Search

Photoacoustic imaging (PAI) is a rapidly growing imaging modality which offers the advantages of good optical contrast and high ultrasound resolution. Although PAI provides imaging depth beyond the optical diffusion limit, penetration depth in biological samples is limited due to absorption and scattering of light in tissues. Improvement in imaging depth has been achieved by irradiating the sample with laser pulses of near infrared-I (NIR-I) region (700 nm-900 nm) due to decreased scattering of light in tissues within this optical window. Recently, further improvement in imaging depth has been reported by irradiating the sample in near infrared-II (NIR-II) region (900 nm-1700 nm). In this work, imaging depth in breast tissues when samples were irradiated by wavelengths in different optical windows has been compared. Initially, Monte Carlo simulation for light propagation in biological tissues was performed to compute imaging depth for excitation wavelengths of 532 nm, 800 nm, and 1064 nm. Further, photoacoustic tomography at 532 nm, 740 nm, and 1064 nm and acoustic resolution photoacoustic microscopy at 570 nm and 1064 nm were conducted to validate the results. We have shown that maximum imaging depth is achieved by NIR-I (740 nm/800 nm) when surface energy for all wavelengths is kept constant. However, when the energy density is proportional to maximum permissible exposure (MPE) at corresponding wavelength, maximum imaging depth is achieved by 1064 nm (NIR-II window). Therefore, we conclude that increased MPE in NIR-II window is responsible for the improved penetration depth in breast tissue in this region. National Medical Research Council (NMRC) Accepted version The authors would like to thank Singapore Ministry of Health’s National Medical Research Council (No. NMRC/OFIRG/0005/2016:M4062012) for funding this research. Srishti would also like to thank NTU-India Connect Research program for providing her an opportunity to visit NTU, Singapore. The authors have no relevant financial interest in the manuscript and no other potential conflicts of interest to disclose.

We designed and fabricated a telecentric f-theta imaging lens (TFL) to improve the imaging performance of spectral domain optical coherence tomography (SD-OCT). By tailoring the field curvature aberration of the TFL, the flattened focal surface was well matched to the detector plane. Simulation results showed that the spot in the focal plane fitted well within a single pixel and the modulation transfer function at high spatial frequencies showed higher values compared with those of an achromatic doublet imaging lens, which are commonly used in SD-OCT spectrometers. The spectrometer using the TFL had an axial resolution of 7.8 μm, which was similar to the theoretical value of 6.2 μm. The spectrometer was constructed so that the achromatic doublet lens was replaced by the TFL. As a result, the SD-OCT imaging depth was improved by 13% (1.85 mm) on a 10 dB basis in the roll-off curve and showed better sensitivity at the same depth. The SD-OCT images of a multi-layered tape and a human palm proved that the TFL was able to achieve deeper imaging depth and better contrast. This feature was seen very clearly in the depth profile of the image. SD-OCT imaging performance can be improved simply by changing the spectrometer’s imaging lens. By optimizing the imaging lens, deeper SD-OCT imaging can be achieved with improved sensitivity.

We established a photoacoustic imaging (PAI) system that can provide variable gain at different depths. The PAI system consists of a pulsed laser with an optical parametric oscillator working at a 728[Formula: see text]nm wavelength and an imaging-acquisition-and-processing unit with an ultrasound transducer. A voltage-controlled attenuator was used to realize variable gain at different depths when acquiring PAI signals. The proof-of-concept imaging results for variable gain at different depths were achieved using specific phantoms. Both resolution and optical contrast obtained through the results of variable gain for a targeted depth range are better than those of constant gain for all depths. To further testify the function, we imaged the sagittal section of the body of in vivo nude mice. In addition, we imaged an absorption sample embedded in a chicken breast tissue, reaching a maximum imaging depth of [Formula: see text]4.6[Formula: see text]cm. The results obtained using the proposed method showed better resolution and contrast than when using 50[Formula: see text]dB gain for all depths. The depth range resolution was [Formula: see text]1[Formula: see text]mm, and the maximum imaging depth of our system reached [Formula: see text]4.6[Formula: see text]cm. Furthermore, blood vessels can be revealed and targeted depth range can be selected in nude mice imaging.

Influence of acoustic inhomogeneities and resulting reflection artifacts is an important problem in reflection-mode photoacoustic imaging. Absorption of light by skin and superficial optical absorbers will generate photoacoustic transients, which traverse into the tissue and get reflected from structures having different acoustic impedance. These reflected photoacoustic signals, when reconstructed, may appear in the region of interest, which causes difficulties in image interpretation. We propose a novel method to identify and potentially eliminate reflection artifacts in photoacoustic images using photoacoustic-guided focused ultrasound [PAFUSion]. Our method uses focused ultrasound pulses to mimic the wave field produced by photoacoustic sources and thus provides a way to identify reflection artifacts in clinical combined photoacoustic and pulse-echo ultrasound. Simulation and phantom results are presented to demonstrate the validity and impact of this method. Results show that PAFUSion can identify reflections in photoacoustic images and thus envisages potential for improving photoacoustic imaging of acoustically inhomogeneous tissue.

Epi-style optoacoustic (OA) imaging provides flexibility by integrating the irradiation optics and ultrasound receiver, yet clutter generated by optical absorption near the probe obscures deep OA sources. Localised vibration tagging (LOVIT) retrieves OA signal from images that are acquired with and without a preceding ultrasonic pushing beam: Radiation force leads to a phase shift of signals coming from the focal area resulting in their visibility in a difference image, whereas clutter from outside the pushing beam is eliminated. Disadvantages of a single-focus approach are residual clutter from inside the pushing beam above the focus, and time-intensive scanning of the focus to retrieve a large field-of-view. To speed up acquisition, we propose to create multiple foci in parallel, forming comb-shaped ARF patterns. By subtracting OA images obtained with interleaved combs, this technique moreover results in greatly improved clutter reduction in phantoms mimicking optical, acoustic and elastic properties of breast tissue. Keywords: Photoacoustic, Imaging depth, Handheld, Ultrasound, Radiation force, Shear wave, Image contrast

Reflection artifacts caused by acoustic inhomogeneities are a main challenge to deep-tissue photoacoustic imaging. Photoacoustic transients generated by the skin surface and superficial vasculature will propagate into the tissue and reflect back from echogenic structures to generate reflection artifacts. These artifacts can cause problems in image interpretation and limit imaging depth. In its basic version, PAFUSion mimics the inward travelling wave-field from blood vessel-like PA sources by applying focused ultrasound pulses, and thus provides a way to identify reflection artifacts. In this work, we demonstrate reflection artifact correction in addition to identification, towards obtaining an artifact-free photoacoustic image. In view of clinical applications, we implemented an improved version of PAFUSion in which photoacoustic data is backpropagated to imitate the inward travelling wave-field and thus the reflection artifacts of a more arbitrary distribution of PA sources that also includes the skin melanin layer. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. We present a phantom experiment and initial in vivo measurements on human volunteers where we demonstrate significant reflection artifact reduction using our technique. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can reduce these artifacts significantly to improve the deep-tissue photoacoustic imaging.

Recently, a variety of tissue optical clearing techniques have been developed to reduce light scattering for imaging deeper and three-dimensional reconstruction of tissue structures. Combined with optical imaging techniques and diverse labeling methods, these clearing methods have significantly promoted the development of neuroscience. Each of them has its own characteristics with certain advantages and disadvantages. Though there are some comparison results, the clearing methods covered are limited and the evaluation indices lack uniformity, which made it difficult to select a best-fit protocol from numerous methods for clearing in practical applications. Hence, it is necessary to systematically assess and compare these clearing methods. We evaluated the performance of seven typical clearing methods, including 3-D imaging of solvent-cleared organs (3DISCO), ultimate DISCO (uDISCO), see deep brain (SeeDB), ScaleS, [Formula: see text] , clear, unobstructed brain imaging cocktails and computational analysis, and passive CLARITY technique (PACT), on mouse brain samples. First, we compared the clearing effect and clearing time as well as size deformation on brain tissues. Further, we evaluated the fluorescence preservation and the increase of imaging depth induced by different methods. The results showed that 3DISCO, uDISCO, and PACT possessed excellent clearing capability on mouse brains, ScaleS and SeeDB rendered moderate transparency, whereas [Formula: see text] performed the worst. uDISCO and 3DISCO induced substantial size reduction on brain sections, and PACT expanded the mouse brain most seriously. Among those methods, ScaleS performed best on fluorescence retention, 3DISCO induced the biggest decline of the fluorescence. PACT achieved the highest increase of imaging depth, and SeeDB and [Formula: see text] possessed the shallowest imaging depth. This study is expected to provide important reference for users in choosing the most suitable brain optical clearing method.

Optoacoustic (OA) imaging in combination with diagnostic pulse-echo ultrasound is most flexibly implemented with irradiation optics and acoustic probe integrated in epi-style in a combined probe. Unfortunately, clinical epi-OA imaging depth is typically limited to one centimetre owing to clutter signals that originates from the site of tissue irradiation. In past years we have developed displacement-compensated averaging (DCA) for clutter reduction, based on the clutter decorrelation that occurs when palpating the tissue using the ultrasound probe. This method has now been implemented on a research ultrasound system for real time scanning with freehand guidance of the linear probe. Volunteer results confirm that clutter is significant in clinical OA imaging, and that DCA significantly improves image contrast as compared to conventional averaging. Clutter reduction is therefore a basic requirement for a successful combination of OA imaging with pulse-echo ultrasound.

A technique capable of capturing the dynamic of the reservoir fluids in the proximity of production wells would provide enormous benefit to the reservoir management; in fact, monitoring can be used to develop a feedback loop between measurements and control technologies to optimize the production. This paper examines the feasibility of a borehole radar tool for near-wellbore imaging. Modeling results show that the maximum imaging range depends mainly on the conductivity of the formation where the radar borehole system lies. Another constraint is given by the operating frequency of the system. Too low frequencies would compromise the electromagnetic wave propagation in favor of diffusion phenomena and too high frequencies would drastically attenuate the signal. In case of a relatively low conductive reservoir and a limited band of frequencies higher than 100 MHz we have defined the optimal imaging capacity of the radar system in the order of tens of meters. We suggest borehole radar measurements as a promising approach to monitor steam chamber growth in Steam Assisted Gravity Drainage (SAGD) processes and to prevent water encroachment in thin oil rims. The penetration capacity of a radar system fits the imaging depth necessary for these environments.

International audience; We present a multiphoton microscope using a Cr4 + :forsterite femtosecond laser with an emission wavelength of 1260 nm for the excitation of the multiphoton processes. This wavelength is well adapted to the "optical window" in biological tissues and permits to reach higher imaging depths than systems using more conventional titanium:sapphire laser sources. The paper describes the experimental set-up and reports on first results on human cornea and skin samples.