mean recovered reduced scattering coefficients.

Fig. 7

Mean optical properties for selected regions of interest, Subject #2. a: Color photo of tattoo with selected regions of interest highlighted. b: Unmodulated diffuse reflectance. c: Mean recovered absorption coefficients. d: Mean recovered reduced scattering coefficients.

Fig. 8

Mean optical properties for selected regions of interest, Subject #3. a: Color photo of tattoo with selected regions of interest highlighted. b: Unmodulated diffuse reflectance. c: Mean recovered absorption coefficients. d: Mean recovered reduced scattering coefficients.

Fig. 9

2-D false color, grayscale absorption map and accompanying histogram, showing from top to bottom: 650, 750, 850, 950 nm. The arrow points to the bin that includes the recovered mean absorption value of the non-tattooed skin.

Fig. 10

2-D false color, grayscale reduced scattering map and accompanying histogram, showing from top to bottom:650, 750, 850, 950 nm. The arrow points to the bin that includes the recovered mean reduced scattering value of the non-tattooed skin.