Occlusion Therapy

 

Acoustic Droplet Vaporization for Occlusion Therapy


Paul L. Carson, J. Brian Fowlkes, Oliver D. Kripfgans, Man (Maggie) Zhang, Kevin J. Haworth, Mario L. Fabiilli

Department of Radiology, University of Michigan, Ann Arbor, MI 48109-0553 USA



BACKGROUND / OBJECTIVE

Acoustic droplet vaporization (ADV) refers to a technique where encapsulated superheated liquids are phase-transitioned into gas bubbles using acoustic fields.  More on this process can be found here or in the papers referenced below.  The resulting gas bubbles can subsequently be used for a variety of medical purposes.  This research focuses on using the bubbles to transiently occlude tissue.


Typically a feed vessel to the tissue to be occluded is targeted using ultrasound image guidance.  Figure 1 shows a linear array used to provide imaging feed back and a single-element transducer (therapy transducer) to phase-transition the droplets.  after targeting, ADV droplets are injected intravenously and the therapy transducer is fired.  The liquid droplets are converted into stable gas bubbles which then travel downstream, eventually lodging in capillary beds and occluding the flow.



Figure 1: Photograph of one of the image guided therapy setups.



RESULTS

Figure 2 shows a duplex B-mode and pulse-wave image.  The externalized kidney is easy to visualize and the arterial flow in the renal artery is clearly identified.  After vaporizing droplets, the kidney becomes occluded.  In figure 3, the flow in the renal artery is now indicative of no net flow.  Additionally, the kidney cortex is filled with bubbles resulting in an increase in echogenicity near the surface of the kidney and a complete shadowing of structures deeper in the kidney.


         

Figure 2: Duplex of kidney pre-ADV treatement                Figure 3: Duplex image of kidney post-ADV

                                                                                           treatement


An ultrasonic flow-cuff is also placed on the renal artery to provide a second feedback mechanism for measuring changes in flow.  Figure 4 shows the mean echo power (a measure of echogenicity) and the flow as measured by the ultrasonic flow-cuff.  An inverse relationship can be seen, indicating that echogenicity may be an indicator for flow reduction.



Figure 4: (Top) Mean echo power from a region-of-interest in the kidney cortex.  (Bottom) Normalized flow from an ultrasonic flow-cuff placed on the renal artery upstream of the therapy focus.





Figure 5: Flow reduction. as measured using colored micro-spheres, in a selected portion of a treated kidney. 


CONCLUSIONS AND FUTURE WORK

The work completed thus far indicates that ADV is a promising occlusion therapy technique.  Continued work must be done to produce appropriate occlusion durations and study other bioeffects.  Work is also being done to demonstrate targeted reduction of a select portion of the kidney.  Figure 5 demonstrates this effect in the anterior portion of a treated kidney (figure 5).


ACKNOWLEDGEMENTS

This work is supported in part by NIH Grant 5R01EB000281.


REFERENCES

Lo AH, Kripfgans OD, Carson PL, Rothman ED, Fowlkes JB.  Acoustic droplet vaporization threshold: effects of pulse duration and contrast agent.  IEEE Trans Ultrason Ferroelectr Freq Control. 2007 May;54(5):933-46.


Lo AH, Kripfgans OD, Carson PL, Fowlkes JB.  Spatial control of gas bubbles and their effects on acoustic fields.  Ultrasound Med Biol. 2006 Jan;32(1):95-106.


Kripfgans OD, Orifici CM, Carson PL, Ives KA, Eldevik OP, Fowlkes JB.  Acoustic droplet vaporization for temporal and spatial control of tissue occlusion: a kidney study.  IEEE Trans Ultrason Ferroelectr Freq Control. 2005 Jul;52(7):1101-10.


Kripfgans OD, Fabiilli ML, Carson PL, Fowlkes JB.  On the acoustic vaporization of micrometer-sized droplets.  J Acoust Soc Am. 2004 Jul;116(1):272-81.


Kripfgans OD, Fowlkes JB, Woydt M, Eldevik OP, Carson PL.  In vivo droplet vaporization for occlusion therapy and phase aberration correction.  IEEE Trans Ultrason Ferroelectr Freq Control. 2002 Jun;49(6):726-38.


Kripfgans OD, Fowlkes JB, Miller DL, Eldevik OP, Carson PL.  Acoustic droplet vaporization for therapeutic and diagnostic applications.  Ultrasound Med Biol. 2000 Sep;26(7):1177-89.