Andries Gisolf

Motion compensation for intravascular ultrasound palpography

Rupture of vulnerable plaques in coronary arteries is the major cause of acute coronary syndromes. Most vulnerable plaques consist of a thin fibrous cap covering an atheromous core. These plaques can be identified using intravascular ultrasound (IVUS) palpography, which measures radial strain by cross-correlating RF signals at different intraluminal pressures. Multiple strain images (i.e., partial palpograms) are averaged per heart cycle to produce a more robust compounded palpogram. However, catheter motion due to cardiac activity causes misalignment of the RF signals and thus of the partial palpograms, resulting in less valid strain estimates. To compensate for in-plane catheter rotation and translation, we devised four methods based on block matching. The global rotation block matching (GRBM) and contour mapping (CMAP) methods measure catheter rotation, and local block matching (LBM) and catheter rotation and translation (CRT) estimate displacements of local tissue regions. These methods were applied to nine in vivo pullback acquisitions, made with a 20 MHz phased-array transducer. We found that all these methods significantly increase the number of valid strain estimates in the partial and compounded palpograms (P < 0.008). The best method, LBM, attained an average increase of 17% and 15%, respectively. Implementation of this method should improve the information coming from IVUS palpography, leading to better vulnerable plaque detection.

Acoustic imaging in enclosed spaces: Analysis of room geometry modifications on the impulse response

Sound propagation in enclosed spaces is characterized by reflections at the boundaries of the enclosure. Reflections can be wanted in the case when they support the direct sound or give a feeling of envelopment or they can be unwanted when they lead to echoes and colouration. When measuring multiple impulse responses in an enclosed space along an array the reflections can be mapped to the reflecting objects. Similar to seismic exploration, medical diagnostics, and underwater acoustics, an image of the reflecting objects is obtained in terms of reflected energy. The imaging process is based on inverse wave field extrapolation with the Kirchhoff–Helmholtz and Rayleigh integrals. The inverse of the imaging process recreates the measured impulse responses from the image and it allows one to remove or alter reflecting objects in the image and investigate their influence on the wave field in the enclosed space in a physically correct way. This can be verified by reimaging the altered wave field. Preliminary res...

A 20-40 MHz ultrasound transducer for intravascular harmonic imaging

Recent studies have suggested the feasibility of tissue harmonic imaging (THI) with intravascular ultrasound (IVUS). This paper describes the design, fabrication and characterization of a piezoelectric transducer optimized for tissue harmonic IVUS. Ideally, such a transducer should efficiently transmit a short acoustic pulse at the fundamental transmission frequency and should be sensitive to its second harmonic echo, for which we have chosen 20 MHz and 40 MHz, respectively. The intravascular application limits the transducer dimensions to 0.75 mm by 1 mm. The transducer comprises of a single piezoelectric layer design with additional passive layers for tuning and efficiency improvement, and the Krimholtz-Leedom-Matthaei (KLM) model was used to find iteratively optimal material properties of the different layers. Based on the optimized design a prototype of the transducer was built. The transducer was characterized by water-tank hydrophone measurements and pulse-echo measurements. These measurements showed the transducer to have two frequency bands around 20 MHz and 40 MHz with -6dB fractional bandwidths of 30% and 25%, and round-trip insertion losses of -19 dB and -34 dB, respectively.

Motion compensation for intravascular ultrasound palpography for in vivo vulnerable plaque detection

Intravascular ultrasound (IVUS) palpography assesses the mechanical properties of coronary arteries in vivo by radial strain measurements. This is accomplished by cross- correlating RF signals acquired at different systemic pressures. However, catheter motion due to cardiac activity causes misalignment of these signals, so that less strain estimates are obtained. Four motion compensation methods have been studied for correcting in-plane catheter rotation and translation. The best method, local block matching, achieved an increase of 15% in the number of strain estimates. Application of motion compensation methods improves IVUS palpography, resulting in better vulnerable plaque detection.

An axial array for three-dimensional intravascular ultrasound

With intravascular ultrasound, high resolution cross-sectional images of arterial walls can be obtained. By pulling a catheter through the artery in the axial direction and stacking consecutive cross-sectional images, a three-dimensional image is obtained. However, cardiac and respiratory motion, combined with a high pull-back rate and a narrow axial radiation pattern, result in poor axial resolution and significant image distortion. To avoid these limitations, in this work the design of an axial linear transducer array is presented with which volumetric rather than cross-sectional images are obtained in every pullback position. The prototype consists of eight elements of dimensions 100 μm by 350 μm, with a kerf of 100 μm, operating at a center frequency of 21 MHz with a bandwidth of more than 80 %. For both an ex vivo bovine artery and a stent-emulating phantom, the image quality in the circumferential and radial directions is similar to that of a conventional catheter. However, in the axial direction a significant increase in image quality is achieved. With the array, it is possible to image both the struts of a stent phantom and the underlying structures, whereas with the conventional catheter the struts are not properly localised and the underlying tissue is not distinguishable from the stent.

The design and fabrication of a linear array for three-dimensional intravascular ultrasound

Current intravascular ultrasound catheters generate high resolution cross-sectional images of arterial walls. However, the elevational resolution, in the direction of the catheter, is limited, introducing image distortion. To overcome this limitation, we designed and fabricated a linear array which can be rotated to image a three-dimensional volume at each pullback position. The array consists of eight rectangular piezo-electric elements of 350 µm by 100 µm operating at a center frequency of 21 MHz with a fractional bandwidth of 80 %, separated by a kerf of 100 µm. The array has been tested on both an ex vivo bovine artery and phantoms and, using the real aperture of the array, axially densely sampled images of the artery are obtained in every position. The array consistently yields significantly higher resolution in longitudinal images and more detail in radial images compared to a conventional catheter.