Eva M. Sevick

Quantitation of time- and frequency-resolved optical spectra for the determination of tissue oxygenation

The recent development of near-infrared time- and frequency-resolved tissue spectroscopy techniques to probe tissue oxygenation and tissue oxygenation kinetics has led to the need for further quantitation of spectroscopic signals. In this paper, we briefly review the theory of light transport in strongly scattering media as monitored in the time and frequency domains, and use this theory to develop algorithms for quantitation of hemoglobin saturation from the photon decay rate (delta log R/delta t) obtained using time-resolved spectroscopy, and from the phase-shift (theta) obtained from frequency-resolved, phase-modulated spectroscopy. To test the relationship of these optical parameters, we studied the behavior of delta log R/delta t and theta as a function of oxygenation in model systems which mimicked the optical properties of tissue. Our results show that deoxygenation at varying hemoglobin concentrations can be monitored with the change in the photon decay kinetics, delta delta log R/delta t in the time-resolved measurements, and with the change in phase-shift, delta theta, in the frequency-resolved technique. Optical spectra of the adult human brain obtained with these two techniques show similar characteristics identified from the model systems.

Time-dependent optical spectroscopy and imaging for biomedical applications

While optical spectroscopy and imaging are essential tools in science and engineering, their application in living tissue is complicated by multiple scattering of light. In spectroscopy, this scattering causes uncertainty in the pathlength traveled by photons in the tissue, while images suffer reduced resolution and contrast. Picosecond light sources and fast detectors have made it possible to address these problems by direct measurement of the photon time-of-flight

Localization of absorbers in scattering media by use of frequency-domain measurements of time-dependent photon migration

Frequency-domain studies of time-dependent light propagation in tissuelike phantoms that contain optical heterogeneities are described. Specifically the phase shift and amplitude modulation of reemergent light were measured when illuminated by an amplitude-modulated light source. Changes in the phase angle and the extent of modulation revealed the presence of a light-absorbing object. Furthermore the magnitude and direction of these changes were sensitive to the absorber depth and the light modulation frequency in a manner that could be used to infer the location of the heterogeneity. These data suggest the feasibility of optical imaging by frequency-domain methods.

Frequency domain imaging of absorbers obscured by scattering

Multiple pixel, frequency domain measurements of phase shift, theta, and modulation, m, in a phantom containing an absorber obscured by a relatively non-absorbing scattering solution are presented in combination with a theory of photon migration imaging. Results employing a single point source show that two dimensional theta measurements made in the presence (theta presence) and in the absence (theta absence) of an absorber can be used to create delta theta images. delta theta (theta absence-theta presence) images can be used to detect as well as locate the three dimensional position of the absorber. Images of mpresence measured in the presence of the absorber normalized by mabsence also provided detection and two dimensional location of its position. Images of % mpresence/mabsence at higher modulation frequencies provided greater resolution as predicted by photon migration theory. Neither theta nor m images alone could be used to detect or locate the presence of the absorber.

The CaMKK2/CaMKIV Relay Is an Essential Regulator of Hepatic Cancer

Hepatic cancer is one of the most lethal cancers worldwide. Here, we report that the expression of Ca2+/calmodulin‐dependent protein kinase kinase 2 (CaMKK2) is significantly up‐regulated in hepatocellular carcinoma (HCC) and negatively correlated with HCC patient survival. The CaMKK2 protein is highly expressed in all eight hepatic cancer cell lines evaluated and is markedly up‐regulated relative to normal primary hepatocytes. Loss of CaMKK2 function is sufficient to inhibit liver cancer cell growth, and the growth defect resulting from loss of CaMKK2 can be rescued by ectopic expression of wild‐type CaMKK2 but not by kinase‐inactive mutants. Cellular ablation of CaMKK2 using RNA interference yields a gene signature that correlates with improvement in HCC patient survival, and ablation or pharmacological inhibition of CaMKK2 with STO‐609 impairs tumorigenicity of liver cancer cells in vivo. Moreover, CaMKK2 expression is up‐regulated in a time‐dependent manner in a carcinogen‐induced HCC mouse model, and STO‐609 treatment regresses hepatic tumor burden in this model. Mechanistically, CaMKK2 signals through Ca2+/calmodulin‐dependent protein kinase 4 (CaMKIV) to control liver cancer cell growth. Further analysis revealed that CaMKK2 serves as a scaffold to assemble CaMKIV with key components of the mammalian target of rapamycin/ribosomal protein S6 kinase, 70 kDa, pathway and thereby stimulate protein synthesis through protein phosphorylation. Conclusion: The CaMKK2/CaMKIV relay is an upstream regulator of the oncogenic mammalian target of rapamycin/ribosomal protein S6 kinase, 70 kDa, pathway, and the importance of this CaMKK2/CaMKIV axis in HCC growth is confirmed by the potent growth inhibitory effects of genetically or pharmacologically decreasing CaMKK2 activity; collectively, these findings suggest that CaMKK2 and CaMKIV may represent potential targets for hepatic cancer. (Hepatology 2015;62:505–520

Near-Infrared Optical Imaging of Tissue Phantoms with Measurement in the Change of Optical Path Lengths

Using 2-D Monte Carlo simulations, we have demonstrated that values of delta < L > at varying delta t, T1 and T2 can contain significant information concerning the presence and location of a light absorbing volume in scattering media such as tissue. Specifically, we have illustrated that relationships exist between ZPW measured in x,y reflectance geometries and the absorber x,y,z position. These relationships are predictable yet can be expected to furthermore vary with (i) absorber z-dimensions, (ii) the optical properties of the surrounding media, and (iii) the source/detector separation, rho. In addition, while we have reported absorber positions located within 1 cm of tissue thickness for rho = 2 cm, One can expect interrogation of absorbers located at greater tissue thicknesses with greater rho 4. Most importantly, it is noteworthy that there exists greater opportunity to monitor specific population of photons in time-domain PMI than in frequency-domain PMI. Therefore, time-domain localization may be more sensitive than in the frequency-domain. From comparison to PMI, Figure 8 illustrates the values of delta I* computed from equation (1) versus absorber position for the same values T1 and T2 as in Figure 3. Upon inspection of Figures 3 and 8, one can see that it is easier to infer the relationship between the PSV and the absorber position from delta < L > than from delta I*. As a consequence, the measurement of delta < L > may enable creation of an inverse localization algorithm for photon migration imaging.(ABSTRACT TRUNCATED AT 250 WORDS)

Microvascular network architecture in a mammary carcinoma

The distribution of blood flow in a given tumor regulates the exchange and uptake of relevant molecules in chemo-, immuno-, and radiation therapy, thereby determining the efficacy of present day cancer treatments. Because it plays a significant role in heat transfer, the distribution of blood flow is also important in both thermographic detection and hyperthermia treatment [1, 2]. But what determines the distribution of blood flow in a microvascular network? In both normal and pathologic tissues, blood flow through a vascular bed is determined by several factors including the vascular network topology and the dimensions of the blood vessels. Previous workers have made qualitative comparisons of host and tumor vasculature including both animal tumors (for review see [3]) as well as human tumor xenotransplants [4]. To date, however, there exists no quantitative data describing the microvascular network architecture of any tumor type. If such data can be obtained, then a network model capable of predicting the blood pressure and flow distribution in a given tumor can be constructed, thus allowing insight into growth rate variability among tumors and the mechanisms governing current cancer therapies. Below is a brief presentation of the results of such a quantitative analysis of the vascular network topology in a mammary adenocarcinoma from Less et al. [5].

Frequency-domain measurements of changes of optical pathlength during spreading depression in a rodent brain model

Previously, we have shown that time-resolved spectroscopy can monitor changes in the distribution of photon migration pathlengths which are reflective of the changes in the tissue absorption due primarily to oxygenated or deoxygenated hemoglobin. In this study, we have monitored mean photon migration pathlengths in the frequency domain in the rodent brain insulted by hypoxia, ischemia and spreading depression (SD) using phase modulated spectroscopy (PMS). This technique consisted of monitoring light which emerged from the exposed rodent skull at 8 mm form an incident light source of 754 nm and 816 nm whose intensity was modulated at 220 MHz. The changes in phase-shift, (theta), of the emergent light with respect to the incident light are reflective of the photon pathlengths and hemoglobin absorbance. A multiprobe assembly holding PMS source fiber, nicotinamide dinucleotide (NADH) fluorometric probe, electrocortigraph (ECoG) electrodes, and doppler blood flow probe was placed on the rodent brain to simultaneously monitor brain metabolism, electrical cortical activity (ECoG) and blood flow. The PMS detector fiber was placed 8 mm posterior to the multiprobe assembly. Correlations between changes in intracellular deoxygenation (NADH) and hemoglobin deoxygenation as measured by PMS changes at 754 nm and 816 nm during hypoxia, and ischemia were found. The depolarization phase of spreading depression resulted in a similar increase at both 754 nm and 816 nm. We attribute this result to vasoconstriction and/or the decrease of extracellular space due to water shift in the rodent brain.

Experimental determination and mathematical model of the transient incorporation of cholesterol in the arterial wall

Experimental data of the radial incorporation of labeled cholesterol [14C-4] into the artery wall is regressed against a mathematical model that predicts macromolecular transport in this biological system. Data is obtained using excised canine carotid arteries that are perfusedin vitro under pulsatile hemodynamic conditions for 2 hr. Vessels are exposed to either normotensive hemodynamics, hypertensive hemodynamics, or simulations in which the rate of flow or vessel compliance is deliberately altered. Several arteries are studied under normotensive conditions following balloon catheter deendothelialization. Transmural concentration profiles of [14C-4] activity are determined by microcryotomy of longitudinal sections of perfused vessels. Nonlinear Marquardt regression on 12 experimental cases yields parameter estimates of effective diffusivity,D and solute filtration velocity,V. Results of this experimental investigation support our hypothesis that hemodynamics and the endothelial lining influence wall flux in intact vessels. Exposure to altered (vs normotensive) hemodynamics is associated with increased incorporation of labeled cholesterol. A similar observation is made for deendothelialized vessels (e.g. a greater accumulation of label and a rise in convective flux). Based upon our companion measurements of vessel wall forces and endothelial cellular morphology accompanying hemodynamic simulations, we suggest that hemodynamically induced alterations to endothelial structures lead to the increased permeability, convection and incorporation that we observe in this work.

Time-dependent photon migration imaging

Recently, the application of both time- and frequency-resolved fluorescence techniques for the determination of photon migration characteristics in strongly scattering media has been used to characterize the optical properties in strongly scattering media. Specifically, Chance and coworkers have utilized measurement of photon migration characteristics to determine tissue hemoglobin absorbance and ultimately oxygenation status in homogeneous tissues. In this study, we present simulation results and experimental measurements for both techniques to show the capacity of time-dependent photon migration characteristics to image optically obscure absorbers located in strongly scattering media. The applications of time-dependent photon imaging in the biomedical community include imaging of light absorbing hematomas, tumors, hypoxic tissue volumes, and other tissue abnormalities. Herein, we show that the time-resolved parameter of mean photon path length, , and the frequency- resolved parameter of phase-shift, (theta) , can be used similarly to obtain three dimensional information of absorber position from two-dimensional measurements. Finally, we show that unlike imaging techniques that monitor the intensity of light without regard to the migration characteristics, the resolution of time-dependent photon migration measurements is enhanced by tissue scattering, further potentiating their use for biomedical imaging.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.