This study noninvasively evaluated the development of left ventricular (LV) dyssynchrony following reperfused myocardial infarction (MI) in mice using an ultrasonic speckle-tracking method. Eight C57BL/6J mice were assessed by high-resolution echocardiography at baseline and at eight time-points following MI. Images were acquired at 1mm elevational intervals encompassing the entire LV to determine chamber volumes and radial strain. Receiver-operating characteristic (ROC) analysis of regional radial strain was used to segment the three-dimensional (3-D) LV into infarct, adjacent and remote zones. This in vivo segmentation was correlated to histologic infarct size (R = 0.89, p < 0.01) in a short-axis, slice-by-slice comparison. The onset of dyssynchrony during LV remodeling was assessed by standard deviation of time to peak radial strain in the infarct, adjacent and remote zones. It was discovered that the form of LV dyssynchrony that develops in the remote zone late after MI does so in concert with the progression of LV remodeling (R = 0.70, p < 0.05).

Images produced by ultrasound systems are adversely hampered by a stochastic process known as speckle. A despeckling method based upon removing outlier is proposed. The method is developed to contrast enhance B-mode ultrasound images. The contrast enhancement is with respect to decreasing pixel variations in homogeneous regions while maintaining or improving differences in mean values of distinct regions. A comparison of the proposed despeckling filter is compared with the other well known despeckling filters. The evaluations of despeckling performance are based upon improvements to contrast enhancement, structural similarity, and segmentation results on a Field II simulated image and actual B-mode cardiac ultrasound images captured in vivo .

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 (T peak ). 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_T peak 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.

The active surface technique using gradient vector flow allows semi-automated segmentation of ventricular borders. The accuracy of the algorithm depends on the optimal selection of several key parameters. We investigated the use of conservation of myocardial volume for quantitative assessment of each of these parameters using synthetic and in vivo data. We predicted that for a given set of model parameters, strong conservation of volume would correlate with accurate segmentation. The metric was most useful when applied to the gradient vector field weighting and temporal step-size parameters, but less effective in guiding an optimal choice of the active surface tension and rigidity parameters.

Cardiac dyssynchrony in the left ventricles of murine hearts late (>= 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 C571B1/6 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 Student's 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. (E-mail: jh7fj@virginia.edu) (C) 2008 World Federation for Ultrasound in Medicine & Biology.

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.

Left ventricular (LV) contractile dyssynchrony in murine hearts late (>=28 days) after reperfused myocardial infarction (post-MI) was assessed using high frequency 30 MHz B-Mode ultrasound imaging. Nine post-MI C57BL/6 mice were analyzed in both the short- and long-axis views for cardiac dyssynchrony, and six normal mice were examined for controls. Regional time to peak displacement (Tpeak_d) and time to peak strain (Tpeak_s) were calculated in 36 sectors along the myocardial circumference. The standard deviations (SD_Tpeak_d and SD_Tpeak_s) were computed among non-infarcted myocardial regions for each mouse, and were compared between the normal and post-MI mouse groups with student's t test. The comparison revealed that SD_Tpeakd and SD_Tpeaks were significantly larger in the post-MI hearts than in the normal hearts. The displacement uniformity ratio was determined to be 0.97 plusmn 0.01 and 0.85 plusmn 0.07 for radial and circumferential displacements in the normal hearts, respectively, and 0.59 plusmn 0.17 and 0.64 plusmn 0.24 in the post- Mi hearts.

High frequency (25-35 MHz), transthoracic, adult mouse echocardiography images sometimes exhibit low echogenecity in segments of the myocardium, resulting in degraded regional motion tracking. In this work, we performed high frequency (30 MHz) transthoracic ultrasound scanning on the left ventricle (LV) of normal C57BL/6 mice. Large-diameter microbubbles (range=2-8 mum, median~=5 mum) were used to enhance myocardial image signal via microbubbles lodging in the myocardial capillary bed [1]. Microbubbles produced an image intensity enhancement of 5-10 dB throughout the myocardium - especially in known signal dropout susceptible locations such as the septal wall. The motion analysis was performed using various filtering and tracking block size parameters, and the results were compared between analysis on microbubble-enhanced images and control echocardiographic images where no microbubbles were employed. The use of microbubbles allowed motion tracking without image pre-filtering and thus reduced the smoothing effect on motion tracking that filtering can cause. Motion tracking using smaller tracking block size (0.2 x 0.2 mm2 vs. 0.4 x 0.4 mm2) on microbubble- enhanced images yielded finer tracking accuracy and better resolution than when tracking was performed on control images. Microbubble enhancement also facilitated motion tracking measurements using "real-time" image sequences (requiring an acquisition lasting a few seconds) versus the previously preferred EKV data acquisition method (i.e., an ECG-gated EKV acquisition mode) that required 2-3 minutes.

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

Active contours have been used in a wide variety of image processing applications due to their ability to effectively distinguish image boundaries with limited user input. In this paper, we consider 3D gradient vector field (GVF) active surfaces and their application in the determination of the volume of the mouse heart left ventricle. The accuracy and efficacy of a 3D active surface is strongly dependent upon the selection of several parameters, corresponding to the tension and rigidity of the active surface and the weight of the GVF. However, selection of these parameters is often subjective and iterative. We observe that the volume of the cardiac muscle is, to a good approximation, conserved through the cardiac cycle. Therefore, we propose using the degree of conservation of heart muscle volume as a metric for assessing optimality of a particular set of active surface parameters. A synthetic dataset consisting of nested ellipsoids of known volume was constructed. The outer ellipsoid contracted over time to imitate a heart cycle, and the inner ellipsoid compensated to maintain constant volume. The segmentation algorithm was also investigated in vivo using B-mode data sets obtained by scanning the hearts of three separate mice. Active surfaces were initialized using a broad range of values for each of the parameters under consideration. Conservation of volume was a useful predictor of the efficacy of the model for the range of values tested for the GVF weighting parameter, though it was less effective at predicting the efficacy of the active surface tension and rigidity parameters.

Contractile dyssynchrony in the left ventricles (LVs) of murine hearts post myocardial infarction (MI) was assessed using high resolution 30 MHz ultrasound. Long axis and mid-ventricular short axis image sequences were obtained from six post-Mi C57B1/6 mice. Two normal C57B1/6 mice were also scanned as non-infarcted controls. The ultrasound images had an axial resolution of approximately 50 mum, a lateral resolution of approximately 100 mum, and a temporal resolution of 1000 frames/s (achieved via retrospective compilation of LCG-gated acquisitions). Myocardial tissue displacement was measured using a 2D image tracking algorithm with sub-pixel resolution. Regional myocardial displacement analyses revealed prolonged regional systole and delay to peak displacement, as well as reduced displacement, in regions adjacent to infarcted zones. The regional delays in time to peak displacement ranged from 12 to 20 ms in mouse hearts with large MI. Thus, high resolution 2D ultrasound imaging followed by 2D image tracking analysis provides an efficient and cost-effective method for characterizing LV dyssynchrony in murine hearts post-MI

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

Two tracking methods that enable 3D volumetric analysis of the murine heart using acquisition of free-hand ultrasound 2D scans are investigated in this paper. The first approach uses an instrumented, articulated arm (immersion Mcroscribe 3DX) to track the position and orientation of the transducer head. The second approach uses an "I-Beam" transducer (a modified array configuration, with linear trucking arrays mounted on each end of a perpendicularly oriented central imaging array). Both systems performed well in a phantom validation study. However, only the Microscribe system yielded acceptable results in vivo. A new tracking array-based transducer is currently being developed to address the known shortcomings of the current I-Beam transducer.