Adi Sheinfeld

Photoacoustic Doppler measurement of flow using tone burst excitation.

In this paper a novel technique for flow measurement which is based on the photoacoustic (PA) Doppler effect is described. A significant feature of the proposed approach is that it can be implemented using tone burst optical excitation thus enabling simultaneous measurement of both velocity and position. The technique, which is based on external modulation and heterodyne detection, was experimentally demonstrated by measurement of the flow of a suspension of carbon particles in a silicon tube and successfully determined the particles mean velocity up to values of 130 mm/sec, which is about 10 times higher than previously reported PA Doppler set-ups. In the theoretical part a rigorous derivation of the PA response of a flowing medium is described and some important simplifying approximations are highlighted.

Simultaneous spatial and spectral mapping of flow using photoacoustic Doppler measurement

We demonstrate the use of tone-burst excitation and time-gated spectral analysis for photoacoustic Doppler mapping of flow in an unperturbed vessel phantom and in a vessel with a spatially varying lumen. The method, which mimics pulsed Doppler ultrasound, enables simultaneous measurement of axial position and flow as well as complete characterization of the Doppler spectrum over a wide range of mean velocities (3.5 to 200 mm∕s). To generate the required optical excitation, a continuous cw laser source followed by an external electro-optic modulator is used. Stenoses at various levels are emulated in a C-flex tube with a flowing suspension of micrometer-scale carbon particles. Two-dimensional maps of spectral content versus axial position at different points along the vessel and for various levels of perturbations demonstrate the potential use of the method for characterization of flow irregularities.

Photoacoustic thermal diffusion flowmetry

Thermal Diffusion Flowmetry (TDF) (also called Heat Clearance Method or Thermal Clearance Method) is a longstanding technique for measuring blood flow or blood perfusion in living tissues. Typically, temperature transients and/or gradients are induced in a volume of interest and the temporal and/or spatial temperature variations which follow are measured and used for calculation of the flow. In this work a new method for implementing TDF is studied theoretically and experimentally. The heat deposition which is required for TDF is implemented photothermally (PT) and the measurement of the induced temperature variations is done by photoacoustic (PA) thermometry. Both excitation light beams (the PT and the PA) are produced by directly modulated 830 nm laser diodes and are conveniently delivered to the volume under test by the same optical fiber. The method was tested experimentally using a blood-filled phantom vessel and the results were compared with a theoretical prediction based on the heat and the photoacoustic equations. The fitting of a simplified lumped thermal model to the experimental data yielded estimated values of the blood velocity at different flow rates. By combining additional optical sources at different wavelengths it will be possible to utilize the method for non-invasive simultaneous measurement of blood flow and oxygen saturation using a single fiber probe.

Time-resolved photoacoustic Doppler characterization of flow using pulsed excitation

A new approach for implementing pulsed excitation enables time-resolved characterization of flow, using the photoacoustic Doppler effect. The method yields two-dimensional maps of the Doppler shift vs. axial position of flowing absorbing particles. It takes advantage of the unique flexibility and accuracy of external modulation which offers excellent control over the parameters of the pulsed optical excitation. The experimental setup comprised a CW tunable laser source operating in the fiber optic communications band (1510-1620nm) followed by an electro-optic modulator, electronically driven by an arbitrary waveform generator. Using the technique the flow of a suspension of carbon particles in a C-flex tube was measured over a wide range of velocities from 18 mm/sec up to 200mm/sec.

The use of pulse synthesis for optimization of photoacoustic measurements

In this paper the use of pulse shaping in photoacoustic (PA) measurements is presented. The benefits of this approach are demonstrated by utilizing it for optimization of either the responsivity or the sensitivity of PA measurements. The optimization is based on the observation that the temporal properties of the PA effect can be represented as a linear system which can be fully characterized by its impulse response. Accordingly, the response of the PA system to an input optical pulse, whose instantaneous power is arbitrarily shaped, can be analytically predicted via a convolution between the pulse envelope and the PA impulse response. Additionally, the same formalism can be used to show that the response of the PA system to a pulse whose instantaneous power is a reversed version of the impulse response, i.e. a matched pulse, would exhibit optimal peak amplitude when compared with all other pulses with the same energy. Pulses can also be designed to optimize the sensitivity of the measurement to a variation in a specific system parameter. The use of the matched pulses can improve SNR and enable a reduction in the total optical energy required for obtaining a detectable signal. This may be important for applications where the optical power is restricted or for dynamical measurements where long integration times are prohibited. To implement this new approach, a novel PA optical setup which enabled synthesis of excitation waveforms with arbitrary temporal envelopes was constructed. The setup was based on a tunable laser source, operating in the near-IR range, and an external electro-optic modulator. Using this setup, our approach for system characterization and response prediction was tested and the superiority of the matched pulses over other common types of pulses of equal energy was demonstrated.

Doppler photoacoustic and Doppler ultrasound in blood with optical contrast agent

Photoacoustic Doppler flowmetry as well as Doppler ultrasound were performed in acoustic resolution regime on tubes filled with flowing blood with indocyanine green (ICG) at different concentrations. The photoacoustic excitation utilized a pair of directly-modulated fiber-coupled 830nm laser-diodes, modulated with either CW or tone-bursts for depthresolved measurements. The amplitude of the Doppler peak in photoacoustic Doppler measurements was found to be proportional to the ICG concentration. Photoacoustic Doppler was measured in ICG at human safe concentrations, but not in whole blood. Comparing the results between the two modalities implied that using a wavelength with higher optical absorption may improve the photoacoustic signal in blood.

Flow-dependant photothermal modulation of the photoacoustic response

The temperature dependence of photoacoustic generation is utilized for monitoring the temperature in flowing blood. A phantom blood vessel is probed with photoacoustic (PA) excitation from a 830nm laser diode whose intensity is sinusoidally modulated at ultrasound frequencies. A second laser diode at the same wavelength is used to photothermally (PT) induce sinusoidal temperature fluctuations in the probed volume. The temperature oscillations lead to modulation sidebands in the PA response. Measurement of the sidebands amplitude as a function of the PT modulation frequency, for different flow rates, reveals a strong dependence of the PT modulation frequency response (MFR) on the flow rate. This is attributed to the thermal properties of the volume under test, and in particular to the heat clearance rate, which is strongly affected by the flow. A simplified lumped model based on the similarity between the system temporal behavior and that of an RC circuit is used to analyze the resulting MFRs. With the addition of an appropriate calibration protocol and by using multispectral PA and/or PT excitation the proposed approach can be used for simultaneous in-vivo measurement of both flow and oxygenation level.

Coded photoacoustic Doppler excitation with near-optimal utilization of the time and frequency domains

We propose and experimentally demonstrate a new photoacoustic (PA) excitation and analysis method which achieves an almost complete utilization of the available time and frequency windows. The method, which enables spectral and spatial characterization of flow, is based on interleaving tens of tone-burst sequences at equally spaced frequencies. Depending on the application, the interleaved signals can be generated by a single optical source or by multiple sources, possibly at different wavelengths. Upon reception, the responses corresponding to the different tone-burst sequences are spectrally de-multiplexed. As demonstrated in the current work, this method can be used to improve the SNR of PA systems based on optical sources with limited peak power. Alternatively, if the interleaved excitation signals are at different wavelengths, the PA responses can be used for multispectral characterization of the medium.

Photoacoustic thermal diffusion flowmetry in tissue-mimicking phantoms

Photoacoustic Thermal Diffusion Flowmetry (PA-TDF) utilizes photothermal heating and photoacoustic temperature monitoring to measure the tissue heat clearance time constants from which blood velocity can be inferred. We extended our study of PA-TDF to tissue-mimicking phantoms with vessels at various diameters, configurations and depths and experimentally verified the relations between the estimated time constants and the vessels and the illuminating beam dimensions. We also demonstrated, for the first time, depth-resolved PA-TDF measurement using tone-burst photoacoustic excitation. The excitation utilized two fiber-coupled 830nm laser diodes, one induced slow temperature oscillations and the other induced the PA excitation.

A highly flexible pulsed photoacoustic setup based on a tunable laser source and external modulation

In this paper performance enhancement in PA measurements through optical pulse shaping is explored and demonstrated. A recently introduced setup, which is based on a CW tunable laser source operating in the optical communications band (1510-1620nm) and an electro-optic modulator, offers exceptional flexibility in controlling the temporal and spectral characteristics of the excitation waveform. Despite its being remarkably simple to construct and to operate, the unique optical configuration of this setup opens the possibility for a range of attractive applications which cannot be implemented by the commonly used pulsed lasers: responsivity and sensitivity optimization through pulse-shaping, enhancement of spatial resolution through pulse compression and simple implementation of high-resolution quantitative spectroscopy.