Evaluating Targeting Accuracy in the Focal Plane for an Ultrasound-guided High-intensity Focused Ultrasound Phased-array System

Accurate targeting is essential to ensure safety and efficacy for image-guided, high-intensity focused ultrasound systems. Our protocol can evaluate in targeting accuracy of ultrasound-guided, high-intensity focused ultrasound phase-array systems. This technique can be used to evaluate the targeting accuracy in the treatment plane while taking into consideration the clinically relevant tissue paths.

This method can be applied to ultrasound-guided, high-intensity focused ultrasound systems with a self-focused transducer using different ultrasound image reconstruction methods. Visual demonstration of this method can help others fully understand the protocol, especially how to determine the location of the target in the reconstructed ultrasound image. Before making the phantom, fabricate the model.

This 3D-printed model is a square with side lengths of 40 millimeters. A solid ball is at each corner. With the model at hand, make the phantom holder.

For this, get an acrylic baseboard and an eight-centimeter-diameter plastic cylinder that is three centimeters tall. Use silica gel to attach them to form a holder. Let the assembly sit for one hour.

Now, move on to make the phantom from fresh bovine muscle. Slice the muscle into a square with sides of 30 millimeters and 10-millimeter thickness. Ventilate the tissue with a fan for two hours to evaporate moisture.

Next, prepare to work with hazardous chemicals. Pour degassed and deionized water into a beaker. Add acrylamide, and stir until it is dissolved.

Then, add bis-acrylamide, and stir until dissolved. Finally, add tetramethylethylenediamine, and stir until the solution is uniform. This is solution one.

In another beaker with degassed and deionized water, add ammonium persulfate, and stir to dissolve. This is solution two. When the phantom has solidified, retrieve the 3D-printed model, and place it on the phantom surface.

With the model in place, the next step is to put the sliced bovine muscle in the middle of the model. Pour the rest of solution one into the phantom holder. Move the bovine muscle to remove air at the interface of the phantom and the slice.

Continue by pouring the remainder of the solution two into the phantom holder. Then, stir for five seconds. Adjust the location of the muscle to be at the center of the phantom, and let everything sit for 20 minutes to solidify.

Once it has solidified, use a screwdriver to remove the silica gel, holding the cylinder and base together, and slowly detach the two. Start the ultrasound-guided, high-intensity focused ultrasound system. Turn on the water-processing module, and set the circulation speed.

To continue, have a cylindrical tank filled with degassed water at room temperature. The tank diameter is 30 centimeters, and its height is 13 centimeters. Get the phantom holder, and place it in the degassed water.

Take measures to ensure the holder is fixed tightly in place. Next, move the treatment bed and the therapeutic unit to allow imaging the phantom. The balloon of the therapeutic unit should be immersed in the degassed water.

Be prepared with a sample of swine muscle before continuing. The sample should be about 100 millimeters long and be about 30 millimeters thick. Put the sample aside, and slowly move the therapeutic unit up and down.

The goal is to make sure the treatment plane is at the upper interface of the sliced bovine muscle and phantom. With the ultrasound imaging probe at zero degrees, move the water tank to make the imaging axis pass through the middle of the two parallel lines in the image. Next, rotate the imaging probe to 90 degrees.

Then, move the tank to make the axis pass through the middle of the two parallel lines in the image. Reconstruct the ultrasound image in the treatment plane at the depth of geometric focus. Check that the four balls are clearly shown in the reconstructed image.

Also, check if the target is located at the center of the square model. When ready, lift the therapeutic unit in order to place the swine muscle above the phantom. Here, the swine muscle is in position and ready for the next steps.

Move the therapeutic unit back into position for imaging. Arrange the depth of geometric focus to be three millimeters beneath the upper surface of the bovine muscle. Continue by setting the parameters for high-intensity focused ultrasound sonication, including the pulse duration and power, among others.

Set the exposure time for the chosen hexagonal virtual target and a two-second exposure time for the focal spot at the geometric center of the phase array. Start the sonication, and put a foot on the foot pedal of the sonication instrument. During sonication, observe and record the ultrasound image to allow comparison of the echogenicity of different samples.

Remove the phantom before using similarly prepared phantoms to repeat the sonication steps with the other virtual targets. This is an ultrasound image at an angle of zero degrees. An ultrasound image at an angle of 90 degrees is similar.

Both images show clear interfaces between the swine muscle, phantom, and bovine muscle. In addition, the sticks of the square model are bright. This is the reconstructed image in the treatment plane.

The blue circles encompass the highest average gray value in the red squares and identify the positions of the four balls in the model. The blue circles also determine the center of the square model, which is the center of the targets. The brown squares indicate the focal spots in the largest regular hexagonal target.

Determining the location of the four balls in the reconstructed ultrasound image is important for matching the target center with the model center. The chemical materials used for phantom preparation are slightly hazardous. A mask and gloves should be worn during the procedure.