Current methods of cardiac strain imaging at high frame rate suffer from motion matching artifacts or poor lateral resolution. Coherent compounding has been shown to improve echocardiographic image quality while maintaining high frame rate but has never been used to image cardiac strain. However, myocardial velocity can have an impact on coherent compounding due to displacements between frames. The objective of this study was to investigate the feasibility and performance of coherent compounding for cardiac strain imaging at a low and a high myocardial velocity. Left ventricular contraction in short-axis view was modeled as an annulus with radial thickening and circumferential rotation. Simulated radiofrequency channel data with a cardiac phased array were obtained using three different beamforming methods: single diverging wave, coherent compounding of diverging waves and conventional focusing. Axial and lateral displacements and strains as well as radial strains were estimated and compared to their true value. In vivo feasibility of cardiac strain imaging with coherent compounding was performed and compared to single diverging wave imaging. At low myocardial velocities, the axial, lateral and radial strain relative error for 9 compounded waves (16.3%, 40.4%, 18.9%) were significantly lower than those obtained with single diverging wave imaging (19.9%, 80.3%, 30.6%) and closer to that obtained with conventional focusing (16.7%, 43.7%, 16.0%). In vivo left-ventricular radial strains exhibited higher quality with 9 compounded waves than with single diverging wave imaging. These results indicate that cardiac strain can be imaged using coherent compounding of diverging waves with a better performance than with single diverging wave imaging while maintaining a high frame rate and therefore has the potential to improve diagnosis of myocardial strain-based cardiac diseases.

Coherent compounding methods using the full or partial transmit aperture have been investigated as a possible means of increasing strain measurement accuracy in cardiac strain imaging; however, the optimal transmit parameters in either compounding approach have yet to be determined. The relationship between strain estimation accuracy and transmit parameters-specifically the subaperture, angular aperture, tilt angle, number of virtual sources, and frame rate-in partial aperture (subaperture compounding) and full aperture (steered compounding) fundamental mode cardiac imaging was thus investigated and compared. Field II simulation of a 3-D cylindrical annulus undergoing deformation and twist was developed to evaluate accuracy of 2-D strain estimation in cross-sectional views. The tradeoff between frame rate and number of virtual sources was then investigated via transthoracic imaging in the parasternal short-axis view of five healthy human subjects, using the strain filter to quantify estimation precision. Finally, the optimized subaperture compounding sequence (25-element subperture, 90° angular aperture, 10 virtual sources, 300-Hz frame rate) was compared to the optimized steered compounding sequence (60° angular aperture, 15° tilt, 10 virtual sources, 300-Hz frame rate) via transthoracic imaging of five healthy subjects. Both approaches were determined to estimate cumulative radial strain with statistically equivalent precision (subaperture compounding E(SNRe %) = 3.56, and steered compounding E(SNRe %) = 4.26).

Radio frequency (RF) ablation of the myocardium is used to treat various cardiac arrhythmias. The size, spacing, and transmurality of lesions have been shown to affect the success of the ablation procedure; however, there is currently no method to directly image the size and formation of ablation lesions in real time. Intracardiac myocardial elastography (ME) has been previously used to image the decrease in cardiac strain during systole in the ablated region as a result of the lesion formation. However, the feasibility of imaging multiple lesions and identifying the presence of gaps between lesions has not yet been investigated. In this paper, RF ablation lesions ( ) were generated in the left ventricular epicardium in three anesthetized canines. Two sets of two lesions each were created in close proximity to one another with small gaps (1.5 and 4 cm), while one set of two lesions was created directly next to each other with no gap. A clinical intracardiac echocardiography system was programmed to transmit a custom diverging beam sequence at 600 Hz and used to image the ablation site before and after the induction of ablation lesions. Cumulative strains were estimated over systole using a normalized cross-correlational displacement algorithm and a least-squares strain kernel. Afterward, lesions were excised and subjected to tetrazolium chloride staining. Results indicate that intracardiac ME was capable of imaging the reduction in systolic strain associated with the formation of an ablation lesion. Furthermore, lesion sets containing gaps were able to be distinguished from lesion sets created with no gaps. These results indicate that the end-systolic strain measured using intracardiac ME may be used to image the formation of lesions induced during an RF ablation procedure, in order to provide critical assessment of lesion viability during the interventional procedure.

Myocardial Elastography (ME) is an ultrasound-based strain imaging method. A survival canine model (n=11) was employed to investigate ME's ability to image myocardial infarction (MI) formation and recovery. Infarcts were generated by ligation of the left anterior descending coronary artery. Canines were survived and imaged for four days (n=7) or four weeks (n=4), allowing sufficient time for recovery via collateral perfusion. A radial strain-based metric, percentage of healthy myocardium by strain (PHM ε ), was developed as a marker for healthy myocardial tissue. PHM ε was strongly linearly correlated with actual infarct size as determined by gross pathology (R 2 = 0.80). Mean PHM ε was reduced 1-3 days postinfarction (p<0.05) at the papillary and apical short-axis levels; full recovery was achieved by day 28, with mean PHM ε returning to baseline levels. ME was capable of diagnosing individual myocardial segments as non-infarcted or infarcted with high sensitivity (82%), specificity (92%), and precision (85%) (ROC AUC = 0.90).

Myocardial elastography (ME) is an ultrasound-based technique that can image 2-D myocardial strains. The objectives of this study were to show that 2-D myocardial strains can be imaged with diverging wave imaging and are different in average between normal and coronary artery disease (CAD) patients. In this study, 66 patients with symptoms of CAD were imaged with ME prior to a nuclear stress test or an invasive coronary angiography. Radial cumulative strains were estimated in all patients. The end-systolic radial strain in the total cross-section of the myocardium in normal subjects (17.9 ± 8.7%) was significantly higher than in patients with reversible perfusion defect (6.2 ± 9.3%, p<0.001) and than in patients with significant (−0.9 ± 7.4%, p<0.001) and non-significant (3.7 ± 5.7%, p <0.01) lesions. End-systolic radial strain in the left anterior descending (LAD), the left circumflex (LCX) and the right coronary artery (RCA) territory was found to be significantly higher in normal subjects than in CAD patients. These preliminary findings indicate that end-systolic radial strain measured with ME is higher on average in healthy subjects compared to CAD patients and indicates that ME has the potential to be used for non-invasive, radiation free early detection of CAD.