Objective: Currently, intravascular ultrasound (US) imaging catheters are developed and produced to provide a complementary diagnostic method in the treatment of blood vessel obstructive disease. Typical catheter dimensions are a diameter of 1-2.5 mm and a length of 1-1.5 m. A real-time 360° US scan of the surrounding of a catheter tip can be obtained by mechanical rotation of a US beam, approximately perpendicular to the catheter axis and originating from a single element. It is the purpose of this paper to make the reader aware of the various image artifacts that might occur with flex-shaft mechanical scanning and of the limitations of the technique, and to discuss the possible causes as well as alternative drive mechanisms. Methods: The study is based on definition and discussion of various error mechanisms caused by improper transmission of rotation flex-shaft drives. Adequate ultrasonic imaging implies a one-to-one relationship between the catheter tip scan rotation angle and the angle of polar image line deflection on the display. This can be achieved by applying a predictable, uniform scan rotation at the catheter tip. Deviation from an intentionally uniform scan rotation gives rise to image artifacts, causing either loss of absolute orientation (due to a constant error angle), loss of quality of the real-time aspect (due to a stochastic error angle), or the occurrence of deceptive image distortions (due to a periodic error angle). The magnitude of an acceptable constant tip rotation error (actual tip rotation angle minus expected angle) depends on the need for maintaining an absolute orientation of the image. The stochastic error should be kept to a minimum: random misplacement of image lines in the order of degrees already is disturbing to the interpretation of the image. As a practical arbitrary criterion for the periodic error angle, a maximum peak-to-peak value of 20° can be defined. Results: Catheter tip driving mechanisms have to be evaluated, considering their potential to maintain a predictable (often uniform) catheter tip scan rotation. A 0.8 mm OD double-layer spiral type flexible drive-shaft has been developed, which meets the requirements, exhibiting a bending rigidity of 1.5 Nmm2 and a torsional rigidity of more than 100 Nmm2. This drive-shaft shows a sensitivity to influences, inducing rotation error angles, like pre-curvature of the shaft and friction, of about 1% compared to a simple wire of e.g. stainless steel. Conclusion: For practical purposes, today's technologies in the production of flex-shafts have improved and these devices can be very usefully applied in clinical situations. Flex-shaft problems can be avoided if, alternatively, rotation power could be provided in the catheter tip itself. To this effect, an electromagnetic micromotor has been developed, with a diameter of 1 mm, exhibiting adequate running behaviour. Implementation of this motor in a catheter tip is in progress.
|Number of pages||11|
|Journal||European Journal of Ultrasound|
|Publication status||Published - Jul 1995|
Bibliographical noteFunding Information:
This research has been financially supported by the Netherlands Technology Foundation and the Dutch Ministry of Economic Affairs. Practical support by and fruitful co-operation with the division of Precision Engineering of the Technical University of Delft and Du-MED B.V., Rotterdam, both in The Netherlands and with USCI, C.R. Bard Inc., Billerica MA, USA, are gratefully acknowledged.