Muyinatu A. Lediju

Short-lag spatial coherence of backscattered echoes: imaging characteristics

Conventional ultrasound images are formed by delay-and-sum beamforming of the backscattered echoes received by individual elements of the transducer aperture. Although the delay-and-sum beamformer is well suited for ultrasound image formation, it is corrupted by speckle noise and challenged by acoustic clutter and phase aberration. We propose an alternative method of imaging utilizing the short-lag spatial coherence (SLSC) of the backscattered echoes. Compared with matched B-mode images, SLSC images demonstrate superior SNR and contrast-to-noise ratio in simulated and experimental speckle-generating phantom targets, but are shown to be challenged by limited point target conspicuity. Matched B-mode and SLSC images of a human thyroid are presented. The challenges and opportunities of real-time implementation of SLSC imaging are discussed.

Lesion Detectability in Diagnostic Ultrasound with Short-Lag Spatial Coherence Imaging

We demonstrate a novel imaging technique, named short-lag spatial coherence (SLSC) imaging, which uses short distance (or lag) values of the coherence function of backscattered ultrasound to create images. Simulations using Field II are used to demonstrate the detection of lesions of varying sizes and contrasts with and without acoustical clutter in the backscattered data. B-mode and SLSC imaging are shown to be nearly equivalent in lesion detection, based on the contrast-to-noise ratio (CNR) of the lesion, in noise-free conditions. The CNR of the SLSC image, however, can be adjusted to achieve an optimal value at the expense of image smoothness and resolution. In the presence of acoustic clutter, SLSC imaging yields significantly higher CNR than B-mode imaging and maintains higher image quality than B-mode with increasing noise. Compression of SLSC images is shown to be required under high-noise conditions but is unnecessary under no- and low-noise conditions. SLSC imaging is applied to in vivo imaging of the carotid sheath and demonstrates significant gains in CNR as well as visualization of arterioles in the carotid sheath. SLSC imaging has a potential application to clutter rejection in ultrasonic imaging.

Quantitative Assessment of the Magnitude, Impact and Spatial Extent of Ultrasonic Clutter

Clutter is a noise artifact in ultrasound images that appears as diffuse echoes overlying signals of interest. It is most easily observed in anechoic or hypoechoic regions, such as in cysts, blood vessels, amniotic fluid, and urine-filled bladders. Clutter often obscures targets of interest and complicates anatomical measurements. An analytical expression that characterizes the extent to which clutter degrades lesion contrast was derived and compared to the measured contrast loss due to clutter in a bladder phantom. Simulation and phantom studies were performed to determine ideal and achievable signal-to-clutter ratios. In vivo clutter magnitudes were quantified in simultaneously-acquired fundamental and harmonic bladder images from five volunteers. Clutter magnitudes ranged from −30 dB to 0 dB, relative to the mean signal of the bladder wall. For this range of clutter magnitudes, the analytical expression predicts a contrast loss of 0–45 dB for lesions with clutter-free contrasts of 6–48 dB. A pixel-wise comparison of simultaneously-acquired fundamental and harmonic bladder images from each volunteer revealed an overall signal reduction in harmonic images, with average reductions ranging from 11–18 dB in the bladder interior and 9–11 dB in the tissue surrounding the bladder. Harmonic imaging did not reduce clutter in all volunteers.

A motion-based approach to abdominal clutter reduction

In ultrasound images, clutter is a noise artifact most easily observed in anechoic or hypoechoic regions. It appears as diffuse echoes overlying anatomical structures of diagnostic importance, obscuring tissue borders and reducing image contrast. A novel clutter reduction method for abdominal images is proposed, wherein the abdominal wall is displaced during successive-frame image acquisitions. A region of clutter distal to the abdominal wall was observed to move with the abdominal wall, and finite impulse response (FIR) and blind source separation (BSS) motion filters were implemented to reduce this clutter. The proposed clutter reduction method was tested in simulated and phantom data and applied to fundamental and harmonic in vivo bladder and liver images from 2 volunteers. Results show clutter reductions ranging from 0 to 18 dB in FIR-filtered images and 9 to 27 dB in BSS-filtered images. The contrast-to-noise ratio was improved by 21 to 68% and 44 to 108% in FIR- and BSS-filtered images, respectively. Improvements in contrast ranged from 4 to 12 dB. The method shows promise for reducing clutter in other abdominal images.

Sources and characterization of clutter in cardiac B-mode images

In echocardiography, clutter is one of the most problematic image artifacts, often obscuring ventricular borders and introducing stationary noise in blood flow measurements. Clutter in transthoracic cardiac images is widely understood to originate from reverberations and off-axis echoes. The objective of this work is to investigate the sources of clutter in cardiac images and their relative contributions. Real-time 3D raw echo data was acquired at a volumetric frame rate of 1 kHz and speckle tracking was applied to resulting images to determine the motion characteristics of clutter and adjacent myocardium. When clutter adjacent to the myocardial wall was tracked, the clutter and adjacent myocardium had similar displacements. When clutter farther from the myocardial wall was tracked, displacements were temporally and spatially complex and did not correlate well with any portion of the myocardium. In addition, principal component analysis (PCA) was applied to the raw echo data and resulting eigenvectors were used to isolate various motion patterns in the cardiac data. Results support the hypothesis that echoes from stationary structures, such as the ribcage and chest wall, are contributors to stationary clutter noise, while the myocardium is a dominant source of nonstationary clutter.

Magnitude, origins, and reduction of abdominal ultrasonic clutter

Clutter is a noise artifact in ultrasound images, arising from multiple sources. Experiments were conducted with urine-filled in vivo bladders to differentiate among various clutter sources. Successive-frame image acquisitions with varying transmit PRFs were used to determine the clutter contributions from echoes of previously sent pulses and random electronic and/or acoustic noise. Images acquired during axial displacement of the abdominal wall were assessed to distinguish clutter due to near-field reverberation from clutter due to off-axis scattering. The results indicate that clutter contributions from random noise and echoes of previously sent pulses are weak, while clutter arising from sound reverberation in abdominal tissues are dominant distal to the abdominal wall. Clutter adjacent to the distal and lateral bladder walls is mainly due to off-axis scattering. Clutter was reduced in fundamental and harmonic bladder images by applying Finite Impulse Response (FIR) and Blind Source Separation (BSS) motion filters to images acquired during axial displacement of the abdominal wall.

Simulation and experimental analysis of ultrasonic clutter in fundamental and harmonic imaging

Harmonic imaging has been shown to yield significant improvements in image quality over conventional ultrasound imaging. It has been proposed that harmonic imaging generates these improvements by the reduction in clutter from reverberation in the tissue layers underlying the transducer, a reduction in beam distortion from aberration, and a reduction in clutter due to suppressed sidelobes. There is little research indicating the exact sources of clutter and how they may relate to the improvements observed with in vivo harmonic imaging. We describe simulation and experimental studies in human bladders describing the sources and characteristics of clutter and discuss their relationship to the above proposed mechanisms. The results indicate that a large source of clutter is the product of reverberation in the abdominal layers. Experimental and simulated harmonic images indicate a 3-5 and 3-8 dB reduction in clutter over fundamental images, respectively, in the upper bladder cavity, lending support for the first mechanism described above. Scattering was also observed from off-axis sources in both the fundamental and harmonic images. Simulations of the fundamental point-spread-function (PSF) showed clutter magnitudes of -43 dB in the isochronous volume. Harmonic imaging marginally improved clutter magnitude to -47 dB in this same region. When aberration was removed from the simulation while keeping the impedance constant, the isochronous volume in the fundamental PSF marginally improved to -47 dB, while harmonic imaging improved this region to -58 dB, a reduction of 11 dB. This indicates that the image quality improvements seen with harmonic imaging are more dependent on the reduction in clutter from near-field layers than with reductions in clutter due to aberration.

Short-lag spatial coherence imaging

Conventional ultrasound images are formed by delay-and-sum beamforming of the backscattered echoes received by the transducer aperture. Although the delay-and-sum beamformer is well suited for ultrasound image formation, it is challenged under non-ideal circumstances, such as the presence of acoustic clutter and phase aberration. We propose an alternative method of imaging utilizing the short-lag spatial coherence of the backscattered echoes. Compared to matched B-mode images, short-lag spatial coherence (SLSC) images demonstrate superior SNR and CNR in simulated speckle-generating phantom targets. Similar results are achieved in matched experimental B-mode and SLSC images of a speckle-generating phantom target, a human thyroid, and a sheep heart.

A Novel Imaging Technique Based on the Spatial Coherence of Backscattered Waves: Demonstration in the Presence of Acoustical Clutter

In the last 20 years, the number of suboptimal and inadequate ultrasound exams has increased. This trend has been linked to the increasing population of overweight and obese individuals. The primary causes of image degradation in these individuals are often attributed to phase aberration and clutter. Phase aberration degrades image quality by distorting the transmitted and received pressure waves, while clutter degrades image quality by introducing incoherent acoustical interference into the received pressure wavefront. Although significant research efforts have pursued the correction of image degradation due to phase aberration, few efforts have characterized or corrected image degradation due to clutter. We have developed a novel imaging technique that is capable of differentiating ultrasonic signals corrupted by acoustical interference. The technique, named short-lag spatial coherence (SLSC) imaging, is based on the spatial coherence of the received ultrasonic wavefront at small spatial distances across the transducer aperture. We demonstrate comparative B-mode and SLSC images using full-wave simulations that include the effects of clutter and show that SLSC imaging generates contrast-to-noise ratios (CNR) and signal-to-noise ratios (SNR) that are significantly better than B-mode imaging under noise-free conditions. In the presence of noise, SLSC imaging significantly outperforms conventional B-mode imaging in all image quality metrics. We demonstrate the use of SLSC imaging in vivo and compare B-mode and SLSC images of human thyroid and liver.