Ultrasound waveguiding of light is a recently introduced technique aiding light transport in scattering media, effectively reducing scattering strength. This technique locally, transiently, reversibly modifies the refractive index of the medium, recollecting and guiding some of the scattered photons to increase light intensity in depth. Here we use transient transversal ultrasound light waveguiding to increase the strength of the signal received from fluorescent target hidden behind a 3 mm thick scattering phantom. We use a common linear array transducer and transmission geometry and show waveguiding-induced increase of fluorescence, excited by a pulsed 532 nm light, typical for photoacoustic imaging setting.
Photoacoustic (PA) wavefront shaping (WS; PAWS) could allow focusing light deep in biological tissue. This could enable increasing the penetration depth of biomedical optical techniques including PA imaging. However, focussing at depth requires a light source of long coherence length (CL), presenting a challenge because the CLs of typical PA excitation lasers are short. To address this challenge, we developed a PAWS system based on an externally modulated external cavity laser with a long CL. The system was demonstrated by focussing light through rigid scattering media using both PAWS and optical WS. PAWS enabled focussing through diffusers with 8 × enhancements, while all-optical WS enabled focussing through various scattering media including a 5.8 mm thick tissue phantom. By enabling PAWS with increased coherence, the system could facilitate exploring the practical depth limits of PAWS, paving the way to focussing light deep in tissue.
There has been considerable interest in extending photoacoustic imaging techniques to endoscopic devices, which would enable a diverse range of applications, e.g. assessment of coronary artery disease or surgical guidance.
However, the difficulty of miniaturising traditional piezoelectric sensors has mostly prevented tomography-mode endoscopic imaging, where an array of sensors is used to reconstruct the full ultrasound field to centimeter-scale depths.
In this work we demonstrate how wavefront shaping through multimode fibres onto a Fabry-Perot optical ultrasound sensor can overcome this limitation, producing an endoscopic imaging system with a footprint an order of magnitude smaller than the state of the art.
We analyze transducer-matched multipulse excitation as a method for improving of the signal-to-noise ratio (SNR) for diode laser-based photoacoustic systems. We discuss the principle of the technique, its advantages, and potential drawbacks and perform measurements to analyze the obtainable SNR increase. We show in experiment and computationally that a lower boundary estimate of 1.2 to 1.8 fold SNR improvement can be provided using transducer-matched pulse bursts, depending on the transducer and particular arrangement. Finally, we analyze implications that the transducer resonance effects may have on the recently introduced advanced photoacoustic techniques. The findings are of immediate interest to modalities utilizing dense pulse sequences and systems possessing limited pulse energy. In particular, transducer-matched multipulse excitation may be beneficial for diode-based photoacoustic systems operated with transducers in the range of 1 to 5 MHz since the required hardware is readily available.
It has been shown recently, that varying the excitation sequence could deliver additional benefits for photoacoustic
imaging, for instance, bringing additional information on the sample under study, or reducing the total acquisition time.
However, for the typically used solid state laser systems, such modification requires significant increase of the systems’
complexity.
We are taking an advantage of high pulse repetition rates that semiconductor laser diodes could offer. That allows the
usage of dense pulse bursts with varied number of pulses and inter-pulse delays in the range of the transducer waveform
duration to study the effects of the overlay of the single pulse photoacoustic responses.
In this study, we conduct a pump-probe experiment, using multi-pulse excitation sequences with varied inter-pulse
delays while registering the acoustic response. We show that pulse burst excitation can be beneficial for increasing the
registered amplitude and suitable inter-pulse delay values can be obtained from the transducer transfer function, either
known or measured. Additionally, we examine the frequency content of the multi-pulse photoacoustic response and
show that it is dominated by the pulse repetition rate used. We focus on low central frequency transducers as being
widely used for clinical applications.
Achieving a good signal-to-noise ratio at increased depths remains a challenge, even for photoacoustic imaging, which stimulates the search for possible contrast improvements. Both double-pulse and pulse burst excitation are shown beneficial for increasing the signal-to-noise ratio or acquiring additional information about the sample. We use the advantage of semiconductor laser diodes offering great opportunities regarding both number of pulses in the burst and inter-pulse delay times to study the dynamics of the pulse burst excitation responses of the single-element PZT transducers, looking for possibilities towards contrast improvement. We concentrate on inter-pulse delay ranges of few hundred nanoseconds and low central frequency transducers as they are mainly used for clinical applications We show that using pulse burst excitation with up to five pulses per burst and transducer-matched inter-pulse delays can increase the registered maximum amplitude, leading to signal-to-noise ratio improvement. The multi-pulse response amplitude increase amounted to 20% of the amplitude of a single-pulse response in the performed measurement.
KEYWORDS: Transducers, Photoacoustic microscopy, Semiconductor lasers, Photoacoustic imaging, Data processing, Data modeling, Data analysis, Glasses, Oscilloscopes, Signal detection
In contrast to the well-established and widely used theory of photoacoustic signal generation by single delta-like pulses, the field of multiple pulse excitation is not yet studied well. Using double-pulse excitation can be beneficial, but as ultrasound transducers have a certain waveform duration, the inter-pulse delays used might be limited.
In order to assess the strength of the transducer influence at short delay times and develop data analysis procedure, we investigate the photoacoustic responses of a phantom sample to double-pulse excitation measured with different transducers. Both focused and flat surface single element transducers are used in the study. The central frequencies are chosen in the low-frequency band as they are most widely used in clinical ultrasound and one higher frequency transducer is taken for comparison.
Despite not observing signal amplification due to Grueneisen relaxation effect, we show that transducer influence is not exceeding measurement error. Additionally we prove that single pulse subtraction procedure can be used to restore the second pulse waveform in double pulse excitation scheme. We believe using this procedure can be beneficial when transducer’s waveform duration is longer than used inter-pulse delays.
In this paper we investigate the possibilities to use a pulsed laser diode based setup to achieve the photoacoustic signal
amplification via the Grueneisen relaxation effect. It is shown that the system is capable of producing the required
multiple pulses burst with pulse widths of 12 ns and pulse inter-delays down to approximately 135 ns. With additional
fluence considerations we expect no improvement from this technique for photoacoustic tomography setting, while our
laser diode based setup is a highly promising compact alternative for Grueneisen relaxation related studies in
photoacoustic microscopy.
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