Andreas Heimdal

Real-Time Strain Rate Imaging of the Left Ventricle by Ultrasound

The regional function of the left ventricle can be visualized in real-time using the new strain rate imaging method. Deformation or strain of a tissue segment occurs over time during the cardiac cycle. The rate of this deformation, the strain rate, is equivalent to the velocity gradient, and can be estimated using the tissue Doppler technique. We present the strain rate as color-coded 2-dimensional cine-loops and color M-modes showing the strain rate component along the ultrasound beam axis. We tested the method in 6 healthy subjects and 6 patients with myocardial infarction. In the healthy hearts, a spatially homogeneous distribution of the strain rate was found. In the infarcted hearts, all the infarcted areas in this study showed up as hypokinetic or akinetic, demonstrating that this method may be used for imaging of regional dysfunction. Shortcomings of the method are discussed, as are some possible future applications of the method.

Strain Rate Imaging by Ultrasound in the Diagnosis of Regional Dysfunction of the Left Ventricle

Background: Regional strain rate in the left ventricle can be assessed by tissue Doppler velocity gradient and color mapped in real time. Regional contractility thus can be visualized and graded. To validate the method, we made a comparison with standard echocardiography. Methods and Results: Fifteen patients with recent myocardial infarction were examined with the use of strain rate imaging (SRI). Velocity gradients were mapped by color. Gray‐scale imaging was performed using the second harmonic mode. Cine loops of two‐dimensional echocardiography (2‐D echo) and SRI images from three standard apical planes were analyzed offline. A four‐grade scale in 16 segments was used to score wall motion by 2‐D echo and by SRI. Of a total of 236 segments, 235 segments were analyzable by 2‐D echo and 218 segments were analyzable by SRI. Correlation of wall motion score index with ejection fraction was – 0.84 by 2‐D echo and – 0.92 by SRI. One hundred fourteen segments had an equal score by the two methods: 51 segments differed by 1 degree and 14 segments differed by 2 degrees (K = 0.45). Conclusions: SRI agrees well with echocardiography in grading regional wall function, and the method can be seen as validated in a clinical setting for assessment of regional systolic wall function and is demonstrated to be applicable for semiquantitative wall motion assessment. SRI has theoretical advantages and may be a valuable addition to standard echocardiography, especially in the field of stress echocardiography.

Ultrasound doppler measurements of low velocity blood flow: limitations due to clutter signals from vibrating muscles

Skeletal muscles vibrate under sustained contraction, and generate low frequency side band clutter in the doppler signal. Both shivering in the hand of the operator and muscle vibrations in the patient itself give rise to the clutter. Clutter rejection filters are commonly used to remove the low frequency components, but the doppler signal from low velocity blood flow is then also lost. This paper describes a model for the pulsed wave (PW) doppler signal from vibrating muscles, reviews a model for the PW doppler signal from moving blood, and by comparing these models presents a theoretical minimum for detectable blood velocity in small vessels, being typically 6.4 mm/s for 6 MHz doppler. The limit has a nonlinear relation to the ultrasound frequency. The model also shows that the radial component of the muscle vibrations can be estimated from the phase of the doppler signal.

Strain during gastric contractions can be measured using Doppler ultrasonography

This study was undertaken to explore if strain of the muscle layers within the gastric wall could be measured by transabdominal strain rate imaging (SRI), a novel Doppler ultrasound (US) method. A total of 9 healthy fasting subjects (8 women, 1 man; ages 22 to 55 years) were studied and both grey-scale and Doppler US data were acquired with a 5- to 8-MHz linear transducer in cineloops of 97 to 256 frames. Rapid stepwise inflation (5 to 60 mL) of an intragastric bag was carried out and bag pressure and SRI were measured simultaneously. SRI enabled detailed studies of layers within the gastric wall in all subjects. Great variations in strain distribution of the muscle layers were found. Radial strain was much higher in the circular than in the longitudinal muscle layer. Strains derived from SRI correlated well with strains obtained with B-mode measurements (r = 0.98, p < 0.05). During balloon distension, we found an inverse correlation between pressure and radial strain (r = -0.87, p < 0.05). Intraobserver correlation of strain estimation was r = 0.98 (p < 0.05) and intraobserver agreement was 0.2% +/- 18.6% (mean difference +/- 2SD, % strain). Interobserver correlation was r = 0.84 (p < 0.05) and interobserver agreement was 6.9% +/- 56.8%. SRI enables detailed mapping of radial strain distribution of the gastric wall and correlates well with B-mode measurements and pressure increments.

High frame rate strain rate imaging of the interventricular septum in healthy subjects.

OBJECTIVE In the present study the feasibility was assessed of a new strain rate imaging method with a very high frame rate of around 300 frames per second. METHODS Digital radio-frequency (RF) data were obtained in nine healthy subjects using a sector of 20-30 degrees in an apical four chamber view. The RF data were analysed using a dedicated software package that displays strain rate images and profiles and calculates strain rate values. With the new method, it is possible to study events and spatial-temporal differences in the heart cycle with duration down to 3.5-3 ms, including the pre-ejection period and the isovolumic relaxation period. Since the interventricular septum (IVS) is of crucial importance for the left and right ventricular function, we assessed changes through the heart cycle of the strain rate in the IVS. RESULTS Mean peak systolic strain rate in the healthy subjects was -1.65+/-0.13 s(-1). Mean peak diastolic strain rate during early filling was 3.14+/-0.50 s(-1) and during atrial systole 0.99+/-0.09 s(-1). We found individual differences in the strain rate patterns, but in all subjects, the ventricular contraction started simultaneously in all parts of the septum. After the ejection period, the elongation started before aortic valve closure, in the midinferior septum and propagated towards the apex. CONCLUSION High frame rate strain rate imaging makes it possible to study rapid deformation patterns in the heart walls.

Real-time strain velocity imaging (SVI)

The viability of a segment of the cardiac muscle is related to the amount and temporal behavior of the strain that is performed by, or is imposed on the muscle segment. The vessel wall thickening is commonly used as an indication of the strain. In this paper a method to estimate the strain velocity directly from the ultrasound RF data is presented. The component of the strain velocity along an ultrasound beam can be found as the spatial gradient of the tissue velocity over a small distance along the beam. The result is presented as a color overlaid on the ultrasound image. Strain velocity images generated by post processing of ultrasound RF data recorded with the Vingmed Sound System Five ultrasound scanner, as well as results from a real time implementation on the scanner are presented.

Vessel wall detection and blood noise reduction in intravascular ultrasound imaging

Scattering from blood (blood noise) limits the contrast between the vessel wall and the lumen in intravascular ultrasound imaging. This makes it difficult to localize the vessel wall, especially in still images. This paper presents a method for automatic detection of vessel walls and reduction of blood noise based on correlation of the RF-signal between adjacent frames. Regions detected as vessel wall are displayed with no modifications, while regions detected as blood noise undergo a hard rejection

Effect of stationary reverberations and clutter filtering in strain rate imaging

Stationary reverberations introduce a bias and an increased variance in velocity magnitude estimates based on Doppler ultrasound imaging. Clutter filtering is commonly used to remove the reverberations. For low velocities though, clutter filtering increases the variance and introduces a bias. The newly developed Strain Rate Imaging (SRI) technique is based on estimating the tissue velocity gradient. To study the effect of stationary reverberations and clutter filtering a the strain rate estimate the authors performed an experiment where RF-data from a dynamic in vitro model of myocardial tissue was obtained. Stationary reverberations were simulated and added to the RF-data at different signal-to-clutter ratios (SCR). The results showed that zero order clutter filtering only reduced the variance for SCR=0 dB. For higher SCR the variance increased. Similarly, clutter filtering reduced the fractional bias for SCR=0 and increased it for SCR above 10 dB.

Ultrasound clutter signal from vibrating muscles limits low velocity blood flow measurements

Skeletal muscle tension vibrates under sustained contraction, and will thereby generate low frequency clutter in the Doppler signal. Both muscles in the hand of the operator holding the ultrasound probe, and in the patient itself can give rise to the clutter. In this paper a model for the Doppler signal from vibrating muscles is presented. For small vibration amplitudes, the signal is a phase modulated signal, and the vibration amplitude can be estimated from the phase of the signal. Clutter rejection filters, which are commonly used to remove the low frequency components in the Doppler signal, will also remove the signal from low velocity blood flow. By comparing the model for the clutter signal with a previously presented model for the Doppler signal from moving blood, a theoretical minimum detectable blood velocity is found for different ultrasound frequencies. For 6 MHz Doppler this limit is 6.4 mm/s, which indicates that capillary blood perfusion is not measurable with conventional techniques.

Spectral strain rate imaging

This paper presents estimators for spectral strain rate display. The estimators were tested on ultrasonic radio-frequency (RF) data acquired at a frame rate of 323 Hz from the beating heart of a healthy volunteer. The strain rate spectrum from a region of interest was calculated using circular convolution of velocity spectra, or equivalently, Fourier transform of products of RF data packets. An extended method combining several frequency scaled spectra was also implemented. Both processing methods produced strain rate spectrograms which compared well with the conventional strain rate. In some time frames, the strain rate spectrum showed a broad bandwidth, indicating a poor signal.