Transcranial ADV for Aberration Correction

 

Ultrasonic Aberration Correction using Acoustic Droplet Vaporization and Time-Reversal Acoustics


Kevin J. Haworth, J. Brian Fowlkes, Paul L. Carson, Oliver D. Kripfgans

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

OVERVIEW

Ultrasonic imaging and therapy both suffer from aberrations, typically due to the varying speed of sound in different tissues.  The aim of this research is to correct for the aberrations.  In particular, this research focuses on correcting for aberrations due to the skull.  The skull is a strong aberrator due to its varying thickness and high speed of sound.  For this reason, ultrasound can only be used in very limited cases for imaging the brain.  Figure 1 schematically demonstrates how signals that are originally lined up do not sum in phase after passing through the skull.




Figure 1: Schematic drawing showing that the receive echo (the sum of all the transmitted pulses after being summed) no longer looks like the transmit pulses due to the misalignment of phases after passing through the skull.


Several different approaches have been discussed in the literature for correcting aberrations.  One approach of particular interest is time-reversal acoustics.  This approach entails recording the ultrasonic echo of a point-reflector (Figure 2) and then retransmitting the recorded echos in a time-reversed fashion as demonstrated (Figure 3).















Figure 2.Figure 3.



METHODS

In order to create point-reflectors we use a technique called Acoustic Droplet Vaporization (ADV).  ADV uses an acoustic pulse to convert superheated liquid droplets into gas bubbles.  These droplets can be formulated such that the resulting bubbles are tens of microns in diameter and provide suitable point-reflectors at typical diagnostic frequencies.




Figure 2: By transmitting onto a point-scatter and recording the resulting waveforms, the aberration due to the skull is easily visualized.



Figure 3: By retransmitting the waveforms in a time-reversed fashion, the waveforms will all arrive at the point-scatterer at the same time and sum together constructively.




RESULTS



The first portion of this work demonstrates that it is possible to create ADV bubbles through the skull.  This was demonstrated using using the setup seen in Figure 4.  The production of stable gas bubbles was observed with ultrasonic imaging and is seen in Figure 5.


     


Figure 4:  Photograph of transcranial vaporization setup    Figure 5: B-mode images before (left), during (middle), and

                                                                                                            after (right) the vaporization pulse.



Then using a setup similar to figures 2 and 3, the echos from a single gas bubble held in ultrasound gel were recorded through the skull.  The echoes were time reversed and retransmitted.  The echos from the time-reversal focused signals were seen to realign (figure 6).




Figure 6: (Top) aberrated waveforms (analogous to figure 1) after reflecting off of a point-scatterer and then traveling through the skull.  (Bottom) when the aberrated waveforms are time-reversed and retransmitted, they realign.


To quantify the ‘goodness’ of the refocusing, a focusing quality factor F was defined.









  



Several different bubbles sizes ranging from 37 to 1000 microns were investigated.  The mean ratio of the focusing factor with corrected rf-lines to aberrated rf-lines was 1.9.  This was obtained going through one portion of a single skull.  A single data point at another portion of the skull had a ratio of 10.  It is anticipated that this ratio will change substantially depending on the ultrasonic array parameters (element-size, pitch, etc.) and portion of the skull used.


CONCLUSIONS / FUTURE WORK


The initial phases of this research indicate that ADV bubbles can act as suitable point targets in vitro.  Current work focuses on developing the appropriate equipment to translate the above results to more in vivo like conditions.  This includes improving the receive focusing for diagnostic imaging and the transmit focusing for both imaging and therapeutic purposes.  More extensive work on multiple skulls will also be performed.


REFERENCES

Haworth KJ, Fowlkes JB, Carson PL, Kripfgans OD.  Towards aberration correction of transcranial ultrasound using acoustic droplet vaporization. Ultrasound Med Biol. 2008 Mar;34(3):435-45.



Acknowledgement:  This work is supported in part by NIH Grants 5R01EB000281 and 1R21CA116043.