Towards Quantitative Whole Organ Thermoacoustics with a Clinical Array plus One Very Low Frequency Channel Applied to Prostate Cancer Imaging

Thermoacoustics has the potential to provide quantitative images of intrinsic tissue properties, most notably electrical conductivity in Siemens/meter, much as shear wave elastography provides tissue stiffness in kilopascal. Although thermoacoustic imaging with optical excitation has been commercialized for small animals, it has not yet made the transition to clinic for whole organ imaging in humans. The purpose of this work was to develop and validate specifications for a clinical ultrasound array for quantitative whole organ thermoacoustic imaging. Imaging a large organ requires exciting thermoacoustic pulses throughout the volume and broadband detection of those pulses because tomographic image reconstruction preserves frequency content. Applying the half-wavelength limit to a $200 \text{-}\upmu \text{m}$ inclusion inside a 7.5-cm diameter organ requires measurement sensitivity to frequencies ranging from 4 MHz to 10 kHz, respectively. A dual-transducer system utilizing a P4-1 array connected to a Verasonics V1 system as well as a focused single-element transducer sensitive to lower frequencies was developed. Very high-frequency (VHF) irradiation generated thermoacoustic pulses throughout a $6 \times 6 \times 5\ \text{cm}^{3}$ volume. In the VHF regime, electrical conductivity drives thermoacoustic signal production. Simultaneous acquisition of thermoacoustic pulses by both transducers enabled comparison of transducer performance. Data from the clinical array generated a stack of 96 images with a separation of 0.3 mm, whereas the single-element transducer imaged only in a single plane. In-plane resolution and quantitative accuracy were quantified at isocenter. The array provided volumetric imaging capability with superior resolution whereas the single-element transducer provided superior quantitative accuracy in axial images. Combining axial images from both transducers preserved resolution of the P4-1 array and improved image contrast. Neither transducer was sensitive to frequencies below 50 kHz, resulting in a dc offset and low-frequency shading over fields of view exceeding 15 mm. Fresh human prostates were imaged ex vivo and volumetric reconstructions reveal structures rarely seen in diagnostic images. In conclusion, quantitative whole-organ thermoacoustic tomography will be feasible by sparsely interspersing transducer elements sensitive to the low end of the ultrasonic range.