Browsing by Author "von Ramm, Olaf T"
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Item Open Access Automatic Volumetric Analysis of the Left Ventricle in 3D Apical Echocardiographs(2015) Wald, Andrew JamesApically-acquired 3D echocardiographs (echoes) are becoming a standard data component in the clinical evaluation of left ventricular (LV) function. Ejection fraction (EF) is one of the key quantitative biomarkers derived from echoes and used by echocardiographers to study a patient's heart function. In present clinical practice, EF is either grossly estimated by experienced observers, approximated using orthogonal 2D slices and Simpson's method, determined by manual segmentation of the LV lumen, or measured using semi-automatic proprietary software such as Philips QLab-3DQ. Each of these methods requires particular skill by the operator, and may be time-intensive, subject to variability, or both.
To address this, I have developed a novel, fully automatic method to LV segmentation in 3D echoes that offers EF calculation on clinical datasets at the push of a button. The solution is built on a pipeline that utilizes a number of image processing and feature detection methods specifically adopted to the 3D ultrasound modality. It is designed to be reasonably robust at handling dropout and missing features typical in clinical echocardiography. It is hypothesized that this method can displace the need for sonographer input, yet provide results statistically indistinguishable from those of experienced sonographers using QLab-3DQ, the current gold standard that is employed at Duke University Hospital.
A pre-clinical validation set, which was also used for iterative algorithm development, consisted of 70 cases previously seen at Duke. Of these, manual segmentations of 7 clinical cases were compared to the algorithm. The final algorithm predicts EF within ± 0.02 ratio units for 5 of them, and ± 0.09 units for the remaining 2 cases, within common clinical tolerance. Another 13 of the cases, often used for sonographer training and rated as having good image quality, were analyzed using QLab-3DQ, in which 11 cases showed concordance (± 0.10) with the algorithm. The remaining 50 cases retrospectively recruited at Duke and representative of everyday image quality showed 62% concordance (± 0.10) of QLab-3DQ with the algorithm. The fraction of concordant cases is highly dependent on image quality, and concordance improves greatly upon disqualification of poor quality images. Visual comparison of the QLab-3DQ segmentation to my algorithm overlaid on top of the original echoes also suggests that my method may be preferable or of high utility even in cases of EF discordance. This paper describes the algorithm and offers justifications for the adopted methods. The paper also discusses the design of a retrospective clinical trial now underway at Duke with 60 additional unseen cases intended only for independent validation.
Item Open Access Harmonic source wavefront aberration correction for ultrasound imaging.(2010) Dianis, Scott W.Aberration is a correctable phenomenon that degrades diagnostic quality in a significant number of ultrasound images. Previous aberration correction studies have focused on development of aberration estimation algorithms or on aberration reduction by using harmonic imaging. In the past, a major drawback of aberration estimation algorithms has been the assumptions required about the imaging target, assumptions that can limit clinical application where correction for multiple locations within a scan may be required. Harmonic imaging attempts to reduce the effect of aberration, without making assumptions about the imaging target, by using a lower-frequency transmit beam that is less prone to aberration. However, harmonic imaging does not correct for any aberration that may remain. It is hypothesized that a harmonic source wavefront correction technique is capable of creating a point-like acoustical source that allows for estimation and correction of two-dimensional aberration in a clinical setting. Harmonic source wavefront correction utilizes the reduced aberration of harmonic imaging to create a known acoustical source to satisfy the assumptions of the aberration estimation algorithms, thus improving their clinical application. Generation of a point-like acoustical source in the presence of aberration is demonstrated using both spatially correlated and spatially uncorrelated electronic aberrators varying in strength from 0.25π radians to 1.16π radians RMS focusing error. Beam properties of the 2.08 MHz fundamental, 4.16 MHz generated harmonic, and 4.17 MHz imaging beams were compared; in the presence of aberration, relative peak beam amplitude of the 4.16 MHz generated harmonic beam was up to 81% higher than the 4.17 MHz imaging beam, while -6 dB beam width indicated the 4.16 MHz generated harmonic beam was 88% narrower and more point-like than the 2.08 MHz fundamental beam. The feasibility of harmonic source wavefront correction was demonstrated by correcting for spatially uncorrelated electronic aberrators in a water tank using a point target, specular reflector, and speckle region as correction targets. Harmonic source wavefront correction was paired with a cross-correlation algorithm to estimate corrective delays and was most effective in correcting peak amplitude of the 4.17 MHz imaging beam using a point target (up to 94% improvement), followed by use of a specular reflector (up to 83% improvement), followed by use of a speckle region (up to 47% improvement). Aberration correction is sensitive to signal-to-noise ratio (SNR),and correction utilizing the 2.08 MHz fundamental, which provided higher SNR, was more effective than correction utilizing the more point-like 4.16 MHz harmonic for the experimental setup used. A harmonic SNR of 14 dB was estimated as necessary for harmonic-based correction performance to equal or surpass fundamental-based correction, regardless of fundamental SNR. Finally, performance of harmonic source wavefront correction was quantified in a clinical setting. Correction of spatially correlated electronic aberrators was performed using both ex vivo porcine kidneys and the left kidneys of 11 human volunteers as correction targets. Correction utilizing porcine kidney resulted in 10 dB greater improvement in peak beam amplitude than correction utilizing the left kidney of human volunteers. Body wall aberration present in the human volunteers was not accounted for during correction and likely caused the disparity in correction performance. An average upper limit for body wall aberration for the human subjects was estimated at 65 ns (±9 ns) RMSItem Open Access Piezoelectric Micromachined Ultrasound Transducers for Medical Imaging(2011) Chou, Derrick RenyuPiezoelectric micromachined ultrasound transducer (pMUT) two-dimensional (2D) arrays have been proposed as an alternative to conventional bulk-PZT thickness-mode transducers for high frequency, forward-looking, catheter-based ultrasound imaging of the cardiovascular system. The appeal of pMUTs is based on several key advantages over conventional transducer technologies, including high operational frequencies, small element size, and low cost due to their microelectromechanical system (MEMS) silicon-based fabrication. While previous studies have demonstrated acoustic performance characteristics suitable for ultrasound image formation, pulse-echo B-mode imaging of tissue and tissue-like phantoms using 2D pMUT arrays small enough for forward-looking catheter-based applications have been demonstrated only at Duke University by Dausch et al.
Having demonstrated the suitability of 2D pMUT arrays for tissue imaging, an important step is to demonstrate effective design control. The frequency of operation is a fundamental component of transducer design. Previous modeling efforts for pMUT vibration have used classical/Kirchoff thin plate theory (CPT) or Mindlin thick plate theory, however pMUTs with geometric dimensions similar to those explored here, have not been modeled with experimental comparison to physical devices.
It is hypothesized that the frequency of vibration of pMUTs can be predictively modeled based on experimental data from various pMUT configurations. Experimental frequency results were acquired and used to develop an empirical model based on a modified Mindlin thick plate theory. This dissertation presents the development of the frequency design theory culminating in a set of predictive design equations for the frequency of vibration of 2D pMUT arrays aimed at improving their use in high-frequency, forward-looking, catheter-based ultrasound imaging applications.
Item Open Access Techniques for Quantitation of Left Ventricular Volume in Ultrasound Using 4DViz(2012) Guo, YuanIn the United States, heart failure is a leading cause of hospitalization. The medical industry places great emphasis on diagnosing heart disease through cardiac metrics like ejection fraction. Left ventricular ejection fraction is a commonly used diagnostic indicator for heart efficiency and is measured with echocardiography through different volume calculation techniques. However, ejection fraction results can drastically vary from one examiner to another. Generally cardiologists still give ejection fraction measurements a plus or minus 10 percent error range.
A program developed at Duke called 4DViz is robust enough for users to process 3D ultrasound data. 4DViz allows examiners to determine heart chamber volumes by constructing a surface model over an imaged heart chamber with many mouse click inputs. Through 4DViz programming, a viable approach for calculating ejection fraction is attempted in this thesis. Using feature tracking, surface drawing, and voxel filling, the new approach aims to reduce examiner input and improve ejection fraction consistency. Water filled balloons were used to calibrate the algorithm's parameters. In testing, several volunteers were asked to use the 4DViz. Their results are compared to volume measurements where user input was standard. The results show promise and may remove some of the inconsistency behind ejection fraction measurements.
Item Open Access Temporal Processing of High Frame Rate Ultrasound Images(2017) Moore, John CooperHigh frame rate ultrasound imaging is a nascent field that holds the potential to bridge the gap between strictly anatomic imaging and functional, or physiologic, imaging. Other currently available functional imaging methods in ultrasound, such as Doppler, ARFI, and SWEI, require transmit-receive sequences different than used in standard B mode or volumetric imaging, requiring a reduction in imaging rate. Using the Duke University Phased Array System, T5, a high frame rate imaging scheme was developed for live acquisition of adult echocardiographic images at rates up to 1000 per second.
It is hypothesized that with higher imaging rates low signal level, rapidly moving blood flowing through vessels can be differentially extracted from B mode or volumetric images and provide enhanced contrast with respect to high signal surrounding or overlapping stationary or slowly moving structures. TO achieve this goal, an online method of subtracting sequential images was developed for the T5 system. The origin of the increase in contrast is investigated, and the statistical model of speckle in B mode images is extended to include the changes in brightness in a fixed location in the image field as targets pass through that location. Further studies were directed at increasing the contrast between blood and surrounding tissues by summation of difference images and the limitations of such summing for cyclical blood flow.
It is concluded that:
A) With the T5 system, high images rates are feasible without significant reduction of image resolution or signal to noise.
B) Statistical targets, or speckle, have a pixel-by-pixel linear relationship between successive image frames between changes in brightness and amount of translation up to the diffraction limited resolution in azimuth and proportional to the pulse length in range. Then, brightness within a given pixel is statistically independent from the brightness with the same pixel in temporally subsequent images.
C) The point at which the brightness in a given pixel from frame to frame is statistically independent in a given pixel from frame to frame is directly related to the resolution of the configured ultrasound system.
D) Subtracted images of independent speckle patterns are statistically independent with each other and summation of such images within physiological constraints in time will further increase the contrast of blood with respect to vessel walls and surrounding tissue.
E) Within physiologic constraints, time domain processing methods can be used to increase image contrast of rapidly moving targets and suppress the appearance of slowly moving, transiently moving, or stationary targets.