Synopsis

Thermo-acoustic ultrasound uses the acoustic waves generated by thermoelastic expansion to detect peaks in local SAR and could be used to assess the risk of RF heating for patients with implanted or interventional devices. We developed simulations to characterize the generation and propagation of thermo-acoustic waves from device lead tips, and we validated the simulations with experimental results. The simulations provide insight into the properties of thermo-acoustic signals and can aid in the development of a thermo-acoustic ultrasound system for the assessment of RF safety.

Purpose

RF tip heating of implanted and interventional device leads is a safety concern that restricts patients’ access to MRI.^1,2 The heating has many dependencies³ and cannot be easily predicted prior to scanning. A pre-scan procedure to identify the risk of excess heating would allow more patients to receive clinically useful scans.

Thermo-acoustic ultrasound(TAUS) is an emerging technique in which the reception of acoustic waves generated by thermoelastic expansion is used to identify peaks in local SAR. Prior work has demonstrated the ability of a TAUS system integrated with an MRI scanner to detect RF tip heating induced by the scanner’s body coil,⁴ making the prospect of a TAUS-based pre-scan to assess RF safety promising. Here, we discuss the development and experimental validation of finite-difference time-domain(FDTD) simulations that provide insight into the TAUS signal generated by lead tips.

Theory & Methods

In TAUS, the pressure generated in a region is proportional to the temporal derivative of the local SAR,⁵ which can be approximated as a separable function of position and time. Our simulations of the TAUS signal are performed through two distinct steps. First, an electromagnetic simulation determines the spatial profile of the local SAR. This serves as the pressure source distribution for an acoustic simulation that uses the temporal derivative of the transmit power as an excitation signal. These simulations are designed to determine the relative magnitudes of the acoustic signals through space, not the absolute magnitude of pressure generated.

The electric field transfer function is commonly used to model RF heating of leads in MRI.⁶ Transfer function theory indicates that the spatial profile of the E-field at a lead tip not does strongly depend on the incident E-field distribution along the lead.⁷ Therefore, an arbitrary placement of an excitation source along a straight lead results in an accurate approximation of the SAR profile at the lead tip for any incident field and lead geometry. We utilized the FDTD electromagnetic solver in Sim4Life(ZMT,Zurich,Switzerland) to estimate the SAR profiles of lead tips immersed in a conductive medium(Figure 1).

To simulate acoustic wave propagation from cylindrical lead tips, we created a 2D FDTD simulation in cylindrical coordinates with azimuthal symmetry in Matlab. SAR data from the electromagnetic simulation was interpolated onto the acoustic simulation grid to determine the pressure source profile(Figure 2a). A wideband Gaussian pulse was used for the transmit signal to characterize acoustic wave propagation at a range of excitation frequencies, and frequency-domain data was tracked at a series of positions.

The simulation results were used to estimate the signal magnitude observed by a transducer at a given position. The frequency-domain simulation data within the desired excitation bandwidth was averaged over the FDTD grid points covered by the circular transducer surface, and the resulting magnitude data was summed to determine the signal level.

To validate the simulations, TAUS measurements were taken using the setup shown in Figure 3. A Medusa MRI Console⁸ was used to transmit and receive a frequency-modulated continuous wave(FMCW)⁴ signal that amplitude-modulated a 64MHz carrier. An ultrasonic transducer was attached to a 2D plotter(Makeblock,Shenzhen,China) to enable measurements at 1mm increments over a plane(Figure 3b,c). Measurements were conducted using an FMCW center frequency of 0.5MHz with a 0.5MHz transducer(Olympus V318-SU) and using a center frequency of 1MHz with a 1MHz transducer(Olympus V303-SU). Measurements were done on two leads, one with 7mm of exposed conductor and the other with 14mm(Figure 1a). They were coupled to a toroid transmitter⁹ and had their tips immersed in saline solution.

Results & Discussion

Figure 1c shows the simulated SAR profiles. The SAR is highest along the exposed conductor, with peaks near the start and end of the conductor. The different SAR profiles for the two lead tips result in different spatial patterns for the simulated TAUS signal(Figure 2).

Figure 4 shows the simulated signal pattern in the measurement plane, while Figure 5 shows the simulated and measured transducer signal levels as a function of transducer position in the plane. These appear significantly different because the transducers are large relative to the acoustic wavelengths. Use of a transducer array with smaller element size or lower measurement frequencies could mitigate this effect. The simulated transducer signal patterns in Figure 5 appear to closely resemble the measured ones, indicating that the FDTD simulations provide accurate results.

Conclusions

Acknowledgements

References

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