Photoacoustic (PA) imaging can be used to monitor flowing blood inside the microvascular and capillary bed. Ultrasound speckle decorrelation based velocimetry imaging was previously shown to accurately estimate blood flow velocity in mouse brain (micro-)vasculature. Translating this method to photoacoustic imaging will allow simultaneous imaging of flow velocity and extracting functional parameters like blood oxygenation. In this study, we use a pulsed laser diode and a quantitative method based on normalized first order field autocorrelation function of PA field fluctuations to estimate flow velocities in an ink tube phantom and in the microvasculature of the chorioallantoic membrane of a chicken embryo. We demonstrate how the decorrelation time of signals acquired over frames are related to the flow speed and show that the PA flow analysis based on this approach is an angle independent flow velocity imaging method.@en

Intravascular ultrasound (IVUS) is a well-established diagnostic method that provides images of the vessel wall and atherosclerotic plaques. We investigate the potential for phased-array IVUS utilizing coded excitation (CE) for improving the penetration depth and image signal-to-noise ratio (SNR). It is realized on a new experimental broadband capacitive micromachined ultrasound transducer (CMUT) array, operated in collapse mode, with 96 elements placed at the circumference of a catheter tip with a 1.2- {mm} diameter. We characterized the array performance for CE imaging and showed that the -6-dB device bandwidth at a 30-V dc biasing is 25 MHz with a 20-MHz center frequency, with a transmit sensitivity of 37 kPa/V at that frequency. We designed a linear frequency modulation code to improve penetration depth by compensating for high-frequency attenuation while preserving resolution by a mismatched filter reconstruction. We imaged a wire phantom and a human coronary artery plaque. By assessing the image quality of the reconstructed wire phantom image, we achieved 60- and 70- mu{mathrm {m}} axial resolutions using the short pulse and coded signal, respectively, and gained 8 dB in SNR for CE. Our developed system shows 20-frames/s, pixel-based beam-formed, real-time IVUS images.@en

Understanding and controlling the ultrasound contrast agent (UCA)'s response to an applied ultrasound pressure field are crucial when investigating ultrasound imaging sequences and therapeutic applications. The magnitude and frequency of the applied ultrasonic pressure waves affect the oscillatory response of the UCA. Therefore, it is important to have an ultrasound compatible and optically transparent chamber in which the acoustic response of the UCA can be studied. The aim of our study was to determine the in situ ultrasound pressure amplitude in the ibidi μ -slide I Luer channel, an optically transparent chamber suitable for cell culture, including culture under flow, for all microchannel heights (200, 400, 600, and 800 μm). First, the in situ pressure field in the 800- μm high channel was experimentally characterized using Brandaris 128 ultrahigh-speed camera recordings of microbubbles (MBs) and a subsequent iterative processing method, upon insonification at 2 MHz, 45° incident angle, and 50-kPa peak negative pressure (PNP). Control studies in another cell culture chamber, the CLINIcell, were compared with the obtained results. The pressure amplitude was -3.7 dB with respect to the pressure field without the ibidi μ -slide. Second, using finite-element analysis, we determined the in situ pressure amplitude in the ibidi with the 800- μm channel (33.1 kPa), which was comparable to the experimental value (34 kPa). The simulations were extended to the other ibidi channel heights (200, 400, and 600 μm) with either 35° or 45° incident angle, and at 1 and 2 MHz. The predicted in situ ultrasound pressure fields were between -8.7 and -1.1 dB of the incident pressure field depending on the listed configurations of ibidi slides with different channel heights, applied ultrasound frequencies, and incident angles. In conclusion, the determined ultrasound in situ pressures demonstrate the acoustic compatibility of the ibidi μ -slide I Luer for different channel heights, thereby showing its potential for studying the acoustic behavior of UCAs for imaging and therapy.@en

A dual frequency probe using a multi-layer piezoelectric material is proposed for simultaneous ultrasound and photoacoustic imaging of the carotid artery with a high resolution ultrasound and a high sensitivity photoacoustic image. The probe consists of lead zirconate titanate (PZT) for ultrasound stack and and polyvinylidene difluoride (PVDF) array for photoacoustic signal reception, which is placed on top of the PZT stack. We used 3D finite element analysis to evaluate a quarter of the full aperture of the dual frequency array, having 48 elements diced PZT-5H for ultrasound pulse-echo and 16 elements of 28µm thick, kerfless PVDF for photoacoustic receiving. We showed that considering the PVDF array as the second matching layer of the ultrasound stack minimized its loading effect at the cost of operating in a higher operation frequency of 9.9 MHz. We modeled a design with and without sub-dicing, where sub-dicing and subsequent suppression of lateral modes allows larger elements and thus larger aperture. The -3dB bandwidth of the ultrasound stack with and without sub-dicing are 87% and 75% relative to the center frequencies. We found a transmit sensitivity of 17 kPa/V and 21 kPa/V for those two realizations respectively.@en

Photoacoustic signals can have very large bandwidth and large dynamic range. With appropriate electrical impedance matching, off-resonance PVDF transducers offer the bandwidth and sensitivity to fully capture these signals. Minimizing pitch and kerf in ultrasound transducer to improve lateral resolution is of great importance. Kerfless arrays, where the electrodes are patterned onto the piezoelectric material, fulfill such a demand and are simpler to manufacture. We have manufactured and characterized a kerfless off resonance PVDF array of 10 elements with dimensions of 1mm×1.5mm×28μm(w×h×t) both numerically and experimentally for PA imaging. We matched the high impedance of the PVDF elements with low impedance of acquisition system through an appropriate LNA (26 dB amplification over 10 MHz BW). PA signals from a tungsten wire with a diameter of 35 μm were received with our device, and compared to those recorded with a 0.2 mm needle hydrophone. Based on simulation and measurement results, the phase change in the off-resonance PVDF is negligible. The crosstalk between adjacent elements is −30dB. We measured a sensitivity for our array of about 1.38 μV/Pa at 3 MHz. The RMS output noise level is 240 uV over the entire bandwidth (10Hz-10 MHz).@en

Photoacoustic signals can have very large bandwidth and large dynamic range. With appropriate electrical impedance matching, off-resonance PVDF transducers offer the bandwidth and sensitivity to fully capture these signals. Minimizing pitch and kerf in ultrasound transducer to improve lateral resolution is of great importance. Kerfless arrays, where the electrodes are patterned onto the piezoelectric material, fulfill such a demand and are simpler to manufacture. We have manufactured and characterized a kerfless off resonance PVDF array of 10 elements with dimensions of 1mm×1.5mm×28μm(w×h×t) both numerically and experimentally for PA imaging. We matched the high impedance of the PVDF elements with low impedance of acquisition system through an appropriate LNA (26 dB amplification over 10 MHz BW). PA signals from a tungsten wire with a diameter of 35 μm were received with our device, and compared to those recorded with a 0.2 mm needle hydrophone. Based on simulation and measurement results, the phase change in the off-resonance PVDF is negligible. The crosstalk between adjacent elements is −30dB. We measured a sensitivity for our array of about 1.38 μV/Pa at 3 MHz. The RMS output noise level is 240 uV over the entire bandwidth (10Hz-10 MHz).@en