Compound scanning with a phased array

Compound scans made with a dynamically focussed phased array system have been produced in real time. The scanner, intended for abdominal imaging, has a large field of view. The compounding improves the acquisition of echoes from specular targets by changing the orientation of the insonifying beam and also reduces the speckle noise in grey scale images. These gains are achieved while maintaining the high resolution and flexibility of a computer controlled phased array sector scanner. The configuration of the compound scanner is described, and in vivo abdominal scans are presented.     

Transfontanelle photoacoustic imaging: ultrasound transducer selection analysis

Transfontanelle ultrasound imaging (TFUI) is the conventional approach for diagnosing brain injury in neonates. Despite being the first stage imaging modality, TFUI lacks accuracy in determining the injury at an early stage due to degraded sensitivity and specificity. Therefore, a modality like photoacoustic imaging that combines the advantages of both acoustic and optical imaging can overcome the existing TFUI limitations. Even though a variety of transducers have been used in TFUI, it is essential to identify the transducer specification that is optimal for transfontanelle imaging using the photoacoustic technique. In this study, we evaluated the performance of 6 commercially available ultrasound transducer arrays to identify the optimal characteristics for transfontanelle photoacoustic imaging. We focused on commercially available linear and phased array transducer probes with center frequencies ranging from 2.5MHz to 8.5MHz which covers the entire spectrum of the transducer arrays used for brain imaging. The probes were tested on both in vitro and ex vivo brain tissue, and their performance in terms of transducer resolution, size, penetration depth, sensitivity, signal to noise ratio, signal amplification and reconstructed image quality were evaluated. The analysis of selected transducers in these areas allowed us to determine the optimal transducer for transfontanelle imaging, based on vasculature depth and blood density in tissue using ex vivo sheep brain. The outcome of this evaluation identified the two most suitable ultrasound transducer probes for transfontanelle photoacoustic imaging.     

Reflection-mode in vivo photoacoustic microscopy with subwavelength lateral resolution

We developed a reflection-mode subwavelength-resolution photoacoustic microscopy system capable of imaging optical absorption contrast in vivo. The simultaneous high-resolution and reflection-mode imaging capacity of the system was enabled by delicately configuring a miniature high-frequency ultrasonic transducer tightly under a water-immersion objective with numerical aperture of 1.0. At 532-nm laser illumination, the lateral resolution of the system was measured to be ~320 nm. With this system, subcellular structures of red blood cells and B16 melanoma cells were resolved ex vivo; microvessels, including individual capillaries, in a mouse ear were clearly imaged label-freely in vivo, using the intrinsic optical absorption from hemoglobin. The current study suggests that, the optical-absorption contrast, subwavelength resolution, and reflection-mode ability of the developed photoacoustic microscopy may empower a wide range of biomedical studies for visualizing cellular and/or subcellular structures.OCIS codes: (170.5120) Photoacoustic imaging, (180.0180) Microscopy, (170.3880) Medical and biological imaging     

Video-rate high-resolution single-pixel nonscanning photoacoustic microscopy

Optical-resolution photoacoustic microscopy (OR-PAM) is widely utilized in biomedical applications because of its ability to noninvasively image biological tissues in vivo while providing high-resolution morphological and functional information. However, one drawback of conventional OR-PAM is its imaging speed, which is restricted by the scanning technique employed. To achieve a higher imaging frame rate, we present video-rate high-resolution single-pixel nonscanning photoacoustic microscopy (SPN-PAM), which utilizes Fourier orthogonal basis structured planar illumination to overcome the above-mentioned limitations. A 473 × 473 µm² imaging field of view (FOV) with 3.73 µm lateral resolution and video-rate imaging of 30 Hz were achieved. In addition, in both in vitro cell and in vivo mouse vascular hemodynamic imaging experiments, high-quality images were obtained at ultralow sampling rates. Thus, the proposed high-resolution SPN-PAM with video-rate imaging speed provides new insights into high-speed PA imaging and could be a powerful tool for rapid biological imaging.     

Frequency domain photoacoustic and fluorescence microscopy

We report on simultaneous frequency domain optical-resolution photoacoustic and fluorescence microscopy with sub-µm lateral resolution. With the help of a blood smear, we show that photoacoustic and fluorescence images provide complementary information. Furthermore, we compare theoretically predicted signal-to-noise ratios of sinusoidal modulation in frequency domain with pulsed excitation in time domain.OCIS codes: (170.5120) Photoacoustic imaging, (180.2520) Fluorescence microscopy, (110.5125) Photoacoustics, (170.3880) Medical and biological imaging, (140.2020) Diode lasers     

In vivo demonstration of noninvasive thermal surgery of the liver and kidney using an ultrasonic phased array

A 256-element, continuous-wave ultrasonic phased array has been used to thermally coagulate deep-seated liver and kidney tissue. The array elements were formed on a 1-3 piezocomposite bowl with a 10-cm radius of curvature and 12-cm diameter. The 0.65 x 0.65 cm2 projection elements were driven at 1.1 MHz by a custom-built amplifier system. A series of in vivo porcine experiments demonstrated the ability to coagulate liver and kidney tissue using the large-scale phased array. The temperature response of the treatment was guided and monitored using magnetic resonance (MR) images. Focal lesion volumes greater than 0.5 cm3 in kidney and 2 cm3 in liver were formed from a single 20-s sonication.     

Tissue-mimicking phantoms for photoacoustic and ultrasonic imaging

In both photoacoustic (PA) and ultrasonic (US) imaging, overall image quality is influenced by the optical and acoustical properties of the medium. Consequently, with the increased use of combined PA and US (PAUS) imaging in preclinical and clinical applications, the ability to provide phantoms that are capable of mimicking desired properties of soft tissues is critical. To this end, gelatin-based phantoms were constructed with various additives to provide realistic acoustic and optical properties. Forty-micron, spherical silica particles were used to induce acoustic scattering, Intralipid(?) 20% IV fat emulsion was employed to enhance optical scattering and ultrasonic attenuation, while India Ink, Direct Red 81, and Evans blue dyes were utilized to achieve optical absorption typical of soft tissues. The following parameters were then measured in each phantom formulation: speed of sound, acoustic attenuation (from 6 to 22 MHz), acoustic backscatter coefficient (from 6 to 22 MHz), optical absorption (from 400 nm to 1300 nm), and optical scattering (from 400 nm to 1300 nm). Results from these measurements were then compared to similar measurements, which are offered by the literature, for various soft tissue types. Based on these comparisons, it was shown that a reasonably accurate tissue-mimicking phantom could be constructed using a gelatin base with the aforementioned additives. Thus, it is possible to construct a phantom that mimics specific tissue acoustical and/or optical properties for the purpose of PAUS imaging studies.     

Visualization of blood-brain barrier disruption with dual-wavelength high-resolution photoacoustic microscopy

The blood-brain barrier (BBB) strictly regulates the substance exchange between the vascular network and the central nervous system, and plays a critical role in maintaining normal brain homeostasis. Impaired BBB is often accompanied with the emergence of cerebral diseases and probably further leads to severe neuroinflammation or even neurological degeneration. Hence, there is an urgent need to precisely monitor the impaired BBB to understand its pathogenesis and better guide the enactment of therapeutic strategies. However, there is a lack of high-resolution imaging techniques to visualize and evaluate the large-scale BBB disruption in pre-clinical and clinical aspects. In this study, we propose a dual-wavelength photoacoustic imaging (PAI) methodology that simultaneously reveals the abnormal microvasculature and impaired BBB within the cerebral cortex. In in vivo studies, BBB disruption in both mice and rats were induced by local hot-water stimulation and unilateral carotid arterial perfusion of hyperosmolar mannitol, respectively. Subsequently, the exogenous contrast agent (CA) was injected into the microcirculation via the tail vein, and photoacoustic (PA) images of the microvasculature and leaked CA within the cerebral cortex were obtained by dual-wavelength photoacoustic microscopy to evaluate the BBB disruption. Besides, analysis of distribution and concentration of leaked CA in lesion region was further conducted to quantitatively reveal the dynamic changes of BBB permeability. Furthermore, we exploited this approach to investigate the reversibility of BBB disruption within the two distinct models. Based on the experimental results, this new proposed approach presents excellent performance in visualizing microvasculature and leaked CA, and enabling it possesses great potential in evaluating the abnormal microvasculature and impaired BBB result from cerebrovascular diseases.     

A low-cost photoacoustic microscopy system with a laser diode excitation

Photoacoustic microscopy (PAM) is capable of mapping microvasculature networks in biological tissue and has demonstrated great potential for biomedical applications. However, the clinical application of the PAM system is limited due to the use of bulky and expensive pulsed laser sources. In this paper, a low-cost optical-resolution PAM system with a pulsed laser diode excitation has been introduced. The lateral resolution of this PAM system was estimated to be 7 µm by imaging a carbon fiber. The phantoms made of polyethylene tubes filled with blood and a mouse ear were imaged to demonstrate the feasibility of this PAM system for imaging biological tissues.OCIS codes: (110.5120) Photoacoustic imaging, (140.2020) Diode lasers, (170.3880) Medical and biological imaging     

Histopathology for Mohs micrographic surgery with photoacoustic remote sensing microscopy

Mohs micrographic surgery (MMS) is a precise oncological technique where layers of tissue are resected and examined with intraoperative histopathology to minimize the removal of normal tissue while completely excising the cancer. To achieve intraoperative pathology, the tissue is frozen, sectioned and stained over a 20- to 60-minute period, then analyzed by the MMS surgeon. Surgery is continued one layer at a time until no cancerous cells remain, meaning MMS can take several hours to complete. Ideally, it would be desirable to circumvent or augment frozen sectioning methods and directly visualize subcellular morphology on the unprocessed excised tissues. Employing photoacoustic remote sensing (PARS) microscopy, we present a non-contact label-free reflection-mode method of performing such visualizations in frozen sections of human skin. PARS leverages endogenous optical absorption contrast within cell nuclei to provide visualizations reminiscent of histochemical staining techniques. Presented here, is the first true one to one comparison between PARS microscopy and standard histopathological imaging in human tissues. We demonstrate the ability of PARS microscopy to provide large grossing scans (>1 cm², sufficient to visualize entire MMS sections) and regional scans with subcellular lateral resolution (300 nm).     

Ultrahigh resolution photoacoustic microscopy via transient absorption

We have developed a novel, hybrid imaging modality, Transient Absorption Ultrasonic Microscopy (TAUM), which takes advantage of the optical nonlinearities afforded by transient absorption to achieve ultrahigh-resolution photoacoustic microscopy. The theoretical point spread function for TAUM is functionally equivalent to confocal and two-photon fluorescence microscopy, potentially enabling cellular/subcellular photoacoustic imaging. A prototype TAUM system was designed, built, and used to image a cross-section through several capillaries in the excised cheek pouch of a Syrian Hamster. The well-resolved capillaries in the TAUM image provided experimental evidence of the spatial resolution. These results suggest that TAUM has excellent potential for producing volumetric images with cellular/subcellular resolution in three dimensions deep inside living tissue.     

Verification of human fetal breathing with phased array ultrasound imaging.

Continuous observation of fetal movement patterns is a novel and useful capability of new high-speed ultrasound imaging systems. Confirmatory documentation of human fetal breathing as a component of these movement patterns has been achieved using a phased-array real time ultrasound sector scanning system. This system permits direct visualization of fetal breathing movements. Permanent recordings can be made on videotape. Recordings can also be made as a time-motion echocardiograms during continuous observation of the full sector. Fetal breathing movements were seen in five patients. The earliest period of gestation at which fetal breathing was seen was 20 weeks.     

In-Vivo functional optical-resolution photoacoustic microscopy with stimulated Raman scattering fiber-laser source

In this paper a multi-wavelength optical-resolution photoacoustic microscopy (OR-PAM) system using stimulated Raman scattering is demonstrated for both phantom and in vivo imaging. A 1-ns pulse width ytterbium-doped fiber laser is coupled into a single-mode polarization maintaining fiber. Discrete Raman-shifted wavelength peaks extending to nearly 800 nm are generated with pulse energies sufficient for OR-PAM imaging. Bandpass filters are used to select imaging wavelengths. A dual-mirror galvanometer system was used to scan the focused outputs across samples of carbon fiber networks, 200μm dye-filled tubes, and Swiss Webster mouse ears. Photoacoustic signals were collected in transmission mode and used to create maximum amplitude projection C-scan images. Double dye experiments and in vivo oxygen saturation estimation confirmed functional imaging potential.OCIS codes: (170.3880) Medical and biological imaging, (110.5120) Photoacoustic imaging, (180.5810) Scanning microscopy, (290.5860) Scattering, Raman