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Toward the Goal of Signal Quantification in Contrast-Enhanced Optical Imaging
BRIEF ABSTRACTS
Academic Radiology, Vol 12, Suppl 1, May 2005
pear to be intrinsically surface-biased and make assumptions about the optical scattering coefficient of the tissue. As with endogenous optical imaging, time domain offers the potential to supersede CW for optical molecular imaging applications. The wealth of information contained in the fluorescence excitation and emission TPSFs permits the depth of the fluorophore to be obtained by even simple sonar-like approaches. Once the fluorophore depth is known, the intensity can be normalized to provide relative fluorophore concentration. In addition to the potential of TD to provide more accurate quantitative estimates of fluorophore depth and concentration than CW, TD enables the measurement of the fluoresence lifetime which CW is unable to provide. This permits the discrimination of fluorophores which may have significantly different fluoresence lifetimes, despite having similar emission spectra which challenges discrimination with intensity-based CW methods. Furthermore, it is known that the fluorescence lifetime can be influenced by the local environment of the fluorophore which provides additional physiological information. Time domain optical molecular imaging is the technological basis for the pre-clinical optical molecular imaging device (SAMI™) developed by ART.
Toward the Goal of Signal Quantification in Contrast-Enhanced Optical Imaging Robert Brasch MD1, Laure Fournier-Dujardin MD, Vincenzo Lucidi MD1, Kirill Berejnoi1, Yanjun Fu PhD1, Jonathan Palley2, Stavros Demos PhD3, 1Center for Pharmaceutical and Molecular Imaging, University of California San Francisco; and 2Center for Biophotonics, Science, and Technology, UC Davis; 3Lawerence Livermore National Laboratory;
proaching the levels of radioactive tracer detectability in nuclear imaging. A major goal for contrast-enhanced OI, a goal that may define the ultimate diagnostic utility of the method, will be to accurately measure the concentration of the contrast agent present within the tissue. Without contrast agent quantification OI would be severely limited; for example, comparisons of disease status and treatment responses over time or kinetic analyses of dynamic responses would be difficult or impossible. Publications of contrast-enhanced OI studies have reported results as ratios of signal from one region to another without true quantification of contrast medium concentration. Thus the purpose of this study was to investigate if the emitted signal from a NIR contrast agent could be readily and accurately measured, if the signal was directly proportional to the concentration, and if the signal measurements could be used to generate a reasonable estimate of tissue physiology, specifically of plasma volume in a tumor.
METHODS: A custom OI system with OPO-tunable laser and liquid-nitrogen-cooled CCD camera was used for optical imaging of in vitro contrast media test solutions, Indocyanine Green (ICG), ranging in concentration from 0 to 104 ug/L and of athymic rats bearing human MB-MDA-231 breast cancers. Tumor rats were imaged postcontrast, ICG 0.9 ug/kg, dynamically for 90 minutes with high temporal resolution; percent tumor plasma volume estimates were based on signal intensity enhancement of tumor and whole blood. The same animals were also imaged by macromolecular-enhanced MRI with % plasma volume assays. OI and MRI results were compared.
RESULTS: PURPOSE: Contrast enhanced diagnostic imaging using incident laser-generated light photons to interrogate living tissues is a relatively new but intensively pursued field. Most investigators have focused their attention on light in the near-infrared (NIR) portion of the electromagnetic spectrum because of its relatively greater depth of tissue penetration; a 4 to 5 centimeter maximum for penetration and detection has been cited. A major appeal for contrast enhanced optical imaging (OI) is the potential to detect NIR-fluorescing compounds in extremely low concentrations, 10⫺11 or 10⫺12 molar, ap-
The OI signal intensity (SI) response of in vitro samples was near-linear only over a limited range of lower ICG concentrations, 0 to 500 ug/L, and became grossly non-linear at higher concentrations. Mean tumor plasma volume estimates by OI from all animals was 68 ⫾ 9%, an unreasonably high value and significantly (p⬍.05) higher than the MRI plasma volume estimate of 5 ⫾ 1%.
CONCLUSIONS: The results failed to confirm an easy and straight-forward means for the OI measurement in vivo of NIR contrast medium
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BRIEF ABSTRACTS
Academic Radiology, Vol 12, Suppl 1, May 2005
concentration, but the data were nonetheless useful by defining an important obstacle to the development of OI for diagnostic application and by providing insights into solving the problem. By using relatively lower but readily detectable concentrations of NIR fluorescing contrast media, quantification seems possible, with the requirement that the contrast medium be at the surface of the imaged object, as for the in vitro test solutions. Clearly one or more confounding factors occur in vivo. Determining the depth of origin for the fluorescent signal and thus, the position of the contrast medium molecule appears to be essential and is currently being addressed by OI tomographic and mathematical modeling techniques. The results highlight the critical importance of distinguishing signal arising from different tissues or structures. Near infrared light originating from a highly emissive structure such as contrast enhanced tumor, blood, or liver, can travel relatively great distances in tissue to emerge in a location that is less emissive, such as soft tissue. This influences the measured parameters of interest. We call this the “light bulb” effect. ACKNOWLEDGMENT
Supported by Grant, CLC-01-28, from The Office of the President University of California
A Receptor-Binding Optical Agent for In Vivo Measurement of Receptor Biochemistry David Vera, PhD, Robert Mattrey, MD, Carl Hoh, MD, Laura McIntosh, PhD University of California, San Diego, and Advanced Research Technologies
PURPOSE: Our objective was to demonstrate receptor-binding properties of an optically labeled receptor-binding agent, Cy5.5-DTPA-galactosyl-dextran.
METHODS: The optical reporter, Cy5.5, was covalently coupled to DTPA-galactosyl-dextran (Figure) or DTPA-dextran. DMSO was added to 0.15 mg of the mono N-succinimidyl-ester of the dye. Cy5.5 solution was added dropwise to a 0.1 ml DMSO solution of DTPA-galactosyl-dextran
Figure
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Academic Radiology, Vol 12, Suppl 1, May 2005
pear to be intrinsically surface-biased and make assumptions about the optical scattering coefficient of the tissue. As with endogenous optical imaging, time domain offers the potential to supersede CW for optical molecular imaging applications. The wealth of information contained in the fluorescence excitation and emission TPSFs permits the depth of the fluorophore to be obtained by even simple sonar-like approaches. Once the fluorophore depth is known, the intensity can be normalized to provide relative fluorophore concentration. In addition to the potential of TD to provide more accurate quantitative estimates of fluorophore depth and concentration than CW, TD enables the measurement of the fluoresence lifetime which CW is unable to provide. This permits the discrimination of fluorophores which may have significantly different fluoresence lifetimes, despite having similar emission spectra which challenges discrimination with intensity-based CW methods. Furthermore, it is known that the fluorescence lifetime can be influenced by the local environment of the fluorophore which provides additional physiological information. Time domain optical molecular imaging is the technological basis for the pre-clinical optical molecular imaging device (SAMI™) developed by ART.
Toward the Goal of Signal Quantification in Contrast-Enhanced Optical Imaging Robert Brasch MD1, Laure Fournier-Dujardin MD, Vincenzo Lucidi MD1, Kirill Berejnoi1, Yanjun Fu PhD1, Jonathan Palley2, Stavros Demos PhD3, 1Center for Pharmaceutical and Molecular Imaging, University of California San Francisco; and 2Center for Biophotonics, Science, and Technology, UC Davis; 3Lawerence Livermore National Laboratory;
proaching the levels of radioactive tracer detectability in nuclear imaging. A major goal for contrast-enhanced OI, a goal that may define the ultimate diagnostic utility of the method, will be to accurately measure the concentration of the contrast agent present within the tissue. Without contrast agent quantification OI would be severely limited; for example, comparisons of disease status and treatment responses over time or kinetic analyses of dynamic responses would be difficult or impossible. Publications of contrast-enhanced OI studies have reported results as ratios of signal from one region to another without true quantification of contrast medium concentration. Thus the purpose of this study was to investigate if the emitted signal from a NIR contrast agent could be readily and accurately measured, if the signal was directly proportional to the concentration, and if the signal measurements could be used to generate a reasonable estimate of tissue physiology, specifically of plasma volume in a tumor.
METHODS: A custom OI system with OPO-tunable laser and liquid-nitrogen-cooled CCD camera was used for optical imaging of in vitro contrast media test solutions, Indocyanine Green (ICG), ranging in concentration from 0 to 104 ug/L and of athymic rats bearing human MB-MDA-231 breast cancers. Tumor rats were imaged postcontrast, ICG 0.9 ug/kg, dynamically for 90 minutes with high temporal resolution; percent tumor plasma volume estimates were based on signal intensity enhancement of tumor and whole blood. The same animals were also imaged by macromolecular-enhanced MRI with % plasma volume assays. OI and MRI results were compared.
RESULTS: PURPOSE: Contrast enhanced diagnostic imaging using incident laser-generated light photons to interrogate living tissues is a relatively new but intensively pursued field. Most investigators have focused their attention on light in the near-infrared (NIR) portion of the electromagnetic spectrum because of its relatively greater depth of tissue penetration; a 4 to 5 centimeter maximum for penetration and detection has been cited. A major appeal for contrast enhanced optical imaging (OI) is the potential to detect NIR-fluorescing compounds in extremely low concentrations, 10⫺11 or 10⫺12 molar, ap-
The OI signal intensity (SI) response of in vitro samples was near-linear only over a limited range of lower ICG concentrations, 0 to 500 ug/L, and became grossly non-linear at higher concentrations. Mean tumor plasma volume estimates by OI from all animals was 68 ⫾ 9%, an unreasonably high value and significantly (p⬍.05) higher than the MRI plasma volume estimate of 5 ⫾ 1%.
CONCLUSIONS: The results failed to confirm an easy and straight-forward means for the OI measurement in vivo of NIR contrast medium
S73
BRIEF ABSTRACTS
Academic Radiology, Vol 12, Suppl 1, May 2005
concentration, but the data were nonetheless useful by defining an important obstacle to the development of OI for diagnostic application and by providing insights into solving the problem. By using relatively lower but readily detectable concentrations of NIR fluorescing contrast media, quantification seems possible, with the requirement that the contrast medium be at the surface of the imaged object, as for the in vitro test solutions. Clearly one or more confounding factors occur in vivo. Determining the depth of origin for the fluorescent signal and thus, the position of the contrast medium molecule appears to be essential and is currently being addressed by OI tomographic and mathematical modeling techniques. The results highlight the critical importance of distinguishing signal arising from different tissues or structures. Near infrared light originating from a highly emissive structure such as contrast enhanced tumor, blood, or liver, can travel relatively great distances in tissue to emerge in a location that is less emissive, such as soft tissue. This influences the measured parameters of interest. We call this the “light bulb” effect. ACKNOWLEDGMENT
Supported by Grant, CLC-01-28, from The Office of the President University of California
A Receptor-Binding Optical Agent for In Vivo Measurement of Receptor Biochemistry David Vera, PhD, Robert Mattrey, MD, Carl Hoh, MD, Laura McIntosh, PhD University of California, San Diego, and Advanced Research Technologies
PURPOSE: Our objective was to demonstrate receptor-binding properties of an optically labeled receptor-binding agent, Cy5.5-DTPA-galactosyl-dextran.
METHODS: The optical reporter, Cy5.5, was covalently coupled to DTPA-galactosyl-dextran (Figure) or DTPA-dextran. DMSO was added to 0.15 mg of the mono N-succinimidyl-ester of the dye. Cy5.5 solution was added dropwise to a 0.1 ml DMSO solution of DTPA-galactosyl-dextran
Figure
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