An in vitro study on the influence of limited frequency resolution on contrast agent-enhanced Doppler signals.

Ultrasound contrast agents for vascular use are gas-filled microspheres and will increase the backscatter from blood and therefore enhance the Doppler signal. Studies have shown that Doppler enhancement may result in an increased maximum velocity when observed as the envelope of the spectral Doppler screen. Change in observed velocity may be due to biological or instrumentational effects. The purpose of this study was to investigate the possible instrumentational effects on the Doppler signal after injection of the contrast agent (Infoson). By performing the measurements on a Doppler phantom, the biological effects was eliminated. Computer simulations of limited frequency resolution were compared to measurements. The conclusion was that the increase in detected maximum velocity after Infoson injection is due to the combined effect of signal enhancement and limited frequency resolution. It is shown that a large change in signal strength can change the estimated maximum frequency from the spectral Doppler whether this is due to a contrast agent or instrument settings. It is therefore important in a clinical situation to have an adequate, but not too strong or too weak signal in order to make the best velocity estimates.

Ultrasonic tomography of biological tissue.

In this paper, quantitative tomographic reconstructions of biological tissue are presented. First, the experimental setup and a hybrid filtered backpropagation (FBP) technique are briefly described. Using this technique, which includes exact backpropagation of data prior to reconstruction by means of the classical FBP algorithm, quantitative velocity maps of relatively large biological objects can be obtained. Since the FBP algorithm is based on a first-order scattering approximation, the deteriorating effects of higher-order scattering in diffraction tomography are also discussed. The higher-order scattering limits the size of the biological object to a few centimeters.

Quantitative results in ultrasonic tomography of large objects using line sources and curved detector arrays.

A tomographic reconstruction technique valid for line sources, curved detector arrays, and large object is presented. For acquisitions involving a curved detector array, inverse diffraction is first used to propagate the field back to a straight line and then the standard filtered backpropagation (FBP) algorithm is employed to reconstruct the image. Using inverse diffraction the measured field can be accurately propagated all the way back to the reconstruction area. Thus an essential improvement is obtained compared to using the approximate backpropagation of Rytov data contained in the FBP algorithm, which becomes inaccurate when the distance from the measurement surface to the reconstruction area is large. This technique is applied to measured data and it is shown that it gives reconstructions of high quality, both with respect to geometry and velocity. It is also shown that, when the illuminating wave is cylindrical rather than plane, segmentation of the image can be used in combination with inverse diffraction and FBP reconstruction to obtain high-quality images of large objects.

Experimental results in ultrasonic tomography using a filtered backpropagation algorithm.

This paper describes an experimental study in diffraction tomography whose main goal was to determine experimentally the performance of the filtered backpropagation algorithm in ultrasound tomography. We have built a simple water tank with a plane wave source and a scanning hydrophone. The test objects have been cylindrical and of a relatively simple structure and made of acrylic and silicone rubber. The resulting reconstructions show that the structure and position of the objects are well reproduced. The velocity of the objects can also be well reproduced if the velocity contrast is not too large. In medical imaging, it is most important to see the structure, and since the velocity change in biological tissue is small, the method should be well suited for medical imaging.