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The history of intraluminal echography dates back to the very beginning of diagnostic ultrasound. Over the years many fascinating ideas and applications of catheter tip or gastroscopic tube tip mounted transducers have been described. This chapter surveys these methods, subdividing them into a) measurements; b) Doppler and c) imaging. The survey ranges from early work of Cieszynski on the feasibility of echocardiography to more recent intra-arterial catheter tip Doppler with guidewire and balloon as described by Serruys. Examples of ultrasound catheter tip echography in combination with other techniques such as angioscopy, laser ablation and spark erosion are also described. Today practical approaches are limited to imaging only. The three major approaches for catheter tip echo imaging are described and compared. This paper concludes with the results of automatic contour analysis of the inner arterial boundaries.

Contrast arteriography is currently the principal method for assessment of the presence and severity of both peripheral and coronary vascular anatomy. With the rapid progress in interventional radiology and cardiology, there is an increasing need for more specific information on vascular pathology and better understanding of its pathophysiology. Noninvasive ultrasound imaging may allow cross-sectional visualization of certain but limited portions of the peripheral arterial system [1, 2] but has serious limitations for imaging the coronary arteries [3,4]. The intraoperative epicoronary application of highfrequency transducers has confirmed the abnormal findings of earlier pathologic studies and further corroborated the insensitivity of coronary arteriography in appraising the extent and distribution of atherosclerotic wall disease [5]. Fiberoptic angioscopy allows visualization of the inner surface of the arterial wall and has significantly added to our understanding of acute ischemic syndromes. The method, however, has practical limitations and no information on the vessel wall under the endothelium is obtained [6].Clearly, there is a need for an imaging method allowing the study of the vessel wall and both the characterization and quantification of its pathology. Ultrasonic real-time intravascular echography allows cross-sectional i.e. circumferential imaging providing information on the arterial wall under the endothelial surface [7]. This technique has unique and fundamental advantages over presently available techniques. In this contribution we will describe the principles of intravascular real-time, two-dimensional echocardiography and give a perspective of the potential applications of this exciting new method.

Mechanically driven catheter tip echo systems presently operate with a flexible shaft. Rotation power from a proximally mounted motor is transferred via this shaft to the rotating echo tip element. In practice, the tip does not identically 'follow' the rotation of the motor due to low torsional rigidity of the shaft, which creates artifacts in the displayed cross-sectional image. In order to visualize curved arteries such as the coronary arteries, a compromise is necessary between the required low flexural rigidity and a high torsional rigidity. In this report the image artifacts of mechanically driven systems are presented that are related to catheter tip motion. The properties of a spiral drive-shaft and a solid drive-shaft have been compared for rotational speed of 1000 and 3000 revolutions per minute (rpm), and for straight as well as strongly curved catheters. By way of example, the periodic angle error varies from 25 degrees top-top in a straight catheter to 80 degrees top-top when the catheter is curved with R = 20 mm, using a spiral drive-shaft at 1000 rpm.

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 oneto-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 Nmm 2 and a torsional rigidity of more than 100 Nmm 2. This drive-shaft shows a sensitivity to influences, inducing Hoff et al. /European Journal of Ultrasound 2 (1995) [227][228][229][230][231][232][233][234][235][236][237] 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 ram, exhibiting adequate running behaviour. Implementation of this motor in a catheter...