KEYWORDS: Absorption, Molecules, In vitro testing, Molecular aggregates, Mass attenuation coefficient, In vivo imaging, Luminescence, Tissue optics, Magnesium, Tumors
We have designed a protease-sensitive imaging probe for optoacoustic imaging whose absorption spectrum changes upon cleavage by a protease of interest. The probe comprises an active site, a derivative of chlorophyll or natural photosynthetic bacteriochlorophyll that absorbs in the near infrared, conjugated to a peptide backbone specific to the protease being imaged. The uncleaved molecules tend to aggregate in dimers and trimers, causing a change in the absorption spectrum relative to that of the monomer. Upon cleavage, the probe molecules deaggregate, giving rise to a spectrum characteristic of monomers. We show using photospectrometry that the two forms of the probe have markedly different absorption spectra, which could allow for in vivo optoacoustic identification using a multiwavelength imaging strategy. Optoacoustic measurements using a narrow-band dye laser find spectral peaks in the two forms of the probe at the expected location. The optoacoustic signal from the uncleaved probe is found to be considerably weaker than that of the cleaved probe, perhaps due to poor optical-acoustic coupling in the aggregated molecules. However, ultimately, it is detection of the cleaved probe that is of the greatest import, since it reports on the protease activity of interest.
We investigate the saturation effect, which describes the violation of the linearity between the measured photoacoustic amplitude and the object's optical absorption coefficient in functional photoacoustic imaging when the optical absorption in the object increases. We model the optical energy deposition and photoacoustic signal generation and detection in a semi-infinite optical absorbing object. Experiments are carried out by measuring photoacoustic signals generated from an ink-filled plastic tube. The saturation effect is studied by varying the optical absorption coefficient in the model and the ink concentration in the photoacoustic experiments. By changing the center frequency of the ultrasonic detector, the requirement to minimize the saturation effect in functional photoacoustic imaging is established.
KEYWORDS: Absorption, Molecules, Mass attenuation coefficient, Molecular aggregates, Signal attenuation, In vivo imaging, Tumors, In vitro testing, Dye lasers, Near infrared
We have designed a protease-sensitive imaging probe for optoacoustic imaging whose absorption spectrum changes
upon cleavage by a protease of interest. The probe comprises an active site, a derivative of chlorophyll or natural
photosynthetic bacteriochlorophyll that absorbs in the near infrared, conjugated to a peptide backbone specific to the
protease being imaged. The uncleaved molecules tend to aggregate in dimers and trimers causing a change in the
absorption spectrum relative to that of the monomer. Upon cleavage, the probe molecules de-aggregate giving rise to a
spectrum characteristic of monomers. We show using photospectrometry that the two forms of the probe have markedly
different absorption spectra, which could allow for in vivo optoacoustic identification using a multiwavelength imaging
strategy. Optoacoustic measurements using a narrow-band dye laser find spectral peaks in the two forms of the probe at
the expected location. The optoacoustic signal from the uncleaved probe is found to be considerably weaker than that of
the cleaved probe, perhaps due to poor optical-acoustic coupling in the aggregated molecules. However, ultimately, it is
detection of the cleaved probe that is of the greatest import since it reports on the protease activity of interest. PUBLISHERS NOTE 9/1/2010: The figure numbers are corrected. If you downloaded the incorrect version of Paper 75641T and no longer have access to download the correct paper, please contact CustomerService@SPIEDigitalLibrary.org for assistance.
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