Helen Routh

Ultrasonic doppler measurement of blood flow velocities by array transducers

A Doppler signal processor is provided for an ultrasonic diagnostic imaging system which identifies the peak and mean flow velocities by Doppler interrogation of a sample volume within the body. The effects of spectral broadening resulting from the use of an array transducer are compensated by producing a peak velocity value which is a function of the dimension of the array aperture and its relation to the location of the sample volume. Mean velocity values calculated from received Doppler echoes are combined with an array distortion function to produce accurate mean velocity values. The entire spectrum may be corrected by deconvolving the received signal spectrum with an array distortion function. Alternatively the effects of spectral broadening are compensated by use of Doppler reference signals for each element of the array which are a function of the position of the individual elements in the array aperture.

Automatic flow angle correction by ultrasonic vector

An ultrasound system produces an image including a blood vessel. A blood flow direction indicator is displayed over the blood vessel to indicate the direction of blood flow within the vessel. The direction of blood flow is used to correct the Doppler estimate for angle of insonation. The orientation of the blood flow direction indicator is set automatically by a vector processor. The automatically calculated angle is used to display the correct flow velocity without user intervention.

Velocity fluctuation reduction in vector Doppler ultrasound using a hybrid single/dual-beam algorithm

In order to reduce the fluctuations in the velocity magnitude estimate, we propose a modification to the standard algorithm for reconstructing the (two component) vector velocity from the measured Doppler shifts in two directions. This uses the standard dual-beam algorithm, combined with temporal smoothing, to find only the velocity angle, then uses the single-beam algorithm to estimate the velocity magnitude. We present initial data showing the significant reduction in velocity estimate fluctuation that this hybrid method achieves compared to the standard algorithm.

Real time vector Doppler for tissue motion

Tissue Doppler Imaging (TDI) can assess tissue motion in vascular and cardiac imaging. However, a major drawback of these measurements is that the motion estimation is limited to the component along the ultrasound beam axis. Cardiac and vessel wall motion studies have shown that complex three-dimensional motions can be observed, and that there is a clinical need to fully assess the three components of the vector motion. This work describes how TDI can be extended by acquiring a real time two-component velocity vector via a dual beam vector Doppler technique. A vector Doppler velocity estimator using a small interbeam angle can suffer from both bias and large variance. This estimator is also strongly dependent on the settings of the echographic system. To reduce the large bias and variance, most vector velocity techniques use a very large ensemble length (EL) (>20), which does not allow real time implementation. We propose a new processing technique, which reduces the bias and the standard deviation of the vector velocity estimate. The new method assumes that the vector velocity angle varies slowly over the cardiac cycle. The angle can then be estimated using a large time window. The performance of this new technique has been tested experimentally using a tissue mimicking rotating phantom. It is shown that the factors influencing the results are the EL, the precision of the TDI estimates and the time window. The results indicate that the variance and bias of velocity magnitude and orientation estimates decrease with increasing EL, increasing precision of the TDI estimates and increasing time window. Using an EL of 9, 8 bits for the velocity estimate, and an observation time of one second, a 5-degree bias of the angle estimate is observed, with a variance below 7 degree averaged over all angles. A 10% bias of the velocity magnitude is observed, with a variance of 1%. In conclusion, TDI can be improved with vector Doppler providing two-dimensional tissue motion estimation, enabling more accurate biomechanical tissue property assessment.

Harmonic imaging in tissue

In the last few years it has become clear that harmonic imaging in tissue (or tissue harmonic imaging) will improve image quality in many ‘‘difficult to image’’ patients. This talk will focus on the practical and clinical aspects of this imaging method which makes use of the nonlinear propagation of ultrasound in tissue. By filtering the received signal around the second harmonic, much of the haze and clutter in the image is reduced. Although there has been much research on the propagation of nonlinear waves in tissue, the exact mechanisms of this image quality improvement are still unclear. One hypothesis is based on the fact that the intensity of the second harmonic signal increases as the signal pass deeper into tissue. As the majority of artifacts arise from structures close to the transducer, the artifacts in the second harmonic signal are significantly lower than those in the fundamental. By imaging the second harmonic signal, much of the haze and clutter in the image can then be reduced. This talk ...