Multispectral imaging of tissue absorption and scattering using spatial frequency domain imaging and a computed-tomography imaging spectrometer

Abstract

We present an approach for rapidly and quantitatively mapping tissue absorption and scattering spectra in a wide-field, noncontact imaging geometry by combining multifrequency spatial frequency domain imaging (SFDI) with a computed-tomography imaging spectrometer (CTIS). SFDI overcomes the need to spatially scan a source, and is based on the projection and analysis of periodic structured illumination patterns. CTIS provides a throughput advantage by simultaneously diffracting multiple spectral images onto a single CCD chip to gather spectra at every pixel of the image, thus providing spatial and spectral information in a single snapshot. The spatial-spectral data set was acquired 30 times faster than with our wavelength-scanning liquid crystal tunable filter camera, even though it is not yet optimized for speed. Here we demonstrate that the combined SFDI-CTIS is capable of rapid, multispectral imaging of tissue absorption and scattering in a noncontact, nonscanning platform. The combined system was validated for 36 wavelengths between 650-1000 nm in tissue simulating phantoms over a range of tissue-like absorption and scattering properties. The average percent error for the range of absorption coefficients (μa) was less than 10% from 650-800 nm, and less than 20% from 800-1000 nm. The average percent error in reduced scattering coefficients (μs') was less than 5% from 650-700 nm and less than 3% from 700-1000 nm. The SFDI-CTIS platform was applied to a mouse model of brain injury in order to demonstrate the utility of this approach in characterizing spatially and spectrally varying tissue optical properties.

Figure 1

A raw CTIS snapshot. The center rectangle is the undiffracted image, surrounded by the diffracted orders of the image after it passes through the 2D grating. The snapshot is passed through a computed-tomography (CT) algorithm to produce a data cube of images (x,y) at each desired wavelength.

Figure 2

Combined structured illumination and CTIS system.

Figure 3

Scattering titration results. Each color represents a phantom with different concentration of scatterer. Absorption was held constant. X’s mark the fit or recovered data points, while solid lines represent the expected values. (a) Fit and expected absorption coefficient versus wavelength, held constant. Colors correspond to the scattering titration phantoms of the same color. (b) Fit and expected reduced scattering coefficient versus wavelength for each of five different scatterer concentrations. (c) The absolute value of the average % error over the range of the titration at each wavelength. Calibration phantom values at 650 nm: ${\mu }_{\mathrm{s}}^{\prime }$ = 1.67 mm⁻¹ and μ_a = 0.0056 mm⁻¹. Number of pixels, n = 10 × 13 (cropped and binned), standard deviation of scattering = <4% at 650 nm.

Figure 4

Absorption titration results. Each color represents a phantom with different concentration of absorber. Scattering was held constant. X’s mark the fit or recovered data; solid lines represent the expected values. (a) Fit and expected absorption coefficient versus wavelength for each of five different absorber concentrations. (b) Fit and expected reduced scattering coefficient versus wavelength, held constant. Colors correspond to the absorption titration phantom of the same color. (c) The absolute value of the average % error over the range of the titration at each wavelength appears below the plots. Calibration phantom values at 650 nm: ${\mu }_{\mathrm{s}}^{\prime }$ = 0.97 mm⁻¹ and μ_a = 0.01 mm⁻¹. Number of pixels, n = 10 × 13 (cropped and binned), standard deviation of absorption at 650 nm = <6.5%.

Figure 5

Mouse cortex with a region of interest (dashed rectangle) chosen to include one side of the cortex containing the site of the small bleed and surrounding normal tissue. The top of the image is the anterior side of the brain.

Figure 6

Optical property maps (top) and histograms (bottom) of mouse cortex at and surrounding the site of injury at 650 nm. (a) Absorption map and (b) reduced scattering map. The image is 6.49 × 4.02 mm, 63 × 39 pixels.

Figure 7

(a) Diffuse reflectance map of mouse cortex at and surrounding the site of injury at 650 nm, (b) absorption, and (c) reduced scattering maps. Regions of interest detailing the absorption and scattering spectra over normal cortex (green), a small pool of blood (blue). The image is 6.49 × 4.02 mm, 63 × 39 pixels. (Color online only.)

Figure 8

Chromophore maps for mouse cortex at and surrounding site of injury, fit from the absorption spectra. (a) Oxy-hemoglobin concentration (HbO₂), (b) deoxy-hemoglobin concentration (Hb), (c) water percentage (H₂O), (d) total hemoglobin concentration (HbT), and (e) oxygen saturation percentage (S_TO₂). Each image is 6.49 × 4.02 mm, 63 × 39 pixels.