Randall L. Barbour

Three-dimensional optical tomography of hemodynamics in the human head

We report on the first three-dimensional, volumetric, tomographic localization of vascular reactivity in the brain. To this end we developed a model-based iterative image reconstruction scheme that employs adjoint differentiation methods to minimize the difference between measured and predicted data. The necessary human-head geometry and optode locations were determined with a photogrammetric method. To illustrate the performance of the technique, the three-dimensional distribution of changes in the concentration of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin during a Valsalva maneuver were visualized. The observed results are consistent with previously reported effects concerning optical responses to hemodynamic perturbations.

Instrumentation for fast functional optical tomography

In this article, we describe the design rationale and performance features of an integrated multichannel continuous wave (cw) near-infrared (NIR) optical tomographic imager capable of collecting fast tomographic measurements over a large dynamic range. Fast data collection (∼70 Hz/channel/wavelength) is achieved using time multiplexed source illumination (up to 25 illumination sites) combined with frequency encoded wavelength discrimination (up to four-wavelength capability) and parallel detection (32 detectors). The described system features a computerized user interface that allows for automated system operation and is compatible with various previously described measuring heads. The results presented show that the system exhibits a linear response over the full dynamic measuring range (180 dB), and has excellent noise (∼10 pW noise equivalent power) and stability performance (<1% over 30 min). Recovered images of laboratory vessels show that dynamic behavior can be accurately defined and spatially loca...

MRI-guided optical tomography: prospects and computation for a new imaging method

Living tissue scatters near-infrared light randomly, can this problem be overcome to make NIR optical tomography possible? If so, it could be more accurate and less damaging than other medical imaging techniques. Computational experiments using combined MRI-optical methods show promise.

Design and implementation of dynamic near-infrared optical tomographic imaging instrumentation for simultaneous dual-breast measurements

Dynamic near-infrared optical tomographic measurement instrumentation capable of simultaneous bilateral breast imaging, having a capability of four source wavelengths and 32 source-detector fibers for each breast, is described. The system records dynamic optical data simultaneously from both breasts, while verifying proper optical fiber contact with the tissue through implementation of automatic schemes for evaluating data integrity. Factors influencing system complexity and performance are discussed, and experimental measurements are provided to demonstrate the repeatability of the instrumentation. Considerations in experimental design are presented, as well as techniques for avoiding undesirable measurement artifacts, given the high sensitivity and dynamic range (1:10(9)) of the system. We present exemplary clinical results comparing the measured physiologic response of a healthy individual and of a subject with breast cancer to a Valsalva maneuver.

Characterization of a New Ion Selective Electrode for Ionized Magnesium in Whole Blood, Plasma, Serum, and Aqueous Samples

Results from a novel ion selective electrode (ISE) for ionized magnesium (Mg2+) correlate well with atomic absorption spectroscopy on aqueous solutions containing from 0.1-3.0 mmol MgCl2/L. Day to day precision (coefficient of variation) of the electrode on protein-based controls is < 4%; aqueous-based controls < 6%. The new ISE is selective for Mg2+ with a selectivity constant for Ca2+ (KMgCa) of 8 x 10(-2). Adding pathophysiologic concentrations of Cd2+, Ca2+, Cu2+, Fe3+, K+, Na+, or Zn2+ to serum and aqueous solutions gave negligible to minimal changes in measured Mg2+. Ligand binding studies in aqueous solution indicate that pathophysiologic concentrations of different anions (e.g. heparin, lactate, bicarbonate, phosphate, acetate and sulfate) bind to Mg2+, effectively reducing its concentration in solution. Likewise, silicon (as either found in Vacuutainer tubes or as chlorosilane) failed to exert any significant effect on measured Mg2+. Addition of Intralipid (up to 500 mg/dL) gave negligible to minimal changes in Mg2+. Mg2+ measurements on whole blood, plasma, and serum for a given human subjects samples are virtually identical, at least within the reference range for Mg2+. Typically, Mg2+ is 71% of TMg, but varies from subject to subject; i.e. Mg2+ cannot be predicted from TMg. Clinical studies revealed that the Mg2+/TMg ratio could be remarkably consistent in sequential samples (e.g., throughout the course of coronary bypass surgery) taken from one patient, but that this ratio could differ dramatically from the ratio in sequential samples taken from another. Mg2+ is held within a narrow range (0.53-0.67 mmol/L) in normal, healthy subjects when compared to TMg (0.70-0.96 mmol/L).

Frequency-domain optical imaging of absorption and scattering distributions by a Born iterative method

We presents a Born; iterative method, for reconstructing optical properties of turbid media by means of frequency-domain data. The approach is based on iterative solution of a linear perturbation equation, which is derived from the integral from of the Helmholtz wave equation for photon-density waves in each iteration the total field and the associated weight matrix are recalculated based on the previous reconstructed image. We then obtain a new estimate by solving the updated perturbation equation. The forward solution, also based on a Helmholtz equation, is obtained by a multigrid finite difference method. The inversion is carried out through a Tikhonov regularized optimization process by the conjugate gradient descent method. Using this method, we first reconstruct the distribution of the complex wave numbers in a test medium, from which the absorption and the scattering distributions are then derived. Simulation results with two-dimensional test media have shown that this method can yield quantitatively (in terms of coefficient valued) as well as qualitatively (in terms of object location and shape) accurate reconstructions of absorption and scattering distributions in cases in which the first-order Born approximation cannot work well. Both full-angle and limited-angle measurement schemes have been simulated to examine the effect of the location of detectors and sources. The robustness of the algorithm to noise has also been evaluated.

Instrumentation and calibration protocol for imaging dynamic features in dense-scattering media by optical tomography

Instrumentation is described that is suitable for acquiring multisource, multidetector, time-series optical data at high sampling rates (up to 150 Hz) from tissues having arbitrary geometries. The design rationale, calibration protocol, and measured performance features are given for both a currently used, CCD-camera-based instrument and a new silicon-photodiode-based system under construction. Also shown are representative images that we reconstructed from data acquired in laboratory studies using the described CCD-based instrument.

A wavelet-based multiresolution regularized least squares reconstruction approach for optical tomography

The authors present a wavelet-based multigrid approach to solve the perturbation equation encountered in optical tomography. With this scheme, the unknown image, the data, as well as the weight matrix are all represented by wavelet expansions, thus yielding a multiresolution representation of the original perturbation equation in the wavelet domain. This transformed equation is then solved using a multigrid scheme, by which an increasing portion of wavelet coefficients of the unknown image are solved in successive approximations. One can also quickly identify regions of interest (ROIs) from a coarse level reconstruction and restrict the reconstruction in the following fine resolutions to those regions. At each resolution level a regularized least squares solution is obtained using the conjugate gradient descent method. This approach has been applied to continuous wave data calculated based on the diffusion approximation of several two-dimensional (2-D) test media. Compared to a previously reported one grid algorithm, the multigrid method requires substantially shorter computation time under the same reconstruction quality criterion.

Influence of systematic errors in reference states on image quality and on stability of derived information for dc optical imaging

Optical measurements of tissue can be performed in discrete, time-averaged, and time-varying data collection modes. This information can be evaluated to yield estimates of either absolute optical coefficient values or some relative change in these values compared with a defined state. In the case of time-varying data, additional analysis can be applied to define various dynamic features. Here we have explored the accuracy with which such information can be recovered from dense scattering media using linear perturbation theory, as a function of the accuracy of the reference medium that serves as the initial guess. Within the framework of diffusion theory and a first-order solution, we have observed the following inequality regarding the sensitivity of computed measures to inaccuracy in the reference medium: Absolute measures ? relative measures > dynamic measures. In fact, the fidelity of derived dynamic measures was striking; we observed that accurate measures of dynamic behavior could be defined even if the quality of the image data from which these measures were derived was comparatively modest. In other studies we identified inaccuracy in the estimates of the reference detector values, and not to corresponding errors in the image operators, as the primary factor responsible for instability of absolute measures. The significance of these findings for practical imaging studies of tissue is discussed.

Normalized–constraint algorithm for minimizing inter–parameter crosstalk in DC optical tomography

In this report, we present a method for reducing the inter-coefficient crosstalk problem in optical tomography. The method described is an extension of a previously reported normalized difference method that evaluates relative detector values, and employs a weight matrix scaling technique together with a constrained CGD method for image reconstruction. Results from numerical and experimental studies using DC measurement data demonstrate that the approach can effectively isolate absorption and scattering heterogeneities, even for complex combinations of perturbations in optical properties. The significance of these results in light of recent theoretical findings is discussed.