E.I. Cespedes

Advancing intravascular ultrasonic palpation toward clinical applications

This paper describes the first reported attempt to develop a real-time intravascular ultrasonic palpation system. We also report on our first experience in the catherization laboratory with this new elastographic imaging technique. The prototype system was based on commercially available intravascular ultrasound (US) scanner that was equipped with a 20-MHz array catheter. Digital beam-formed radiofrequency (RF) echo data (i.e., 12 bits, 100 Hz) was captured at full frame rate from the scanner and transferred to personal computer (PC) memory using a fast data-acquisition system. Composite palpograms were created by applying a one-dimensional (1-D) echo tracking technique in combination with global motion compensation and multiframe averaging to several pairs of RF echo frames that were obtained in the diastolic phase of the cardiac cycle. The quality of palpograms was assessed by conducting experiments on vessel phantoms and on patients. The results demonstrated that robust and consistent palpograms could be generated in almost real-time using the proposed system. Good correlation was observed between low strain values and regions of calcification as identified from the intravascular US (IVUS) sonograms. Although the clinical results are clearly preliminary, it was concluded that the prototype system performed sufficiently well to warrant further and more in-depth clinical investigation.

Intraluminal ultrasonic palpation: assessment of local and cross-sectional tissue stiffness.

Many intravascular therapeutic techniques for the treatment of significant atherosclerotic lesions are mechanical in nature: examples are angioplasty, stenting and atherectomy. The selection of the most adequate treatment would be advantageously aided by knowledge of the mechanical properties of the lesion and surrounding tissues. Based on the success of intravascular ultrasound (IVUS) in accurately depicting the morphology of atheromatous lesions, ultrasonic tissue characterisation has been proposed as a tool to determine the composition of atheroma. We describe the addition of local compliance information to the IVUS image in the form of a colour-coded line congruent with the lumen perimeter. The technique involves analysis of echo signals obtained at two or more states of incremental intravascular pressure. Using vessel phantoms and specimens, we demonstrate the utility of intravascular compliance imaging. The palpograms are able to identify lesions of different elasticity independently of the echogenicity contrast, because the information provided by the elastograms is generally independent of that obtained from the IVUS image. Thus, the palpogram can complement the characterisation of lesion from the IVUS image. We also describe cross-sectional measures of elasticity that are based on the elastogram. Finally, natural extensions of intravascular palpation to other endoluminal ultrasound applications are proposed.

Elastic and Acoustic Properties of Vessel Mimicking Material for Elasticity Imaging

The mechanical and acoustic properties of agar-gelatin gels, used to construct vessel mimicking phantoms for ultrasonic elasticity studies, were investigated. Gels with varying compression moduli were made using a gelatin solution (8% by weight) with a variable amount of agar(1%-3% by weight). Carborundum particles were added as scattering material. The compression modulus was determined using a dynamic mechanical analyzer. The dependence of the compression modulus and the acoustic parameters on the agar concentration, as well as on the age and the temperature of the samples, was investigated. The results show that the compression modulus is strongly influenced by these factors, while the effect on the acoustic parameters is less. Compression moduli spanning a useful range for vascular phantom construction with realistic acoustic parameters can be achieved by varying the amount of agar. Phantoms constructed from these gels are well suited to serve as a model for plaque containing vessels.

Echo decorrelation from displacement gradients in elasticity and velocity estimation

Several ultrasonic techniques for the estimation of blood velocity, tissue motion and elasticity are based on the estimation of displacement through echo time-delay analysis. A common assumption is that tissue displacement is constant within a short observation time used for time delay estimation (TDE). The precision of TDE is mainly limited by noise sources corrupting the echo signals. In addition to electronic and quantization noise, a substantial source of TDE error is the decorrelation of echo signals because of displacement gradients within the observation time. The authors present a theoretical model that describes the mean changes of the crosscorrelation function as a function of observation time and displacement gradient. The gradient is assumed to be small and uniform within the observation time; the decorrelation introduced by the lateral and elevational displacement components is assumed to be small compared with the decorrelation caused by the axial component. The decorrelation model predicts that the expected value of the crosscorrelation function is a low-pass filtered version of the autocorrelation function (i.e., the crosscorrelation obtained without gradients). The filter is a function of the axial gradient and the observation time. This theoretical finding is corroborated experimentally. Limitations imposed by decorrelation in displacement estimation and potential uses of decorrelation in medical ultrasound are discussed.

Novel developments in intravascular imaging

In the development of intravascular ultrasound (IVUS), a serious emphasis has been given to the improvement of the image quality in terms of resolution. The image quality is indeed a very important issue, but there is lot more information hidden in the ultrasound signals than is currently exploited by commercially available IVUS equipment. Over the past few years, at the Thorax centre we have been exploring the possibilities of analysing sequences of radiofrequency (RF) traces. This could provide a significant extension of the functionality of the IVUS machines. It gives possibilities for local elasticity assessment, flow estimation and enhanced lumen detection. This paper is an up to date impression of where RF-data analysis has taken us.

New developments in intravascular ultrasound imaging.

IntraVascular Ultrasound Imaging (IVUS) has already been proposed in the early days of diagnostic ultrasound. Today, it has come under further full attention as a result of minimal invasive techniques. Not only excellent intravascular two-dimensional (2D) images are presently obtained, also three-dimensional (3D) reconstructed images show their diagnostic value. Based on 3D information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based on image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values, which in turn may indicate sites prone to stenosis. With a decorrelation technique for radiofrequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as in vivo experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain, used as a measure of local tissue hardness, can be estimated to identify hard or soft plaques independently of the echogenicity contrast between plaque and vessel wall.

Decorrelation characteristics of transverse blood flow along an intravascular array catheter

In recent years, a new method to measure transverse blood flow based on the decorrelation of radio frequency (RF) signals has been introduced. In this paper, we investigated the decorrelation characteristics of transverse blood flow measurement using an intravascular ultrasound (IVUS) array catheter by means of computer modeling. Blood was simulated as a collection of randomly located point scatterers. Moving this scattering medium transversally across the acoustical beam represented flow. First-order statistics were evaluated, and the signal-to-noise ratio from the signals was measured. The correlation coefficient method was used to present the results. The decorrelation patterns for RF and for RF-envelope signals were studied. The decorrelation patterns from the RF signals were in good agreement with those obtained from theoretical beam profiles. This agreement suggests that the decorrelation properties of an IVUS array catheter for measuring quantitative transverse blood flow can be assessed by measuring the ultrasound beam. A line of point scatterers, moved transversally across the acoustical beam (line spread function), can determine this decorrelation behaviour.

Flow estimation using an intravascular imaging catheter

Coronary flow assessment can be useful for determining the hemodynamic severity of a stenosis and to evaluate the outcome of interventional therapy. We developed a method for measuring the transverse flow through the imaging plane of an intravascular ultrasound (IVUS) catheter. This possibility has raised great clinical interest since it permits simultaneous assessment of vessel geometry and function with the same device. Furthermore, it should give more accurate information than combination devices because lumen diameter and velocity are determined at the same location. Flow velocity is estimated based on decorrelation estimation from sequences of radiofrequency (RF) traces acquired at nearly the same position. Signal gating yields a local estimate of the velocity. Integrating the local velocity over the lumen gives the quantitative flow. This principle has been calibrated and tested through computer modeling, in vitro experiments using a flow phantom and in vivo experiments in a porcine animal model, and validated against a Doppler element containing guide wire (Flowire) in humans. Originally the method was developed and tested for a rotating single element device. Currently the method is being developed for an array system. The great advantage of an array over the single element approach would be that the transducer has no intrinsic motion. This intrinsic motion sets a minimal threshold in the detectable velocity components. Although the principle is the same, the method needs some adaptation through the inherent different beamforming of the transducer. In this paper various aspects of the development of IVUS flow are reviewed.

Intravascular ultrasonic palpation: assessment of local wall compliance

The availability of IVUS imaging has lead to interest in the development of complementary ultrasonic characterization techniques such as the assessment of the mechanical properties of arterial tissues. The authors present a technique to incorporate local compliance information onto the IVUS image. The technique involves analysis of echo signals obtained at two or more states of incremental intravascular pressure. Elasticity information is added in the form of a color coded line congruent with the lumen perimeter. Using vessel phantoms and a specimen the authors demonstrate the utility of intravascular palpation. The palpograms are able to identify lesions of different elasticity independently of the echogenicity contrast. Thus, palpograms may complement the information provided by the image with additional characterization of the lesion.

In vivo validation of blood flow estimation using the decorrelation of radiofrequency intravascular echo signals

Last year we reported on a method for flow estimation from the decorrelation properties of radio frequency intravascular ultrasound signals from blood. This year optimization of the method and validation in an animal study and the first human applications are reported. In vivo validation results obtained in various flow conditions indicate that this method is able to provide accurate and reproducible measure of blood volume flow rate, offering a unique opportunity to simultaneously assess physiologic and anatomic parameters in humans.