D. A. Boas and Dunn, A. K., “Laser speckle contrast imaging in biomedical optics.,” Journal of biomedical optics, vol. 15, pp. 011109, 2010. Publisher's VersionAbstract
First introduced in the 1980s, laser speckle contrast imaging is a powerful tool for full-field imaging of blood flow. Recently laser speckle contrast imaging has gained increased attention, in part due to its rapid adoption for blood flow studies in the brain. We review the underlying physics of speckle contrast imaging and discuss recent developments to improve the quantitative accuracy of blood flow measures. We also review applications of laser speckle contrast imaging in neuroscience, dermatology and ophthalmology.
R. T. Zaman, Parthasarathy, A. B., Vargas, G., Chen, B., Dunn, A. K., Rylander, H. G., and Welch, A. J., “Perfusion in hamster skin treated with glycerol.,” Lasers in surgery and medicine, vol. 41, pp. 492–503, 2009. Publisher's VersionAbstract
The objective of this article is to quantify the effect of hyper-osmotic agent (glycerol) on blood velocity in hamster skin blood vessels measured with a dynamic imaging technique, laser speckle contrast imaging (LSCI).
M. S. Starosta and Dunn, A. K., “Three-dimensional computation of focused beam propagation through multiple biological cells.,” Optics express, vol. 17, pp. 12455–69, 2009. Publisher's VersionAbstract
The FDTD method was used to simulate focused Gaussian beam propagation through multiple inhomogeneous biological cells. To our knowledge this is the first three dimensional computational investigation of a focused beam interacting with multiple biological cells using FDTD. A parametric study was performed whereby three simulated cells were varied by organelle density, nuclear type and arrangement of internal cellular structure and the beam focus depth was varied within the cluster of cells. Of the organelle types investigated, it appears that the cell nuclei are responsible for the greatest scattering of the focused beam in the configurations studied. Additional simulations to determine the optical scattering from 27 cells were also run and compared to the three cell case. No significant degradation of two-photon lateral imaging resolution was predicted to occur within the first 40 microm of imaging depth.
T. J. Huppert, Jones, P. B., Devor, A., Dunn, A. K., Teng, I. C., Dale, A. M., and Boas, D. A., “Sensitivity of neural-hemodynamic coupling to alterations in cerebral blood flow during hypercapnia.,” Journal of Biomedical Optics, vol. 14, pp. 044038, 2009. Publisher's VersionAbstract
The relationship between measurements of cerebral blood oxygenation and neuronal activity is highly complex and depends on both neurovascular and neurometabolic biological coupling. While measurements of blood oxygenation changes via optical and MRI techniques have been developed to map functional brain activity, there is evidence that the specific characteristics of these signals are sensitive to the underlying vascular physiology and structure of the brain. Since baseline blood flow and oxygen saturation may vary between sessions and across subjects, functional blood oxygenation changes may be a less reliable indicator of brain activity in comparison to blood flow and metabolic changes. In this work, we use a biomechanical model to examine the relationships between neural, vascular, metabolic, and hemodynamic responses to parametric whisker stimulation under both normal and hypercapnic conditions in a rat model. We find that the relationship between neural activity and oxy- and deoxyhemoglobin changes is sensitive to hypercapnia-induced changes in baseline cerebral blood flow. In contrast, the underlying relationships between evoked neural activity, blood flow, and model-estimated oxygen metabolism changes are unchanged by the hypercapnic challenge. We conclude that evoked changes in blood flow and cerebral oxygen metabolism are more closely associated with underlying evoked neuronal responses.
P. B. Jones, Shin, H. K., Boas, D. A., Hyman, B. T., Moskowitz, M. A., Ayata, C., and Dunn, A. K., “Simultaneous multispectral reflectance imaging and laser speckle flowmetry of cerebral blood flow and oxygen metabolism in focal cerebral ischemia,” Journal of Biomedical Optics, vol. 13, pp. 44007–44011, 2008. Publisher's VersionAbstract
A. B. Parthasarathy, Tom, W. J., Gopal, A., Zhang, X., and Dunn, A. K., “Robust flow measurement with multi-exposure speckle imaging,” Optics Express, vol. 16, pp. 1975–1989, 2008. Publisher's VersionAbstract
Laser Speckle Contrast Imaging \{(LSCI)\} is a minimally invasive full field optical technique used to generate blood flow maps with high spatial and temporal resolution. The lack of quantitative accuracy and the inability to predict flows in the presence of static scatterers such as an intact or thinned skull have been the primary limitation of \{LSCI.\} We present a new \{Multi-Exposure\} Speckle Imaging \{(MESI)\} instrument that has potential to obtain quantitative baseline flow measures. We show that the \{MESI\} instrument extends the range over which relative flow measurements are linear. We also present a new speckle model which can discriminate flows in the presence of static scatters. We show that in the presence of static scatterers the new model used along with the new \{MESI\} instrument can predict correlation times of flow consistently to within 10% of the value without static scatterers compared to an average deviation of more than 100% from the value without static scatterers using traditional \{LSCI.\} We also show that the new speckle model used with the \{MESI\} instrument can maintain the linearity of relative flow measurements in the presence of static scatterers.
J. Park, Estrada, A., Sharp, K., Sang, K., Schwartz, J. A., Smith, D. K., Coleman, C., Payne, J. D., Korgel, B. A., Dunn, A. K., and Tunnell, J. W., “Two-photon-induced photoluminescence imaging of tumors using near-infrared excited gold nanoshells,” Optics Express, vol. 16, pp. 1590–1599, 2008. Publisher's VersionAbstract
Gold nanoshells (dielectric silica core/gold shell) are a novel class of hybrid metal nanoparticles whose unique optical properties have spawned new applications including more sensitive molecular assays and cancer therapy. We report a new photo-physical property of nanoshells \{(NS)\} whereby these particles glow brightly when excited by near-infrared light. We characterized the luminescence brightness of \{NS,\} comparing to that of gold nanorods \{(NR)\} and fluorescent beads \{(FB).\} We find that \{NS\} are as bright as \{NR\} and 140 times brighter than \{FB.\} To demonstrate the potential application of this bright two-photon-induced photoluminescence \{(TPIP)\} signal for biological imaging, we imaged the \{3D\} distribution of gold nanoshells targeted to murine tumors.
A. Devor, Hillman, E. M. C., Tian, P., Waeber, C., Teng, I. C., Ruvinskaya, L., Shalinsky, M. H., Zhu, H., Haslinger, R. H., Narayanan, S. N., Ulbert, I., Dunn, A. K., Lo, E. H., Rosen, B. R., Dale, A. M., Kleinfeld, D., and Boas, D. A., “Stimulus-induced changes in blood flow and 2-deoxyglucose uptake dissociate in ipsilateral somatosensory cortex,” Journal of Neuroscience, vol. 28, pp. 14347–14357, 2008. Publisher's VersionAbstract
The present study addresses the relationship between blood flow and glucose consumption in rat primary somatosensory cortex \{(SI)\} in vivo. We examined bilateral neuronal and hemodynamic changes and 2-deoxyglucose \{(2DG)\} uptake, as measured by autoradiography, in response to unilateral forepaw stimulation. In contrast to the contralateral forepaw area, where neuronal activity, blood oxygenation/flow and \{2DG\} uptake increased in unison, we observed, in the ipsilateral \{SI,\} a blood oxygenation/flow decrease and arteriolar vasoconstriction in the presence of increased \{2DG\} uptake. Laminar electrophysiological recordings revealed an increase in ipsilateral spiking consistent with the observed increase in \{2DG\} uptake. The vasoconstriction and the decrease in blood flow in the presence of an increase in \{2DG\} uptake in the ipsilateral \{SI\} contradict the prominent metabolic hypothesis regarding the regulation of cerebral blood flow, which postulates that the state of neuroglial energy consumption determines the regional blood flow through the production of vasoactive metabolites. We propose that other factors, such as neuronal (and glial) release of messenger molecules, might play a dominant role in the regulation of blood flow in vivo in response to a physiological stimulus.
A. T. N. Kumar, Raymond, S. B., Dunn, A. K., Bacskai, B. J., and Boas, D. A., “A time domain fluorescence tomography system for small animal imaging.,” IEEE transactions on medical imaging, vol. 27, pp. 1152–63, 2008. Publisher's VersionAbstract
We describe the application of a time domain diffuse fluorescence tomography system for whole body small animal imaging. The key features of the system are the use of point excitation in free space using ultrashort laser pulses and noncontact detection using a gated, intensified charge-coupled device (CCD) camera. Mouse shaped epoxy phantoms, with embedded fluorescent inclusions, were used to verify the performance of a recently developed asymptotic lifetime-based tomography algorithm. The asymptotic algorithm is based on a multiexponential analysis of the decay portion of the data. The multiexponential model is shown to enable the use of a global analysis approach for a robust recovery of the lifetime components present within the imaging medium. The surface boundaries of the imaging volume were acquired using a photogrammetric camera integrated with the imaging system, and implemented in a Monte-Carlo model of photon propagation in tissue. The tomography results show that the asymptotic approach is able to separate axially located fluorescent inclusions centered at depths of 4 and 10 mm from the surface of the mouse phantom. The fluorescent inclusions had distinct lifetimes of 0.5 and 0.95 ns. The inclusions were nearly overlapping along the measurement axis and shown to be not resolvable using continuous wave (CW) methods. These results suggest the practical feasibility and advantages of a time domain approach for whole body small animal fluorescence molecular imaging, particularly with the use of lifetime as a contrast mechanism.
M. Atlan, Forget, B. \^ıotC., Boccara, A. C., Vitalis, T., Rancillac, A., Dunn, A. K., and Gross, M., “Cortical blood flow assessment with frequency-domain laser Doppler microscopy.,” Journal of biomedical optics, vol. 12, pp. 024019, 2007. Publisher's VersionAbstract
We report the assessment of cerebral blood flow (CBF) changes with a wide-field laser Doppler imager based on a CCD camera detection scheme, in vivo, in mice. The setup enables the acquisition of data in minimally invasive conditions. In contrast with conventional laser Doppler velocimeters and imagers, the Doppler signature of moving scatterers is measured in the frequency domain, by detuning a heterodyne optical detection. The quadratic mean of the measured frequency shift is used as an indicator of CBF. We observe a significant variability of this indicator in an experiment designed to induce blood flow changes.
E. M. C. Hillman, Devor, A., Bouchard, M. B., Dunn, A. K., Krauss, G. W., Skoch, J., Bacskai, B. J., Dale, A. M., and Boas, D. A., “Depth-resolved optical imaging and microscopy of vascular compartment dynamics during somatosensory stimulation.,” NeuroImage, vol. 35, pp. 89–104, 2007. Publisher's VersionAbstract
The cortical hemodynamic response to somatosensory stimulus is investigated at the level of individual vascular compartments using both depth-resolved optical imaging and in-vivo two-photon microscopy. We utilize a new imaging and spatiotemporal analysis approach that exploits the different characteristic dynamics of responding arteries, arterioles, capillaries and veins to isolate their three-dimensional spatial extent within the cortex. This spatial delineation is validated using vascular casts. Temporal delineation is supported by in-vivo two-photon microscopy of the temporal dynamics and vascular mechanisms of the arteriolar and venous responses. Using these techniques we have been able to characterize the roles of the different vascular compartments in generating and controlling the hemodynamic response to somatosensory stimulus. We find that changes in arteriolar total hemoglobin concentration agree well with arteriolar dilation dynamics, which in turn correspond closely with changes in venous blood flow. For 4-s stimuli, we see only small changes in venous hemoglobin concentration, and do not detect measurable dilation or ballooning in the veins. Instead, we see significant evidence of capillary hyperemia. We compare our findings to historical observations of the composite hemodynamic response from other modalities including functional magnetic resonance imaging. Implications of our results are discussed with respect to mathematical models of cortical hemodynamics, and to current theories on the mechanisms underlying neurovascular coupling. We also conclude that our spatiotemporal analysis approach is capable of isolating and localizing signals from the capillary bed local to neuronal activation, and holds promise for improving the specificity of other hemodynamic imaging modalities.
H. K. Shin, Dunn, A. K., Jones, P. B., Boas, D. A., Lo, E. H., Moskowitz, M. A., and Ayata, C., “Normobaric hyperoxia improves cerebral blood flow and oxygenation, and inhibits peri-infarct depolarizations in experimental focal ischaemia,” Brain, vol. 130, pp. 1631, 2007.Abstract
A. J. Strong, Anderson, P. J., Watts, H. R., Virley, D. J., Lloyd, A., Irving, E. A., Nagafuji, T., Ninomiya, M., Nakamura, H., Dunn, A. K., and Graf, R., “Peri-infarct depolarizations lead to loss of perfusion in ischaemic gyrencephalic cerebral cortex.,” Brain, vol. 130, pp. 995–1008, 2007. Publisher's VersionAbstract
In the light of accumulating evidence for the occurrence of spontaneous cortical spreading depression and peri-infarct depolarizations in the human brain injured by trauma or aneurysmal subarachnoid haemorrhage, we used DC electrode recording and laser speckle imaging to study the relationship between depolarization events and perfusion in the ischaemic, gyrencephalic brain. In 14 adult male cats anaesthetized with chloralose, one cerebral hemisphere was exposed and the middle cerebral artery occluded. Surface cortical perfusion in core and penumbral territories was imaged semiquantitatively at intervals of 13 s for 4 h. Cortical surface DC potential was recorded. Time interval between changes in DC potential and in perfusion was examined, and this comparison was repeated using microelectrodes for DC potential in five similar experiments in a second laboratory. Mean pre-occlusion perfusion was 11707 +/- 4581 units (equivalent to CBF (cerebral blood flow) approximately 40.5 +/- SD 14.4 ml/100 g/min), and fell on occlusion to 5318 +/- 2916 (CBF approximately 17.1 +/- 8.3), 5291 +/- 3407 (CBF approximately 17.0 +/- 10.1), and 6711 +/- 3271 (CBF approximately 22.2 +/- 9.6), quickly recovering to 8704 +/- 4581 (CBF approximately 29.5 +/- 14.4), 9741 +/- 4499 (CBF approximately 33.3 +/- 14.1) and 10 314 +/- 3762 (CBF approximately 35.4 +/- 11.4) on the core, intermediate and outer penumbral gyri, respectively. Mean perfusion later fell secondarily on core and intermediate gyri but, overall, was preserved on the outer (upper level of perfusion) gyrus during the period of observation. Pattern and severity of transient changes in perfusion associated with depolarization events varied with gyral location; falls in perfusion were sometimes profound and irreversible, and followed rather than preceded depolarization. In this model of occlusive stroke, reductions in perfusion linked to peri-infarct depolarization events contribute to secondary deterioration in penumbral areas. The findings suggest that such events play a central rather than a subsidiary role in cerebral infarction in the gyrencephalic brain.
M. Atlan, Gross, M., Forget, B. C., Vitalis, T., Rancillac, A., and Dunn, A. K., “Frequency-domain wide-field laser Doppler in vivo imaging.,” Optics letters, vol. 31, pp. 2762–4, 2006. Publisher's VersionAbstract
We present a new instrument, based on a low-frame-rate (8 Hz) CCD camera used in a heterodyne optical-mixing configuration, that can create wide-field laser Doppler maps. As an illustration, we show results obtained in a mouse brain, in vivo, showing the Doppler signature of blood flow. The instrument is based on a frequency-shifting digital holography scheme.
Evaluation of laser speckle flowmetry for imaging cortical perfusion in experimental stroke studies: quantitation of perfusion and detection of peri-infarct depolarisations,” Journal of Cerebral Blood Flow & Metabolism, vol. 26, pp. 645–653, 2006.Abstract
{Laser speckle imaging of the exposed cerebral cortex allows detailed examination of the time course and topography of perfusion under different experimental conditions. Here we examine the quantitative capacity of the method and its sensitivity for the detection of peri-infarct depolarisations \{(PIDs).\} In four cats anaesthetised with chloralose, the right hemisphere was exposed and the right middle cerebral artery was occluded. The brain was illuminated with a laser diode, the speckle pattern was imaged, and images of inverse speckle correlation time \{(ICT)\} were derived from the calculated speckle contrast images. We examined the relationship of İCT\} with perfusion, as imaged quantitatively using umbelliferone clearance \{(CBF(umb)).\} Values of İCT\
A. Devor, Ulbert, I., Dunn, A. K., Narayanan, S. N., Jones, S. R., Andermann, M. L., Boas, D. A., and Dale, A. M., “Coupling of the cortical hemodynamic response to cortical and thalamic neuronal activity.,” Proceedings of the National Academy of Sciences of the United States of America, vol. 102, pp. 3822–7, 2005. Publisher's VersionAbstract
Accurate interpretation of functional MRI (fMRI) signals requires knowledge of the relationship between the hemodynamic response and the neuronal activity that underlies it. Here we address the question of coupling between pre- and postsynaptic neuronal activity and the hemodynamic response in rodent somatosensory (Barrel) cortex in response to single-whisker deflection. Using full-field multiwavelength optical imaging of hemoglobin oxygenation and electrophysiological recordings of spiking activity and local field potentials, we demonstrate that a point hemodynamic measure is influenced by neuronal activity across multiple cortical columns. We demonstrate that the hemodynamic response is a spatiotemporal convolution of the neuronal activation. Therefore, positive hemodynamic response in one cortical column might be explained by neuronal activity not only in that column but also in the neighboring columns. Thus, attempts at characterizing the neurovascular relationship based on point measurements of electrophysiology and hemodynamics may yield inconsistent results, depending on the spatial extent of neuronal activation. The finding that the hemodynamic signal observed at a given location is a function of electrophysiological activity over a broad spatial region helps explain a previously observed increase of local vascular response beyond the saturation of local neuronal activity. We also demonstrate that the oxy- and total-hemoglobin hemodynamic responses can be well approximated by space-time separable functions with an antagonistic center-surround spatial pattern extending over several millimeters. The surround "negative" hemodynamic activity did not correspond to observable changes in neuronal activity. The complex spatial integration of the hemodynamic response should be considered when interpreting fMRI data.
S. Yuan, Devor, A., Boas, D. A., and Dunn, A. K., “Determination of optimal exposure time for imaging of blood flow changes with laser speckle contrast imaging,” Applied Optics, vol. 44, pp. 1823–1830, 2005.Abstract
Laser speckle contrast imaging is becoming an established method for full-field imaging of cerebral blood flow dynamics in animal models. The sensitivity and noise in the measurement of blood flow changes depend on the camera exposure time. The relation among sensitivity, noise, and camera exposure time was investigated experimentally by imaging the speckle contrast changes in the brain after electrical forepaw stimulation in rats. The sensitivity to relative changes in speckle contrast was found to increase at longer exposure times and to reach a plateau for exposure times greater than approximately 2 ms. However, the speckle contrast noise also increases with exposure time and thus the contrast-to-noise ratio was found to peak at an exposure time of approximately 5 ms. Our results suggests that approximately 5 ms is an optimal exposure time for imaging of stimulus-induced changes in cerebral blood flow in rodents.
A. T. Kumar, Skoch, J., Bacskai, B. J., Boas, D. A., and Dunn, A. K., “Fluorescence-lifetime-based tomography for turbid media,” Optics Letters, vol. 30, pp. 3347–3349, 2005.Abstract
We derive a novel algorithm to recover the in vivo distributions of fluorophores based on an asymptotic life-time analysis of time-domain fluorescence measurements with turbid tissue. We experimentally demonstrate the advantage offered by this method in localizing fluorophores with distinct lifetimes. This algorithm has wide applicability for diagnostic fluorescence imaging in the presence of several-centimeter-thick biological tissue, since fluorescence lifetime is a sensitive indicator of local tissue environment and interactions at the molecular level.
M. Gross, Goy, P., Forget, B. C., Atlan, M., Ramaz, F., Boccara, A. C., and Dunn, A. K., “Heterodyne detection of multiply scattered monochromatic light with a multipixel detector,” Optics Letters, vol. 30, pp. 1357–1359, 2005.Abstract
A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a \{CCD\} as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.
Spatial extent of oxygen metabolism and hemodynamic changes during functional activation of the rat somatosensory cortex,” Neuroimage, vol. 27, pp. 279–290, 2005.Abstract
{The spatial extent of the changes in oxy-hemoglobin \{(HbO),\} deoxy-hemoglobin \{(HbR),\} total hemoglobin concentration \{(HbT),\} cerebral blood flow \{(CBF),\} and the cerebral metabolic rate of oxygen \{(CMRO(2))\} in response to forepaw and whisker stimulation were compared in the rat somatosensory cortex using a combination of multi-wavelength reflectance imaging and laser speckle contrast imaging of cerebral blood flow. The spatial extents of the response of each hemodynamic parameter and \{CMRO(2)\} were found to be comparable at the time of peak response, and at early times following stimulation onset, the spatial extent of the change in \{HbR\} was smaller than that of \{HbO,\} \{HbT,\} \{CBF,\} and \{CMRO(2).\} In addition, a slight spatial dependence was found in the power law coefficient relating changes in \{CBF\} and \{HbT.\} Although the \{CMRO(2)\} response is a metabolic measure and thus expected to have a more localized response than the hemodynamic parameters, the results presented here suggest that this may not be the case in general, possibly due to the increased sensitivity of optical imaging techniques to superficial cortical layers where the lateral extent of the metabolic and neuronal activation is larger compared to that in layer İV.\} In addition, we found that the measured spatial extent of the \{CMRO(2)\} changes was insensitive to assumptions made in the calculation of the \{CMRO(2)\} changes such as baseline hemoglobin concentrations, vascular weighting constants, and wavelength dependence of tissue scattering. Multi-parameter full field imaging of the functional response provides a more complete picture of the hemodynamic response to functional activation including the spatial and temporal estimation of \{CMRO(2)\} changes.