Bistatic passive mapping of the field distribution of single element transducer in agar phantom

High intensity focused ultrasound (HIFU) can provide a means of noninvasive ablation or hyperthermia of tissues such as tumors. Real-time monitoring of the progression of the field distribution of the HIFU transducer during treatment is important for localizing the intersection of the beam with the tissue. By continuously visualizing the HIFU field in a tissue, better positioning of the HIFU beam for therapy can be obtained during treatment. To visualize the HIFU field in a tissue, a passive listening technique was employed using beamforming approaches and a linear array system co-aligned with the HIFU source. The passive array made use of the weakly scattered signal from the medium to reconstruct the field pattern of the HIFU field in the medium. The focus of a 6-MHz single-element transducer (f/3) was aligned perpendicular to the field of view from a linear array (L14-5) operated by a SonixRP system equipped with a Sonix- DAQ. A homogeneous tissue-mimicking phantom made of agar containing glass beads was placed at the focus between the 6-MHz source and the linear array. The 6-MHz source was excited with a pulse and the field scattered from the phantom was received by each element of the linear array. Beam forming techniques were used to focus the received field of the linear array around the focal region of the 6-MHz source. The intensity field pattern of the 6-MHz source was reconstructed from the scattered field. Next, a wire target was placed in the field and the intensity field pattern was reconstructed by moving the wire throughout the focal region. The intensity pattern from the phantom was compared to the nominal field characteristics of the 6-MHz source and to the field characterized by a wire. The beam width at the focus of the reconstructed intensity field pattern was estimated to be 1.7 mm. The nominal estimate of the beam width was approximately 1.4 mm (-6 dB) and the beam width as estimated from the wire target field mapping was 1.5 mm. Therefore, the novel passive reconstruction technique can visualize the field of a focused source in a weakly scattering medium.