Background

Intravascular ultrasound elastography is a technique that assesses the local strain in the artery wall and plaque [1]. The technique was developed by Ophir and colleagues in 1990 [2] and is based on (quasi-) static deformation of a linear isotropic elastic material. The tissue under inspection is deformed and the strain (direct or indirect using displacement) between pairs of ultrasound signals with and without deformation is determined. The primary goal was the identification and characterization of breast lesions. For intravascular purposes, the compression can be obtained from the systemic pressure difference that is already available in intravascular applications. Additionally, well-controlled deformation is possible by using a transducer positioned in a compliant intravascular balloon.

Principle

The principle of intravascular elastography is illustrated in figure 1. An ultrasound image of a vessel-phantom with a hard vessel wall and a soft eccentric plaque is acquired at a low pressure. In this case, there is no difference in echogenicity between the vessel wall and the plaque resulting in a homogeneous IVUS echogram. A second acquisition at a higher intraluminal pressure (pressure differential is approximately 5 mmHg) is performed. The elastogram (image of the radial strain) is plotted as a complimentary image to the IVUS echogram. The elastogram reveals the presence of an eccentric region with increased strain values thus identifying the soft eccentric plaque [3].

Figure 1: Echogram (left) and elastogram (right) of a vessel mimicking phantom containing an isoechoic soft lesion between 7 and 11 o’clock. The lesion is invisible in the echogram, while it is clearly depicted in the echogram

Detection Technique

The deformation is obtained using cross-correlation analysis. First the boundary between lumen and vessel-wall is determined. Next the signal is divided into windows representing 300 microns of tissue. The windows have an ovelap of 50%. Each window of the signal acquired at the low intra-luminal pressure is cross-correlated with the corresponding window of the signal acquired at the high intra-luminal pressure. The position of the peak of the cross-correlation function correponds with the displacement of the tissue. If the discplacement for all windows is known, the differential displacement between successive windows is determined. After normalizing for the window lenght and overlap, the strain is obtained.

The calculation of the time delay as a function of echodepth is illustrated in fig 2. In the upper part of the figure, subsequent windows of 50 sampling points of both the rf traces are shown. In this figure both traces are interpolated and the second scan is preshifted for improved visual inspection of the shape of the signals Comparison of the 2 signals, before and after compression, shows that the correlation between the signals is high, thus allowing the use of the proposed technique. The cross correlation function was estimated using these windows of the two traces and are shown in the bottom part of the figure. Comparison of the subsequent cross correlation function windows in echodepth shows a decreasing position of the peak of this function, due to the compression of the material. The radial strain profile is calculated using a one dimensional finite difference algorithm.

Figure 2: Principle of time delay estimation using the peak of the crosscorrelation coefficient function. In the upper part, both the rf-traces (with the 2nd trace preshifted for better visual comparison) are shown for windows with increasing echodepth. In the lower part, the corresponding crosscorrelation coefficient function for each window is plotted, showing a decreasing position of the peak with increasing echodepth.

Palpography

Intravascular ultrasonic palpation is a one-dimensional elasticity imaging technique, which we have developed to measure the elastic properties of vascular tissues within the inner layer of the arterial wal1. The technique has the distinct advantage of being simple enough to be implemented in real-time using relatively inexpensive hardware. Additionally, it is generally more robust compared to standard intravascular elastography, and thus more suited for clinical elasticity imaging.

General Principle

The technique is similar to intravascular elastography in that radial strain is estimated by performing cross-correlation analysis on pairs of RF echo frames that are acquired at different intra-coronary pressure. However, compound images that are known as strain palpograms are created by colour coding the measured radial strain profile and superimposing it on the IVUS echogram at the lumen vessel interface, as illustrated in figure 3.

Figure 3: Examples of strain palpograms corresponding to an elastically homogeneous vessel phantom. Showing the strain palpograms produced using pressure differences of 2 and 7 mmHg.

References

[1] C. L. de Korte, A. F. W. van der Steen, E. I. Céspedes, and G. Pasterkamp. “Intravascular ultrasound elastography of human arteries: initial experience in vitro” Ultrasound in Medicine and biology 1998; 24: 401-408
[2] J Ophir, E. I. Céspedes, H. Ponnekanti, Y. Yazdi, X. Li. "Elastography: a method for imaging the elasticity in biological tissues" Ultrasonic Imaging 1991; 13: 111-134
[3] C. L. de Korte, E. I. Céspedes, A. F. W. van der Steen, and C. T. Lancée, “Intravascular elasticity imaging using ultrasound: feasibility studies in phantoms” Ultrasound in Medicine and Biology 1997; 23: 735-746