Vaibhav Kakkad

In Vivo Application of Short-Lag Spatial Coherence and Harmonic Spatial Coherence Imaging in Fetal Ultrasound

Fetal scanning is one of the most common applications of ultrasound imaging and serves as a source of vital information about maternal and fetal health. Visualization of clinically relevant structures, however, can be severely compromised in difficult-to-image patients due to poor resolution and the presence of high levels of acoustical noise or clutter. We have developed novel coherence-based beamforming methods called Short-Lag Spatial Coherence (SLSC) imaging and Harmonic Spatial Coherence imaging (HSCI), and applied them to suppress the effects of clutter in fetal imaging. This method is used to create images of the spatial coherence of the backscattered ultrasound as opposed to images of echo magnitude. We present the results of a patient study to assess the benefits of coherence-based beamforming in the context of first trimester fetal exams. Matched fundamental B-mode, SLSC, harmonic B-mode, and HSCI images were generated using raw radio frequency data collected on 11 volunteers in the first trimester of pregnancy. The images were compared for qualitative differences in image texture and target conspicuity as well as using quantitative imaging metrics such as signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and contrast. SLSC and HSCI showed statistically significant improvements across all imaging metrics compared with B-mode and harmonic B-mode, respectively. These improvements were greatest for poor quality B-mode images where contrast of anechoic targets was improved from 15 dB in fundamental B-mode to 27 dB in SLSC and 17 dB in harmonic B-mode to 30 dB in HSCI. CNR improved from 1.4 to 2.5 in the fundamental images and 1.4 to 3.1 in the harmonic case. These results exhibit the potential of coherence-based beamforming to improve image quality and target detectability, especially in high noise environments.

In vivo performance evaluation of short-lag spatial coherence and harmonic spatial coherence imaging in fetal ultrasound

Ultrasonography is the primary imaging modality used for diagnostic assessment of fetal and maternal health during pregnancy. However, a large proportion of fetal ultrasound exams suffer from inadequate or lack of visualization of clinically relevant structures due to acoustical noise or clutter. These undesirable effects are even more pronounced in overweight and obese mothers. In previous work our group has introduced short-lag spatial coherence (SLSC) and harmonic spatial coherence imaging (HSCI) as methods for clutter reduction. These have been shown to be successful in improving image contrast and border delineation in both simulations and in vivo experiments on liver and cardiac imaging. We extend the application of these techniques to fetal ultrasound imaging. We collected individual channel RF data on 11 volunteers in their first trimester of pregnancy. The amount of intrinsic clutter varied across the volunteers. These data sets were used to generate matched fundamental B-mode, SLSC, harmonic B-mode, and HSCI images. These images were compared qualitatively by assessing image texture and target detectability and quantitatively using SNR, CNR, and contrast. SLSC and HSCI images showed significant improvements across all imaging metrics compared to B-mode and harmonic B-mode, respectively. The improvements in clutter suppression and improved conspicuity of target structures were greatest for poor quality B-mode images. Clutter levels in these images, measured in a region of amniotic fluid compared to a region of uniform speckle, were measured to be 15 dB for B-mode versus 27 dB for SLSC, and 17 dB for harmonic B-mode versus 30 dB for HSCI. CNR for the same set of images improved from 1.4 to 2.5 for B-mode to SLSC and from 1.4 to 3.1 for harmonic B-mode to HSCI.

Effect of Transmit Beamforming on Clutter Levels in Transthoracic Echocardiography

Transmit beamforming has a strong impact on several factors that govern image quality, field-of-view, and frame-rate in ultrasound imaging. For cardiac applications, the visualization of fine structures and the ability to track their motion is equally important. Consequently, beamforming choices for echocardiography aim to optimize these trade-offs. Acoustic clutter can dramatically impact image quality and degrade the diagnostic value of cardiac ultrasound imaging. Clutter levels, however, are closely tied to the choice of beamforming configuration. This study aims to quantify the impact of transmit beamforming on clutter levels under in vivo conditions. The performance of focused as well as plane wave transmit configurations in fundamental and harmonic modes is evaluated under matched conditions. Contrast between the cardiac chambers and the interventricular septum is used as a surrogate for the level of clutter in a given imaging scenario. Under in vivo conditions, contrast was found to improve incrementally across the four beamforming configurations in the following order: fundamental-plane, fundamental-focused, harmonic-plane, and harmonic-focused. Using the fundamental-focused configuration as a reference, the harmonic-plane and harmonic-focused cases showed improvements in median contrast of 2.97 dB and 6.1 dB, respectively, while the fundamental-plane case showed a contrast deterioration of 1.23 dB. Contrast was also found to vary systematically as a function of imaging depth. Median contrast for the right ventricle (shallow chamber) was measured to be 2.96 dB lower than that in the left ventricle (deep chamber).

A novel strain-based drift compensation algorithm for improved beat-to-beat repeatability of myocardial strain imaging: Preliminary in vivo results

Strain imaging is gaining traction as a means to assess cardiac function by tracking the cyclic deformation of the myocardium. Compared to traditional measures such as ejection fraction, global myocardial strain has been shown to be an earlier and more sensitive measure of overall ventricular function. Similarly, regional myocardial strain has been shown to be useful for identifying ischemia and myocardial infarction. However, accurately quantifying myocardial strain over several cardiac cycles has proven challenging due to complications with drift and heart rate variability.

Clinical feasibility of a noninvasive method to interrogate myocardial function via strain and acoustic radiation force-derived stiffness

Myocardial elastance, derived from pressure-volume (PV) loops in the left ventricle (LV), can be used to assess LV function and myocardial performance. This method requires an invasive intracardiac pressure-volume catheter to be inserted in the LV, limiting the methods utility in clinical screening and monitoring. Strain echocardiography and cardiac acoustic radiation force impulse (ARFI) imaging are ultrasonic techniques to noninvasively asses myocardial function by tracking the deformation through the cardiac cycle and the tissue response to repeated impulsive acoustic radiation force, respectively. Strain and ARFI imaging have largely been used in separate contexts. Our group has previously demonstrated the use of cardiac ARFI and strain in an invasive open-chest animal study, but this is the first work to examine the relationship between ARFI-derived stiffness and strain in a noninvasive in vivo clinical study.

Parameters impacting accuracy of ARFI-derived stiffness ratios: A simulation study with implications on measurement of dynamic myocardial stiffness

Measurement of myocardial stiffness using ultrasound-based transient elastography has received significant attention in recent years. The mechanical properties of myocardium not only undergo cyclic dynamic changes over the cardiac cycle but also change on a slower scale with physiological factors. They are also closely tied to several cardiovascular disorders such as heart failure, cardio-toxicity and transplant rejection. SWEI allows for quantitative assessment of tissue mechanical properties. However, in the context of cardiac imaging it has only been shown to be feasible in diastole. ARFI, on the other hand, provides a relative estimate of tissue mechanical properties and has been shown to be viable over the entire cardiac cycle. ARFI-derived myocardial stiffness ratios have been investigated as a potential clinical metric of myocardial function. While this ratio is indicative of the change in stiffness of the myocardium over the cardiac cycle, its quantitative relationship to absolute material properties has yet to be thoroughly investigated.

Spatio-temporal consistency of transthoracic ARFI-derived metrics of myocardial function

Cardiac function has traditionally been studied using pressure-volume (PV) loop analysis. PV loops can be used to derive functional indices such as end-systolic pressure volume relationship (ESPVR)—a measure of myocardial contractility, end-diastolic pressure volume relationship (EDPVR) — a measure of myocardial compliance and relaxation time constant (τ) — a measure of left ventricular relaxation. However, PV loop analysis is largely an invasive proedure; it requires catherization and only provides a global assessment of cardiac function. Transthoracic ARFI (TTE ARFI) has been shown to capture the dynamic trends of myocardial stiffness over the cardiac cycle in a noninvasive manner. In this work we explore the potential of using myocardial stiffness measurements made using TTE ARFI to derive functional indices similar to the ones measured using PV loops. We studied parameters impacting the spatio-temporal stability and reliability of TTE ARFI-derived measures of myocardial function.

In vivo transthoracic measurements of acoustic radiation force induced displacements in the heart over the cardiac cycle

Myocardial elasticity is an important indicator of cardiac function and is affected in many disorders associated with heart failure. Ultrasound based interrogation of cardiac stiffness has been extensively studied in ex-vivo, open chest and intracardiac imaging environments. The ability to make these measurements robustly through non-invasive means such as transthoracic imaging would make them more clinically viable and widely applicable. However, transthoracic imaging is a challenging environment for displacement estimation due to poor SNR, acoustic clutter and complex cardiac motion. This work aims to address some of those challenges on a clinical ultrasound system. Sequences to make M-mode measurements of acoustic radiation force induced displacements in the heart over the entire cardiac cycle were implemented on the Siemens SC2000 and a cardiac phased array probe. Pulse inversion harmonic tracking was employed on the tracking beams to suppress the effect of stationary clutter on displacement estimation. Two families of motion filters, high pass filters and polynomial fit filters were analyzed for their performance in being able to remove the background cardiac motion and isolate the radiation force induced tissue response. Clinical data was acquired on 4 subjects and analyzed for repeatability of diastolic vs. systolic displacements. A high pass filter with a cutoff of 100 Hz and a 2nd order polynomial fit filter were found to be equally effective in suppressing intrinsic motion. Diastolic-to-systolic displacement ratios measured in the interventricular septum ranged from 1.3 to 2.2 across subjects but were found to be fairly consistent between the parasternal long axis and the parasternal short axis views for each subject.