Showing papers by "Roger J. Zemp published in 2019"

Transparent capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic applications.

Abstract: Integration of acoustic and optical techniques prompted the need for transparent ultrasonic transducers to guide the light through the transducer and improve the signal to noise ratio. In the presented paper, capacitive micromachined ultrasound transducers (CMUTs) using glass substrate and indium-tin-oxide electrodes were fabricated by adhesive wafer bonding technique presenting a transparency of up to 82% in the visible range. A receive sensitivity of 65.5 μV/Pa was measured with noise equivalent sensitivity of 95 Pa. Capacity of the produced CMUTs for photoacoustic imaging was also demonstrated by successfully detecting the photoacoustic signal from an aluminum foil target, which was irradiated by a 532-nm pulse laser transmitted through the CMUT. The centre frequency of the detected photoacoustic signal was at 2 MHz with 52.3% -6-dB fractional bandwidth.

Ultraviolet photoacoustic remote sensing microscopy.

Abstract: Traditional histopathology involves fixing, sectioning, and staining protocols that are time consuming and subject to staining variability. In this Letter, we present ultraviolet photoacoustic remote sensing microscopy, capable of imaging cell nuclei without the need for exogenous stains or labelling. Our reflection mode approach is non-contact and has the potential to provide useful histological information without laborious sample preparation steps. Tumor cell cultures and excised tissue samples were imaged with the 0.7 μm resolution and signal-to-noise ratios as high as 53 dB, with close agreement to traditional hematoxylin and eosin staining.

Real-time functional photoacoustic remote sensing microscopy.

Abstract: A fiber-tetherable non-contact photoacoustic remote sensing microscopy system capable of multiplex functional imaging is reported. By utilizing stimulated Raman scattering within an over-pumped polarization-maintaining single-mode optical fiber, rapid pulse-to-pulse switching (500 kHz) of excitation spectral content is demonstrated and utilized as a photoacoustic excitation source. These rapid acquisitions aim to reduce motion artifacts and facilitate high frame rates appropriate for real-time feedback to users. The system is characterized by estimating blood oxygen saturation in blood-flow phantoms and within a mouse ear in vivo.

Fast Orthogonal Row–Column Electronic Scanning Experiments and Comparisons

Abstract: Three-dimensional ultrasound imaging presents the technical challenges of addressing large numbers of elements in 2-D array transducers. Top-orthogonal-to-bottom electrode (TOBE) 2-D transducer arrays can simplify addressing but typical imaging methods with such arrays enable only one-way focusing in azimuth and elevation. Here, experimental results are reported for the fast orthogonal row–column electronic scanning (FORCES) imaging scheme implemented on a $64 \times 64$ element bias-sensitive electrostrictive relaxor TOBE array. The FORCES imaging scheme involves transmitting along rows to form an elevational transmit focus, while biasing columns with bias patterns selected from a Hadamard matrix. Channel data from columns is received and decoded for synthetic-aperture beamforming in azimuth. This scheme offers two-way azimuthal focusing. The volumetric imaging experiments were conducted using wire phantoms as well as on rat hearts using two different TOBE imaging schemes: Scheme 1 (transmit focusing in elevation and receive focusing in azimuth) and FORCES. Wire phantom experiments at a depth of 2 cm showed an azimuthal resolution of 0.42 and 0.31 mm with Scheme 1 and FORCES, respectively. We also compared the elevational imaging performance of these imaging schemes with a mechanically scanned linear array. The FORCES imaging displayed an elevational resolution of 0.46 mm at a depth of 2 cm, and the linear array displayed an elevational resolution of 0.72 cm. The novel TOBE array architecture and FORCES imaging scheme thus enable high-quality 3-D ultrasound imaging using only row–column addressing and bias control and may prove an enabling technology for many future 3-D imaging platforms.

A Nonlinear Lumped Equivalent Circuit Model for a Single Uncollapsed Square CMUT Cell

Abstract: An accurate nonlinear lumped equivalent circuit model is used for modeling of capacitive micromachined ultrasonic transducers (CMUTs). Finite-element analysis (FEA) is a powerful tool for the analysis of CMUT arrays with a small number of cells while with the harmonic balance (HB) analysis of the lumped equivalent circuit model, the entire behavior of a large-scale arbitrary CMUT array can be modeled in a very short time. Recently, an accurate nonlinear equivalent circuit model for uncollapsed single circular CMUT cells has been developed. However, the need for an accurate large-signal circuit model for CMUT cells with square membranes motivated us to produce a new nonlinear large-signal equivalent circuit model for uncollapsed CMUT cells. In this paper, using analytical calculations and FEA as the tuning tool, a precise large signal equivalent circuit model of square CMUT dynamics was developed and showed excellent agreement with finite-element modeling (FEM) results. Then, different CMUT single cells with square and circular membranes were fabricated using a standard sacrificial release process. Model predictions of resonance frequencies and displacements closely matched experimental vibrometer measurements. The framework presented here may prove valuable for future design and modeling of CMUT arrays with square membranes for ultrasound imaging and therapy applications.

A New 3D Imaging Technique Integrating Ultrafast Compounding, Hadamard Encoding, and Reconfigurable Fresnel Lensing, demonstrated on a 128-Element, Crossed Electrode Endoscope

Abstract: Crossed electrode arrays address some the challenges associated with 3D ultrasound imaging because of the significant reduction in the number of elements (2N vs. N2). However, creating a two-way focused 3D image in real-time is difficult with these arrays because azimuth and elevation dimensions cannot be beamformed at the same time. We have developed a new 3D imaging approach that uses the flexibility of bias sensitive substrates to create a high-quality elevation focus on a crossed electrode array. The principle behind this technique is to perform conventional compound imaging with an azimuth set of electrodes, while implementing a bias controllable elevation lens with an elevation set of electrodes. On transmit, the biases are chosen to mimic a Fresnel lens. Then, on receive, Hadamard coding is implemented along the elevation dimension. After decoding, we gain the RF data for each element across the elevation aperture even though there is effectively only one channel in that dimension. A 30MHz, 64x64 element crossed electrode relaxor array was fabricated on a semi-kerfed, electrostrictive substrate and was used to demonstrate the performance of the imaging technique. The −6dB beamwidths were simulated to be 155µm and 170µm in the azimuth and elevation direction respectively and the secondary lobe levels were suppressed below −50dB. Images were generated of a wire phantom to confirm the performance of the elevational focus with good agreement between simulation and experiment.

Label-free non-contact imaging of cell nuclei using ultraviolet photoacoustic remote sensing microscopy (Conference Presentation)

Abstract: Despite advancements in imaging and surgical methodology surgeons continue to face challenges in differentiating suitable margins in tumor resection sites and assessing the extent of cancer proliferation. Presently, histology is the gold standard used for determining margins post-operatively. However, currently no intraoperative tools can analyze entire resection sites. This results in unnecessary repeated surgeries and is especially critical to patient outcome for certain cancers. To address this critical need, we have developed a next-generation microscopy system intended to allow accurate intraoperative virtual histopathology for margin assessment. The modality, named ultraviolet photoacoustic remote sensing microscopy (UV-PARS), takes advantage of the intrinsic optical absorption contrast of DNA at 266 nm and a non-contact PARS technique. This approach measures time-dependent refractive index modulations at their subsurface origin resulting from thermo-elastic excitation from a pulsed excitation source. This enables the possibility of real time non-contact label-free visualization of cell nuclei in vivo. Preliminary results are presented including studies with live cell cultures, excised tissue samples, phantoms, and characterization of system parameters. Lateral-resolution was found to be 0.7 µm with a signal-to-noise ratio of 50 dB achieved in phantoms and 30 dB for HeLa cells. Simultaneously collected confocal reflectance microscopy using 1310 nm light provided cell body morphology. HeLa and PC3 cell cultures were imaged with good agreement to conventional H&E stained images of the same samples.

Camera-based photoacoustic remote sensing microscopy

Abstract: Photoacoustic remote sensing microscopy is a recently developed optical non-contact imaging method that provides optical absorption contrast in reflection mode. Previously, this was performed by co-scanning of tightly co-focused excitation and interrogation beams. We have demonstrated the proof of principle that superficial optical absorption information can be measured from a scattering sample by a camera in reflection mode using a pulsed excitation and interrogation beams. This allows wide field-of-view absorption imaging in scattering samples in real-time. Using a wire-bonding wire embedded in a phantom, the photoacoustic effect is first induced by a 532-nm pulsed excitation beam which alters the optical property of the wire that is illuminated with a 1064-nm pulsed interrogation beam with 80 ns delay. The scattering of the interrogation beam with and without the excitation beam is captured by the camera and the difference is calculated. Increasing contrast in difference images can be observed as the fluence rate of the excitation beam is set to 5.28 mJ/cm2, 12.8 mJ/cm2, 19.5 mJ/cm2 and 26.0 mJ/cm2. The mean relative difference is increased from 0.92 %, 2.10 %, 2.64 % and 3.27%, respectively.