John W. Pickering

Double-integrating-sphere system for measuring the optical properties of tissue

A system is described and evaluated for the simultaneous measurement of the intrinsic optical properties of tissue: the scattering coefficient, the absorption coefficient, and the anisotropy factor. This system synthesizes the theory of two integrating spheres and an intervening scattering sample with the inverse adding-doubling algorithm, which employs the adding-doubling solution of the radiative transfer equation to determine the optical properties from the measurement of the light flux within each sphere and of the unscattered transmission. The optical properties may be determined simultaneously, which allows for measurements to be made while the sample undergoes heating, chemical change, or some otherexternal stimulus. An experimental validation of the system with tissue phantoms resulted in the determination of the optical properties with a < 5% deviation when the optical density was between 1 and 10 and the albedo was between 0.4 and 0.95.

In vitro double-integrating-sphere optical properties of tissues between 630 and 1064 nm

The optical properties (absorption and scattering coefficients and the scattering anisotropy factor) were measured in vitro for cartilage, liver, lung, muscle, myocardium, skin, and tumour (colon adenocarcinoma CC 531) at 630, 632.8, 790, 850 and 1064 nm. Rabbits, rats, piglets, goats, and dogs were used to obtain the tissues. A double-integrating-sphere setup with an intervening sample was used to determine the reflectance, and the diffuse and collimated transmittances of the sample. The inverse adding-doubling algorithm was used to determine the optical properties from the measurements. The overall results were comparable to those available in the literature, although only limited data are available at 790-850 nm. The results were reproducible for a specific sample at a specific wavelength. However, when comparing the results of different samples of the same tissue or different lasers with approximately the same wavelength (e.g. argon dye laser at 630 nm and HeNe laser at 632.8 nm) variations are large. We believe these variations in optical properties should be explained by biological variations of the tissues. In conclusion, we report on an extensive set of in vitro absorption and scattering properties of tissues measured with the same equipment and software, and by the same group. Although the accuracy of the method requires further improvement, it is highly likely that the other existing data in the literature have a similar level of accuracy.

Modeling the effect of wavelength on the pulsed dye laser treatment of port wine stains

To determine the influence of wavelength on the depth of vascular injury in port wine stains following pulsed dye laser treatment, we calculated fluence rates at wavelengths varying from 415 to 590 nm in a two-layer Monte Carlo model representing the epidermis and the dermis. Calculations were made for four different volumetric fractions of blood in the dermis: 0%, 1%, 5%, and 10%. The depth of the selective vascular injury was determined to be the depth at which the rate of temperature rise at some point within the vessel just equals that at the epidermal-dermal junction. This was maximal between 577 and 590 nm with the maximum shifted toward 590 nm for a greater dermal blood content and for larger vessels. The effect of greater epidermal pigmentation was not only to reduce the depth of vascular injury but to shift slightly the wavelength of the maximum vascular injury to a shorter wavelength.

Changes in the optical properties (at 632.8 nm) of slowly heated myocardium

The three transport equation optical properties, the absorption coefficient, the scattering coefficient, and the average cosine of the scattering angle, or anisotropy factor have been measured (at 632.8 nm) for canine myocardium after it is heated in a water bath at room temperature and at 37-75 degrees C for 1000 s. The measurement system was a double integrating sphere with collimated light and utilized the adding-doubling solution to the equation of radiative transfer. The absorption coefficient (room temperature control, 2.0 +/- 0.4 cm(-1)) began to increase and the anisotropy factor (room temperature control, 0.93 +/- 0.02) to decrease at above 45 degrees C. At 75 degrees C the changes were significant at the p < 0.0005 level (absorption, 4.5 +/- 1.3 cm(-1); anisotropy, 0.78 +/- 0.05). There was no significant change in the scattering coefficient (room temperature controls, 161 +/- 33 cm(-1)).