Modified weighting method for forward-looking ring-annular arrays.

Side-looking Intravascular Ultrasound Systems (IVUS) systems provide high resolution cross-sectional images of vascular structure. However, blood velocity estimation is hampered by the orthogonality of the primary blood flow direction to the imaging plane, and do not image in front of the catheter. A forward-looking aperture would improve the ability to guide interventions and allow Doppler processing for improved blood speed estimates; however, catheter-based systems delivered over a guide wire limit forward-looking array geometry to annular variations. Unfortunately, the imaging characteristics of annular arrays are inferior to a full aperture disk. A modified weighting method for a practical synthetic phased, ring-annular array is presented that achieves a transmit-receive aperture and on-axis performance comparable to a full aperture disk array.

Quantitative blood speed imaging with intravascular ultrasound.

Previously, we presented a method of real-time arterial color flow imaging using an intravascular ultrasound (IVUS) imaging system, where real-time RF A-scans were processed with an FIR (finite-impulse response) filter bank to estimate relative blood speed. Although qualitative flow measurements are clinically valuable, realizing the full potential of blood flow imaging requires quantitative flow speed and volume measurements in real time. Unfortunately, the rate of RF echo-to-echo decorrelation is not directly related to scatterer speed in a side-looking IVUS system because the elevational extent of the imaging slice varies with range. Consequently, flow imaging methods using any type of decorrelation processing to estimate blood speed without accounting for spatial variation of the radiation pattern will have estimation errors that prohibit accurate comparison of speed estimates from different depths. The FIR filter bank approach measures the rate of change of the ultrasound signal by estimating the slow-time spectrum of RF echoes. A filter bank of M bandpass filters is applied in parallel to estimate M components of the slow-time DFT (discrete Fourier transform). The relationship between the slow-time spectrum, aperture diffraction pattern, and scatterer speed is derived for a simplified target. Because the ultimate goal of this work is to make quantitative speed measurements, we present a method to map slow time spectral characteristics to a quantitative estimate. Results of the speed estimator are shown for a simulated circumferential catheter array insonifying blood moving uniformly past the array (i.e., plug flow) and blood moving with a parabolic profile (i.e., laminar flow).

Blood speed imaging with an intraluminal array.

In applications in which Doppler processing is not possible, such as side-looking intravascular imaging systems, decorrelation methods can be used to estimate blood speed. Here, a method is presented measuring relative blood speed using an FIR filter bank to estimate temporal decorrelation rates. It can be implemented in a modern commercially available ultrasound imaging system with no additional hardware. Both simulations and experiments using an intraluminal scanner appropriate for coronary artery applications have tested the system. In this study, the FIR filter bank is contrasted with previous methods, and its utility is further demonstrated with real-time color flow images from a pig model.

Strain imaging of coronary arteries with intraluminal ultrasound: experiments on an inhomogeneous phantom.

In coronary arteries, knowing the relative stiffness of atherosclerotic lesions can help physicians select the most appropriate therapeutic modality. Because soft material supports larger strains than hard, measurements of this quantity can distinguish tissue of differing stiffness. In a previous paper, we described techniques for computing displacements and strains in coronary arteries using an integrated angioplasty and imaging catheter. Here, we demonstrate that hard and soft materials in a tissue-mimicking phantom can be differentiated with this device. Because tissue motion cannot be distinguished from catheter motion a priori, we perform all computations in the coordinate system centered at the balloon's geometric center. This reference frame depends only on balloon shape and is independent of catheter motion. A specialized correlation-based, phase-sensitive speckle tracking algorithm has been developed to compute strain. Maximum phantom displacement was about 25 microns, and the maximum radial, normal strain was about 1.5 percent.