Increasing the transmission efficiency of transcranial ultrasound using a dual-mode conversion technique based on Lamb waves

Technical note: High-efficient and wireless transcranial ultrasound excitation based on electromagnetic acoustic transducer

BACKGROUND

The generation of transcranial ultrasound is usually based on the piezoelectric effect, so it is necessary to attach transducers around the skull. However, the skull will cause serious attenuation and scattering of ultrasound, which makes it particularly difficult for transcranial ultrasound imaging and modulation.

PURPOSE

In transcranial ultrasound imaging, there is significant attenuation and scattering of ultrasound waves by the skull bone. To mitigate this influence and enable precise imaging and high-efficient transcranial ultrasound for specific patients (such as stroke patients who already require craniotomy as part of their surgical care), this paper proposes to use EMAT to excite metal plates placed inside the skull based on the excellent penetration characteristics of EM waves into the skull, generating ultrasound signals, which can completely avoid the influence of skull on ultrasound transmission.

METHODS

Based on an efficient wireless transcranial ultrasound experimental platform, we first verified that the skull would not affect the propagation of electromagnetic waves generated by EMAT. In addition, the distribution of the transcranial sound field generated by EMAT was measured.

RESULTS

EMAT can generate 1.0 MHz ultrasound by wireless excitation of a 0.1 mm thick copper plate through an adult skull with a thickness of ∼1 cm, and the frequency and amplitude of the generated ultrasound are not affected by the skull. The results indicated that the electromagnetic waves successfully penetrated the skull, with a recorded strength of approximately 2 mV. We also found that the ultrasound signals generated by the EMAT probe through the skull remained unaffected, measuring around 2 mV. In addition, the measurement of the transcranial sound field distribution (80*50 mm2 ) generated by EMAT shows that compared with the traditional extracranial ultrasound generation method, the sound field distribution generated by the wireless excitation of the intracranial copper plate based on EAMT is no longer affected by the uneven and irregular skull.

CONCLUSION

Our experiments involved validating the penetration capabilities of electromagnetic waves utilizing the EMAT probe through a 7 (5+2) mm thick organic glass plate and a real human skull ranging from 8 to 15 mm in thickness. The efficient and wireless transcranial ultrasound excitation proposed in this paper may be possible for transcranial ultrasound imaging and therapy.

Effect of skull porosity on ultrasound transmission and wave mode conversion at large incidence angles

BACKGROUND

Transcranial ultrasound imaging and therapy depend on the efficient transmission of acoustic energy through the skull. Multiple previous studies have concluded that a large incidence angle should be avoided during transcranial-focused ultrasound therapy to ensure transmission through the skull. Alternatively, some other studies have shown that longitudinal-to-shear wave mode conversion might improve transmission through the skull when the incidence angle is increased above the critical angle (i.e., 25° to 30°).

PURPOSE

The effect of skull porosity on the transmission of ultrasound through the skull at varying incidence angles was investigated for the first time to elucidate why transmission through the skull at large angles of incidence is decreased in some cases but improved in other cases.

METHODS

Transcranial ultrasound transmission at varying incidence angles (0°-50°) was investigated in phantoms and ex vivo skull samples with varying bone porosity (0% to 28.54% ± 3.36%) using both numerical and experimental methods. First, the elastic acoustic wave transmission through the skull was simulated using micro-computed tomography data of ex vivo skull samples. The trans-skull pressure was compared between skull segments having three levels of porosity, that is, low porosity (2.65% ± 0.03%), medium porosity (13.41% ± 0.12%), and high porosity (26.9%). Next, transmission through two 3D-printed resin skull phantoms (compact vs. porous phantoms) was experimentally measured to test the effect of porous microstructure alone on ultrasound transmission through flat plates. Finally, the effect of skull porosity on ultrasound transmission was investigated experimentally by comparing transmission through two ex vivo human skull segments having similar thicknesses but different porosities (13.78% ± 2.05% vs. 28.54% ± 3.36%).

RESULTS

Numerical simulations indicated that an increase in transmission pressure occurs at large incidence angles for skull segments having low porosities but not for those with high porosity. In experimental studies, a similar phenomenon was observed. Specifically, for the low porosity skull sample (13.78% ± 2.05%), the normalized pressure was 0.25 when the incidence angle increased to 35°. However, for the high porosity sample (28.54% ± 3.36%), the pressure was no more than 0.1 at large incidence angles.

CONCLUSIONS

These results indicate that the skull porosity has an evident effect on the transmission of ultrasound at large incidence angles. The wave mode conversion at large, oblique incidence angles could enhance the transmission of ultrasound through parts of the skull having lower porosity in the trabecular layer. However, for transcranial ultrasound therapy in the presence of highly porous trabecular bone, transmission at a normal incidence angle is preferable relative to oblique incidence angles due to the higher transmission efficiency.