Yinbo Li

High frequency ultrasound imaging detects cardiac dyssynchrony in noninfarcted regions of the murine left ventricle late after reperfused myocardial infarction.

Cardiac dyssynchrony in the left ventricles of murine hearts late (> or =28 d) after reperfused myocardial infarction (post-MI) was assessed using high frequency 30 MHz B-mode ultrasound imaging. Nine post-MI and six normal C57Bl/6 mice were studied in both short- and long-axis views. Regional time to peak displacement (T(peak_d)) and time to peak strain (T(peak_s)) were calculated in 36 sectors along the myocardial circumference; then their standard deviations (SD_T(peak_d) and SD_T(peak_s)) were computed among noninfarcted myocardial regions for each mouse and were compared between the normal and post-MI mouse groups with Students t-test. The comparison revealed that SD_T(peak_d) and SD_T(peak_s) were significantly larger in the post-MI hearts than in the normal hearts. The displacement uniformity ratio was determined to be 0.97 +/- 0.01 and 0.85 +/- 0.07 for radial and circumferential displacements in the normal hearts, respectively; and 0.59 +/- 0.17 and 0.64 +/- 0.24 in the post-MI hearts. In conclusion, this high resolution ultrasound image tracking method provides for the detection of cardiac dyssynchrony in the noninfarcted regions in the murine left ventricles late after MI by identifying the temporal and spatial disparity of regional myocardial contraction.

High resolution quantification of myocardial motion in mice using 2d speckle tracking

Mid-ventricular myocardial tissue motion in mice was examined using high resolution ultrasound B- mode images. The mouse heart was imaged at 35MHz, with approximately 40μm axial resolution and 100μm lateral resolution. Over 100 image frames per cardiac cycle were used in the analysis. Myocardial motion was tracked between successive image frames using a 2D minimum sum of absolute differences (MSAD) applied to 2D pixel kernels. The method is computationally efficient and a processing speed of 0.11 second per tracked image frame was achieved on a 2.9GHz Pentium IV PC, nearly 100X faster than in MATLAB. In the normal mouse heart, physiologically typical values for left ventricular myocardial deformation were obtained: the peak radial strain was approximately 40%, the peak rate of systolic wall endocardial velocity was 23 mm/s and the peak rate of diastolic wall velocity was 39 mm/s in the lateral wall. The preliminary mouse results correlate well with human MRI results.

Combined elasticity and 3d imaging of the prostate

A 3D volumetric elasticity reconstruction method is proposed for prostate elastography. The method uses crosscorrelation based I/Q data tracking technique in 2D with I-Beam transducer to reconstruct 3D volumetric elastograms. The elevational motion is tracked using block matching based on minimum of the sum-of-absolute differences between the successive elevational frames. 3D elasticity reconstruction estimates the 3D shape and size of cancers, which may be an important step towards their classification as a malignant or a benign.

Free-hand ultrasound scanning approaches for volume quantification of the mouse heart left ventricle

Two approaches for free-hand motion tracking that enable volumetric quantification of the murine heart were investigated. One approach used an instrumented, multijointed articulated arm attached to a 14 MHz ultrasound transducer array. A second approach used an E-beam transducer - a modified linear transducer array containing a main imaging array adjacent to three perpendicular tracking arrays. Motion between successive B-mode image frames was computed by tracking image speckle in each tracking array. Both tracking systems produced accurate results in a phantom validation study (4.50% error and 3.75% error for estimates derived using the articulated arm and E-beam tracking techniques, respectively). The tracking approaches also were tested in vivo on three mice. Results were compared to values obtained by mounting each mouse on a micromanipulator, adjusting its position by 0-5-mm increments, and acquiring B-mode images using a high-resolution ultrasound scanner. Left ventricular end diastolic volume (LVEDV) estimates differed from values obtained using the high-resolution scanner by a mean error of 18.2% and 2.60% for eight scans conducted on each of two mice using the articulated arm, and a mean error of 13.6%, 6.53%, and 12.58% for eight scans conducted on each of three mice using the E-beam

2I-2 Quantification and MRI Validation of Regional Cardiac Contractile Dysfunction in Mice Post Myocardial Infarction Using High Resolution Ultrasound

High resolution 30 MHz ultrasound imaging was performed on the left ventricles (LV) of C57B1/6 mice post myocardial infarction (MI). Normal (no MI) C57B1/6 mice were examined as control experiments. Ultrasound imaging was performed at 5-7 short axis slices and 4-5 long axis slices at 1 mm intervals that encompass the LV. Myocardial tissue displacement vectors were measured using a 2D ultrasonic speckle tracking algorithm with sub-pixel resolution. The pixel block window size was approximately 0.4 mm times 0.4 mm and the search region was 0.6 mm times 0.6 mm. The extent of contractile dysfunction and cardiac dyssynchrony were revealed in post-MI mouse LVs by regional myocardial displacement and strain analyses. Strain analyses from the mid-ventricular short axis based upon the ultrasound images were compared to those obtained using MRI, and high levels of correlation were obtained (R = 0.91 and R = 0.84 for radial and circumferential strain, respectively). Displacement and strain analyses were conducted on multiple short axis and long axis slices, and 3D displacement vectors were determined at the lines of intersection by summing the vector components derived from each of the orthogonal slices

Assessment of transient myocardial perfusion defects in intact mice using a microbubble contrast destruction/refill approach

The mouse provides a very useful model for human cardiovascular disease. Consequently, there is significant need for noninvasive approaches for assessing left ventricular function in the setting of various cardiovascular pathophysiological conditions induced in normal and genetically-modified mice. In this work, myocardial contrast echocardiography (MCE) was used to identify regions of ischemia within ultrasound images of the left ventricular myocardium in closed-chest, intact mice. Contrast agent was infused at a constant rate and, periodically, high intensity, contrast destructive frames were applied. Thereafter, an exponential refill curve was fitted to the mean gray scale value within the selected region of interest. Perfusion was investigated before, during and immediately after an induced ischemic event. As anticipated, a significantly reduced refill rate was observed during ischemia. The refill exponential rate constant was observed to decline by 35% in the region of myocardium affected by the induced ischemia.

3D cardiac motion estimation using RF signal decorrelation

Mouse cardiac ultrasound imaging is generally acquired as sets of 2D B-mode video or RF data. The high signal bandwidth and frame rate (>100 Hz) required for real-time 3D mouse heart scanning presents a major challenge making it unlikely that direct capture of finely sampled real-time 3D data will be achieved in the near future. Collecting and registering image sets from intersecting orthogonal 2D scan planes enables the estimation of 3D motion, but only at points along lines of intersection between acquired image frames. We propose the use of RF signal decorrelation to estimate elevational motion from 2D data at points other than the lines of intersection. RF data was collected and processed using a 30 MHz VisualSonics Vevo 770 scanner. A mouse heart was scanned along short and long axis imaging planes, producing RF data throughout the entire cardiac cycle. RF decorrelations were computed along all A-lines. Orthogonal displacement measurements were used to compute functions which map decorrelation to displacement at lines of intersection between planes. Combining in-plane speckle tracked estimates and out of plane decorrelation based displacement estimates yielded a full 3D displacement data set for the entire cardiac cycle. Displacements estimated using RF decorrelation were highly correlated with independent (orthogonal) speckle-tracked estimates. Out-of-plane decorrelation provides a method for computing 3D displacements from 2D scan-planes of in vivo mouse heart RF data. 3D displacement vectors may be used to compute 3D strain that may be useful when analyzing ischemic, normal and ldquoborder zonerdquo regions post myocardial infarct.

Synthetic axial acquisition-full resolution, low-cost C-scan ultrasonic imaging

The synthetic axial acquisition (SAA) approach presented here is designed to produce C-scan images using low-cost, low bandwidth, front end electronics. We exploit plane wave transmission and shallow C-scan imaging of non-moving, or slowly moving, tissue regions. Between each transmit/receive cycle, the receive sampling trigger is offset by one sampling interval so that over a sequence of acquisitions a sufficiently long data record is synthesized to enable a high-quality approximation to conventional delay and sum beamforming. FIELD II simulations were performed to model the next generation of our sonic window C-scan imaging system using a 5 MHz center frequency, 50% bandwidth, and a 60 X 60 fully sampled two-dimensional (2-D) array with a 0.3-mm element pitch. These simulations, which include analysis of the impact of target motion both parallel and perpendicular to the acoustic beam, indicate that SAA is robust with respect to target motion no faster than 10 mm/s. The impact of electronic noise on SAA also is considered.

High resolution 2D quantification of myocardial motion abnormalities in mice using high resolution ultrasound with MRI validation

High resolution ultrasound image sequences of left-ventricular motion abnormalities in mice were analyzed on a regional basis using an optimized 2D pixel block tracking technique. The mouse heart was imaged at 35 MHz with approximately 40 mum axial resolution and 100 mum lateral resolution. Myocardial regional motion was tracked using high frame rate mouse heart image sequences using 2D minimum sum of absolute differences (MSAD) block matching. Eight pixel parallel processing was achieved using single instruction multiple data (SIMD) instructions, resulting in a processing speed of 0.11 seconds per frame on a 2.9 GHz Pentium4 PC. Significant differences were observed in the regional displacement, strain and velocity between healthy and infarcted myocardial tissue. Myocardial strains were compared to those obtained via magnetic resonance imaging (MRI). The correlation between the assessment using ultrasound and MRI was R = 91% for radial strain and R = 84% for circumferential strain. Thus, high resolution ultrasound followed by 2D image motion post-processing offers the potential to derive quantitative measures of mouse heart function with the low cost, fine temporal resolution and ease-of-mobility of ultrasound instrumentation

A four-dimensional model-based method for assessing cardiac contractile dyssynchrony in mice

Four-dimensional (4D), or equivalently, 3D + time, analysis is useful for comprehensive assessment cardiac function, especially in the asymmetric left ventricle (LV) after myocardial infarction (MI). This paper presents a 4D-model-based method for ultrasound assessment of cardiac contractile function in mice. Echocardiographic image sequences were acquired at high frequency (30 MHz) from the hearts of C57Bl/6 mice. Image sequences were acquired at contiguous slice locations encompassing the entire 3D LV. In order to reconstruct continuous, dynamic 3D LVs from the images using a 4D mathematical cardiac model, endocardial and epicardial contours were segmented for all image slice locations through one cardiac cycle. In the 4D model, shape and continuity constraints were applied in order to normalize irregularities caused by noise or non-uniform distribution of image data. Root mean square error (RMSE) was calculated between the model-fitted 4D LV and the actual LV surface measured from image data. RMSE was 0.23 mm (~4.5% of epicardial diameter) for the epicardial surface, and 0.20 mm (6.4% of endocardial diameter) for the endocardial surface. 3D regional wall thickening was calculated from the 4D LV surface, and LV dyssynchrony was assessed by analyzing the time to peak strain (Tpeak). This 3D analysis of contractile function in post-MI mouse hearts revealed >80% reduction of peak radial displacement, a 10-15 ms delay in Tpeak in the infarct zone, and a SD_Tpeak of 6-10 ms over the entire 3D LV. In summary, the 4D-model-based method was successfully used for analyzing cardiac dyssynchrony in the 3D murine LV, and it proved advantageous over conventional 2D methods because it was more comprehensive and noise-robust.