All optical photoacoustic tomography (PAT) using a planar Fabry-Perot interferometer polymer film sensor has been demonstrated for in vivo human palm imaging with an imaging penetration depth of 5 mm. The relatively larger vessels in the superficial plexus and the vessels in the dermal plexus are visible in PAT. However, due to both resolution and sensitivity limits, all optical PAT cannot reveal the smaller vessels such as capillary loops and venules. Melanin absorption also sometimes causes difficulties in PAT to resolve vessels. Optical coherence tomography (OCT) based angiography, on the other hand, has been proven suitable for microvasculature visualization in the first couple millimeters in human. In our work, we combine an all optical PAT system with an OCT system featuring a phase stable akinetic swept source. This multimodal PAT/OCT/OCT-angiography system provides us co-registered human skin vasculature information as well as the structural information of cutaneous. The scanning units of the sub-systems are assembled into one probe, which is then mounted onto a portable rack. The probe and rack design gives six degrees of freedom, allowing the multimodal optical imaging probe to access nearly all regions of human body. Utilizing this probe, we perform imaging on patients with various skin disorders as well as on healthy controls. Fused PAT/OCT-angiography volume shows the complete blood vessel network in human skin, which is further embedded in the morphology provided by OCT. A comparison between the results from the disordered regions and the normal regions demonstrates the clinical translational value of this multimodal optical imaging system in dermatology.
Studies have proven the relationship between cutaneous vasculature abnormalities and dermatological disorders, but to image vasculature noninvasively in vivo, advanced optical imaging techniques are required. In this study, we imaged a palm of a healthy volunteer and three subjects with cutaneous abnormalities with photoacoustic tomography (PAT) and optical coherence tomography with angiography extension (OCTA). Capillaries in the papillary dermis that are too small to be discerned with PAT are visualized with OCTA. From our results, we speculate that the PA signal from the palm is mostly from hemoglobin in capillaries rather than melanin, knowing that melanin concentration in volar skin is significantly smaller than that in other areas of the skin. We present for the first time OCTA images of capillaries along with the PAT images of the deeper vessels, demonstrating the complementary effective imaging depth range and the visualization capabilities of PAT and OCTA for imaging human skin in vivo. The proposed imaging system in this study could significantly improve treatment monitoring of dermatological diseases associated with cutaneous vasculature abnormalities.
The combination of photoacoustic tomography (PAT) with optical coherence tomography (OCT) has seen steady progress over the past few years. With the benchtop and semi-benchtop configurations, preclinical and clinical results have been demonstrated, paving the way for wider applications using dual modality PAT/OCT systems. However, as for the most updated semi-benchtop PAT/OCT system which employs a Fabry-Perot polymer film sensor, it is restricted to only human palm imaging due to the limited flexibility of the probe. The passband limit of the polymer film sensor further restricts the OCT source selection and reduces the sensitivity of the combined OCT system. To tackle these issues, we developed an articulated PAT/OCT probe for both preclinical and clinical applications. In the probe design, the sample arm of OCT sub-system and the interrogation part of the PAT sub-system are integrated into one compact unit. The polymer film sensor has a quick release function so that before each OCT scan, the sensor can be taken off to avoid the sensitivity drop and artefacts in OCT. The holding mechanism of the sensor is also more compact compared to previous designs, permitting access to uneven surfaces of the subjects. With the help of the articulated probe and a patient chair, we are able to perform co-registered imaging on human subjects on both upper and lower extremities while they are at rest positions. An increase in performance characteristics is also achieved. Patients with skin diseases are currently being recruited to test its clinical feasibility.
We present a novel akinetic optical ultrasound sensor, consisting of a rigid, fiber-coupled Fabry-P´erot interferometer (FPI) with a central opening. The sensing principle relies exclusively on the detection of pressure-induced changes of the refractive index in the liquid located between the cavity mirrors. This enables resonance-free, inherently linear signal detection over a large bandwidth. We demonstrate that the sensor allows to realize exceedingly low noise equivalent pressure (NEP) values of 2 Pa over a 20 MHz measurement bandwidth, while maintaining a large full field of view of 2,7mm × 1,3mm as well as a flat frequency response. Imaging tests on phantoms and biological tissue show the suitability of the XARION-sensor for optical resolution photoacoustic microscopy (OR-PAM) applications. Transparent in axial direction, the sensor facilitates the implementation of highly sensitive fast-scanning reflection-mode OR-PAM setups, as well as easy integration with other imaging modalities such as confocal microscopy or OCT.
To obtain the initial pressure from the collected data on a planar sensor arrangement in photoacoustic tomography, there exists an exact analytic frequency-domain reconstruction formula. An efficient realization of this formula needs to cope with the evaluation of the data’s Fourier transform on a nonequispaced mesh. We use the nonuniform fast Fourier transform to handle this issue and show its feasibility in three-dimensional experiments with real and synthetic data. This is done in comparison to the standard approach that uses linear, polynomial, or nearest neighbor interpolation. Moreover, we investigate the effect and the utility of flexible sensor location to make optimal use of a limited number of sensor points. The computational realization is accomplished by the use of a multidimensional nonuniform fast Fourier algorithm, where nonuniform data sampling is performed both in frequency and spatial domain. Examples with synthetic and real data show that both approaches improve image quality.
Elastography is implemented by applying a mechanical force to a specimen and visualizing the resulting displacement. As a basis of elastographic imaging typically ultrasound, optical coherence tomography or magnetic resonance imaging are used. Photoacoustics has not been viewed as a primary imaging modality for elastography, but only as a complementary method to enhance the contrast in ultrasound elastography. The reason is that photoacoustics is considered speckle free [3], which hinders application of speckle tracking algorithms. However, while conventional ultrasound only uses a single frequency, photoacoustics utilizes a broad frequency spectrum. We are therefore able to generate artificial texture by using a frequency band limited part of the recorded data. In this work we try to assess the applicability of this technique to photoacoustic tomography. We use Agar phantoms with predefined Young's moduli and laterally apply a 50μm static compression. Pre- and post compression data are recorded via a Fabry Pérot interferometer planar sensor setup and reconstructed via a non-uniform-FFT reconstruction algorithm. A displacement vector field, between pre- and post compressed data is then determined via optical flow algorithms. While the implementation of texture generation during post processing reduces image quality overall, it turns out that it improves the detection of moving patterns and is therefore better suited for elastography.
We present an ultrahigh resolution dual modality optical resolution photoacoustic microsopy (OR-PAM) and spectral domain optical coherence microscopy (SD-OCM) system. The ultrahigh sub-micron lateral resolution is provided by the high numerical aperture of the objective lens used while the ultrahigh axial resolution is provided by the broadband OCT laser that covers 107 nm with a central wavelength of 840 nm. The synchronized simultaneous acquisition for the two modalities is achieved using a 40MHz FPGA. 2D-scanning is realized by two orthogonal translation stages (PI, 400 nm resolution). The transversal resolution of the system is 0.5 μm, the axial resolutions are 30 μm (PAM) and 4 μm (OCM), respectively. The values have been determined experimentally using nanospheres (diameter 10-200nm). For a demonstration of the imaging capability we present images from thin slices of different biological samples as well as in vivo imaging in the zebrafish embryo.
Fluorescent proteins brought a revolution in life sciences and biological research in that they make a powerful tool for researchers to study not only the structural and morphological information, but also dynamic and functional information in living cells and organisms. While green fluorescent proteins (GFP) have become a common labeling tool, red-shifted or even near infrared fluorescent proteins are becoming the research focus due to the fact that longer excitation wavelengths are more suitable for deep tissue imaging. In this study, E2-Crimson, a far red fluorescent protein whose excitation wavelength is 611 nm, was genetically expressed in the exocrine pancreas of adult zebrafish. Using spectroscopic all optical detection photoacoustic tomography, we mapped the distribution of E2-Crimson in 3D after imaging the transgenic zebrafish in vivo using two different wavelengths. With complementary morphological information provided by imaging the same fish using a spectral domain optical coherence tomography system, the E2-Crimson distribution acquired from spectroscopic photoacoustic tomography was confirmed in 2D by epifluorescence microscopy and in 3D by histology. To the authors’ knowledge, this is the first time a far red fluorescent protein is imaged in vivo by spectroscopic photoacoustic tomography. Due to the regeneration feature of zebrafish pancreas, this work preludes the longitudinal studies of animal models of diseases such as pancreatitis by spectroscopic photoacoustic tomography. Since the effective penetration depth of photoacoustic tomography is beyond the transport mean free path length, other E2-Crimson labeled inner organs will also be able to be studied dynamically using spectroscopic photoacoustic tomography.
Spectroscopic photoacoustic microscopy (PAM) requires a pulsed nanosecond laser with tunable wavelength, but such
lasers are expensive and have poor wavelength switching speed. We are developing a rapidly tunable system based on a
high repetition rate supercontinuum source. A supercontinuum is produced by propagating 0.6 ns duration pulses from
an 7.5 kHz Q-switched Nd:YAG microchip laser through 7 meters of photonic crystal fiber (PCF). Wavelength
selection is achieved with a rapidly tunable prism-based monochromator, where an actuator-controlled mask selects the
desired wavelength band. Ten different wavelength bands (570 to 930 nm) are acquired in less than 1 second for each
image pixel. Each wavelength has a bandwidth of 40 nm. The PAM system employs optical focusing of the excitation
beam and detection with a 25 MHz spherically focused f/3 transducer. Multiwavelength imaging is tested on phantoms
with different color inks. The inks were correctly identified by processing the multiwavelength images with a linear
discriminant analysis. A major advantage of our tunable source is the high repetition rate and rapid access to widely
separated wavelengths. These promising results suggest the potential of our wavelength agile source for spectroscopic
photoacoustic microscopy.
Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. Microchip lasers are
attractive optical sources for PAM, as they are compact and provide nanosecond pulse durations at several kHz
repetition rates. However, spectroscopic imaging is not possible with microchip lasers due to their fixed wavelength
output. We are investigating multispectral PAM with a supercontinuum source based on a photonic crystal fiber (PCF)
pumped with a microchip laser. The Q-switched Nd:YAG microchip laser produces 0.6 ns duration pulses at 1064 nm
with 8 uJ of energy at a 6.6 kHz repetition rate. These pulses are sent through 7 meters of PCF with a 5 um diameter core
and a zero dispersion wavelength of 1040 nm. The supercontinuum is sent through a tunable band-pass filter before
being focused into the object. Photoacoustic detection is performed with a 25 MHz spherically focused f/2 transducer.
En-face imaging experiments were performed on ink phantoms. Images are acquired at seven different wavelengths from
575 to 875 nm. A simple discriminant analysis of the multispectral photoacoustic data produces images that clearly
distinguish the different absorbing regions of the sample. These preliminary results suggest the potential of the
supercontinuum PCF source for multispectral PAM.
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