Purpose: Our purpose is to develop and test a molecular probe that can detect the expression of neutropilin-1 receptor
(NPR-1) in vivo using fluorescence imaging and photoacoustic spectroscopy.
Introduction: NPR-1 is expressed on endothelial cells and some breast cancer cells, and binds to vascular endothelial
growth factor VEGF165, a growth factor associated with pathological tumor angiogenesis. This receptor is coexpressed
with VEGFR2 and shown to enhance the binding of VEGF165; therefore, it has the potential to be used as a
marker of angiogenic activity and targeted for therapy.
Material and Methods: A peptide specific to NPR-1 receptor was synthesized and conjugated to a NIR fluorochrome
(IRDye800CW) and was intravenously injected into mice with breast tumors (MCF7VEGF). Probe kinetics was
monitored in vivo via near infrared fluorescence (NIRF) within an optical imager for up to 72 hours within the tumor
and compared to other organs (liver, muscle) for binding specificity. A multivariate fitting algorithm was used to
spectrally deconvolve the IRDye800CW from endogenous hemoglobin signature (hemoglobin concentration and
oxygen saturation).
Results: Dynamics of the NIR fluorescence signal within the first hour after injection indicates specific binding
compared to muscle, with an average tumor-to-muscle ration of 2.00 (+/- 0.27). Spectral analysis clearly indentified
the presence of the NPR-1 probe. Based on calibration data, the average tumor concentration from both NIRF and
PCT-S was measured to be ~200-300nM.
Conclusion: These preliminary results show the capability of PCT to image an exogenous probe in vivo in addition to
its hemoglobin state.
Purpose: The purpose of this study is to calibrate the PCT scanner to quantify the hemoglobin status utilizing a blood
flow phantom.
Materials and Methods: A blood circulation system was designed and constructed to control the oxygen saturation and
hemoglobin concentration of blood. As a part of the circulation system, a 1.1mm FEP tube was placed in the center of
imaging tank of PCT scanner as the imaging object. Photoacoustic spectra (690-950 nm) was acquired for different
hemoglobin concentrations (CtHb) and oxygen saturation levels (SaO2), where the formers was formed by diluting
blood samples with PBS and the latter by mixing blood with gases at different oxygen content. Monte Carlo simulations
were performed to calculate the photon energy depositions in the phantom tube, which took into account photon losses in
water and blood. A Kappa value which represents the energy transfer efficiency of hemoglobin molecule was calculated
based on the PCT measurement and simulation result. The final SaO2 value of each blood sample was calculated based
on the PCT spectrum and Kappa value. These oxygen saturation results were compared with co-oximeter measurements
to obtain systematic errors.
Results and Conclusion: The statistic error of calculating Kappa value from hemoglobin concentration experiment was
less than 5%. The systematic error between PCT spectra analysis and co-oximeter analysis for hemoglobin oxygen
saturation was -4.5%. These calibration techniques used to calculate Kappa and hemoglobin absorption spectra would be
used in hypoxia measurements in tumors as well as for endogenous biomarkers studies.
Purpose: The purpose of this study is to use PCT spectroscopy scanner to monitor the hemoglobin concentration and
oxygen saturation change of living mouse by imaging the artery and veins in a mouse tail.
Materials and Methods: One mouse tail was scanned using the PCT small animal scanner at the isosbestic wavelength
(796nm) to obtain its hemoglobin concentration. Immediately after the scan, the mouse was euthanized and its blood was
extracted from the heart. The true hemoglobin concentration was measured using a co-oximeter. Reconstruction
correction algorithm to compensate the acoustic signal loss due to the existence of bone structure in the mouse tail was
developed. After the correction, the hemoglobin concentration was calculated from the PCT images and compared with
co-oximeter result. Next, one mouse were immobilized in the PCT scanner. Gas with different concentrations of oxygen
was given to mouse to change the oxygen saturation. PCT tail vessel spectroscopy scans were performed 15 minutes
after the introduction of gas. The oxygen saturation values were then calculated to monitor the oxygen saturation change
of mouse.
Results: The systematic error for hemoglobin concentration measurement was less than 5% based on preliminary
analysis. Same correction technique was used for oxygen saturation calculation. After correction, the oxygen saturation
level change matches the oxygen volume ratio change of the introduced gas.
Conclusion: This living mouse tail experiment has shown that NIR
PCT-spectroscopy can be used to monitor the oxygen
saturation status in living small animals.
Purpose: The purpose of this study is to monitor in vivo the IR dose dependent response of NF-κB and tumor
hemodynamics as a function of time.
Material and Methods: An MDA-231 breast cancer cell line was stably transfected with a firefly luciferase gene
within the NF-kappaB promoter. Tumors on the right flank irradiated with a single fractionated dose of 5Gy or 10Gy.
Over two weeks, photoacoustic spectroscopy (PCT-S), bioluminescence imaging (BLI), and dynamic contrast
enhanced CT (DCE-CT) was used to monitor hemoglobin status, NF-kappaB expression, and physiology, respectively.
Results: From the BLI, an increase in NF-kappaB expression was observed in both the right (irradiation) and left (nonirradiated)
tumors, which peaked at 8-12 hours, returned to basal levels after 24 hours, and increased a second time
from 3 to 7 days. This data identifies both a radiation-induced bystander effect and a bimodal longitudinal response
associated with NF-κB-controlled luciferase promoter. The physiological results from DCE-CT measured an increase
in perfusion (26%) two days after radiation and both a decrease in perfusion and an increase in fp by week 1 (10Gy
cohort). PCT-S measured increased levels of oxygen saturation two days post IR, which did not change after 1 week.
Initially, NF-κB would modify hemodynamics to increase oxygen delivery after IR insult. The secondary response
appears to modulate tumor angiogenesis.
Conclusions: A bimodal response to radiation was detected with NF-kappaB-controlled luciferase reporter with a
concomitant hemodynamic response associated with tumor hypoxia. Experiments are being performed to increase
statistics.
Purpose: The purpose of this study is to calibrate the PCT small animal scanner system with a blood phantom and to measure the blood hemoglobin concentration and oxygen saturation level in mouse tail vein and tumors.
Methods and Materials: A blood phantom with variable blood flow and oxygen content was integrated into the PCT scanner with a circulation system. The circulation system consisted of a pump, an oxygen electrode detector and a tonometer. The SaO2 values were determined based on a linear combination model of oxy-hemoglobin and deoxy-hemoglobin absorption spectrum curves. Hemoglobin concentration (CHb) was determined by measuring the linear relationship for different blood dilutions. SaO2 and CHb as measured in vivo for the artery and vein in a mouse tail were also measured.
Results: The PCT spectra of blood phantom samples were measured and compared with hemoglobin optical absorption spectra. The linear relationship between hemoglobin concentration and PCT intensities were observed by phantom study. The hemoglobin concentration of mouse is ~9.7g/dL. The saturation difference between arteries and veins in mouse tail is also measured by PCT scan.
Conclusions: Both the phantom and living mouse tail vessel scans have shown that NIR PCT-spectroscopy can be used to measure the hemoglobin saturation level and hemoglobin concentration in small animal for future tumor hypoxia study.
KEYWORDS: Monte Carlo methods, Tumors, Blood, Photoacoustic spectroscopy, Absorption, Near infrared, Tissue optics, Photon transport, Acoustics, Signal attenuation
Purpose: The purpose of this study is to evaluate the influence of photon propagation on the NIR spectral features associated with photoacoustic imaging.
Introduction: Photoacoustic CT spectroscopy (PCT-S) has the potential to identify molecular properties of tumors while overcoming the limited depth resolution associated with optical imaging modalities (e.g., OCT and DOT). Photoacoustics is based on the fact that biological tissue generates high-frequency acoustic signals due to volume of expansion when irradiated by pulsed light. The amplitude of the acoustic signal is proportional to the optical absorption properties of tissue, which varies with wavelength depending on the molecular makeup of the tissue. To obtain quantifiable information necessitate modeling and correcting for photon and acoustic propagation in tumors.
Material and Methods: A Monte Carlo (MC) algorithm based on MCML (Monte Carlo for Multi-Layered edia) has been developed to simulate photon propagation within objects comprised of a series of complex 3D surfaces (Mcml3D). This code has been used to simulate and correct for the optical attenuation of photons in blood, and for subcutaneous tumors with homogenous and radially heterogeneous vascular distributions.
Results: The NIR spectra for oxygenated and deoxygenated blood as determined from Monte Carlo simulated photoacoustic data matched measured data, and improving oxygen saturation calculations. Subcutaneous tumors with a homogeneous and radially heterogeneous distribution of blood revealed large variations in photon absorption as a function of the scanner projection angle. For select voxels near the periphery of the tumor, this angular profile between the two different tumors appeared similar.
Conclusions: A Monte Carlo code has been successfully developed and used to correct for photon propagation effects in blood phantoms and restoring the integrity of the NIR spectra associated with oxygenated and deoxygenated blood. This code can be used to simulate the influence of intra-tumor heterogeneity on the molecular identification via NIR spectroscopy.
Purpose: To evaluate photoacoustic CT spectroscopy (PCT-S) and dynamic contrast-enhanced CT (DCE-CT) ability to measure parameters - oxygen saturation and vascular physiology - associated with the intra-tumor oxygenation status.
Material and Methods: Breast (VEGF165 enhance MCF-7) and ovarian (SKOV3x) cancer cells were implanted into the fat pads and flanks of immune deficient mice and allowed to grow to a diameter of 8-15 mm. CT was used to determine physiological parameters by acquiring a sequence of scans over a 10 minute period after an i.v. injection of a radio-opaque contrast agent (Isovue). These time-dependent contrast-enhanced curves were fit to a two-compartmental model determining tumor perfusion, fractional plasma volume, permeability-surface area produce, and fractional interstitial volume on a voxel-by-voxel basis. After which, the tumors were imaged using photoacoustic CT (Optosonics, Inc., Indianapolis, IN 46202). The near infrared spectra (700-910 nm) within the vasculature was fit to linear combination of measured oxy- and deoxy-hemoglobin blood samples to obtain oxygen saturation levels (SaO2).
Results: The PCT-S scanner was first calibrated using different samples of oxygenated blood, from which a statistical error ranging from 2.5-6.5% was measured and a plot of the hemoglobin dissociation curve was consistent with empirical formula. In vivo determination of tumor vasculature SaO2 levels were measurably tracked, and spatially correlated to the periphery of the tumor. Tumor depend variations in SaO2 - 0.32 (ovarian) and 0.60 (breast) - and in vascular physiology - perfusion, 1.03 and 0.063 mL/min/mL, and fractional plasma volume, 0.20 and 0.07 - were observed.
Conclusion: Combined, PCT-S and CED-CT has the potential to measure intra-tumor levels of tumor oxygen saturation and vascular physiology, key parameters associated with hypoxia.
Purpose: The purpose of this study is to evaluate PCT Imaging technique to classify tissue and extract kidney cysts in pcy mice model of human adolescent nephronophthisis. Method: Four mice with late stages of nephronophthisis with polycystic kidney disease-PKD and one normal mouse were scanned in the PCT Small Animal Scanner. Both vivo and ex-vivo images of mice kidney were taken at wavelength from 680 nm to 940 nm. The ex-vivo PCT images were compared with histology photographs to check the sensitivity of detecting cysts. Histograms of kidney images were generated over slices and fitted to Gaussian-curve model for volumetric analysis. The portions of cysts in kidneys were estimated and kidney images were segmented by three different colors to present the distribution of different tissues. Result: A good correspondence between PCT imaging findings and PKD histology result was observed. Histogram curves from images of pcy kidneys and normal kidneys were fitted to Gaussian-curve model. Portions of cysts, parenchyma and area of high level hemoglobin were estimated according to the curve fit result. A growth of cysts associated with relatively volume decrease of parenchyma and tissues with high perfusion of hemoglobin was observed. Conclusion: The PCT enabled visualization of renal cysts for mouse model and had the potential for volumetric measurements of kidney.
Purpose. To evaluate photoacoustic spectroscopy as a potential imaging modality capable of measuring intra-tumor heterogeneity and spectral features associated with hemoglobin and the molecular probe indocyanine green (ICG). Material and Methods. Immune deficient mice were injected with wildtype and VEGF enhanced MCF-7 breast cancer cells or SKOV3x ovarian cancer cells, which were allowed to grow to a size of 6-12 mm in diameter. Two mice were imaged alive and after euthanasia for (oxy/deoxy)-hemoglobin content. A 0.4 mL volume of 1 μg/mL concentration of ICG was injected into the tail veins of two mice prior to imaging using the photoacoustic computed tomography (PCT) spectrometer (Optosonics, Inc., Indianapolis, IN 46202) scanner. Mouse images were acquired for wavelengths spanning 700-920 nm, after which the major organs were excised, and similarly imaged. A histological study was performed by sectioning the organ and optically imaging the fluorescence distribution. Results. Calibration of PCT-spectroscopy with different samples of oxygenated blood reproduced a hemoglobin dissociation curve consistent with empirical formula with an average error of 5.6%. In vivo PCT determination of SaO2 levels within the tumor vascular was measurably tracked, and spatially correlated to the periphery of the tumor. Statistical and systematic errors associated with hypoxia were estimated to be 10 and 13%, respectively. Measured ICG concentrations determined by contrast-differential PCT images in excised organs (tumor, liver) were approximately 0.8 μg/mL, consistent with fluorescent histological results. Also, the difference in the ratio of ICG concentration in the gall bladder-to-vasculature between the mice was consistent with excretion times between the two mice. Conclusion. PCT spectroscopic imaging has shown to be a noninvasive modality capable of imaging intra-tumor heterogeneity of (oxy/deoxy)-hemoglobin and ICG in vivo, with an estimated error in SaO2 at 17% and in ICG at 0.8 μg/mL in excised tissue. Ongoing development of spectroscopic analysis techniques, probe development, and calibration techniques are being developed to improve sensitivity to both exogenous molecular probes and (oxy/deoxy)-hemoglobin fraction.
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