Near-field radiofrequency thermoacoustic tomography with impulse excitation

Imaging performance of radiofrequency and microwave-based thermoacoustic tomography systems is mainly determined by the ability to deposit a substantial amount of electromagnetic energy within ultrashort time duration. Pulses of nanosecond-range duration that can carry hundreds of millijoules energy are ideal for obtaining good signal-to-noise and spatial resolution in many biological imaging applications. However, existing implementations are based on modulated-carrier-frequency amplification solutions, which are generally costly and cannot achieve ultrahigh-peak-power requirements essential for optimal thermoacoustic signal generation.

Herein the authors suggest and experimentally validate a near-field radiofrequency tomography (NRT) method for high resolution imaging of biological tissues using ultrashort electromagnetic impulses. The solution includes a low-cost pulsing system while the imaged objects are placed in the near field of the energy-emitting aperture for improved coupling using nonradiative fields.

In the current design, the authors were able to achieve excitation impulse energies of hundreds of millijoules with durations in the order of a few nanoseconds, corresponding to peak power levels of multiple megawatts. The phantom imaging experiments demonstrated image features with characteristic sizes of around 170 microm, but the impulse durations used herein allow in principle spatial resolutions in the order of a few tens of microns when using an appropriate ultrasonic detection bandwidth.

The proposed NRT method makes it possible to attain very high spatial resolution without compromising the thermoacoustic signal strength. This makes the imaging performance to be limited by the available bandwidth of the ultrasonic detector rather than by the microwave pulse duration. It is overall expected that the combination of pulsed near-field coupling with optimal choice of energy dissipation elements will generate a practical modality that can scale its application to small and larger volumes alike, while optimally adjusting the resolution to match the acoustic resolution possible. Such an approach should find several applications in small animal and clinical imaging.