Zahra Torbatian

A split-aperture transmit beamforming technique with phase coherence grating lobe suppression

A small element-to-element pitch (~.5λ) is conventionally required for phased array ultrasound transducers to avoid large grating lobes. This constraint can introduce many fabrication difficulties, particularly in the development of high-frequency phased arrays at operating frequencies greater than 30 MHz. In this paper, a new transmit beamforming technique along with sign coherence factor (SCF) receive beamforming is proposed to suppress grating lobes in large-pitch phased-array transducers. It is based on splitting the transmit aperture (N elements) into N/K transmit elements and receive beamforming on all N elements to reduce the temporal length of the transmit grating lobe signal. Therefore, the use of synthetic aperture beamforming, which can introduce relative phase distortions between the echoes received over many transmit events, can be avoided. After each transmit-receive event, the received signals are weighted by the calculated SCF to suppress the grating lobes. After pulsing all sub-apertures, the RF signals are added to generate one line of the image. Simulated 2-way radiation patterns for different K values show that grating lobes can be suppressed significantly at different steering angles. Grating lobes can be suppressed by approximately 20 dB with K = 2 at steering angles greater than 25° and an element pitch greater than 0.75λ. A technique for determining the optimal transmit sub-apertures has been developed.

High-Frequency Ex vivo Ultrasound Imaging of the Auditory System

A 50MHz array-based imaging system was used to obtain high-resolution images of the ear and auditory system. This previously described custom built imaging system (Brown et al. 2004a, 2004b; Brown and Lockwood 2005) is capable of 50 microm axial resolution, and lateral resolution varying from 80 microm to 130 microm over a 5.12 mm scan depth. The imaging system is based on a 2mm diameter, seven-element equal-area annular array, and a digital beamformer that uses high-speed field programmable gate arrays (FPGAs). The images produced by this system have shown far superior depth of field compared with commercially available single-element systems. Ex vivo, three-dimensional (3-D) images were obtained of human cadaveric tissues including the ossicles (stapes, incus, malleus) and the tympanic membrane. In addition, two-dimensional (2-D) images were obtained of an intact cochlea by imaging through the round window membrane. The basilar membrane inside the cochlea could clearly be visualized. These images demonstrate that high-frequency ultrasound imaging of the middle and inner ear can provide valuable diagnostic information using minimally invasive techniques that could potentially be implemented in vivo.

Listening to the Cochlea With High-Frequency Ultrasound

We have developed a high-frequency pulsed-wave Doppler ultrasound probe as a promising minimally-invasive technique for measuring intracochlear mechanics without damaging the cochlea. Using a custom high-frequency ultrasound system, we have measured dynamic motion of intracochlear structures by recording the pulsed-wave Doppler signal resulting from the vibration of the basilar and round window membranes. A 45 MHz needle-mounted Doppler probe was fabricated and placed against the round window membranes of eight different fresh human temporal bones. Pulsed-wave ultrasonic Doppler measurements were performed on the basilar membrane and round window membrane during the application of pure tones to the external ear canal. Doppler vibrational information for acoustic input frequencies ranging from 100-2000 Hz was collected and normalized to the sound pressure in the ear canal. The middle ear resonance, located at approximately 1000 Hz, could be characterized from the membrane velocities, which agreed well with literature values. The maximum normalized mean velocity of the round window and the basilar membrane were 180 μm/s/Pa and 27 μm/s/Pa at 800 Hz. The mean phase difference between the membrane displacements and the applied ear canal sound pressure showed a flat response almost up to 500 Hz where it began to accumulate. This is the first study that reports the application of high frequency pulsed wave Doppler ultrasound for measuring the vibration of basilar membrane through the round window. Since it is not required to open or damage the cochlea, this technique might be applicable for investigating cochlear dynamics, in vivo.

Transmit beamforming techniques for suppressing grating lobes in large pitch ultrasonic phased arrays

To date, clinical implementation of high-frequency ultrasound has been limited due to the difficulties in fabricating sufficiently small micro-array transducers. Specifically, if an array is desired with the ability to beam-steer to large angles, an inter-element pitch of approximately .5λ is required to avoid grating lobe artifacts. At high-frequencies (30-70MHz), this introduces major fabrication challenges since the required element pitch is between 10 and 25 microns. A new technique called Phase Coherence Imaging has been introduced in the literature for suppressing grating lobes in large-pitch arrays by calculating a weighting factor proportional to the instantaneous phase coherence of the received element echoes. If the reflected echoes in the grating lobe region are relatively broadband, only some of the echoes will overlap and the resulting weighting factor will be less. Unfortunately, most beamforming techniques result in relatively narrowband echoes in the grating lobe region, making this technique less effective. We have developed a technique that splits the N-element transmit aperture into N/K transmit elements and N receive elements in order to better suppress grating lobes by increasing the bandwidth of the grating lobe echoes. We have also developed a technique that uses a probing pulse from a virtual point source behind the array in order to pre-calculate weighting factors from broadband echoes before conventional transmit beamforming is used. Radiation patterns have been simulated and the amount of grating lobe suppression has been quantified using the proposed techniques. It has been shown that these techniques are very effective in suppressing grating lobes in large-pitch phased-arrays, potentially simplifying high-frequency array fabrication.

Experimental verification of pulse-probing technique for improving phase coherence grating lobe suppression

Fabrication of high-frequency phased-array ultrasound transducers is challenging because of the small element- to-element pitch required to avoid large grating lobes appearing in the field-of-view. Phase coherence imaging (PCI) was recently proposed as a highly effective technique to suppress grating lobes in large-pitch arrays for synthetic aperture beamforming. Our previous work proposed and theoretically validated a technique called pulse probing for improving grating lobe suppression when transmit beamforming is used with PCI. The present work reports the experimental verification of the proposed technique, in which the data was collected using a high-frequency ultrasound system and the processing was done offline. The data was collected with a 50-MHz, 256-element, 1.26λ-pitch linear array, for which only the central 64-elements were used as the full aperture while the beam was steered to various angles. By sending a defocused pulse, the PCI weighting factors could be calculated, and were subsequently applied to the conventional transmit-receive beamforming. The experimental two-way radiation patterns showed that the grating lobe level was suppressed approximately 40 dB using the proposed technique, consistent with the theory. The suppression of overlapping grating lobes in reconstructed phased array images from multiple wire-phantoms in a water bath and tissue phantoms further validated the effectiveness of the proposed technique. The application of pulse probing along with PCI should simplify the fabrication of large-pitch phased arrays at high frequencies.

Experimental verification of a split-aperture transmit beamforming technique for suppressing grating lobes in large pitch phased arrays

Phased array ultrasound transducers are desirable in some applications where large field-of-view is required from a small aperture. Fabrication of high-frequency phased array transducers is challenging due to the inter-element pitch constraint (~0.5 λ) to avoid large grating lobe artifacts. Phase coherence imaging (PCI) has been introduced as a method for suppressing grating lobes in large-pitch arrays, however, it is not as effective when the grating lobe echoes have large overlap in the time-domain. In previous work we suggested a technique called split-aperture transmit beamforming to increase the effectiveness of PCI in grating lobe suppression of these arrays and used computer simulation to evaluate its effectiveness. In the present work we report on the experimental verification of the technique using a commercially available high-frequency ultrasound linear array system (Vevo 2100, Visualsonics). We demonstrate that by using only two split-aperture transmit beamforming events along with PCI we can effectively suppress the grating lobes resulting from a 50 MHz, 64-element, 1.26 λ pitch phased array to less than 60 db for a wire-phantom placed at 25 degrees from the center of transducer. We further show that this grating lobe suppression greatly improves the contrast in tissue phantom. Finally, we show that the measured beamformed radiation patterns are consistent with simulations.

Direct Measurement of Basilar Membrane Motion Using Pulsed‐Wave Doppler High‐Frequency Ultrasound

We present a preliminary report on the use of a new technique for measuring the motion of the basilar membrane, high‐frequency ultrasound Doppler vibrometry. Using a custom‐built, 1 mm diameter probe, we collected ultrasonic reflections from intracochlear structures and applied pulsed‐wave Doppler vibrometry to measure the basilar membrane response to pressure applied in the ear canal.

High‐frequency ultrasound Doppler velocimetry measurements of intra‐cochlear structures in human temporal bones.

Hearing loss is one of the most prevalent, chronic, and fast growing disorders, affecting approximately one‐tenth of the population. Diagnostic imaging tools currently used in this field such as CT and MRI do not have sufficient spatial/temporal resolution to properly diagnose most of the underlying causes of hearing loss. In this work, we present the first high‐resolution velocimetry measurements of intra‐cochlear structures using high‐frequency (40 MHz) pulsed‐wave Doppler ultrasound. A 40‐MHz single‐element transducer based on PMN‐PT single‐crystal piezoelectric was fabricated in‐house and mounted onto the tip of a needle. The transducer was a side‐looking circular disk with a 1 mm outer diameter. A custom data acquisition system was developed in order to perform pulsed‐wave Doppler and was synchronized with an acoustic stimulus. Velocimetry measurements were performed on the basilar membrane located inside the cochlea of fresh temporal bones by imaging across the round window membrane and stimulating ...

Imaging the auditory system: A new application of high-frequency ultrasound

This work describes an ex-vivo imaging study of the auditory system using high-frequency ultrasound. Tissue structures relevant to hearing disorders were imaged using a realistic in-vivo approach and a custom built 50 MHz annular-array imaging system. Images were generated of the middle ear and features of the tympanic membrane and ossicles could be visualized. Images were also generated of the inner ear where the basilar membrane could be visualized. The comparison of ultrasound images with “post-imaging” microscopic photos showed that features of the ossicles and the cochlea agreed with the ultrasound images. A 1 mm diameter Doppler probe has also been fabricated and pulsed-wave Doppler measurements were performed through the round window membrane on a fresh in-tact temporal bone as sound was applied to the ear canal.

A high-frequency beamformer design based on variable PMN-PT SAW delays

A high-frequency ultrasound beamformer based on 70Pb(Mg<inf>⅓</inf>Nb<inf>⅔</inf>)O<inf>3</inf>–0.30PbTiO<inf>3</inf> (PMN-30%PT) single crystal has been designed. The beamformer delays are created using surface acoustic wave (SAW) delay lines, which can be dynamically tuned using the elongation and compression of the bulk PMN-PT substrate between the send and receive SAW electrodes. The change in SAW delay due to the elongation has been measured experimentally and good agreement with the theoretical predictions was found.