Functional Testing of Subcutaneous Piezoelectrically Actuated Hearing Aid: Comparison With BAHA and Potential for Treating Single-sided Deafness.

OBJECTIVE: To compare the performance of a subcutaneous piezoelectrically actuated hearing aid (SPAHA) with the bone-anchored hearing aid (BAHA) and assess its effectiveness as a treatment option for conductive loss and single-sided deafness (SSD). BACKGROUND: To validate the use of the SPAHA as a bone conduction implant, its performance was compared with a widely used bone conduction implant, the BAHA. Maximum dynamic range, power consumed to deliver standard speech signals and total harmonic distortion (THD) was assessed. The transcranial attenuation was also measured to assess the SPAHA's potential to treat SSD. METHOD: Functional testing of the SPAHA and BAHA was conducted using cadaver heads. Ipsilateral and contralateral promontory velocity and the power consumption by the devices were measured at 111 different frequencies in the range of 200 to 9600 Hz. Performance metrics were derived from these measurements. RESULT: The maximum dynamic range for SPAHA was within 10 dB of that of BAHA. The THD for the SPAHA was at most 3%, slightly better than the BAHA. The power consumption by the SPAHA, whereas highly variable, was not statistically different than that of the BAHA. Transcranical attenuation in case of SPAHA was 5 to 10 dB across the measured frequency range. CONCLUSION: From observed dynamic range and THD, the speech quality delivered by the SPAHA should equal or exceed that delivered by the BAHA. To attain equivalent hearing sensation at lower frequencies, the drive voltage for SPAHA would have to be significantly higher than that for BAHA. For typical speech inputs the power consumption requirements of the SPAHA should be roughly equal to those of the BAHA. Given its performance at high frequencies, the SPAHA seems well-suited to treating SSD.

Mass-spring matching layers for high-frequency ultrasound transducers: a new technique using vacuum deposition.

We have developed a technique of applying multiple matching layers to high-frequency (>30 MHz) imaging transducers, by using carefully controlled vacuum deposition alone. This technique uses a thin mass-spring matching layer approach that was previously described in a low-frequency (1 to 10 MHz) transducer design with epoxied layers. This mass- spring approach is more suitable to vacuum deposition in highfrequency transducers over the conventional quarter-wavelength resonant cavity approach, because thinner layers and more versatile material selection can be used, the difficulty in precisely lapping quarter-wavelength matching layers is avoided, the layers are less attenuating, and the layers can be applied to a curved surface. Two different 3-mm-diameter 45-MHz planar lithium niobate transducers and one geometrically curved 3-mm lithium niobate transducer were designed and fabricated using this matching layer approach with copper as the mass layer and parylene as the spring layer. The first planar lithium niobate transducer used a single mass-spring matching network, and the second planar lithium niobate transducer used a single mass-spring network to approximate the first layer in a dual quarter-wavelength matching layer system in addition to a conventional quarter-wavelength layer as the second matching layer. The curved lithium niobate transducer was press focused and used a similar mass-spring plus quarter-wavelength matching layer network. These transducers were then compared with identical transducers with no matching layers and the performance improvement was quantified. The bandwidth of the lithium niobate transducer with the single mass-spring layer was measured to be 46% and the insertion loss was measured to be -21.9 dB. The bandwidth and insertion loss of the lithium niobate transducer with the mass-spring network plus quarter-wavelength matching were measured to be 59% and -18.2 dB, respectively. These values were compared with the unmatched transducer, which had a bandwidth of 28% and insertion loss of -34.1 dB. The bandwidth and insertion loss of the curved lithium niobate transducer with the mass-spring plus quarter-wavelength matching layer combination were measured to be 68% and -26 dB, respectively; this compared with the measured unmatched bandwidth and insertion loss of 35% and -37 dB. All experimentally measured values were in excellent agreement with theoretical Krimholtz-Leedom-Matthaei (KLM) model predictions.

Fabrication and performance of a miniaturized 64-element high-frequency endoscopic phased array.

We have developed a 40-MHz, 64-element phased-array transducer packaged in a 2.5 x 3.1 mm endoscopic form factor. The array is a forward-looking semi-kerfed design based on a 0.68Pb(Mg(1/3)Nb(2/3))O(3) - 0.32PbTiO3 (PMN-32%PT) single-crystal wafer with an element-to-element pitch of 38 µm. To achieve a miniaturized form factor, a novel technique of wire bonding the array elements to a polyimide flexible circuit board oriented parallel to the forward looking ultrasound beam and perpendicular to the array was developed. A technique of partially dicing into the back of the array was also implemented to improve the directivity of the array elements. The array was fabricated with a single-layer P(VDF-TrFE)-copolymer matching layer and a polymethylpentene (TPX) lens for passive elevation focusing to a depth of 7 mm. The two-way -6-dB pulse bandwidth was measured to be 55% and the average electromechanical coupling (k(eff)) for the individual elements was measured to be 0.62. The one-way -6-dB directivities from several array elements were measured to be ±20°, which was shown to be an improvement over an identical kerfless array. The -3-dB elevation focus resulting from the TPX lens was measured to be 152 µm at the focal depth, and the focused lateral resolution was measured to be 80 µm at a steering angle of 0°. To generate beam profiles and images, the probe was connected to a commercial ultrasound imaging platform which was reprogrammed to allow for phased array transmit beamforming and receive data collection. The collected RF data were then processed offline using a numerical computing script to generate sector images. The radiation pattern for the beamformed transmit pulse was collected along with images of wire phantoms in water and tissue-equivalent medium with a dynamic range of 60 dB. Finally, ex vivo tissue images were generated of porcine brain tissue.

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, 1.26 λ-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.

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.

Cost-effectiveness analysis of continuous-flow left ventricular assist devices as destination therapy.

BACKGROUND: Continuous-flow left ventricular assist devices (LVADs) have become the dominant devices for mechanical circulatory support, but their cost-effectiveness is undetermined. This study assessed the cost-effectiveness of continuous-flow devices for destination therapy versus optimal medical management in advanced heart failure and compared the results with previous estimates for pulsatile devices. METHODS AND RESULTS: A Markov model was developed to assess cost-effectiveness. Survival, hospitalization rates, quality of life, and cost data were obtained for advanced heart failure patients treated medically or with a continuous-flow LVAD. Rates of clinical outcomes for all patients were obtained from clinical trial databases. Medicare prospective payments were used to estimate the cost of heart failure admissions. The cost of LVAD implantation was obtained prospectively from hospital claims within a clinical trial. Compared with medically managed patients, continuous-flow LVAD patients had higher 5-year costs ($360 407 versus $62 856), quality-adjusted life years (1.87 versus 0.37), and life years (2.42 versus 0.64). The incremental cost-effectiveness ratio of the continuous-flow device was $198 184 per quality-adjusted life year and $167 208 per life year. This equates to a 75% reduction in incremental cost-effectiveness ratio compared with the $802 700 per quality-adjusted life year for the pulsatile-flow device. The results were most sensitive to the cost of device implantation, long-term survival, cost per rehospitalization, and utility associated with patients' functional status. CONCLUSIONS: The cost-effectiveness associated with continuous-flow LVADs for destination therapy has improved significantly relative to the pulsatile flow devices. This change is explained by significant improvements in survival and functional status and reduction in implantation costs.

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 highfrequency 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.