http://repub.eur.nl/res/aut/8684/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/14552/ 2008-10-01T00:00:00Z Mechanically scanned transducers are currently used for tissue harmonic imaging (THI) and nonlinear microbubble imaging at high frequencies. The pulse inversion (PI) technique is widely used for suppressing the fundamental signal, but its effectiveness is reduced by relative tissue/transducer motion. In this paper, we investigate multipulse inversion (MPI) sequences that achieve a significant improvement on the fundamental suppression for mechanically scanned single-element transducers. MPI was subsequently applied on simulated and measured RF-data and relative fundamental suppression was compared with the 2-pulse PI technique. Simulations showed, for example, an increased fundamental suppression of 6 and 10 dB for MPI-sequences that combined 3 and 7 pulses, respectively, for a rotating intravascular ultrasound transducer with an interpulse angle of 0.15°. Initial application of MPI sequences on RF-data from in vivo acquisitions resulted in similar fundamental suppression levels. The investigated MPI technique will help to reduce relative tissue/transducer motion effects and might lead to improved sensitivity and spatial resolution in nonlinear tissue imaging and improved microbubble detection in contrast imaging for mechanically scanned transducers. http://repub.eur.nl/res/pub/35990/ 2007-12-01T00:00:00Z The feasibility of subharmonic contrast intravascular ultrasound (IVUS) imaging was investigated using a prototype nonlinear IVUS system and the commercial contrast agent Definity™. The system employed a mechanically scanned commercial catheter with a custom transducer element fabricated to have sensitivity at both 15 and 30 MHz. Experiments were conducted at a fundamental frequency of 30 MHz (F30; 25% bandwidth), with on-axis pressures ranging from 0.12 to 0.79 MPa, as measured with a needle hydrophone. In vitro characterization experiments demonstrated the detection of 15 MHz subharmonic signals (SH15) when pressure levels reached 360 kPa. The formation of SH15 images was shown, with tissue signals suppressed to near the noise floor and contrast to tissue ratios were improved by up to 30 dB relative to F30. In vivo experiments were performed using the atherosclerotic rabbit aorta model. Following the bolus injection of contrast agent upstream of the imaging catheter, agent was detected within the aorta, vena cava and within the perivascular space. These results provide a first in vivo demonstration of subharmonic contrast IVUS and suggest its potential as a new technique for imaging vasa vasorum. (E-mail: goertz@sri.utoronto.ca). http://repub.eur.nl/res/pub/31788/ 2006-12-22T00:00:00Z Recently, the in vivo feasibility of tissue harmonic imaging (THI) with a mechanically-rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, both pulse inversion (PI) and analog filtering were used. In the present paper, we report the development of a simulation tool to investigate nonlinear IVUS beams and the influence of rotation on the efficiency of PI signal processing. Nonlinear 20 MHz beams were simulated in a homogeneous tissue-mimicking medium, resulting in second harmonic pressure fields at 40 MHz. The acoustic response from tissue was simulated by summing radio-frequency (RF) pulse-echo responses from many point-scatterers. When the transducer was rotated with respect to the point-scatterers, the fundamental frequency suppression using PI degraded rapidly with increasing inter-pulse angles. The results of this study will aid in the optimization of harmonic IVUS imaging systems. http://repub.eur.nl/res/pub/7950/ 2006-09-08T00:00:00Z Medical ultrasound is a popular imaging modality in cardiology. Harmonic Imaging is a technique that has been shown to increase the image quality of diagnostic ultrasound at frequencies below 10 MHz. However, Intravascular Ultrasound, which is a technique to acoustically investigate arteries from within the lumen by means of a catheter, typically transmits at higher ultrasound frequencies ( >20 MHz). This dissertation describes studies showing the feasibility and potential of Harmonic Imaging for Intravascular Ultrasound. Harmonic Intravascular Ultrasound can reduce undesired image artefacts, and shows the potential to detect ultrasound contrast agents from within the lumen. This makes Harmonic Intravascular Ultrasound a promising tool in cardiovascular research.