Imaging the Activation of Low-Boiling-Point Phase-Change Contrast Agents in the Presence of Tissue Motion Using Ultrafast Inter-frame Activation Ultrasound Imaging

Dynamic Coronary Blood Flow Velocity and Wall Shear Stress Estimation Using Ultrasound in an Ex Vivo Porcine Heart

PURPOSE

Wall shear stress (WSS) is a critically important physical factor contributing to atherosclerosis. Mapping the spatial distribution of local, oscillatory WSS can identify important mechanisms underlying the progression of coronary artery disease.

METHODS

In this study, blood flow velocity and time-varying WSS were estimated in the left anterior descending (LAD) coronary artery of an ex vivo beating porcine heart using ultrasound with an 18 MHz linear array transducer aligned with the LAD in a forward-viewing orientation. A pulsatile heart loop with physiologically-accurate flow was created using a pulsatile pump. The coronary artery wall motion was compensated using a local block matching technique. Next, 2D and 3D velocity magnitude and WSS maps in the LAD coronary artery were estimated at different time points in the cardiac cycle using an ultrafast Doppler approach. The blood flow velocity estimated using the presented approach was compared with a commercially-available, calibrated single element blood flow velocity measurement system.

RESULTS

The resulting root mean square error (RMSE) of 2D velocity magnitude acquired from a high frequency, linear array transducer was less than 8% of the maximum velocity estimated by the commercial system.

CONCLUSION

When implemented in a forward-viewing intravascular ultrasound device, the presented approach will enable dynamic estimation of WSS, an indicator of plaque vulnerability in coronary arteries.

Fast and Selective Super-Resolution Ultrasound In Vivo With Acoustically Activated Nanodroplets

Perfusion by the microcirculation is key to the development, maintenance and pathology of tissue. Its measurement with high spatiotemporal resolution is consequently valuable but remains a challenge in deep tissue. Ultrasound Localization Microscopy (ULM) provides very high spatiotemporal resolution but the use of microbubbles requires low contrast agent concentrations, a long acquisition time, and gives little control over the spatial and temporal distribution of the microbubbles. The present study is the first to demonstrate Acoustic Wave Sparsely-Activated Localization Microscopy (AWSALM) and fast-AWSALM for in vivo super-resolution ultrasound imaging, offering contrast on demand and vascular selectivity. Three different formulations of acoustically activatable contrast agents were used. We demonstrate their use with ultrasound mechanical indices well within recommended safety limits to enable fast on-demand sparse activation and destruction at very high agent concentrations. We produce super-localization maps of the rabbit renal vasculature with acquisition times between 5.5 s and 0.25 s, and a 4-fold improvement in spatial resolution. We present the unique selectivity of AWSALM in visualizing specific vascular branches and downstream microvasculature, and we show super-localized kidney structures in systole (0.25 s) and diastole (0.25 s) with fast-AWSALM outperforming microbubble based ULM. In conclusion, we demonstrate the feasibility of fast and selective imaging of microvascular dynamics in vivo with subwavelength resolution using ultrasound and acoustically activatable nanodroplet contrast agents.

Perfluorocarbon nanodroplet size, acoustic vaporization, and inertial cavitation affected by lipid shell composition in vitro

Perfluorocarbon nanodroplets (PFCnDs) are ultrasound contrast agents that phase-transition from liquid nanodroplets to gas microbubbles when activated by laser irradiation or insonated with an ultrasound pulse. The dynamics of PFCnDs can vary drastically depending on the nanodroplet composition, including the lipid shell properties. In this paper, we investigate the effect of varying the ratio of PEGylated to non-PEGylated phospholipids in the outer shell of PFCnDs on the acoustic nanodroplet vaporization (liquid to gas phase transition) and inertial cavitation (rapid collapse of the vaporized nanodroplets) dynamics in vitro when insonated with focused ultrasound. Nanodroplets with a high concentration of PEGylated lipids had larger diameters and exhibited greater variance in size distribution compared to nanodroplets with lower proportions of PEGylated lipids in the lipid shell. PFCnDs with a lipid shell composed of 50:50 PEGylated to non-PEGylated lipids yielded the highest B-mode image intensity and duration, as well as the greatest pressure difference between acoustic droplet vaporization onset and inertial cavitation onset. We demonstrate that slight changes in lipid shell composition of PFCnDs can significantly impact droplet phase transitioning and inertial cavitation dynamics. These findings can help guide researchers to fabricate PFCnDs with optimized compositions for their specific applications.

Slow-Flow Ultrasound Localization Microscopy Using Recondensation of Perfluoropentane Nanodroplets

Ultrasound localization microscopy (ULM) is an emerging, super-resolution imaging technique for detailed mapping of the microvascular structure and flow velocity via subwavelength localization and tracking of microbubbles. Because microbubbles rely on blood flow for movement throughout the vascular space, acquisition times can be long in the smallest, low-flow microvessels. In addition, detection of microbubbles in low-flow regions can be difficult because of minimal separation of microbubble signal from tissue. Nanoscale, phase-change contrast agents (PCCAs) have emerged as a switchable, intermittent or persisting contrast agent for ULM via acoustic droplet vaporization (ADV). Here, the focus is on characterizing the spatiotemporal contrast properties of less volatile perfluoropentane (PFP) PCCAs. The results indicate that at physiological temperature, nanoscale PFP PCCAs with diameters less than 100 nm disappear within microseconds after ADV with high-frequency ultrasound (16 MHz, 5- to 6-MPa peak negative pressure) and that nanoscale PFP PCCAs have an inherent deactivation mechanism via immediate recondensation after ADV. This "blinking" on-and-off contrast signal allowed separation of flow in an in vitro flow phantom, regardless of flow conditions, although with a need for some replenishment at very low flow conditions to maintain count rate. This blinking behavior allows for rapid spatial mapping in areas of low or no flow with ULM, but limits velocity tracking because there is no stable bubble formation with nanoscale PFP PCCAs.

Very Low Frequency Radial Modulation for Deep Penetration Contrast-Enhanced Ultrasound Imaging

Contrast-enhanced ultrasound imaging allows vascular imaging in a variety of diseases. Radial modulation imaging is a contrast agent-specific imaging approach for improving microbubble detection at high imaging frequencies (≥7.5 MHz), with imaging depth limited to a few centimeters. To provide high-sensitivity contrast-enhanced ultrasound imaging at high penetration depths, a new radial modulation imaging strategy using a very low frequency (100 kHz) ultrasound modulation wave in combination with imaging pulses ≤5 MHz is proposed. Microbubbles driven at 100 kHz were imaged in 10 successive oscillation states by manipulating the pulse repetition frequency to unlock the frame rate from the number of oscillation states. Tissue background was suppressed using frequency domain radial modulation imaging (F-RMI) and singular value decomposition-based radial modulation imaging (S-RMI). One hundred-kilohertz modulation resulted in significantly higher microbubble signal magnitude (63-88 dB) at the modulation frequency relative to that without 100-kHz modulation (51-59 dB). F-RMI produced images with high contrast-to-tissue ratios (CTRs) of 15 to 22 dB in a stationary tissue phantom, while S-RMI further improved the CTR (19-26 dB). These CTR values were significantly higher than that of amplitude modulation pulse inversion images (11.9 dB). In the presence of tissue motion (1 and 10 mm/s), S-RMI produced high-contrast images with CTR up to 18 dB; however, F-RMI resulted in minimal contrast enhancement in the presence of tissue motion. Finally, in transcranial ultrasound imaging studies through a highly attenuating ex vivo cranial bone, CTR values with S-RMI were as high as 23 dB. The proposed technique demonstrates successful modulation of microbubble response at 100 kHz for the first time. The presented S-RMI low-frequency radial modulation imaging strategy represents the first demonstration of real-time (20 frames/s), high-penetration-depth radial modulation imaging for contrast-enhanced ultrasound imaging.

Arterial Labeling Ultrasound Subtraction Angiography (ALUSA) Based on Acoustic Phase-Change Nanodroplets

Biomedical imaging technology (like digital subtraction angiography (DSA)) based on contrast agents has been widely employed in the diagnosis of vascular-related diseases. While the DSA achieves the high-resolution observation of specified vessels and their downstream perfusion at the cost of invasive, radioactive operation and hepatorenal toxicity. To address those problems, this study develops arterial labeling ultrasound (US) subtraction angiography (ALUSA) based on a new perfluorobutane (PFB) nanodroplets with a lower vaporization threshold through spontaneous nucleation. The nanodroplets can be selectively vaporized to microbubbles, indicating a highly echogenic signal at B-mode images only using a diagnostic transducer. By labeling a single blood vessel for nanodroplets vaporization and tracking its downstream blood perfusion in segmental renal arteries at a frame rate of 500 Hz. The results demonstrate the color-coded super-resolution ALUSA image, exhibiting the downstream arcuate and interlobular arteries of each segmental renal artery with a resolution of 36 µm in a rabbit kidney. Furthermore, ALUSA could offer the vascular structures, blood flow velocity, and direction of their primary supply vessels in the mouse breast tumor. ALUSA fills the gap of noninvasive labeling angiography in US and opens a broad vista in the diagnosis and treatment of tumor and vascular-related diseases.

Forward-viewing estimation of 3D blood flow velocity fields by intravascular ultrasound: Influence of the catheter on velocity estimation in stenoses

Coronary artery disease is the most common type of cardiovascular disease, affecting > 18 million adults, and is responsible for > 365 k deaths per year in the U.S. alone. Wall shear stress (WSS) is an emerging indicator of likelihood of plaque rupture in coronary artery disease, however, non-invasive estimation of 3-D blood flow velocity and WSS is challenging due to the requirement for high spatial resolution at deep penetration depths in the presence of significant cardiac motion. Thus we propose minimally-invasive imaging with a catheter-based, 3-D intravascular forward-viewing ultrasound (FV US) transducer and present experiments to quantify the effect of the catheter on flow disturbance in stenotic vessel phantoms with realistic velocities and luminal diameters for both peripheral (6.33 mm) and coronary (4.74 mm) arteries. An external linear array ultrasound transducer was used to quantify 2-D velocity fields in vessel phantoms under various conditions of catheter geometry, luminal diameter, and position of the catheter relative to the stenosis at a frame rate of 5000 frames per second via a particle imaging velocimetry (PIV) approach. While a solid catheter introduced an underestimation of velocity measurement by > 20% relative to the case without a catheter, the hollow catheter introduced < 10% velocity overestimation, indicating that a hollow catheter design allowing internal blood flow reduces hemodynamic disturbance. In addition, for both peripheral and coronary arteries, the hollow catheter introduced < 3% deviation in flow velocity at the minimum luminal area compared to the control case. Finally, an initial comparison was made between velocity measurements acquired using a low frequency, catheter-based, 3-D intravascular FV US transducer and external linear array measurements, with relative error < 12% throughout the region of interest for a flow rate of 150 mL/min. While further system development is required, results suggest intravascular ultrasound characterization of blood flow velocity fields in stenotic vessels could be feasible with appropriate catheter design.

Transcranial activation and imaging of low boiling point phase-change contrast agents through the temporal bone using an ultrafast interframe activation ultrasound sequence

PURPOSE

As a cavitation enhancer, low boiling point phase-change contrast agents (PCCA) offer potential for ultrasound-mediated drug delivery in applications including intracerebral hemorrhage or brain tumors. In addition to introducing cavitation, ultrasound imaging also has the ability to provide guidance and monitoring of the therapeutic process by localizing delivery events. However, the highly attenuating skull poses a challenge for achieving an image with useful contrast. In this study, the feasibility of transcranial activation and imaging of low boiling point PCCAs through the human temporal bone was investigated by using a low frequency ultrafast interframe activation ultrasound (UIAU) imaging sequence with singular value decomposition-based denoising.

METHODS

Lipid-shelled PCCAs filled with decafluorobutane were activated and imaged at 37°C in tissue-mimicking phantoms both without and with an ex vivo human skull using the new UIAU sequence and a low frequency diagnostic transducer array at frequencies from 1.5 to 3.5 MHz. A singular value decomposition-based denoising filter was developed and used to further enhance transcranial image contrast. The contrast-to-tissue ratio (CTR) and contrast enhancement (CE) of UIAU was quantitatively evaluated and compared with the amplitude modulation pulse inversion (AMPI) and vaporization detection imaging (VDI) approaches.

RESULTS

Image results demonstrate enhanced contrast in the phantom channel with suppressed background when the low boiling point PCCA was activated both without and with an ex vivo human skull using the UIAU sequence. Quantitative results show that without the skull, low frequency UIAU imaging provided significantly higher image contrast (CTR ≥ 18.56 dB and CE ≥ 18.66 dB) than that of AMPI and VDI (P < 0.05). Transcranial imaging results indicated that the CE of UIAU (≥18.80 dB) was significantly higher than AMPI for free-field activation pressures of 5 and 6 MPa. The CE of UIAU is also significantly higher than that of VDI when PCCAs were activated at 2.5 MHz and 3 MHz (P < 0.05). The CTR (23.30 [20.07-25.56] dB) of denoised UIAU increased by 12.58 dB relative to the non-denoised case and was significantly higher than that of AMPI at an activation pressure of 4 MPa (P < 0.05).

CONCLUSIONS

Results indicate that low boiling point PCCAs can be activated and imaged at low frequencies including imaging through the temporal bone using the UIAU sequence. The UIAU imaging approach provides higher contrast than AMPI and VDI, especially at lower activation pressures with additional removal of electronic noise.