Implant, performance, and retrieval of an atrial leadless pacemaker in sheep.

BACKGROUND: Medtronic is developing an atrial Micra Transcatheter Pacing System (Medtronic, Minneapolis, Minnesota) and associated retrieval system. OBJECTIVE: The purpose of this study was to evaluate chronic atrial Micra retrieval, reimplantation, and chronic pacing performance. METHODS: Sheep were implanted in 2 groups: group 1 (n = 6) for 6 months, a second device implanted, and first retrieved and studied for an additional 6 months; group 2 (n = 6) for 6 months, devices were retrieved, and a second device implanted and observed acutely. Both groups underwent histopathological evaluation. Pacing capture thresholds (PCTs), p wave amplitude, and pacing impedances were measured chronically. Device retrieval times were recorded, and intracardiac echocardiography was used. RESULTS: At 24 weeks, PCTs for group 1 were low and stable for both the first device (0.55 ± 0.14 V) and the second device (0.57 ± 0.09 V), in which the average retrieval time was 17:35 minutes. For group 2, the average retrieval time was 6:12 minutes, chronic PCTs in the first device were 0.53 ± 0.11 V, and acute PCTs for the second device were 0.71 ± 0.19 V. Pathological findings were within an expected range of tissue responses for similar Micra acute and chronic implants and device retrievals. p waves and impedances were stable and within an expected range for implant site and electrode design. Complications included 1 early dislodgment and 1 death attributed to a prototype retrieval tool. CONCLUSION: In an animal model, an atrial Micra can be easily implanted with excellent chronic pacing performance and is easily retrievable at 6 months. A second device can successfully be implanted with low, chronic stable thresholds. A developed prototype retrieval tool was easy to use and, with modifications, complication free.

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading: erratum.

Typographical errors in a few equations in [Appl. Opt.58, 6067 (2019)APOPAI0003-693510.1364/AO.58.006067] are corrected.

3D angle-independent Doppler and speckle tracking for the myocardium and blood flow

A technology based on velocity ratio indices is described for application in the myocardium. Angle-independent Doppler indices, such as the pulsatility index, which employ velocity ratios, can be measured even if the ultrasound beam vector at the moving target and the motion vector are not in a known plane. The unknown plane situation is often encountered when an ultrasound beam interrogates sites in the myocardium. The velocities employed in an index calculation must be close to the same or opposite directions. The Doppler velocity ratio indices are independent of angle in 3D space as are ratio indices based on 1D strain and 1D speckle tracking. Angle-independent results with spectral Doppler methods are discussed. Possible future imaging techniques based on velocity ratios are presented. By using indices that involve ratios, several other sources of error cancel in addition to that of angular dependence for example errors due to less than optimum gain settings and beam distortion. This makes the indices reliable as research or clinical tools. Ratio techniques can be readily implemented with current commercial blood flow pulsed wave duplex Doppler equipment or with pulsed wave tissue Doppler equipment. In 70 patients where the quality of the real-time B-mode looked suitable for the Doppler velocity ratio technique, there was only one case where clear spectra could not be obtained for both the LV wall and the septum. A reproducibility study of spectra from the septum of the heart shows a 12% difference in velocity ratios in the repeat measurements.

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading.

The power conversion efficiency of an ultrathin CuIn1-ξGaξSe2 (CIGS) solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane. The model predicts that specially tailored bandgap grading can significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. An efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These high efficiencies arise due to a large electron-hole pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Thus, bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, can be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.

Resolving Ultrasound Contrast Microbubbles Using Minimum Variance Beamforming.

Minimum Variance (MV) beamforming is known to improve the lateral resolution of ultrasound images and enhance the separation of isolated point scatterers. This paper aims to evaluate the adaptive beamformer's performance with flowing microbubbles (MBs) which are relevant to super-resolution ultrasound imaging. Simulations using point scatterer data from single emissions were complemented by an experimental investigation performed using a capillary tube phantom and the Synthetic Aperture Real-time Ultrasound System (SARUS). The MV performance was assessed by the minimum distance that allows the display of two scatterers positioned side-by-side, the lateral Full-Width-at-Half-Maximum (FWHM), and the Peak-Sidelobe-Level (PSL). In the tube, scatterer responses separated by down to [Formula: see text] (or 1.05λ ) were distinguished by the MV method, while the standard Delay-And-Sum (DAS) beamformers were unable to achieve such separation. Up to ninefold FWHM decrease was also measured in favor of the MV beamformer for individual echoes from MBs. The lateral distance between two scatterers impacted on their FWHM value, and additional differences in the scatterers' axial or out-of-plane position also impacted on their size and appearance. The simulation and experimental results were in agreement in terms of lateral resolution. The point scatterer study showed that the proposed MV imaging scheme provided clear resolution benefits compared to DAS. Current super-resolution methods mainly depend on DAS beamformers. Instead, the use of the MV method may provide a larger number of detected, and potentially better localized, MB scatterers.

Study of selected birth defects among American Indian/Alaska Native population: A multi-state population-based retrospective study, 1999-2007.

BACKGROUND: Higher prevalence of selected birth defects has been reported among American Indian/Alaska Native (AI/AN) newborns. We examine whether known risk factors for birth defects explain the higher prevalence observed for selected birth defects among this population. METHODS: Data from 12 population-based birth defects surveillance systems, covering a birth population of 11 million from 1999 to 2007, were used to examine prevalence of birth defects that have previously been reported to have elevated prevalence among AI/ANs. Prevalence ratios (PRs) were calculated for non-Hispanic AI/ANs and any AI/ANs (regardless of Hispanic ethnicity), adjusting for maternal age, education, diabetes, and smoking, as well as type of case-finding ascertainment surveillance system. RESULTS: After adjustment, the birth prevalence of two of seven birth defects remained significantly elevated among AI/ANs compared to non-Hispanic whites (NHWs): anotia/microtia was almost threefold higher, and cleft lip +/- cleft palate was almost 70% higher compared to NHWs. Excluding AI/AN subjects who were also Hispanic had only a negligible impact on adjusted PRs. CONCLUSIONS: Additional covariates accounted for some of the elevated birth defect prevalences among AI/ANs compared to NHWs. Exclusion of Hispanic ethnicity from the AI/AN category had little impact on birth defects prevalences in AI/ANs. NHWs serve as a viable comparison group for analysis. Birth defects among AI/ANs require additional scrutiny to identify modifiable risk and protective factors.

Attenuation Coefficients of the Individual Components of the International Electrotechnical Commission Agar Tissue-Mimicking Material.

Tissue-mimicking materials (TMMs) are widely used in quality assurance (QA) phantoms to assess the performance of ultrasound scanners. The International Electrotechnical Commission (IEC) defines the acoustic parameters of up to 10MHz. To manufacture a TMM that closely mimics the acoustical properties of small animal soft tissue at high frequencies, the acoustic properties of each of the individual component ingredients used in the IEC agar-TMM recipe need to be quantified. This study was aimed at evaluating whether the overall attenuation coefficient of the IEC agar-TMM is the linear sum of the attenuation coefficients of each of its ingredients. Eight batches of agar-based materials were manufactured with different combinations of ingredients from the IEC agar-TMM recipe. The percentage concentration of each ingredient used in the individual mixes was identical to that specified in the IEC recipe. The attenuation of each of these batches was measured over the ultrasound frequency range 12-50MHz, and the attenuation value of the agar component was subtracted from the attenuation values of the other batches. Batch attenuation values, representing the attenuation of individual components within the IEC agar-TMM, were then summated and yielded attenuation values that accurately reproduced the attenuation of the IEC agar-TMM. This information forms a valuable resource for the future development of TMMs with acoustic properties similar to those of soft tissue at high frequencies.

Super-Resolution Axial Localization of Ultrasound Scatter Using Multi-Focal Imaging.

OBJECTIVE: This paper aims to develop a method for achieving micrometre axial scatterer localization for medical ultrasound, surpassing the inherent, pulse length dependence limiting ultrasound imaging. METHODS: The method, directly translated from cellular microscopy, is based on multi-focal imaging and the simple, aberration-dependent, image sharpness metric of a single point scatterer. The localization of a point scatterer relies on the generation of multiple overlapping sharpness curves, created by deploying three foci during receive processing, and by assessing the sharpness values after each acquisition as a function of depth. Each derived curve peaks around the receive focus and the unique position of the scatterer is identified by combining the data from all curves using a maximum likelihood algorithm with a calibration standard. RESULTS: Simulated and experimental ultrasound point scatter data show that the sharpness method can provide scatterer axial localization with an average accuracy down to 10.21 m ( 21) and with up to 11.4 times increased precision compared to conventional localization. The improvements depend on the rate of change of sharpness using each focus, and the signal to noise ratio in each image. CONCLUSION: Super-resolution axial imaging from optical microscopy has been successfully translated into ultrasound imaging by using raw ultrasound data and standard beamforming. SIGNIFICANCE: The normalized sharpness method has the potential to be used in scatterer localization applications and contribute in current super-resolution ultrasound imaging techniques.

Acoustic Properties of Small Animal Soft Tissue in the Frequency Range 12-32 MHz.

Quality assurance phantoms are made of tissue-mimicking materials (TMMs) the acoustic properties of which mimic those of soft tissue. However, the acoustic properties of many soft tissue types have not been measured at ultrasonic frequencies >9 MHz. With the increasing use of high-frequency ultrasound for both clinical and pre-clinical applications, it is of increasing interest to ensure that TMMs accurately reflect the acoustic properties of soft tissue at these higher frequencies. In this study, the acoustic properties of ex vivo brain, liver and kidney samples from 50 mice were assessed in the frequency range 12-32 MHz. Measurements were performed within 6 min of euthanasia in a phosphate-buffered saline solution maintained at 37.2 ± 0.2 °C. The measured mean values for the speed of sound for all organs were found to be higher than the International Electrotechnical Commission guideline recommended value for TMMs. The attenuation coefficients measured for brain, liver and kidney samples were compared with the results of previous studies at lower frequencies. Only the measured kidney attenuation coefficient was found to be in good agreement with the International Electrotechnical Commission guideline. The information provided in this study can be used as a baseline on which to manufacture a TMM suitable for high-frequency applications.

Experimental performance assessment of the sub-band minimum variance beamformer for ultrasound imaging.

Recent progress in adaptive beamforming techniques for medical ultrasound has shown that current resolution limits can be surpassed. One method of obtaining improved lateral resolution is the Minimum Variance (MV) beamformer. The frequency domain implementation of this method effectively divides the broadband ultrasound signals into sub-bands (MVS) to conform with the narrow-band assumption of the original MV theory. This approach is investigated here using experimental Synthetic Aperture (SA) data from wire and cyst phantoms. A 7MHz linear array transducer is used with the SARUS experimental ultrasound scanner for the data acquisition. The lateral resolution and the contrast obtained, are evaluated and compared with those from the conventional Delay-and-Sum (DAS) beamformer and the MV temporal implementation (MVT). From the wire phantom the Full-Width-at-Half-Maximum (FWHM) measured at a depth of 52mm, is 16.7µm (0.08λ) for both MV methods, while the corresponding values for the DAS case are at least 24 times higher. The measured Peak-Side-lobe-Level (PSL) may reach -41dB using the MVS approach, while the values from the DAS and MVT beamforming are above -24dB and -33dB, respectively. From the cyst phantom, the power ratio (PR), the contrast-to-noise ratio (CNR), and the speckle signal-to-noise ratio (sSNR) measured at a depth of 30mm are at best similar for MVS and DAS, with values ranging between -29dB and -30dB, 1.94 and 2.05, and 2.16 and 2.27 respectively. In conclusion the MVS beamformer is not suitable for imaging continuous targets, and significant resolution gains were obtained only for isolated targets.

Broadband Acoustic Measurement of an Agar-Based Tissue-Mimicking-Material: A Longitudinal Study.

Commercially available ultrasound quality assurance test phantoms rely on the long-term acoustic stability of the tissue-mimicking-material (TMM). Measurement of the acoustic properties of the TMM can be technically challenging, and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique was proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range 4.5-50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and attenuation were very similar and within the International Electrotechnical Commission-recommended value. However, the results obtained using the proposed modified technique exhibited greater stability over the 1-y period compared with the results acquired using the standard technique.

Dynamic Enhancement of B-Mode Cardiac Ultrasound Image Sequences.

Limited contrast, along with speckle and acoustic noise, can reduce the diagnostic value of echocardiographic images. This study introduces dynamic histogram-based intensity mapping (DHBIM), a novel approach employing temporal variations in the cumulative histograms of cardiac ultrasound images to contrast enhance the imaged structures. DHBIM is then combined with spatial compounding to compensate for noise and speckle. The proposed techniques are quantitatively assessed (32 clinical data sets) employing (i) standard image quality measures and (ii) the repeatability of routine clinical measurements, such as chamber diameter and wall thickness. DHBIM introduces a mean increase of 120.9% in tissue/chamber detectability, improving the overall repeatability of clinical measurements by 17%. The integrated approach of DHBIM followed by spatial compounding provides the best overall enhancement of image quality and diagnostic value, consistently outperforming the individual approaches and achieving a 401.4% average increase in tissue/chamber detectability with an associated 24.3% improvement in the overall repeatability of clinical measurements.

Small Rodent Cardiac Phantom for Preclinical Ultrasound Imaging.

Imaging phantoms play a valuable role in the quality control and quality assurance of medical imaging systems. However, for use in the relatively new field of small-animal preclinical imaging, very few have been described in the literature, and even less or none at all are available commercially. Yet, preclinical small animal phantoms offer the possibility of reducing the need for live animals for test and measurement purposes. Human scale cardiac phantoms, both reported in the literature and available commercially, are typically complex devices. Their designs include numerous flow control valves, pumps, and servo motors. These devices are coupled to tissue mimicking materials (TMMs) shaped to replicate the form of cardiac chambers and valves. They are then operated in such a way as to cause the replica TMM heart to move in a lifelike manner. This paper describes the design and construction of a small rodent preclinical cardiac phantom, which is both of a simple design and construction. Using only readily available materials and components, it can be manufactured without the use of workshop facilities, using only hand-tools. Drawings and pictures of the design are presented along with images of the phantom in operation, using a high-frequency preclinical ultrasound scanner.

Cancer among American Indians - Identifying Priority Areas in Oklahoma.

BACKGROUND: We describe and compare cancer incidence and mortality among American Indians (AI/ANs) and whites in nine Indian Health Service (IHS) Service Units in Oklahoma. METHODS: Using data from the Oklahoma Central Cancer Registry and the web-based OK2SHARE database, we obtained age-adjusted cancer incidence rates from 1997 to 2012 and cancer mortality rates from 1999 to 2009 for AI/ANs and whites in Oklahoma. We examined differences in primary site, percentage of late stage diagnoses, and trends over time. RESULTS: AI/ANs consistently had higher cancer incidence and mortality compared to whites in Oklahoma. The magnitude of disparity for cancer incidence and mortality varied by IHS Service Unit and by gender. The top three cancer sites were the same for all Service Units. The percentage of late stage diagnosis also varied by region. CONCLUSIONS: We identify priority areas where cancer disparity challenges exist among AI/ANs in Oklahoma.

Elevational spatial compounding for enhancing image quality in echocardiography.

INTRODUCTION: Echocardiography is commonly used in clinical practice for the real-time assessment of cardiac morphology and function. Nevertheless, due to the nature of the data acquisition, cardiac ultrasound images are often corrupted by a range of acoustic artefacts, including acoustic noise, speckle and shadowing. Spatial compounding techniques have long been recognised for their ability to suppress common ultrasound artefacts, enhancing the imaged cardiac structures. However, they require extended acquisition times as well as accurate spatio-temporal alignment of the compounded data. Elevational spatial compounding acquires and compounds adjacent partially decorrelated planes of the same cardiac structure. METHODS: This paper employs an anthropomorphic left ventricle phantom to examine the effect of acquisition parameters, such as inter-slice angular displacement and 3D sector angular range, on the elevational spatial compounding of cardiac ultrasound data. RESULTS AND CONCLUSION: Elevational spatial compounding can produce substantial noise and speckle suppression as well as visual enhancement of tissue structures even for small acquisition sector widths (2.5° to 6.5°). In addition, elevational spatial compounding eliminates the need for extended acquisition times as well as the need for temporal alignment of the compounded datasets. However, moderate spatial registration may still be required to reduce any tissue/chamber blurring side effects that may be introduced.

Assessment of Spectral Doppler for an Array-Based Preclinical Ultrasound Scanner Using a Rotating Phantom.

Velocity measurement errors were investigated for an array-based preclinical ultrasound scanner (Vevo 2100, FUJIFILM VisualSonics, Toronto, ON, Canada). Using a small-size rotating phantom made from a tissue-mimicking material, errors in pulse-wave Doppler maximum velocity measurements were observed. The extent of these errors was dependent on the Doppler angle, gate length, gate depth, gate horizontal placement and phantom velocity. Errors were observed to be up to 172% at high beam-target angles. It was found that small gate lengths resulted in larger velocity errors than large gate lengths, a phenomenon that has not previously been reported (e.g., for a beam-target angle of 0°, the error was 27.8% with a 0.2-mm gate length and 5.4% with a 0.98-mm gate length). The error in the velocity measurement with sample volume depth changed depending on the operating frequency of the probe. Some edge effects were observed in the horizontal placement of the sample volume, indicating a change in the array aperture size. The error in the velocity measurements increased with increased phantom velocity, from 22% at 2.4 cm/s to 30% at 26.6 cm/s. To minimise the impact of these errors, an angle-dependent correction factor was derived based on a simple ray model of geometric spectral broadening. Use of this angle-dependent correction factor reduces the maximum velocity measurement errors to <25% in all instances, significantly improving the current estimation of maximum velocity from pulse-wave Doppler ultrasound.

Temporal compounding: a novel implementation and its impact on quality and diagnostic value in echocardiography.

Temporal compounding can be used to suppress acoustic noise in transthoracic cardiac ultrasound by spatially averaging partially decorrelated images acquired over consecutive cardiac cycles. However, the reliable spatial and temporal alignment of the corresponding frames in consecutive cardiac cycles is vital for effective implementation of temporal compounding. This study introduces a novel, efficient, accurate and robust technique for the spatiotemporal alignment of consecutive cardiac cycles with variable temporal characteristics. Furthermore, optimal acquisition parameters, such as the number of consecutive cardiac cycles used, are derived. The effect of the proposed implementation of temporal compounding on cardiac ultrasound images is quantitatively assessed (32 clinical data sets providing a representative range of image qualities and diagnostic values) using measures such as tissue signal-to-noise ratio, chamber signal-to-noise ratio, tissue/chamber contrast and detectability index, as well as a range of clinical measurements, such as chamber diameter and wall thickness, performed during routine echocardiographic examinations. Temporal compounding (as implemented) consistently improved the image quality and diagnostic value of the processed images, when compared with the original data by: (i) increasing tissue and cavity signal-to-noise ratios as well as tissue/cavity detectability index, (ii) improving the corresponding clinical measurement repeatability and inter-operator measurement agreement, while (iii) reducing the number of omitted measurements caused by data corruption.

High resolution depth-resolved imaging from multi-focal images for medical ultrasound.

An ultrasound imaging technique providing sub-diffraction limit axial resolution for point sources is proposed. It is based on simultaneously acquired multi-focal images of the same object, and on the image metric of sharpness. The sharpness is extracted by image data and presents higher values for in-focus images. The technique is derived from biological microscopy and is validated here with simulated ultrasound data. A linear array probe is used to scan a point scatterer phantom that moves in depth with a controlled step. From the beamformed responses of each scatterer position the image sharpness is assessed. Values from all positions plotted together form a curve that peaks at the receive focus, which is set during the beamforming. Selection of three different receive foci for each acquired dataset will result in the generation of three overlapping sharpness curves. A set of three calibration curves combined with the use of a maximum-likelihood algorithm is then able to estimate, with high precision, the depth location of any emitter fron each single image. Estimated values are compared with the ground truth demonstrating that an accuracy of 28.6 µm (0.13λ) is achieved for a 4 mm depth range.

Wall-less flow phantom for high-frequency ultrasound applications.

There are currently very few test objects suitable for high-frequency ultrasound scanners that can be rapidly manufactured, have appropriate acoustic characteristics and are suitably robust. Here we describe techniques for the creation of a wall-less flow phantom using a physically robust konjac and carrageenan-based tissue-mimicking material. Vessel dimensions equivalent to those of mouse and rat arteries were achieved with steady flow, with the vessel at a depth of 1.0 mm. We then employed the phantom to briefly investigate velocity errors using pulsed wave Doppler with a commercial preclinical ultrasound system. This phantom will provide a useful tool for testing preclinical ultrasound imaging systems.

acoustic assessment of a konjac­carrageenan tissue-mimicking material aT 5­60 MHZ.

The acoustic properties of a robust tissue-mimicking material based on konjac­carrageenan at ultrasound frequencies in the range 5­60 MHz are described. Acoustic properties were characterized using two methods: a broadband reflection substitution technique using a commercially available preclinical ultrasound scanner (Vevo 770, FUJIFILM VisualSonics, Toronto, ON, Canada), and a dedicated high-frequency ultrasound facility developed at the National Physical Laboratory (NPL, Teddington, UK), which employed a broadband through-transmission substitution technique. The mean speed of sound across the measured frequencies was found to be 1551.7 ± 12.7 and 1547.7 ± 3.3 m s21, respectively. The attenuation exhibited a non-linear dependence on frequency, f (MHz), in the form of a polynomial function: 0.009787f2 1 0.2671f and 0.01024f2 1 0.3639f, respectively. The characterization of this tissue-mimicking material will provide reference data for designing phantoms for preclinical systems, which may, in certain applications such as flow phantoms, require a physically more robust tissuemimicking material than is currently available.