James R. McLaughlan

Expanding 3D geometry for enhanced on-chip microbubble production and single step formation of liposome modified microbubbles

Micron sized, lipid stabilized bubbles of gas are of interest as contrast agents for ultra-sound (US) imaging and increasingly as delivery vehicles for targeted, triggered, therapeutic delivery. Microfluidics provides a reproducible means for microbubble production and surface functionalisation. In this study, microbubbles are generated on chip using flow-focussing microfluidic devices that combine streams of gas and liquid through a nozzle a few microns wide and then subjecting the two phases to a downstream pressure drop. While microfluidics has successfully demonstrated the generation of monodisperse bubble populations, these approaches inherently produce low bubble counts. We introduce a new micro-spray flow regime that generates consistently high bubble concentrations that are more clinically relevant compared to traditional monodisperse bubble populations. Final bubble concentrations produced by the micro-spray regime were up to 10(10) bubbles mL(-1). The technique is shown to be highly reproducible and by using multiplexed chip arrays, the time taken to produce one millilitre of sample containing 10(10) bubbles mL(-1) was ∼10 min. Further, we also demonstrate that it is possible to attach liposomes, loaded with quantum dots (QDs) or fluorescein, in a single step during MBs formation.

Ultrasonic enhancement of photoacoustic emissions by nanoparticle-targeted cavitation

A technique to enhance the photoacoustic emissions from laser-heated nanoparticles is presented. Gold nanoparticle-doped phantoms are subjected to pulsed optical and ultrasound fields, resulting in bubble formation and collapse and producing strong acoustic emissions. The applied ultrasound field allows for cavitation at lower laser fluences than using light alone. The acoustic emission associated with bubble collapse well exceeds the direct photoacoustic response and is used to image a nanoparticle-doped region in a tissue phantom. The strong acoustic emission and low-threshold fluence associated with ultrasound-assisted cavitation make the technique well suited for nanoparticle-targeted biological imaging and tissue therapy.

The effect of amplitude modulation on subharmonic imaging with chirp excitation

Subharmonic generation from ultrasound contrast agents depends on the spectral and temporal properties of the excitation signal. The subharmonic response can be improved by using wideband and long-duration signals. However, for sinusoidal tone-burst excitation, the effective bandwidth of the signal is inversely proportional to the signal duration. Linear frequency-modulated (LFM) and nonlinear frequencymodulated (NLFM) chirp excitations allow independent control over the signal bandwidth and duration; therefore, in this study LFM and NLFM signals were used for the insonation of microbubble populations. The amplitude modulation of the excitation waveform was achieved by applying different window functions. A customized window was designed for the NLFM chirp excitation by focusing on reducing the spectral leakage at the subharmonic frequency and increasing the subharmonic generation from microbubbles. Subharmonic scattering from a microbubble population was measured for various excitation signals and window functions. At a peak negative pressure of 600 kPa, the generated subharmonic energy by ultrasound contrast agents was 15.4 dB more for NLFM chirp excitation with 40% fractional bandwidth when compared with tone-burst excitation. For this reason, the NLFM chirp with a customized window was used as an excitation signal to perform subharmonic imaging in an ultrasound flow phantom. Results showed that the NLFM waveform with a customized window improved the subharmonic contrast by 4.35 ± 0.42 dB on average over a Hann-windowed LFM excitation.

REAL-TIME MONITORING OF HIGH-INTENSITY FOCUSED ULTRASOUND LESION FORMATION USING ACOUSTO-OPTIC SENSING

High-intensity focused ultrasound (HIFU) is a promising modality that is used to noninvasively ablate soft tissue tumors. Nevertheless, real-time treatment monitoring with diagnostic ultrasound still poses a significant challenge since tissue necrosis, in the absence of cavitation or boiling, provides little acoustic contrast with normal tissue. In comparison, the optical properties of tissue are significantly altered accompanying lesion formation. A photorefractive crystal-based acousto-optic (AO) sensing system that uses a single HIFU transducer to simultaneously generate tissue necrosis and pump the AO interaction is used to monitor the real-time optical changes associated with thermal lesions induced in chicken breast ex vivo. It is found that the normalized change in AO response increases proportionally with the volume of necrosis. This study demonstrates AO sensing can identify the onset and growth of lesion formation in real time and, when used as feedback to guide exposures, results in more predictable lesion formation.

Increasing the sonoporation efficiency of targeted polydisperse microbubble populations using chirp excitation

The therapeutic use of microbubbles for targeted drug or gene delivery is a highly active area of research. Phospholipid-encapsulated microbubbles typically have a polydisperse size distribution over the 1 to 10 μm range and can be functionalized for molecular targeting and loaded with drug-carrying liposomes. Sonoporation through the generation of shear stress on the cell membrane by microbubble oscillations is one mechanism that results in pore formation in the cell membrane and can improve drug delivery. A microbubble oscillating at its resonant frequency would generate maximum shear stress on a membrane. However, because of the polydisperse nature of phospholipid microbubbles, a range of resonant frequencies would exist in a single population. In this study, the use of linear chirp excitations was compared with equivalent duration and acoustic pressure tone excitations when measuring the sonoporation efficiency of targeted microbubbles on human colorectal cancer cells. A 3 to 7 MHz chirp had the greatest sonoporation efficiency of 26.9 ± 5.6%, compared with 16.4 ± 1.1% for the 1.32 to 3.08 MHz chirp. The equivalent 2.2- and 5-MHz tone excitations have efficiencies of 12.8 ± 2.1% and 15.6 ± 1.1%, respectively, which were all above the efficiency of 4.1 ± 3.1% from the control exposure.

Superharmonic imaging with chirp coded excitation: filtering spectrally overlapped harmonics

Superharmonic imaging improves the spatial resolution by using the higher order harmonics generated in tissue. The superharmonic component is formed by combining the third, fourth, and fifth harmonics, which have low energy content and therefore poor SNR. This study uses coded excitation to increase the excitation energy. The SNR improvement is achieved on the receiver side by performing pulse compression with harmonic matched filters. The use of coded signals also introduces new filtering capabilities that are not possible with pulsed excitation. This is especially important when using wideband signals. For narrowband signals, the spectral boundaries of the harmonics are clearly separated and thus easy to filter; however, the available imaging bandwidth is underused. Wideband excitation is preferable for harmonic imaging applications to preserve axial resolution, but it generates spectrally overlapping harmonics that are not possible to filter in time and frequency domains. After pulse compression, this overlap increases the range side lobes, which appear as imaging artifacts and reduce the Bmode image quality. In this study, the isolation of higher order harmonics was achieved in another domain by using the fan chirp transform (FChT). To show the effect of excitation bandwidth in superharmonic imaging, measurements were performed by using linear frequency modulated chirp excitation with varying bandwidths of 10% to 50%. Superharmonic imaging was performed on a wire phantom using a wideband chirp excitation. Results were presented with and without applying the FChT filtering technique by comparing the spatial resolution and side lobe levels. Wideband excitation signals achieved a better resolution as expected, however range side lobes as high as -23 dB were observed for the superharmonic component of chirp excitation with 50% fractional bandwidth. The proposed filtering technique achieved >50 dB range side lobe suppression and improved the image quality without affecting the axial resolution.

The capture of flowing microbubbles with an ultrasonic tap using acoustic radiation force

The accumulation of 1–10 μm phospholipid-shelled microbubbles was demonstrated by creating an “ultrasonic tap” using acoustic travelling waves. Microbubbles were flowed through a 200 μm cellulose tube at rates ranging between 14–50 ml/h, in order to approximate the velocities and wall shear rates found throughout the human circulatory system. The generated acoustic radiation force directly opposing the flow direction was sufficient to hold microbubbles in a fluid flow up to 28 cm/s. Clusters of microbubbles subject to wall shear rates of up to 9000 s−1 were retained near a pressure null for several seconds.

New performance metrics for ultrasound pulse compression systems

In medical ultrasound, B-mode images are log-compressed and displayed with a grayscale map, typically on a 40-60 dB dynamic range. The image formation process is the same for an ultrasound pulse compression system using coded excitation. Metrics, such as full width at half maximum (FWHM), peak sidelobe level (PSL) and integrated sidelobe level (ISL), used to evaluate pulse compression systems were adopted from radar and communications. These metrics are utilized to evaluate the performance of an auto-correlation function, which is the ideal case. In medical ultrasound imaging however, the combination of frequency and depth dependent attenuation, dispersion, harmonic generation, beamforming errors, and limited transducer bandwidth create a more complicated case for a pulse compressed system that is far from the ideal.

Non-linear harmonic reduction pulse width modulation (HRPWM) for the arbitrary control of transducer-integrated switched excitation electronics

Advances in electronics and transducer fabrication have provided the ultrasound system designer with the opportunity for integrating electronics into the transducer head. Switched mode circuits are miniaturizable, low loss and as such are suited to array excitation. Without careful excitation signal design switched mode waveforms lack combined time varying amplitude and harmonic control. Existing modulation schemes have provided either amplitude or harmonic control. The proposed harmonic reduction pulse width modulation (HRPWM) provides a method of creating a quinary (5-level) switched waveform with both time-varying amplitude control and third harmonic cancellation. HRPWM achieves waveform control through matching the amplitude of energy contained at the fundamental frequency with the desired amplitude, and third harmonic cancellation through the use of a non-linear modulating waveform. The resulting HRPWM excitation signals use multiple bipolar low voltage pulses for the control of signals below 50% magnitude, and a five-level pulse for amplitudes above 50%. In simulations, HRPWM shows an excitation signal third harmonic power of -34.75 dB, a reduction of 19 dB from other 5-level modulation schemes, and a ultrasound pressure waveform third harmonic power of -64.19 dB, a reduction of 19.9 dB from other 5-level modulation schemes, and 25 dB lower than bipolar switched excitation.

Separating the second harmonic response of tissue and microbubbles using bispectral analysis

The second harmonic generation in medical ultrasound is either caused by tissue or ultrasound contrast agents. The conventional signal processing techniques cannot separate the harmonic response from microbubbles and tissue. The second order spectral analysis, commonly known as the frequency analysis, is the most common way of evaluating the microbubble response. Although frequency analysis can estimate the power spectrum effectively, it suppresses the phase relation between the frequency components. In this study, bispectral analysis is used to evaluate the microbubble response and separate the second harmonic generated by tissue and microbubbles via the phase coupling between fundamental and harmonic components.