Toward transcranial ultrasound tomography: Design of a 456-element low profile conformal array

We have been investigating a 'belt-type' conformal transducer array designed for transcranial brain imaging. In precursory simulation work it was estimated that an array consisting of approximately 500 channels would be sufficient to achieve two-dimensional brain imaging utilizing a tomographic approach valid on irregular boundaries [G T Clement 2014 Inverse Probl. 30 1-22], such as the human head. To achieve such an array in practice we describe here a transducer designed and constructed around an assembly of rigid 1-3 composite planar sub-arrays (500 kHz, 1-3 random-fiber, 20 mm × 20 mm) consisting of 19 elements. Sub-arrays are bonded to custom-printed circuit boards wired to shielded ribbon cable. The full transducer is created by ultra violet (UV) bonding the electrodes to a flexible 3D-printed head band. Element performance is assessed by underwater scanned hydrophone characterization in the far-field. Measurements are then numerically back-projected to the transducer face to assess isolation. It is determined that elements function independently with little impact on the nearby elements. The maximum measured pressure amplitude crosstalk of the neighbouring elements to the active element is less than -14 dB, which making appropriate connections to the 1-3 composite tile was a significant consideration satisfies the later tomography imaging requirements. Results also show a maximum 3 dB decrease in the pressure amplitude as referenced to the maximum pressure amplitude of elements. Due to the limited number of channels on the data and control acquisition system (N = 128), an expander was designed and fabricated using 4:1 multiplexers (MUXs) to cover all the required number of transducer elements for the imaging system. The electrical characterization of the expander was addressed in terms of noise and crosstalk. Results show crosstalk of about -58.7 ± 2.9 dB for the MUXs. Average signal to noise ratio (SNR) decreased by about 0.05 dB after adding the expander. Noise levels remained almost the same after increasing the average detected signal power. Overall, the design and measured results are found to satisfy the key transducer requirements for a conformal array designed for transcranial diffraction tomography. This 'work in progress' will be combined with an optical registration technique to form a complete imaging device.