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

Abstract

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 backprojected 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.

An Emerging Era: Conformable Ultrasound Electronics

“…[ 200 ] Therefore, most of the flexible array are based on commercial PZT ceramics or PZT and AlN thin films, including a flexible ultrasound array for structural health monitoring (Figure 5f), [ 190 ] a flexible PZT linear array by the sol‐gel composite spray technique (Figure 5h), [ 191 ] a 3D‐smart flexible phased array with a profilometer for element localization, [ 201 ] piezoelectric fibers with air gap to achieve flexibility, [ 202 ] flexible 2 × 8 array with copper‐cladded flexible printed circuit, [ 203 ] affixable thin flexible ultrasound transducer using PZT mounted on the silicon islands for needle guidance, [ 204 ] a ring‐annular ultrasound array for use in interventional procedures, [ 205 ] MEMs fabricated flexible 4 × 4 transducer, [ 206 ] rubber encapsulated flexible array for inspecting curved components, [ 207 ] 1–3 composites encapsulated by rubber layer for irregular surface, [ 208 ] flexible transducer for imaging aluminum pipe with interior crack, [ 209 ] bendable pMUT array for brain stimulation, [ 210 ] wearable pMUT array for muscle disorder diagnostics, [ 211 ] bendable array for the image‐guided neural therapy, [ 187 ] AlN based flexible pMUT, [ 212 ] PZT with flexible passive rubber imaging on a fractured cortical bovine bone, [ 63a ] AgNWs electrodes for 1–3 bendable composites, [ 63b ] and ‘belt‐type’ conformal transducer array for transcranial brain imaging. [ 213 ] In addition to pMUT, researchers also expanded their studies on cMUT in a flexible configuration, [ 214 ] but continue to emphasize the fabrication method and rarely discuss the imaging application.…”

A Novel Versatile Approach for Underwater Conformal Volumetric Array Design

“…In many other studies, the practical construction of these arrays is carried out by simply bending them around the curvature of a host surface [ 33 , 34 , 35 , 36 ]. Meanwhile, the transducer made in such a manner has been identified with several problems such as shift in resonance frequency, change in performance, and mismatch between the diameter of the transducer and the bending radius [ 37 ].…”