Robust and durable aberrative and absorptive phantom for therapeutic ultrasound applications

Abstract

Phase aberration induced by soft tissue inhomogeneities often complicates high-intensity focused ultrasound (HIFU) therapies by distorting the field and, previously, we designed and fabricated a bilayer gel phantom to reproducibly mimic that effect. A surface pattern containing size scales relevant to inhomogeneities of a porcine body wall was introduced between gel materials with fat- and muscle-like acoustic properties-ballistic and polyvinyl alcohol gels. Here, the phantom design was refined to achieve relevant values of ultrasound absorption and scattering and make it more robust, facilitating frequent handling and use in various experimental arrangements. The fidelity of the interfacial surface of the fabricated phantom to the design was confirmed by three-dimensional ultrasound imaging. The HIFU field distortions-displacement of the focus, enlargement of the focal region, and reduction of focal pressure-produced by the phantom were characterized using hydrophone measurements with a 1.5 MHz 256-element HIFU array and found to be similar to those induced by an ex vivo porcine body wall. A phase correction approach was used to mitigate the aberration effect on nonlinear focal waveforms and enable boiling histotripsy treatments through the phantom or body wall. The refined phantom represents a practical tool to explore HIFU therapy systems capabilities.

FIG. 1.

(Color online) The design and fabrication process to produce the body wall phantom. The (a) surface plot of the phantom's rippled interface containing a range of inhomogeneities relevant to a porcine body wall, (b) 3-D printed mold mounted into metal housing, (c) cross section of BG cast against a 3-D print, (d) patterned BG block mounted into a metal housing with standoff, and (e) cross section of the finished body wall phantom after BG is encapsulated in cured PVA hydrogel are shown.

FIG. 2.

(Color online) The characterization of the body wall phantom's rippled interfacial surface via 3-D ultrasound imaging. The (a) reconstruction of the interfacial surface, (b) 2-D frequency domain plot of the reconstructed surface, (c) surface plot of the mathematical interfacial surface, and (d) 2-D frequency domain plot of the mathematical surface are shown.

FIG. 3.

(Color online) The attenuation coefficient measurement of PVA and BG versus the frequency.

FIG. 4.

(Color online) The measured linear 2-D pressure amplitude distributions acquired in the focal plane are depicted in (a) the free field (water), (b) with the aberrating body wall phantom inserted, and (c) with the porcine body wall inserted. The pressure amplitudes are normalized to the maximum pressure value in each plot.

FIG. 5.

(Color online) The phase correction applied to each of the 256 elements on the HIFU transducer in the cases of the (a) aberrating phantom and (b) porcine body wall are shown.

FIG. 6.

(Color online) The (a) peak positive and (b) peak negative pressures measured in nonlinear focus versus the voltage, including free field data, aberration-corrected, and uncorrected data sets behind the phantom and body wall, as well as (c) the focal waveforms acquired at high drive level (34 V) through the aberrating phantom and porcine body wall with and without aberration correction are shown. Arbitrary phases were added to the waveforms to improve visualization.

FIG. 7.

(Color online) The measurements of the peak positive and peak negative focal pressures versus the system driving voltage are depicted, including the free field data and derated, aberration-corrected data behind the phantom and body wall.

FIG. 8.

(Color online) An overview of the BH treatment. The treatment setup included the HIFU array, organ-mimicking phantom, and inserted (a) aberrating phantom or (d) body wall. The B-mode images from the end of the 30 s treatment indicated bubble activity in the focal region for the (b) phantom and (e) body wall cases. The resulting lesions in the organ-mimicking phantom are shown for the (c) phantom and (f) body wall cases.