1. and Strain Rate: Validation Against Sonomicrometry
Ultrasound Speckle Tracking System Using Radiofrequency Data (RF) for Simultaneous Computation of Strain
and Strain Rate: Validation Against Sonomicrometry
Muhammad Ashraf, MD; Max Carlson; Colleen Newey; Shiza Ashraf; Karen Li; Sarah K Yang; Jamie Hamilton, PhD; David J Sahn, MD
Oregon Health & Science University, Portland, OR; Epsilon Imaging, Inc., Ann Arbor, MI
Background
Abstract
Digital tracking of tissue markers in dynamic ultrasound image loop has made it possible to compute mechanical indices of cardiac function like strain and strain rate non-invasively without angle dependency of Doppler based methods.
Many studies to date have linked these speckle-tracking based mechanical functions ( LV twist and strain) with dynamic function of healthy and diseased heart.
Current commercial B-mode speckle or feature tracking packages use the processed envelop signal that results in loss of information important for consistency of speckles. Also they typically operate at lower sampling rates (70 fps) that may compromise accuracy and reproducibility of these measurements especially at higher heart rates.
Aim of this study was to test the accuracy of a new prototype speckle tracking system [Dynamic Contractility Imaging (DCI)] that was developed utilizing raw radiofrequency (RF) echo data. RF-based speckle tracking uses raw ultrasound signals which provide high fidelity information with higher sensitivity and finer resolution. It is not sensitive to changes in signal magnitude and shows better performance in the presence of noise.
Dynamic Contractility Imaging uses the normalized cross-correlation technique that allows tracking at very high frame rates, which is crucial for optimizing the quality of tracking at a given resolution.
Background: There is significant interest in speckle tracking echo derived mechanical indices to assess myocardial deformation and contractility. Current commercial B-mode feature tracking packages use the envelop signal and operate at lower sampling rates (70 frames/sec). We tested a new prototype speckle tracking system [Dynamic Contractility Imaging (DCI)] that was developed utilizing raw radiofrequency (RF) echo data. Methods: We studied 10 freshly harvested pig hearts connected to a calibrated pulsatile pump through a balloon secured in the LV cavity. Each heart was mounted in a water bath to facilitate ultrasound scanning and driven passively by a pump at a constant rate of 60/min. Studies were conducted at two stroke volumes (40ml & 80ml). Six 2mm sonomicrometry crystals were secured into the myocardium and arranged as two triangles at the apical and mid cavity levels. Cardiac motion was scanned at 3 MHz, 125 frames/sec, along the direction of planes formed by crystals at each stroke volume to acquire two long axis (2ch and 4ch) views and two short axis (apical and mid cavity levels) views. Results: Increase in stroke volume resulted in increased stretching of myocardium that was detected by both sonomicrometry and DCI. DCI slightly underestimated the strain values (4.5 ± 2.5%) when compared to sonomicrometry but both methods showed a strong positive linear correlation (r=0.85). Conclusions: DCI is an accurate imaging based method for computing strain and strain rate..
Methods
Results
We studied 10 freshly harvested pig hearts in a custom designed water tank to facilitate ultrasound imaging. Each heart was mounted on a plastic ring to avoid translational motion and connected to a calibrated pulsatile pump through a balloon secured in the LV cavity. Each heart was driven passively at two known stroke volumes (40ml & 80ml) by pump at a constant rate of 60/min.
Six sonomicrometry crystals were secured into the myocardium and arranged as two triangles at the apical and mid cavity levels to acquire data of crystal motion both in long and short axis planes at sampling rate >250 samples/sec.
Images were collected at 3 MHz, 125 frames/sec, along the direction of planes formed by crystals at each stroke volume to acquire two long axis (2ch and 4ch) views and two short axis (apical and mid cavity levels) views.
Ultrasound data was processed using Epsilon Imaging’s EchoInsighttm UltraLab software for strain and strain rate using RF based speckle tracking methods. Segmental strain measurements were collected regions corresponding to sono crystal pair. Segmental DCI measurements and sono measurements were loaded and compared using SonoCompare tool. For each DCI/sono comparison, a time window was selected from the sono data that best fit the DCI (minimized SR difference).
At the higher stroke volume, the myocardium was stretched more as compared to lower stroke volume. This increased amount of myocardial stretching was detected by both sonomicrometry and DCI derived strain measurements. (p<0.05)
DCI slightly underestimated the strain values (4.5 ± 2.5%) at each stroke volume when compared to sonomicrometry
Both methods showed a strong positive linear correlation when compared to each other. (r>0.85).
DCI derived measurements of strain and strain rate showed high inter-observer (r=0.78) and intra-observer (r=0.82) reproducibility.
Conclusions
Results of our controlled phantom study shows that Dynamic Contractility Imaging is an accurate and reproducible imaging based method for computing strain and strain rate.
Disclosures
No relationships to disclose:
Muhammad Ashraf
Max Carlson
Colleen Newey
Shiza Ashraf
Karen Li
Sarah K Yang
Employed by Epsilon Imaging:
Jamie Hamilton
Occasional consultant to Epsilon Imaging:
David J Sahn, MD