A reduced aperture allows for transcranial focus localization at lower pressure

Abstract

Localizing the focus during transcranial focused ultrasound procedures is important to ensure accurate targeting of specific brain regions and interpretation of results. Magnetic resonance acoustic radiation force imaging uses the displacement induced by the ultrasound focus in the brain to localize the beam, but the high pressure required to displace brain tissue may cause damage or confounds during subsequent neuromodulatory experiments. Here, reduced apertures were applied to a phased array transducer to generate comparable displacement to the full aperture but with 20% lower free field pressure.

Fig. 1.

Left: A side profile of the transducer showing the decreasing aperture as elements are turned off and the f-number is increased. The full array has an f-number of 0.7 and 128 elements. By electronically turning off the outside set of elements (in blue), the f-number is increased to 0.8, and by further turning off the elements in red, the f-number is 1.0. Right: Element positions on the transducer showing which elements were turned off for each f-number configuration.

Fig. 2.

Hydrophone data comparing PNP vs the ratio of magnitude of the second harmonic (2 F₀) to the center frequency (F₀). Measurements of the pressure wave at the focus were performed for each f-number configuration, and the Fourier transform was used to measure the magnitude of the Fourier transform at the center frequency and second harmonic. At high pressure, there is more harmonic content in the wave within each f-number configuration. For larger f-numbers, we observed a higher ratio of harmonic content compared to the center frequency at similar pressures with the lowest ratio at f/0.7 (blue) increasing at f/0.8 (red) and f/1.0 (yellow).

Fig. 3.

Phantom MR-ARFI displacement map showing the short axis view of the beam for f/1.0. The displacement is perpendicular to the image plane. The displacement along the beam (red line) is plotted to the right for each f-number configuration. Using pressure matched sonications, an increase in displacement was measured while using larger f-number apertures.

Fig. 4.

In vivo MR ARFI data. The magnitude image showing the positioning of the transducer over the NHP head and the region where the focus was located (left). A displacement of 1.41  $\mu$ m was measured with the full array (f/0.7) using 5 MPa of free field pressure (top right). With a reduced array (f/1.0), 20% less free field pressure was able to generate 1.54  $\mu$ m of displacement (bottom right).