Research Platform

The research platform (RP) incorporates all the described systems into a unified program that allows for easy control and data collection. At its base, the RP integrates the driver, the acquisition unit, and the beamformer to display a live synthetic aperture image. This image then serves as the template for all further experimentation. Focal points can be added to the image to allow the user to point and click where he or she would like to target. Each time a focal point is moved, a new driving pattern is calculated and downloaded to the FPGA, in addition, a GPU based homogeneous acoustic simulation is performed and displayed as a color overlay on the image. Below you can see a single focus pattern overlaid on a Synthetic Aperture image of a liver sample. For reference, the array would be located below the image.

Steered Ultrasound Focus A phased array is not limited to depositing its energy at a single point in space. By appropriately weighting and phasing the elements, multiple focal point patterns can be synthesized. You can see below the resulting field from a double focus pattern. Something to note is the difference in pre-focal and post-focal fields in the double focus case as compared to the single focus. As you add additional focal points, the interference pattern can become more irregular. It is even possible to get spots that result in a higher pressure than the intended focal points. These hot spots can be controlled during the diving pattern optimization process. I haven’t shown it here but we have the ability to refocus the therapy beam around certain structures to avoid heating critical locations. This is essential when targeting tissue within the abdominal cavity, where care must be taken to avoid the ribs.

Multiple Focus Array PatternOnce the treatment points have been identified, the user can then select what sort of treatment should be delivered. There are four options we typically use:

  1. Uninterrupted. In this case the HIFU is delivered in a continuous burst, there are no interruptions of any sort.
  2. ¬†Fractionated. In this case the HIFU is delivered at some pulse rate with a defined duty cycle. For example, we may deliver a 500 us burst of HIFU every 1 ms. There are a number of reasons this may be desired, (i.e. prevent overheating of the array, induce tissue motion, silence for imaging, etc…).
  3. Fractionated with Imaging. This is really a subset of the previous case, but in this experiment mode we fractionate the therapy to allow for high frame rate imaging. For example, if we deliver 500 us of therapy every 1 ms, this allows the system 500 us to transmit imaging pulses to interrogate the medium.
  4. Controlled. Here we modulate the therapy beam based on some sort of control. The system opens up a TCP/IP port that allows for commands from an external monitoring system to control the course of the therapy. We use this to control the temperature at focal points as well as stop therapy once we’ve detected the tissue change we were hoping for.

The images above and those to follow below are from an experiment in which we fractionated the therapy in order to collect imaging data. In addition, there was a thermocouple placed adjacent to the focus to record the temperature along the periphery of the ablation zone. The red dot below shows the focus of the energy, and the whitish area to the lower right is the location of the thermocouple. The two white areas below the focus, and just behind the front of the tissue are vessels.

Synthetic Aperture Ultrasound Image

Below you can see a single transmit focus image taken during the initial stage of the treatment. At this point in the therapy the temperature is rising as a nice exponential curve.

Single Transmit Focus
After approximately 3 seconds, there was a very visible change at the focus and the heating rate increased dramatically. What the thermocouple and the image are both showing, is that the temperature at the focus had risen enough to cause a change in the ultrasound attenuation dramatically increasing the back-scattered energy and the absorption rate. This tissue damage tends to grow towards the array, and as it does it forms a shield preventing the ultrasound from propagating further into the medium. You can see this effect in the temperature plateau observed by the thermocouple. Once the tissue damage progressed beyond the thermocouple, the temperature actually decreased, even though the therapy continued.Single Transmit Focus ImageBelow you can see the synthetic aperture image taken quickly after therapy stopped. You can see that the therapy has created a very echogenic region at the focus and extending pre-focally. This region has been essentially cooked by the ultrasound, while the surrounding regions are untouched.

Synthetic Aperture Ultrasound ImageI’ve included the overall movie of this experiment below. During this experiment, the array was driven with some what modest power levels. At full power, we can form these types of tissue damage in a few milliseconds, depending on the depth of the target.

The following are some presentations whose content relied on this system or a variant of this system.

International Ultrasound Symposium, Oct. 2010
Acoustical Society of America, Nov. 2010

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