The use of microbubbles in Doppler ultrasound studies

Ultrasonic Traveling Waves for Near-Wall Positioning of Single Microbubbles in a Flowing Channel

Although microbubbles are used primarily in the medical industry as ultrasonic contrast agents, they can also be manipulated by acoustic waves for targeted drug delivery, sonothrombolysis and sonoporation. Acoustic waves can also potentially remove microbubbles from tubing systems (e.g., in hemodialysis) to prevent the negative effects associated with circulating microbubbles. A deeper understanding of the interactions between the acoustic radiation force, the microbubble and the channel wall could greatly benefit these applications. In this study, single air-filled microbubbles were injected into a flowing (polydimethylsiloxane) channel and monitored by a high-speed camera while passing through a pulsed ultrasonic wave zone (0.5 MHz). This study compared various bubble sizes, flow rates and acoustic pressure amplitudes to better understand the three physical regimes observed: free bubble translation (away from the wall); on-wall translation; and bubble-wall attachment. Comparison with a theoretical model revealed that the acoustic radiation force needs to exceed the combined repulsive forces (shear lift, wall lubrication and repulsive Van der Waal forces) for the dead state of bubble-wall attachment. The bubble dynamics revealed through this investigation provide an opportunity for efficient positioning of microbubbles in a channel flow, for either in vivo manipulation or removal in biological applications.

Ultrasound assessment of translation of microbubbles driven by acoustic radiation force in a channel filled with stationary fluid

In this report, a method is proposed to quantify the translation of ultrasound contrast agent (UCA) microbubbles driven by acoustic radiation for the detection of channels filled with stationary fluid. The authors subjected UCA microbubbles in a channel with diameters of 0.1 and 0.5 mm to ultrasound pulses with a center frequency of 14.4 MHz. The translational velocity of the UCA microbubbles increased with the sound pressure and pulse repetition frequency (PRF) of the transmitted ultrasound. The mean translational velocity reached 0.75 mm/s at a negative peak sound pressure of 2.76 MPa and a PRF of 2 kHz. This trend agreed with the theoretical prediction, which indicated that the translational velocity was proportional to the square of the sound pressure and the PRF. Furthermore, an experiment was carried out with a phantom that mimics tissue and found that the proposed method aided in detection of the channel, even in the case of a low contrast-echo to tissue-echo ratio. The authors expect to develop the proposed method into a technique for detecting lymph vessels.

Easy Pulsatile Phantom for Teaching and Validation of Flow Measurements in Ultrasound

PURPOSE

To build a simple model to teach and validate non-pulsatile and pulsatile flow quantification in ultrasound.

MATERIALS AND METHODS

The setting consists of the following connected components: (1) medical syringe pump producing an adjustable constant flow (ml/min), (2) modulator modifying constant flow to a reproducible pulsatile flow, (3) water tank containing a diagonal running silicone tube (0.5 mm inner diameter), and (4) a fixated ultrasound probe (L9 Linear Array 9 MHz, GE Logiq E9) measuring the flow inside the tube. Commercially available microbubbles suspended with physiological saline solution were used for ultrasonic visibility. Spectral Doppler of different flow profiles is performed.

RESULTS

The syringe pump produces an adjustable, constant flow and serves as the reference standard. The filling volume of the tube system is 1.2 ml. Microbubbles are very well detected by ultrasound and can be used as an easy and clean blood mimicking substance. The modulator generates different physiological and pathological flow profiles. Velocities are similar to those found within human blood vessels. Thus, it is possible to train and validate flow measurements in ultrasound.

CONCLUSION

The model produces non-pulsatile and various pulsatile flow profiles and allows validation of flow measurements. The compact size permits easy and economic setup for flow measurements in research, skills lab and continuing education.

Superharmonic microbubble Doppler effect in ultrasound therapy

The introduction of microbubbles in focused ultrasound therapies has enabled a diverse range of non-invasive technologies: sonoporation to deliver drugs into cells, sonothrombolysis to dissolve blood clots, and blood-brain barrier opening to deliver drugs into the brain. Current methods for passively monitoring the microbubble dynamics responsible for these therapeutic effects can identify the cavitation position by passive acoustic mapping and cavitation mode by spectral analysis. Here, we introduce a new feature that can be monitored: microbubble effective velocity. Previous studies have shown that echoes from short imaging pulses had a Doppler shift that was produced by the movement of microbubbles. Therapeutic pulses are longer (>1 000 cycles) and thus produce a larger alteration of microbubble distribution due to primary and secondary acoustic radiation force effects which cannot be monitored using pulse-echo techniques. In our experiments, we captured and analyzed the Doppler shift during long therapeutic pulses using a passive cavitation detector. A population of microbubbles (5  ×  10(4)-5  ×  10(7) microbubbles ml(-1)) was embedded in a vessel (inner diameter: 4 mm) and sonicated using a 0.5 MHz focused ultrasound transducer (peak-rarefactional pressure: 75-366 kPa, pulse length: 50 000 cycles or 100 ms) within a water tank. Microbubble acoustic emissions were captured with a coaxially aligned 7.5 MHz passive cavitation detector and spectrally analyzed to measure the Doppler shift for multiple harmonics above the 10th harmonic (i.e. superharmonics). A Doppler shift was observed on the order of tens of kHz with respect to the primary superharmonic peak and is due to the axial movement of the microbubbles. The position, amplitude and width of the Doppler peaks depended on the acoustic pressure and the microbubble concentration. Higher pressures increased the effective velocity of the microbubbles up to 3 m s(-1), prior to the onset of broadband emissions, which is an indicator for high magnitude inertial cavitation. Although the microbubble redistribution was shown to persist for the entire sonication period in dense populations, it was constrained to the first few milliseconds in lower concentrations. In conclusion, superharmonic microbubble Doppler effects can provide a quantitative measure of effective velocities of a sonicated microbubble population and could be used for monitoring ultrasound therapy in real-time.

Flow Velocity Mapping Using Contrast Enhanced High-Frame-Rate Plane Wave Ultrasound and Image Tracking: Methods and Initial in Vitro and in Vivo Evaluation

Ultrasound imaging is the most widely used method for visualising and quantifying blood flow in medical practice, but existing techniques have various limitations in terms of imaging sensitivity, field of view, flow angle dependence, and imaging depth. In this study, we developed an ultrasound imaging velocimetry approach capable of visualising and quantifying dynamic flow, by combining high-frame-rate plane wave ultrasound imaging, microbubble contrast agents, pulse inversion contrast imaging and speckle image tracking algorithms. The system was initially evaluated in vitro on both straight and carotid-mimicking vessels with steady and pulsatile flows and in vivo in the rabbit aorta. Colour and spectral Doppler measurements were also made. Initial flow mapping results were compared with theoretical prediction and reference Doppler measurements and indicate the potential of the new system as a highly sensitive, accurate, angle-independent and full field-of-view velocity mapping tool capable of tracking and quantifying fast and dynamic flows.

Effect of microbubble contrast on intracranial blood flow velocity assessed by transcranial Doppler

PURPOSE

Ultrasound contrast agents (UCA) salvage a considerable number of transcranial Doppler (TCD) exams which would have failed because of poor bone window. UCA bolus injection causes an undesirable increase in measured blood flow velocity (BFV). The effect of UCA continuous infusion on measured BFV has not been investigated, and some in vitro experiments suggest that gain reduction during UCA administration may also influence measured BFV. This study aimed to investigate the effect of UCA continuous infusion on BFV measured by TCD and the influence of gain reduction on these measurements in a clinical setting.

METHODS

The right middle cerebral artery of ten patients with optimal bone window was insonated using a 2 MHz probe. UCA were administered using an infusion pump. BFV was measured (1) at baseline, (2) during UCA infusion, (3) during UCA infusion with gain reduction, and (4) after UCA wash-out phase. Gain reduction was based on the agreement between two neurosonographers on the degree of gain reduction necessary to restore baseline Doppler signal intensity (DSI). Actual DSI was estimated offline by analysis of raw data.

RESULTS

BFV measured during UCA infusion with no gain adjustment was significantly higher than baseline BFV [peak systolic velocity (PSV): 85.1 ± 19.7 vs. 74.4 ± 19.7 cm/s, p < 0.0001; Mean velocity (MV): 56.5 ± 11.8 vs. 50.2 ± 12.3 cm/s, p < 0.0001]. BFV measured during UCA infusion with gain reduction was not significantly higher than baseline BFV (PSV: 74.3 ± 18.9 vs. 74.4 ± 19.4 cm/s, p = 0.8; MV: 49.4 ± 11.0 vs. 50.2 ± 12.3 cm/s, p = 0.8). Actual DSI during UCA infusion with gain reduction was not significantly higher than baseline DSI (13 ± 1 vs. 13 ± 1 dB).

CONCLUSION

This study shows that UCA continuous infusion leads to an increase in measured BFV which may be counteracted by reducing Doppler gain thus restoring pre-contrast DSI.