KEYWORDS: Transducers, Signal detection, Photoacoustic imaging, Kidney, Ultrasonography, Visualization, Blood vessels, Signal to noise ratio, Photoacoustic microscopy, In vivo imaging
Photoacoustic microscopy with linear array transducers enables fast two-dimensional, cross-sectional photoacoustic imaging. Unfortunately, most ultrasound transducers are only sensitive to a very narrow angular acceptance range and preferentially detect signals along the main axis of the transducer. This often limits photoacoustic microscopy from detecting blood vessels which can extend in any direction. Rotational compounded photoacoustic imaging is introduced to overcome the angular-dependency of detecting acoustic signals with linear array transducers. An integrate system is designed to control the image acquisition using a linear array transducer, a motorized rotational stage, and a motorized lateral stage. Images acquired at multiple angular positions are combined to form a rotational compounded image. We found that the signal-to-noise ratio improved, while the sidelobe and reverberation artifacts were substantially reduced. Furthermore, the rotational compounded images of excised kidneys and hindlimb tumors of mice showed more structural information compared with any single image collected.
The use of near-infrared wavelengths for photoacoustic (PA) imaging takes advantage of the relatively low
inherent absorption of tissues and has encouraged the development of agents which show high contrast in
this range. Here, we describe the modification of a commercially available PA imaging system (Vevo
LAZR, VisualSonics, Toronto) to take advantage of the 532nm and 1064nm wavelengths inherent in the
generation of the currently tuneable range of 680 to 970nm and in the use of these two wavelengths to
assess contrast agents.
The photoacoustic imaging system generated light from a Nd/YAG laser modified to extract the 532 and
1064nm wavelengths in addition to its OPO-derived tuneable range (680 - 970 nm) and deliver this light
through a fiber integrated into a linear array transducer (LZ400, VisualSonics).
Gold nanorods (UT Austin), carbon nanotubes (Stanford U), DyLight 550 (Thermo Fisher) and blood were
imaged in a phantom (PE20 tubing) and in a hindlimb subcutaneous tumor in vivo to determine their
photoacoustic signal intensity at all wavelengths.
In the phantom and in vivo, all agents caused an enhancement of the photoacoustic signal at their respective
peak absorbance wavelengths. These results show that the 532nm and 1064nm wavelengths could prove
useful in biomedical imaging due to the contrast agents customized for them. The 1064nm wavelength in
particular has the advantage of having very low generation of endogenous signal in vivo, making agents
tuned to this wavelength ideal for targeted contrast imaging.
Photoacoustic (PA) Imaging can estimate the spatial distribution of oxygen saturation (sO2) and total hemoglobin
concentration (HbT) in blood, and be co-registered with B-Mode ultrasound images of the surrounding anatomy. This
study will focus on the development of a PA imaging mode on a commercially available array based micro-ultrasound
(μUS) system that is capable of creating such images. The system will then be validated in vivo against a complementary
technique for measuring partial pressure of oxygen in blood (pO2). The pO2 estimates are converted to sO2 values based
on a standard dissociation curve found in the literature. Finally, the system will be used for assessing oxygenation in a
murine model of ischemia, both during injury and recovery.
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