Visualization of the Intensity Field of a Focused Ultrasound Source <italic>In Situ</italic>

Abstract

In an increasing number of applications of focused ultrasound (FUS) therapy, such as opening of the blood brain barrier or collapsing microbubbles in a tumor, elevation of tissue temperature is not involved. In these cases, real-time visualization of the field distribution of the FUS source would allow localization of the FUS beam within the targeted tissue and allow repositioning of the FUS beam during tissue motion. In this work, in order to visualize the FUS beam in situ, a 6- MHz single-element transducer (f/2) was used as the FUS source and aligned perpendicular to a linear array which passively received scattered ultrasound from the sample. An image of the reconstructed intensity field pattern of the FUS source using bistatic beamforming was then superimposed on a registered Bmode image of the sample acquired using the same linear array. The superimposed image is used to provide anatomical context of the FUS beam in the sample being treated. The intensity field pattern reconstructed from a homogeneous scattering phantom was compared to the field characteristics of the FUS source characterized by the wire technique. The beamwidth estimates at the FUS focus using the in situ reconstruction technique and the wire technique were 1.5 mm and 1.2 mm, respectively. The depth of field estimates for the in situ reconstruction technique and the wire technique were 11.8 mm and 16.8 mm, respectively. The FUS beams were also visualized in a two layer phantom and a chicken breast. The novel reconstruction technique was able to accurately visualize the field of an FUS source in the context of the interrogated medium.

Passive acoustic mapping with absolute time‐of‐flight information and delay‐multiply‐sum beamforming

“…23 Combining the DS beamforming algorithm with the absolute time-of -flight information obtained by timing synchronization, better axial resolution can be obtained when the FUS pulse is short. [30][31][32][33] However, the AtPAM image beamformed by the DS algorithm suffers from a large mainlobe size and high sidelobe level. This study proposes an enhanced AtPAM algorithm based on DMS beamforming, to address the shortcomings of DS-based AtPAM.…”

“…The conventional PAM with the relative time‐of‐flight information results in poor axial resolution when utilizing an ultrasound diagnostic transducer (e.g., a linear array) because of its diffraction limit 23 . Combining the DS beamforming algorithm with the absolute time‐of‐flight information obtained by timing synchronization, better axial resolution can be obtained when the FUS pulse is short 30–33 . However, the AtPAM image beamformed by the DS algorithm suffers from a large mainlobe size and high sidelobe level.…”

“…32 Recently, the ability of AtPAM using DS beamforming to visualize the FUS beam in situ has been validated in phantoms and biological tissues. 33 The above indicates that this technique has great development potential and application prospects. However, DS beamforming results in a wide mainlobe and high sidelobe due to its non-adaptiveness and blindness, limiting the abilities of distinguishing targets and suppressing interference.…”

A Microfluidic System of Gene Transfer by Ultrasound

“…Although the focus of a spherical ultrasonic source has been a topic of fundamental interest for many years, several of its aspects, such as non-linear performance, are still not fully understood [ 31 ]. As there is no analytical solution for a spherical focusing field, some approximation models have been derived [ 32 , 33 , 34 ].…”

“…For continuous waves, under the assumptions of source piston movement and plane wave propagation, B.G.Lucas derived an integral model. However, the model is obtained by converting the Helmholtz equation into a parabola equation with the restriction of Fresnel approximation, and is only applicable to shallow curved spherical sources with a feature size much larger than the wavelength of the sound generated [ 31 ]. The MEMS ultrasonic transducer we propose is heavily curved to achieve a strong focusing effect, and its feature size is similar to the ultrasound wavelength; therefore, we cannot use Lucas’ model.…”