Imaging With Therapeutic Acoustic Wavelets-Short Pulses Enable Acoustic Localization When Time of Arrival is Combined With Delay and Sum

Timing-synchronized passive ultrasound imaging of cavitation using eigenspace-based minimum variance beamforming and principal component analysis

BACKGROUND

Passive ultrasound imaging (PUI) allows to spatially resolve cavitation triggered during ultrasound irradiation, its application in therapeutic ultrasound has been gaining attention in recent years. The diffraction mode of the imaging transducer greatly limits the PUI axial resolution, which can be improved by transmit-receive synchronization and employment of delay sum beamforming (DSB) when transmitting short pulses, however, DSB yields poor performance in resolution and anti-interference.

PURPOSE

Inspired by adaptive beamforming and its low-complexity algorithm in active imaging field, this paper aims to develop an improved timing-synchronized PUI (TSPUI) algorithm for detection of short-pulse transmission-induced cavitation.

METHODS

The passive array data collected by timing synchronization is processed by minimum variance beamforming (MVB), whose weights are optimized by projection on the eigendecomposed signal subspace, that is, eigenspace-based MVB (EMVB), with the sum of the flight times on the transmitting and receiving paths as the delay. Applying principal component analysis (PCA) on the pre-collected MVB weight samples, a conversion matrix is constructed to allow the matrix inversion and eigendecomposition involved in weight calculation to be performed in a low dimension. The algorithm performance is confirmed by experiments, where a high-intensity focused ultrasound transducer and a linear-array transducer configured in a common parallel or vertical manner are employed for cavitation induction and cavitation imaging, and evaluated with the established indicators.

RESULTS

Reducing the eigenvalue threshold coefficient allows more sidelobes to be removed, and choosing an appropriate principal component number can reduce the time cost while guaranteeing the reconstruction quality. EMVB-PCA provides high resolution and anti-interference performance relative to DSB, with a reduction of over 60% in the point spread area and over 14 dB in the sidelobe and noise level, meanwhile, its time cost is considerably lower than EMVB, with a reduction of over 80%. Additionally, constructing the conversion matrix by simulation is feasible and valid, providing convenience for real imaging.

CONCLUSIONS

EMVB-PCA allows for high-quality TSPUI reconstruction of cavitation at a fast rate, providing an effective tool for detecting short-duration cavitation and further benefiting short-pulse therapeutic ultrasound applications.

Biological effects of rapid short pulses of focused ultrasound for drug delivery to the brain

Focused ultrasound in combination with intravenously injected microbubbles offers a non-invasive and localised method to deliver drugs across the blood-brain barrier, enabling targeted treatment of brain disorders. Recently, we have shown that applying sequences of Rapid Short-Pulses (RaSP; 5 μs pulses emitted at 1.25 kHz grouped into 10 ms bursts) of ultrasound can deliver drugs with an improved efficacy and safety profile compared with traditionally-used longer pulses (> 10 ms). In this study, we examined the extent to which RaSP sequences allowed the extravasation of endogenous blood proteins, including albumin and immunoglobulin, as well as T cells, into the brain parenchyma. We also investigated the effect of RaSP ultrasound treatments on synaptic connectivity, and the distribution and excretion of fluorescently-labelled 3 kDa dextran delivered to the brain with RaSP. The left hippocampus of mice was sonicated with either a RaSP sequence (5 μs at 1.25 kHz in groups of 10 ms at 0.5 Hz) or a long pulse sequence (10 ms at 0.5 Hz), at 0.35, 0.53 and 0.71 MPa with a 1-MHz center frequency. Significantly less albumin was detected in RaSP-treated brains immediately after treatment and was cleared within 10 min compared to those treated with long pulses, while immunoglobulin was hardly detected in RaSP-treated brains at 0, 10 or 20 min after treatment. No T cells were detected in RaSP-treated brains at 0.35, 0.53 or 0.71 MPa after 0 or 2 h. In long pulse samples, however, T cells did extravasate when using the two higher acoustic pressures, 0.53 and 0.71 MPa, immediately after treatment. Quantification of dendritic spine area revealed no differences between RaSP-treated hippocampi compared to untreated contralateral hippocampi and control mice following three weekly ultrasound treatments. Finally, fluorescently-labelled dextran increasingly moved towards blood vessels and away from the parenchyma once delivered to the brain with both RaSP and long pulse sequences. Uptake of dextran within cells decreased over time with both sequences, and long pulses lead to a larger number of vessels with dextran uptake. This study highlights that RaSP ultrasound sequences can deliver molecules across the blood-brain barrier with minimal extravasation of endogenous proteins and no T cell infiltration, while preserving dendritic spine integrity, thus offering an improved safety profile.

An Ultrasound Array of Emitter-Receiver Stacks for Microbubble-Based Therapy

Most therapeutic ultrasound devices place emitters and receivers in separate locations, so that the long therapeutic pulses (>1 ms) can be emitted while receivers monitor the procedure. However, with such placement, emitters and receivers are competing for the same space, producing a trade-off between emission efficiency and reception sensitivity. Taking advantage of recent studies demonstrating that short-pulse ultrasound can be used therapeutically, we aimed to develop a device that overcomes such trade-offs. The array was composed of emitter-receiver stacks, which enabled both emission and reception from the same location. Each element was made of a lead zirconate titanate (PZT)-polyvinylidene fluoride (PVDF) stack. The PZT (frequency: 500 kHz, diameter: 16 mm) was used for emission and the PVDF (thickness: 28 μm, diameter: 16 mm) for broadband reception. 32 elements were assembled in a 3D-printed dome-shaped frame (focal length: 150 mm; [Formula: see text]-number: 1) and was tested in free-field and through an ex-vivo human skull. In free-field, the array had a 4.5 × 4.5 × 32 mm focus and produced a peak-negative pressure (PNP) of 2.12 MPa at its geometric center. The electronic steering range was ±15 mm laterally and larger than ±15 mm axially. Through the skull, the array produced a PNP of 0.63 MPa. The PVDF elements were able to localize broadband microbubble emissions across the skull. We built the first multi-element array for short-pulse and microbubble-based therapeutic applications. Stacked arrays overcome traditional trade-offs between the transmission and reception quality and have the potential to create a step change in treatment safety and efficacy.

Improved source localization in passive acoustic mapping using delay-multiply-and-sum beamforming with virtually augmented aperture

High-intensity focused ultrasound (HIFU) is a promising non-invasive treatment method whose applications include tissue ablation, hemostasis, thrombolysis and blood-brain barrier opening etc. Its therapeutic effects come from the thermal necrosis and the mechanical destruction associated with acoustic cavitation. Passive acoustic mapping (PAM) is capable of simultaneous monitoring of HIFU-induced cavitation events using only receive beamforming. Nonetheless, conventional time exposure acoustics (TEA) algorithm has poor spatial resolution and suffers from the X-shaped artifacts. These factors lead to difficulties in precise localization of cavitation source. In this study, we proposed a novel adaptive PAM method which combines Delay-Multiply-and-Sum (DMAS) beamforming with virtual augmented aperture (VA) to overcome the problem. In DMAS-VA beamforming, the magnitude of each channel waveform is scaled by p-th root while the phase is multiplied by L. The p and L correspond respectively to the degree of signal coherence in DMAS beamforming and the augmentation factor of aperture size. After channel sum, p-th power is applied to restore the dimensionality of source strength and then the PAM image is reconstructed by accumulating the signal power over the observation time. Based on simulation and experimental results, the proposed DMAS-VA has better image resolution and image contrast compared with the conventional TEA. Moreover, since the VA method may introduce grating lobes into PAM because of the virtually augmented pitch size, DMAS coherent factor (DCF) is further developed to alleviate these image artifacts. Results indicate that, with DCF weighting, the PAM image of DMAS-VA beamforming could be constructed without detectable image artifacts from grating lobes and false main lobes.

Real-Time Passive Acoustic Mapping With Enhanced Spatial Resolution in Neuronavigation-Guided Focused Ultrasound for Blood-Brain Barrier Opening

OBJECTIVE

Passive acoustic mapping (PAM) provides the spatial information of acoustic energy emitted from microbubbles during focused ultrasound (FUS), which can be used for safety and efficacy monitoring of blood-brain barrier (BBB) opening. In our previous work with a neuronavigation-guided FUS system, only part of the cavitation signal could be monitored in real time due to the computational burden although full-burst analysis is required to detect transient and stochastic cavitation activity. In addition, the spatial resolution of PAM can be limited for a small-aperture receiving array transducer. For full-burst real-time PAM with enhanced resolution, we developed a parallel processing scheme for coherence-factor-based PAM (CF-PAM) and implemented it onto the neuronavigation-guided FUS system using a co-axial phased-array imaging transducer.

METHODS

Simulation and in-vitro human skull studies were conducted for the performance evaluation of the proposed method in terms of spatial resolution and processing speed. We also carried out real-time cavitation mapping during BBB opening in non-human primates (NHPs).

RESULTS

CF-PAM with the proposed processing scheme provided better resolution than that of traditional time-exposure-acoustics PAM with a higher processing speed than that of eigenspace-based robust Capon beamformer, which facilitated the full-burst PAM with the integration time of 10 ms at a rate of 2 Hz. In vivo feasibility of PAM with the co-axial imaging transducer was also demonstrated in two NHPs, showing the advantages of using real-time B-mode and full-burst PAM for accurate targeting and safe treatment monitoring.

SIGNIFICANCE

This full-burst PAM with enhanced resolution will facilitate the clinical translation of online cavitation monitoring for safe and efficient BBB opening.

A Novel Approach for the Detection of Every Significant Collapsing Bubble in Passive Cavitation Imaging

Passive cavitation image (PCI) shows the power distribution of the acoustic emissions resulting from cavitation bubble collapses. The conventional PCI convolves the emitted cavitation signals with the point spread function of an imaging system, and it suffers from a low spatial resolution and contrast due to the increased sidelobe artifacts accumulated during the temporal integral process. To overcome the problems, the present study considers a 3-D time history of instantaneous PCIs where cavitation occurs at the local maxima of the main lobes of the beamformed cavitation field surrounded by the sidelobes largely spreading out in a time-space domain. A spatial and temporal gating technique was employed to detect the local maxima so that cavitation bubbles can be identified with their collapsing strength. The proposed approach was verified by the simulation for single and multiple cavitation bubbles, proving that it accurately detects the location and strength of the collapsing bubbles. An experimental test was also carried out for the cavitation bubbles produced by a clinical extracorporeal shock wave therapeutic device, which underpins that the proposed method successfully identifies every individual cavitation bubble.

High-spatial-resolution, instantaneous passive cavitation imaging with temporal resolution in histotripsy: a simulation study

Purpose

In histotripsy, a shock wave is transmitted, and the resulting inertial bubble cavitation that disrupts tissue is used for treatment. Therefore, it is necessary to detect when cavitation occurs and track the position of cavitation occurrence using a new passive cavitation (PC) imaging method.

Methods

An integrated PC image, which is constructed by collecting the focused signals at all times, does not provide information on when cavitation occurs and has poor spatial resolution. To solve this problem, we constructed instantaneous PC images by applying delay and sum beamforming at instantaneous time instants. By calculating instantaneous PC images at all data acquisition times, the proposed method can detect cavitation when it occurs by using the property that when signals from the cavitation are focused, their amplitude becomes large, and it can obtain a high-resolution PC image by masking out side lobes in the vicinity of cavitation.

Results

Ultrasound image simulation confirmed that the proposed method has higher resolution than conventional integrated PC imaging and showed that it can determine the position and time of cavitation occurrence as well as the signal strength.

Conclusion

Since the proposed novel PC imaging method can detect each cavitation separately when the incidence of cavitations is low, it can be used to monitor the treatment process of shock wave therapy and histotripsy, in which cavitation is an important mechanism of treatment.

Passive Cavitation Detection With a Needle Hydrophone Array

Therapeutic ultrasound and microbubble technologies seek to drive systemically administered microbubbles into oscillations that safely manipulate tissue or release drugs. Such procedures often detect the unique acoustic emissions from microbubbles with the intention of using this feedback to control the microbubble activity. However, most sensor systems reported introduce distortions to the acoustic signal. Acoustic shockwaves, a key emission from microbubbles, are largely absent in reported recording, possibly due to the sensors being too large or too narrowband, or having strong phase distortions. Here, we built a sensor array that countered such limitations with small, broadband sensors and a low-phase distorting material. We built eight needle hydrophones with polyvinylidene fluoride (PVDF, diameter: 2 mm) then fit them into a 3-D-printed scaffold in a two-layered, staggered arrangement. Using this array, we monitored microbubbles exposed to therapeutically relevant ultrasound pulses (center frequency: 0.5 MHz, peak-rarefactional pressure: 130-597 kPa, pulselength: four cycles). Our tests revealed that the hydrophones were broadband with the best having a sensitivity of -224.8 dB ± 3.2 dB re 1 V/ μ Pa from 1 to 15 MHz. The array was able to capture shockwaves generated by microbubbles. The signal-to-noise ratio (SNR) of the array was approximately two times higher than individual hydrophones. Also, the array could localize microbubbles (-3 dB lateral resolution: 2.37 mm) and determine the cavitation threshold (between 161 and 254 kPa). Thus, the array accurately monitored and localized microbubble activities, and may be an important technological step toward better feedback control methods and safer and more effective treatments.

A PZT-PVDF Stacked Transducer for Short-Pulse Ultrasound Therapy and Monitoring

Therapeutic ultrasound technologies using microbubbles require a feedback control system to perform the treatment in a safe and effective manner. Current feedback control technologies utilize the microbubble's acoustic emissions to adjust the treatment acoustic parameters. Typical systems use two separated transducers: one for transmission and the other for reception. However, separating the transmitter and receiver leads to foci misalignment. This limitation could be resolved by arranging the transmitter and receiver in a stacked configuration. Taking advantage of an increasing number of short-pulse-based therapeutic methods, we have constructed a lead zirconate titanate-polyvinylidene fluoride (PZT-PVDF) stacked transducer design that allows the transmission and reception of short-pulse ultrasound from the same location. Our design had a piston transmitter composed of a PZT disk (1 MHz, 12.7 mm in diameter), a backing layer, and two matching layers. A layer of PVDF ( [Formula: see text] in thickness, 12.7 mm in diameter) was placed at the front surface of the transmitter for reception. Transmission and reception from the same location were demonstrated in pulse-echo experiments where PZT transmitted a pulse and both PZT and PVDF received the echo. The echo signal received by the PVDF was [Formula: see text] shorter than the signal received by the PZT. Reception of broadband acoustic emissions using the PVDF was also demonstrated in experiments where microbubbles were exposed to ultrasound pulses. Thus, we have shown that our PZT-PVDF stack design has unique transmission and reception features that could be incorporated into a multielement array design that improves focal superimposing, transmission efficiency, and reception sensitivity.