Assessments of Flow Velocity Changes by Phase-Contrast MRI Near Acoustic Radiation Force Aggregated Bubbles
Zhe-Wei Wu1, Wu Chen-Hua2, Hsu Po-Hong3, Hao-Li Liu3, Yeh Chic-Kuang2, and Peng Hsu-Hsia2

1Biomedical Engineering and Environmental Sciences, Nation Tsing Hua University, Hsinchu, Taiwan, 2Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan, 3Electrical Engineering, Chang-Gung University, Taoyuan, Taiwan

Synopsis

The aim of this study was to real-time assess flow velocity changes in the neighboring pixels of acoustic radiation force (ARF) aggregated bubbles by using phase-contrast MRI (PC-MRI). The flow velocity increased with the increase of microbubbles (MBs) concentrations, which might be attributed to the increase of aggregated bubbles size, narrowing the chamber diameter and thus presenting higher flow velocity. In conclusion, we verified the feasibility of using PC-MRI to assess flow velocity changes on pixels near ARF aggregated bubbles. In the future, a systematic investigation shall be conducted to comprehend the association between bubble size and the flow velocity.

Introduction

The acoustic radiation force (ARF) induced by microbubbles (MBs) with focused ultrasound (FUS) was well-known as a scheme of localizing drug delivery by primary radiation force and secondary radiation force, which can respectively propel MBs to the side of tubes or vessels and aggregate MBs to form a large-size bubble [1,2]. At the present of secondary ARF, the larger-size bubbles on one side of the tube or vessel can be seen as a barrier of flow and thereby may be able to change the local flow velocity. A previous study reported the influence of MBs concentrations and acoustic pressure on the aggregated bubble size [3]. In this study, we aim to adopt phase-contrast MRI (PC-MRI) to real-time evaluate the changes of flow velocity nearby bubbles formed by secondary ARF. In the preliminary tests, we performed experiments with different MBs concentrations to clarify its influence on the measured velocity.

Method

All images were acquired with PC-MRI (TR/TE= 26.9/8.42 ms, pixel size= 0.313 x 0.313 x 1 mm3, pixel bandwidth= 260 Hz/pixel, flip angle= 10, Venc = 6 cm/s, temporal resolution = 2.2 s) in a 7 Tesla scanner (Biospec 47/40, Bruker, Ettlingen, Germany). The experimental setup was shown in Fig. 1. MBs were diluted to the concentration of 0.001X and 0.01X. The solutions of MBs (lipid shell with C3F8, mean diameter=1.25 µm (Number %) were injected with a velocity of 2 cm/s into a gel phantom with chamber diameter=6 mm. The FUS pulses were transmitted from a single-element probe (central frequency 1.014276 MHz, 2.5 cm diameter, 2.0 cm curvature, RK300, FUS Instruments, Toronto, Canada). Continuous FUS pulses were transmitted to MBs solutions with acoustic pressure of 100 kPa. The imaging slices were prescribed parallel to the chamber of phantom. We acquired 40 measurements (Pre-FUS=1-9, ARF=10-30, Post=31-40). We analyzed the flow velocity with a self-developed program written in Matlab. To calculate the velocity changes, we determined the region of interest at the side of near field, as shown in Fig. 1. The pixel velocity of forty measurements during the statuses of Pre-FUS, ARF, and Post-FUS (21 pixel/measurement) were put together in a histogram to observe the distribution of velocity within ROI.

Result

Figure 1 displayed the magnitude images and phase images of experiments with MBs concentrations of 0.01X and 0.001X. In the zoom-in plot of ARF in Fig. 1a, the hypointensive region, indicated by arrow, could be attributed to the formation of aggregated bubble. Figure 2a and 2b illustrated the time courses of velocity within the ROIs. The light blue cycles emphasized the velocity changes upon turning-on and turning-off FUS pulses. With 0.01X MBs, the mean velocity increased from 0.45±0.05 cm/s to 0.65±0.05 cm/s. As ceasing FUS pulses, the mean velocity fell back to 0.45±0.05 cm/s. While lowering the MBs concentration to 0.001X, we observed relatively minor increased mean velocity of 0.57±0.05cm/s. Figures 2c and 2d presented the velocity distribution within ROI. Either with 0.01X or 0.001X MBs, the histograms tended to shift to right in ARF status, indicating the increase of pixel velocity.

Discussion and Conclusion

In this study, we used PC-MRI to real-time evaluate the flow velocity changes surrounding bubbles aggregated by secondary ARF. The increased flow velocity values were observable in both MBs concentrations during transmitting FUS pulses. The right-shifting velocity distributions in histograms further verified the increase of velocity (Fig. 2). The higher the MBs concentration, the more significant velocity changes could be observed. Masuda et al previously reported that the size of MBs aggregations by ARF was related to acoustic pressure, MBs concentration, and pulse duration time. Therefore, the more substantial velocity changes in 0.01X MBs might be attributed to the increase of aggregated bubbles size, narrowing the chamber diameter and thus presenting higher flow velocity. In conclusion, we verified the feasibility of using PC-MRI to real-time assess the flow velocity changes on pixels near ARF aggregated bubbles. In the future, a systematic investigation of various MBs and FUS conditions shall be conducted to comprehend the association between the aggregated bubble size and the measured flow velocity changes.

Acknowledgements

No acknowledgement found.

References

[1]Paul A. Dayton et al, “A Preliminary Evaluation Of The Effects Of Primary And Secondary Radiation Forces On Acoustic Contrast Agents”, IEEE, 1997

[2] Paul Dayton et al,” Acoustic Radiation Force In Vivo: A Mechanism To Assist Targeting Of Microbubbles”, ELSEVIER, 1999

[3] Kohji Masuda et al, “Observation Of Flow Variation In Capillaries Of Artificial Blood Vessel By Producing Microbubble Aggregations”, IEEE, 2012

Figures

The experimental setup of FUS transducer and phantom with flowing MBs solutions in the hollow chamber. ROIs for computing velocity are marked with yellow line. (a) 0.01X MBs, (b) 0.001X MBs.

The time courses of velocity with (a) 0.01X MBs and (b) 0.001X MBs. The velocity distributions of 84 pixels (21 pixel/measurement, 4 measurements) within ROI at Pre-FUS, ARF, and Post-FUS were shown for (c) 0.01X MBs and (d) 0.001X MBs.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3609