Timothy L. Proulx

Single crystals vs. pzt ceramics for medical ultrasound applications

The major advantages of single crystals (PMN-PT and PZN-PT) over conventional PZT 5H are their high piezoelectric and electro-mechanical coupling constants, which are attractive for high performance transducers. Other properties that contribute to transducer performance, such as velocity, clamped dielectric constant and coercive field, are generally lower for single crystals than for PZT. This paper reviews the strengths and limitations of single crystals for medical ultrasound applications. It is likely impractical to use this material for devices with center frequencies higher than 6 MHz, because of the exceptional fragility, the low sound velocity and the low coercive field of single crystal material. For very low frequency arrays with small elements, electrical impedance becomes very high, which causes severe mismatch to the cable and system. An example of a 2.5MHz fine-pitch array with PMN-PT and a triple- layer PZT-5H with simulated and experimental data shows that the single layer PMN-PT array underperforms the multi-layer PZT transducer. Our experience suggests that optimal applications for single crystals in medical ultrasound applications are transducers with 3-5MHz center frequencies and preferably large elements. The results for a 300 µm pitch, 3MHz PMN-PT array with 90% fractional bandwidth compared to 75% for the same PZT array are presented.

Advances in catheter-based ultrasound imaging Intracardiac Echocardiography and the ACUSON AcuNavTM Ultrasound Catheter

Conventional transthoracic and transesophageal ultrasound imaging often cannot satisfy imaging requirements for many advanced catheter-based interventional cardiac procedures due to restricted access to the anatomy. Intracardiac echocardiography (ICE) can provide almost unrestricted access and has been shown to be very effective in accurately guiding interventional cardiac procedures by providing high quality visualization of intracardiac anatomy and intracardiac devices. ICE is steadily replacing transesophageal ultrasound as the preferred imaging tool for device guidance during atrial fibrillation treatment, valve repair and closure of atrial septal defects. This article provides a history of the ACUSON AcuNav™ Ultrasound Catheter, a discussion of key technologies employed and a review of the expanding range of clinical applications where ICE technology is utilized. We also discuss recent innovations in catheter-based ultrasound, such as three- dimensional (3D) tools for navigation and volume rendering, new imaging modalities like acoustic radiation force imaging (ARFI) and emerging applications for catheter-based ultrasound in the field of minimally invasive surgery.