Gregory Thomas Clement

We have been investigating a "belt-type" conformal transducer array designed transcranial brain imaging. In precursory simulation work, an array consisting of approximately 500 channels would be sufficient to achieve two-dimensional brain imaging utilizing a tomographic approach valid on irregular boundaries, such as the human head. 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, 20mm X 20mm) 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. Maximum measured pressure amplitude crosstalk of the neighbouring elements to the active element is less than -14 dB, 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 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 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.

In the present proof of principle study, we evaluated the homogenous angular spectrum method for passive acoustic mapping (AS-PAM) of microbubble oscillations using simulated and experimental data. In the simulated data we assessed the ability of AS-PAM to form 3D maps of a single and multiple point sources. Then, in the two dimensional limit, we compared the 2D maps from AS-PAM with alternative frequency and time domain passive acoustic mapping (FD- and TD-PAM) approaches. Finally, we assessed the ability of AS-PAM to visualize microbubble activity in vivo with data obtained during 8 different experiments of FUS-induced blood-brain barrier disruption in 3 nonhuman primates, using a clinical MR-guided FUS system. Our in silico results demonstrate AS-PAM can be used to perform 3D passive acoustic mapping. 2D AS-PAM as compared to FD- PAM and TD-PAM is 10 and 200 times faster respectively and has similar sensitivity, resolution, and localization accuracy, even when the noise was 10-fold higher than the signal. In-vivo, the AS-PAM reconstructions of emissions at frequency bands pertinent to the different types of microbubble oscillations were also found to be more sensitive than TD-PAM. AS-PAM of harmonic-only components predicted safe blood-brain barrier disruption, whereas AS-PAM of broadband emissions correctly identified MR-evident tissue damage. The disparity (3.2 mm) in the location of the cavitation activity between the three methods was within their resolution limits. These data clearly demonstrate that AS-PAM is a sensitive and fast approach for PAM, thus providing a clinically relevant method to guide therapeutic ultrasound procedures.

A penetrable finite region consisting of layers of arbitrary shape, density, and sound speed is considered. Solutions are formulated as a T-matrix operator that is independent of the incident field, thereby allowing scattering due to any external source field to be rapidly calculated in a single operation. In departure from standard methods, boundary terms are written in their Fourier integral forms in polar coordinates. It will be shown that the approach allows scattering terms to be reduced to a single integral equation, a set of which form a block tri-diagonal matrix; a well-studied matrix form with various methods available for efficiently solving the inverse problem. A numeric algorithm is developed around the approach and validated against a previously reported wave-vector approach. Several medically relevant examples will be shown and discussed along with the potential for such methods to aid in the development of diagnostic and therapeutic techniques. [Study funded under National Institutes of Health Grant R01EB014296.]

A tomographic method is considered that forms images from sets of spatially randomized source signals and receiver sensitivities. The method is designed to allow image reconstruction for an extended number of transmitters and receivers in the presence noise and without plane wave approximation or otherwise approximation on the size or regularity of source and receiver functions. An overdetermined set of functions are formed from the Hadamard product between a Gaussian function and a uniformly distributed random number set. It is shown that this particular type of randomization tends to produce well-conditioned matrices whose pseudoinverses may be determined without implementing relaxation methods. When the inverted sets are applied to simulated first-order scattering from a Shepp-Logan phantom, successful image reconstructions are achieved for signal-to-noise ratios (SNR) as low as 1. Evaluation of the randomization approach is conducted by comparing condition numbers with other forms of signal randomization. Image quality resulting from tomographic reconstructions is then compared with an idealized synthetic aperture approach, which is subjected to a comparable SNR. By root-mean-square-difference comparisons it is concluded that - provided a sufficient level of oversampling - the dynamic transmit and dynamic receive approach produces superior images, particularly in the presence of low SNR.

Recently, we reported on our observation that diffuse infrared (IR) light transmitted through human skull bone exhibits an intensity that correlates positively with acoustic (1 MHz) time-of-flight data acquired at equivalent locations [POMA 22, 020002 (2015)]. Presently, we investigate the potential to exploit this correlation as a means of correcting for skull-induced aberration. For measurement, a dual ultrasonic/infrared array capable of illuminating a localized region of the skull bone and recording the backscattered light was constructed. The array design utilized 940 nm IR emitters (TSAL4400, 3-mm-diameter) and a single detector (TEFD4300-3 mm) configured to transmit and record under the face of an ultrasonic PZT ring transducer. Initial tests consisted of a three-emitter configuration, with fiberglass-reinforced foil used to isolate the direct infrared signal between the emitters and the receiver. Effects of skin were considered by attaching a skin-mimicking layer to the skull and comparing results with and without the layer. Data acquired on a set of eight skull samples will be presented. Preliminary results show good correlation between ultrasound and IR intensity. [Work supported by NIH award R01EB014296.]Posters will be on display and contributors will be at their posters during the session break from 2:40 p.m. to 3:05 p.m.

Field characterization methods using a scattering target in the absence of a point-like receiver have been well described, in which scattering is recorded by a relatively large receiver located outside the field of measurement. Unfortunately, such methods are prone to artifacts caused by averaging across the receiver surface. To avoid this problem while simultaneously increasing the gain of a received signal, the present study introduces a binary plate lens designed to focus sphericallyspreading waves onto a planar region having a nearly-uniform phase proportional to that of the target location. The lens is similar to a zone plate, but modified to produce a bi-convexlike behavior, such that it focuses both planar and spherically spreading waves. A measurement device suitable for characterizing narrowband ultrasound signals in air is designed around this lens by coupling it to a target and planar receiver. A prototype device is constructed and used to characterize the field of a highly-focused 400-kHz in-air transducer along 2 radial lines. Comparison of the measurements with numeric predictions formed from nonlinear acoustic simulation showed good relative pressure correlation, with mean differences of 10% and 12% over the center 3-dB full-width at half-maximum drop and 12% and 17% over the 6-dB drop.

Skull-induced distortion and attenuation present a challenge to both transcranial imaging and therapy. Whereas therapeutic procedures have been successful in offsetting aberration using images from prior CTs, this approach is impractical for imaging. In an effort to provide a simplified means for aberration correction, we have been investigating the use of diffuse infrared light as an indicator of acoustic properties. Infrared wavelengths were specifically selected for tissue penetration; however this preliminary study was performed through bone alone via a transmission mode to facilitate comparison with acoustic measurements. The inner surface of a half human skull, cut along the sagittal midline, was illuminated using an infrared heat lamp, and images of the outer surface were acquired with an IR-sensitive camera. A range of source angles was acquired and averaged to eliminate source bias. Acoustic measurements were likewise obtained over the surface with a source (1 MHz, 12.7 mm-diam) oriented parallel to the skull surface and hydrophone receiver (1 mm PVDF). Preliminary results reveal a positive correlation between sound speed and optical intensity, whereas poor correlation is observed between acoustic amplitude and optical intensity.

Hardware for tomographic imaging presents both challenge and opportunity for simplification when compared with traditional pulse-echo imaging systems. Specifically, point diffraction tomography does not require simultaneous powering of elements, in theory allowing just a single transmit channel and a single receive channel to be coupled with a switching or multiplexing network. In our ongoing work on transcranial imaging, we have developed a 512-channel system designed to transmit and/or receive a high voltage signal from/to arbitrary elements of an imaging array. The overall design follows a hierarchy of modules including a software interface, microcontroller, pulse generator, pulse amplifier, high-voltage power converter, switching mother board, switching daughter board, receiver amplifier, analog-to-digital converter, peak detector, memory, and USB communication. Two pulse amplifiers are included, each capable of producing up to 400Vpp via power MOSFETS. Switching is based around mechanical relays that allow passage of 200V, while still achieving switching times of under 2ms, with an operating frequency ranging from below 100kHz to 10MHz. The system is demonstrated through ex vivo human skulls using 1MHz transducers. The overall system design is applicable to planned human studies in transcranial image acquisition, and may have additional tomographic applications for other materials necessitating a high signal output.