Time-gated viewing studies on tissuelike phantoms

Non-tissue-like features in the time-of-flight distributions of plastic tissue phantoms

We measure high-temporal-resolution time-of-flight distributions of picosecond laser pulses in the visible and near-infrared, scattered in the forward direction by solid and liquid phantoms, and compare them to those obtained by using ex vivo tissues. We demonstrate that time-of-flight distributions from solid phantoms made of Delrin, Nylon, and Teflon are modulated by ripples that are absent in the biological samples and disappear when the temporal and/or angular resolution of the measuring apparatus is decreased. This behavior prevents the use of such materials as tissue phantoms when spatial mode and time selection are required, such as in imaging methods exploiting early arriving photons.

A soft deformable tissue-equivalent phantom for diffuse optical tomography

A recipe is presented for the manufacture of highly compressible phantoms for diffuse optical tomography. The recipe is based on polyvinyl alcohol (PVA) slime, a viscoelastic fluid which readily deforms under moderate pressure. Scattering particles and absorbing compounds can be added to provide a uniform material with stable and reproducible optical properties. A linear relationship between the concentration of scattering particles (either titanium dioxide or microspheres) and the transport scatter coefficient is demonstrated. Phantoms of an arbitrary size and shape may be produced by containing the slime within a thin latex shell, and a stability over a period of at least 3 months has been established. The deformable phantoms may be used to test and calibrate optical tomography systems designed for use on patients with irregular or variable geometries.

Pulsed Raman fiber laser and multispectral imaging in three dimensions

Raman scattering in single-mode optical fibers is exploited to generate multispectral light from a green nanolaser with high pulse repetition rate. Each pulse triggers a picosecond camera and measures the distance by time-of-flight in each of the 0.5 Mpixels. Three-dimensional images are then constructed with submillimeter accuracy for all visible colors. The generation of a series of Stokes peaks by Raman scattering in a Si fiber is discussed in detail and the laser radar technique is demonstrated. The data recording takes only a few seconds, and the high accuracy 3D color imaging works at ranges up to approximately 200 m. Applications for optical tomography in highly scattering media such as water and human tissue are mentioned.

Time-resolved optical tomography using short-pulse laser for tumor detection

Our objective is to perform a comprehensive experimental and numerical analysis of the short-pulse laser interaction with a tissue medium with the goal of tumor-cancer diagnostics. For a short-pulse laser source, the shape of the output signal is a function of the optical properties of the medium, and hence the scattered temporal optical signal helps in understanding the medium characteristics. Initially experiments are performed on tissue phantoms embedded with inhomogeneities to optimize the time-resolved optical detection scheme. Both the temporal and the spatial profiles of the scattered reflected and transmitted optical signals are compared with the numerical modeling results obtained by solving the transient radiative transport equation using the discrete ordinates technique. Next experiments are performed on in vitro rat tissue samples to characterize the interaction of light with skin layers and to validate the time-varying optical signatures with the numerical model. The numerical modeling results and the experimental measurements are in excellent agreement for the different parameters studied. The final step is to perform in vivo imaging of anesthetized rats with tumor-promoting agents injected inside skin tissues and of an anesthetized mouse with mammary tumors to demonstrate the feasibility of the technique for detecting tumors in an animal model.

Development of a time-domain optical mammograph and first in vivo applications

We have developed a laser-pulse mammograph capable of recording optical mammograms within approximately 3 min by measuring time-resolved transmittance at each of typically 1500 scan positions, 2.5 mm apart. As a first application two patients who have tumors were investigated successfully. From measured distributions of times of flight of photons corrected for edge effects we derived (1) characteristic quantities, such as photon counts in selected time windows, to generate optical mammograms; (2) effective transport scattering and absorption coefficients of breast tissue at each scan position, assuming the breast to be homogeneous; and (3) optical properties of a selected tumor by applying the theory of diffraction of photon density waves by spherical inhomogeneity. Mammograms recorded at different lateral offsets between source and detector fiber were used to estimate the depth of inhomogeneities.

Tumor contrast in time-domain, near-infrared laser breast imaging

The contrast of breast disease in an image produced by a time-domain, near-infrared laser, imaging system was calculated by use of a finite-difference method. The contrast was investigated for tumors of different sizes, in a range of positions within the breast, and in a range of breast thicknesses. Contrast is greatest for large tumors that are near the breast surfaces and that are measured at short times of flight. The contrast was shown to increase as the scattering and the absorption properties of the tumor increased. The scattering properties of the tumor were shown to make the biggest contribution to the contrast at short times of flight. Both pointlike and large extended source and detector configurations were investigated. The effect of the system size on the signal-to-noise ratio was investigated.

Computer simulation of time-gated transillumination and reflection of biological tissues and tissuelike phantoms

A simulation technique is employed to explore the possibility of locating millimeter-sized objects, immersed in turbid media, from time-gated measurements of the transmitted or reflected light. The simulation results for tissuelike phantoms are compared to experimental transillumination data and excellent agreement is found. Simulations of time-gated reflection experiments show that it is possible to detect objects of 1 mm diameter. This may open new possibilities for medical diagnosis of breast cancer in an early stage.

Evaluation of the temporally extrapolated absorbance method for dual-wavelength imaging through tissuelike scattering media

An independent assessment is described of a dual-wavelength imaging technique, known as the temporally extrapolated absorbance method (TEAM), proposed by Yamada et al. [Opt. Eng. 32, 634-641 (1993)]. The technique involves recording the temporal distribution of light transmitted across a scattering medium at two carefully chosen wavelengths at which the scattering properties of the medium are assumed to be identical. The objective is to image internal structure that absorbs more strongly at one wavelength than it does at the other. A simple theoretical treatment of TEAM is presented that employs a perturbation model of photon transport. This indicates that despite the lack of a secure theoretical basis, the technique may provide a potentially effective ad hoc method of generating images of highly scattering media. The method was also evaluated experimentally by using, for the first time to our knowledge, a single object and two wavelengths. A single-projection, two-dimensional image was obtained of a solid phantom with optical properties representative of breast tissue. The results exhibited good agreement with the theoretical model, and a small embedded feature that absorbs 3.5 times as strongly as the surrounding medium at one wavelength was revealed successfully.