GB2436066A

GB2436066A - Catheter having electromagnetic ablation means and plural, symmetric ultrasonic transducers

Info

Publication number: GB2436066A
Application number: GB0605425A
Authority: GB
Inventors: David Groves; Roger Kevin Geoffrey Moore
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-03-17
Filing date: 2006-03-17
Publication date: 2007-09-19
Also published as: GB0605425D0

Abstract

An imaging catheter 52, which can be advanced to the site of an occlusion 102 in a body conduit 100, receives means for directing substantial forward facing electromagnetic energy into the occlusion and has a distal tip having plural symmetrically arranged ultrasound transducers 62 round and beyond the tip. The electromagnetic source for breaking up the occlusion or blockage may be RF or microwave radiation, or light provided by a laser wire 64. At least three steering wires 54 may be provided to orientate catheter tip by compressing a soft material torus 58 against a rigid backing plate 60. Further low resolution ultrasonic transducers may be provided, symmetrically disposed about the cone shaped catheter tip. The use of plural symmetrically spaced ultrasound transducers allows for improved positioning and image field of view compared with prior catheters as shown in Figures 1-3.

Description

I

CATHETER FOR ACCURATELY POSITIONING AN

ELECTROMAGNETIC ENERGY SOURCE AT

THE SITE OF AN OCCLUDED VESSEL

This invention relates to an imaging catheter for use in accurately positioning an electromagnetic energy source at the site of an occlusion. In particular, this invention relates to a catheter which is capable of being advanced and orientated by an operator to the site of an occlusion in a conduit or blood vessel. The catheter includes at least one forward-facing ultrasound transducer which images around and beyond the distal tip of the catheter. The imaging catheter is also adapted to receive means for directing electromagnetic energy into the occlusion to be ablated which is, in turn, electrically connected to a remote electromagnetic energy source.

Blockages that form in coronary arteries are a significant factor in the occurrence of heart attacks and strokes. If the coronary arteries become blocked, minimally-invasive procedures using a catheter to open them and restore normal blood flow are now well-known. Techniques such as balloon angioplasty and stenting have been used for a number of years for successfully treating partially occluded blood vessels.

Total chronic occlusions are typically characterised by a buildup of cholesterol, white blood cells, calcium, and other substances in the walls of arteries which completely seals off blood flow in the vessel. Balloon catheters are not suitable for crossing total chronic occlusions and, generally, if a total occlusion is encountered in the patient's vasculature, one option available to the physician would be to attempt to poke the coronary guide wire through the occlusion. Of course the problem associated with this approach is that there is a significant risk of damaging the blood vessel. In order to address this problem, catheters that include some form of cutting device have been proposed, with only limited success.

In a further attempt to address this problem, procedures which utilise a laser wire catheter have been proposed. With this approach, a laser wire catheter is advanced through the patient's vasculature using a previously-positioned coronary guide catheter, and which provides an open channel to the site of the occlusion for various other catheters. The tip of the laser wire has a small bend; the angle of which is set by an operator by bending the end of the wire. The tip is guided through the patient's vasculature by the operator who can see the tip in a "live" planar x-ray image of the patient's thorax. To negotiate bends in the blood vessels and avoid or enter vessels at junctions, the operator appropriately rotates the wire which rotates the angled tip towards or away from junctions, as necessary. The wire in this manner being advanced to the site of the occlusion. The operator would then try to position' the angled tip of the laser wire to the centre of the occlusion using the live radiographic image before firing the laser. However, misplacement of the tip frequently results in damage to the walls of the vessel at the boundary between the vessel wall and the occlusion.

In order to address this further, US 6,394,956 relates to a combination ultrasound and RF ablation catheter system; the distal tip of which is shown in Fig. 1. Fig. I shows a sleeved catheter 10 that is capable of being routed through a patient's vasculature to the site of an occlusion (not shown). At the distal end of the sleeved catheter 10 is a combination RF ablation electrode 12 and an ultrasound transducer 14; the echo signals of which are transmitted from the ultrasound transducer 14 back to an ultrasound processor (not shown) to produce an image of the blood vessel on a monitor (not shown). To provide a 360 view of the vessel, the ultrasound transducer 14 is rotated at high-speed, via a conductive drive shaft 16, to build up a complete image of the blood vessel.

The ultrasound device described in US 6,394,956 utilises a linear array of rotating ultrasound transducers 14. For a given line of ultrasound transducers 14, the direction of imaging is orthogonal to the line. The angle of view a around this orthogonal centre is very limited and dependent on the frequency of the ultrasound, the spacing of the transducers 14 and the number of transducers 14 in the line. The ultrasound is directed in a plane or fan' which passes through the centre of rotation and the line of transducers 14 by phasing the output from the transducers 14 in the line. To achieve undistorted images of sufficient angle of view a, the line of transducers 14 required turns out to be longer than the width of coronary vessels. As such, there are plethora of known sideways-propagating ultrasound devices available for substantially-sideways imaging where the required length of the line of transducers can be accommodated only along the length of the catheter.

In US 6,394,956, to enable some of the tip of the RF electrode 12 at the front of the catheter 10 to be imaged, two compromises in the geometry have been made, as is shown in Fig. 1. In particular, (i) the rotating linear array of ultrasound transducers 14 has been tilted forward, and (ii) the RE electrode 12 is mounted eccentrically. This has to be done to enable the eccentric tip of the electrode 12 to be seen at the edge in the ultrasound image.

However this is a serious disadvantage as coronary vessels 100 with occlusions 102 are typically very narrow, as is illustrated in Fig. 2. In this configuration, the tip of the eccentrically-mounted RF electrode 12 can only be brought to the centre of the occlusion 102 by the body of the catheter 10 being held at an angle to the long axis of the vessel 100 immediately before the occlusion 102. This geometry does not allow the centre of the occlusion 102 and the contact point of the RF electrode 12 to be imaged in the vessel 100. It is also worth noting in Fig. 2 that the RF electrode 12 and occlusion 102 point of contact are not imaged, and the proximity of the lower surface of the RF electrode 12 to the vessel wall 100.

As shown in Fig. 3, when the combination catheter 10 is parallel to the long axis of the blood vessel 100 (in typically narrow vessels), imaging does not aid positioning of electrode 12, other than (as the imaging device 14 is rotating) inform the operator when the device in the centre of the vessel 100 and parallel to the long axis of the vessel 100 in the portion of vessel 100 behind the tip which has been imaged. In short, all prior art vascular imaging-based devices for dealing with occlusions make use of conventional "good quality" images of the sides of the vessel 100 behind the tip, which delivers RF energy to destroy the occlusion 102 in front of the tip. The operator has to make the assumption that the direction of the vessel 100 ahead of the tip is the same as the direction of the vessel 100 behind the tip.

Thus, with severe bends in the vessel 100, so-called "side image steering" can result in the tip burning through the vessel wall 100. The limitation of imaging the vessel 100 behind the tip is due to the assumption that minimally distorted and good resolution images are necessary requiring a line of ultrasound transducers 14 which can only fit along, rather than across, the catheter 10.

Clearly, there is a significant need for a catheter which is capable of being advanced and orientated by an operator to the site of a total chronic occlusion in a blood vessel and then utilising a substantially forward-facing ultrasound scanner to accurately position and direct electromagnetic radiation into the occluding tissue to be ablated.

It is the object of the present invention to provide a system for crossing an occlusion in a body conduit that is minimally-invasive and reliable. The system includes a catheter having a forward-facing ultrasound scanner which images around and beyond the tip of the catheter. Due to the small diameter of the blood vessels through which the catheter is to be introduced, high-resolution ultrasound imaging is not possible or indeed practicable and it is another object of the present invention to provide a catheter which provides an image with little distortion or circularly symmetric distortion, so that the operator is still able to accurately position the catheter. The catheter being adapted to receive means for directing electromagnetic energy into said occlusion, which is, in turn, electrically connected to a remote electromagnetic energy source. In use, removal of the occlusion without unacceptable damage to the walls of the blood vessel will require repeated applications of the energy source; the tip of the catheter being advanced, positioned and orientated appropriately by the operator before each application of the energy source.

According to the present invention there is provided a system for crossing an occlusion in a body conduit, comprising an imaging catheter which, in use, can be advanced and orientated to the site of said occlusion in said conduit, the catheter being adapted to receive means for directing substantially forward-facing electromagnetic energy into said occlusion, said means for directing electromagnetic energy being electrically connected to a remote electromagnetic energy source, the distal tip of said catheter characterised by: a plurality of ultrasound transducers for imaging around and beyond said distal tip of said catheter, said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter.

Preferably, said catheter is advanced and orientated through said body conduit by mechanical, electrical or other control means. In use, said catheter further comprises at least three steering wires being symmetrically disposed around the body of said catheter. In use, each of said at least three steering wires orientates said distal tip of said catheter by compressing a soft material torus against a rigid backing plate located on said distal tip of said catheter.

In a preferred embodiment, said means for directing substantially forward-facing electromagnetic energy into said occlusion is a laser wire. In use, said remote electromagnetic energy source provides sufficient power and divergence to ablate said occlusion without unacceptable damage to said body conduit.

Further preferably, said plurality of ultrasound transducers transmit ultrasound signals around and beyond said distal tip of said catheter and receive corresponding echo signals. An ultrasound processor preferably processes said corresponding echo signals to provide an image around and beyond said distal tip of said catheter on a monitor or other display or data capture means.

In use, said ultrasound processor processes said corresponding echo signals in real-time and said image obtained around and beyond said distal tip of said catheter is used to position said catheter substantially in the centre of said body conduit to ablate said occlusion. Said plurality of ultrasound transducers may comprise at least one array of low-resolution ultrasound transducers. Preferably, said plurality of ultrasound transducers further comprises at least three lines of low-resolution ultrasound transducers being symmetrically disposed around a conal-shaped distal tip of said catheter.

Further preferably, said ultrasound signals radiate orthogonally from the central axes of said lines of low-resolution ultrasound transducers being symmetrically disposed around a conal-shaped distal tip of said catheter.

Preferably, the distance between said distal tip of said catheter and said occlusion is determined by sonar measurements of said corresponding echo signals. The distance between said distal tip of said catheter and said occlusion may be determined by phase measurements of said corresponding echo signals. In one embodiment, said sonar measurements of said corresponding echo signals are performed using a focusing device positioned between said plurality of ultrasound transducers and said occlusion.

In a preferred embodiment, said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter reduces image distortion by distributing said plurality of ultrasound transducers spatially about the long axis of said catheter tip or by rotating at least one curvilinear array of ultrasound transducers temporally around said distal tip of said catheter. Rotating at least one curvilinear array of ultrasound transducers temporally around said distal tip of said catheter may be achieved by mechanically or electrically rotating at least one curvilinear array of ultrasound transducers about said long axis of said catheter tip.

In use, said distal tip of said catheter is substantially formed as a cone, truncated cone or other pointed shape to assist advancement through said body conduit. In a further embodiment, said distal tip of said catheter is substantially annular or exhibits substantially hemispherical symmetry.

Also according to the present invention there is provided an imaging catheter which, in use, can be advanced and orientated to the site of an occlusion in a body conduit, the catheter being adapted to receive means for directing substantially forward-facing electromagnetic energy into said occlusion, the distal tip of said catheter characterised by: a plurality of ultrasound transducers for imaging around and beyond said distal tip of said catheter, said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter.

It is believed that a system for crossing an occlusion in a body conduit in accordance with the present invention at least addresses the problems outlined above. In particular, the advantages of the present invention are that a system for crossing an occlusion in a body conduit is provided that is minimally-invasive and reliable. Advantageously, the system includes a catheter having a forward-facing ultrasound scanner which images around and beyond the tip of the catheter. Due to the small diameter of the blood vessels through which the catheter is to be introduced, high-resolution ultrasound imaging is not possible or indeed practicable and, advantageously, the present invention provides a catheter which provides an image with little distortion or circularly symmetric distortion so that the operator is still able to accurately position the catheter. The catheter being adapted to receive means for directing electromagnetic energy into said occlusion, which is, in turn, electrically connected to a remote electromagnetic energy source.

Advantageously, in use, removal of the occlusion without unacceptable damage to the walls of the blood vessel will require repeated applications of the energy source; the tip of the catheter being advanced, positioned and orientated appropriately by the operator before each application of the energy source.

It will be obvious to those skilled in the art that variations of the present invention are possible and it is intended that the present invention may be used other than as specifically described herein.

A specific non-limiting embodiment of the invention will be described by way of example and with reference to the accompanying drawings in which: Fig. I illustrates a side view of a prior art combination ultrasound and RF ablation catheter.

Fig. 2 shows a side view of the prior art combination ultrasound and RE ablation catheter of Fig. I in a blood vessel.

Fig. 3 depicts a further side view of the prior art combination ultrasound and RE ablation catheter of Fig. I in a blood vessel.

Fig. 4 illustrates a side view of a catheter according to the present invention.

Fig. 5 shows a side view of a catheter according to the present invention located at the site of an occluded vessel.

Referring now to the drawings, the implementation of the present invention is illustrated in Figs. 4 and 5. The catheter 50 is shaped as a flexible, substantially elongate tube and formed from a suitable plastics material. The catheter 50 includes a steerable distal tip 52 under the control of steering wires 54, which are also used to advance and orientate the catheter 50 through a patient's vasculature. Alternately, the skilled person will appreciate that providing a catheter 50 with a fixed distal tip at a predetermined angle will still permit accurate orientation control of the catheter 50 by rotation of the steering wires 54.

As shown in Fig. 4, each of the steering wires 54 are contained within an outer sheath 56. The distal tip 52 of the catheter 50 can then be tilted using steering wires 54 which compress a soft material torus 58 against a rigid backing plate 60. For complete control of the tilt direction of the distal tip 52 without rotation, a minimum of three steering wires 54 are required, each of which can be tensioned and locked as required.

Situated at the distal tip 52 of the catheter 50 is at least one substantially forward-facing ultrasound transducer 62 which images around and beyond the distal tip 52 of the catheter 50. The ultrasound transducer 62 transmits ultrasound signals around and beyond the distal tip 52 of the catheter 50 and receives corresponding echo signals which are then taken to an ultrasound processor (not shown) through an ultrasound trailing loom (not shown), situated at the proximal end of the catheter 50. The ultrasound signals are then processed and used to produce an image surrounding distal tip 52 of the catheter 50 using the ultrasound processor (not shown) on a monitor or other display or data capture means (not shown). In use, the ultrasound transducer 16 provides real-time images around and beyond the distal tip 52 of the catheter 50 so that the operator is able to accurately target the catheter 50 in the patient's vasculature, as detailed below in relation to Fig. 5.

Figs. 4 and 5 show that the catheter 50 is adapted to receive means for directing electromagnetic energy into the occluding tissue 102 to be ablated.

As shown in Figs. 4 and 5, this is achieved using a forward-facing laser wire 64, which is used to propagate laser energy outwards towards the occlusion 102 to facilitate ablation of the occlusion 102. The means for directing electromagnetic energy is connected to a remote electromagnetic energy source, via a power loom (not shown), situated at the proximal end of the laser wire 64. The skilled person will appreciate that the electromagnetic energy source and means for directing electromagnetic energy into the occluding tissue 102 could be provided by, for example, RF or microwave radiation, and this is in no way intended to be limiting.

In use, the laser wire 64 is firstly advanced to the site of the occlusion 102 using a previously-positioned coronary guide catheter (not shown). Once the laser wire 64 is approximately positioned in the vicinity of the occlusion 102 using live planar radiographic images, the imaging catheter 50 is then mounted on the laser wire 64 which passes coaxially through its centre with its trailing loom, and is advanced to the end of the laser wire 64. When it is positioned at the end of the laser wire 64, using an end stop' or by a marker on the loom, the imaging catheter 50 is able to image substantially in the forward direction, giving the operator a targeting ultrasound image of the occlusion 102 and its interface with vessel walls 100 in the direction the laser wire 64 will be fired, forward of the tip of the catheter 50, even in the narrowest of vessels 100.

In use, the flexible catheter 50 is advanced and orientated by mechanical, electrical or other means of control, via the steering wires 54, to the site in the blood vessel 100 (or other body conduit) of the total occlusion 102. The laser wire 54 fires, under the control of the operator, electromagnetic laser radiation in a substantially forward direction towards the centre of the face of the occlusion 102 with sufficient power and divergence to remove part or all of the occlusion 102 without unacceptable damage to the walls of the vessel 100. In general, removal of the occlusion 102 without unacceptable damage to the walls of the vessel 100 will require repeated applications of the laser 64, the distal tip 52 of the catheter 50 being advanced, positioned and oriented appropriately by the operator before each application of the laser 64.

To enable accurate positioning and orientation of the catheter tip 52, the ultrasound transducer 62 is configured to provide real-time ultrasound imaging with a direction of view /1 broadly in the forward direction and substantially parallel to the direction in which the laser 64 fires. The ultrasound image obtained from the ultrasound transducer 62 is required to convey to the operator when the distal tip 52 of the catheter 50 is centrally placed in the vessel 100, at the correct distance from the face of the occlusion 102, with the laser wire 64 running parallel to the central axis of the catheter tip 52 pointing at the centre of the face of the occlusion 102 such that the laser 64 can be fired.

In conduits such as blood vessels 100, which have cross-sectional circular symmetry or cross-sectional elliptical symmetry about the ellipse's major and minor axes, the three dimensions of position and orientation can be conveyed to the operator using only low-resolution ultrasound imaging, where high-resolution imaging is not possible or practicable. In both cases, a circular or elliptically symmetric image of minimum eccentricity of the boundary between the face of the occlusion 102 and the inner walls of the vessel 100 will occur only when the tip 52 of the catheter 50 is both at or near the centre of cross-section of the vessel 100 and the distal tip 52 is pointing at the centre of the face of the occlusion 102. Recognition of symmetry by the operator does not depend on resolution of the ultrasound image. Distortion of the ultrasound image will not affect the operator's recognition of the target symmetry in the image, provided the distortion exhibits circular symmetry around the central axis of the catheter tip 52.

In a preferred embodiment, multiple lines of at least three ultrasound transducers 62 are placed symmetrically along the canal tip of the of the catheter tip 52 such that no rotation is required for forward-facing ultrasound imaging. When, and only when, both the catheter 50 is at the centre of the vessel 100 and the central axis of the catheter tip 52 (through which the laser wire 64 is advanced) is parallel to the walls of the vessel 100 ahead of the tip 52, an (unachievable) ideal undistorted ultrasound image of the margin between the occlusion 102 and the vessel waIls 100 would be near circular.

In practice, the shortness of the transducer lines 62, containing as little as three transducers per line, short enough to fit across a canal head of the catheter tip 52 enabling them to fit across the forward surface of the catheter will produce significant distortion and loss of resolution. However, as the individual identical lines of transducers 62 will produce identical distortions in each of the fans' of view radiating orthogonally from the central axis of the cone, the forward looking ultrasound image will contain only circular symmetric distortion. Circular symmetric distortion when imaging a circular symmetric vessel 100 preserves circular symmetry in the resulting image.

Therefore the low resolution and distorted image will have circular symmetry when, and only when, the imaging catheter 50 is correctly positioned both at the centre of the vessel 100 and pointing at the centre of the occlusion 102 ahead of the tip 52.

The result is that the operator can be sure the catheter 50 is placed correctly when, and only when, the image obtained (even though it contains distortions and is of low resolution) exhibits circular symmetry. In addition to having operator viewing of this image, the skilled person will appreciate that it would be possible to apply automatic image processing to guide the position interactively, or even automatically, using the described invention.

In operation, the imaging catheter 50 would be correctly positioned using the symmetry of the image as a guide and the laser wire 64 would be advanced forward and fired directly at the centre of the occlusion 102. Where the vessel 100 bends, the imaging catheter 50 (and hence the direction in which the laser wire 64 will advance through the catheter 50) can be tilted using the steering wires 54 to compress the soft material torus 58.

In practice, the laser wire 64, which is first introduced to the vessels 100, has a small kink at its tip, placed there by the operator to help guide its passage through the vessels 100. The imaging catheter 50, when first introduced onto the laser wire 64, would travel initially to the very end of the wire 64 to straighten out the kink. Then the process of making a path through the occlusion 102 would begin, positioning the imaging catheter 50 and then incrementally advancing and firing the laser wire 64 or other energy delivery device. Once a path through the occlusion 102 has been formed, a conventional balloon catheter could be placed through the new channel and the vessel 100 further widened in the conventional way.

In use, the distance of the occlusion 102 from the catheter tip 52 is determined by sonar measurement of the ultrasound signals or by focusing of the ultrasound signals produced by either phasing of the ultrasound signals transmitted by an array of ultrasound transducers 62 or by including a physical focusing device (not shown) positioned between the array of ultrasound transducers 62 and the face of the occlusion 102.

As mentioned above, forward-facing ultrasound imaging can both be of low resolution and exhibit distortion because of the required small size of the catheter tip 52, thus limiting the number of ultrasound transducers 62 and the required shaping of the tip 52 which limits the possible 2/3 dimensional distributions of ultrasound transducers 62. Circular symmetry of image distortion can be achieved by distributing the ultrasound transducers 62 symmetrically about the long axis of the catheter tip 52 either spatially (a 2/3 dimensional array of ultrasound transducers 62 mapped symmetrically around the tip 52) or temporally (by rotating a curvilinear array or number of curvilinear arrays of ultrasound transducers 62 mapped along the tip 52 in line with the central axis of the catheter 50). The form of the distribution of ultrasound transducers 62 at the tip 52 of the catheter 50 is further confined by a requirement for a conal (or other pointed') shape for the catheter tip 52 to aid its advancement through vessels to the site of the occlusion 102.

However, provided the required shaping of the catheter tip 12 has circular symmetry about the long axis of the tip 52, a symmetric array of ultrasound transducers 62 can be formed around the central axis of the tip 52 or a curvilinear array ultrasound transducers 62 with mechanical, electrical or other circularly symmetric rotation about the central axis of the catheter tip 52.

It is also envisaged that forward-facing, real-time ultrasound imaging of vessels can also be achieved with little distortion or circularly symmetric distortion in a number of proposed ways. These include providing a catheter tip 52 having an annular flat end; phase-steerable ultrasound imaging; distributing the ultrasound transducers 62 on offset annular rings and mapping the corresponding echo signals to a cone, hemisphere or other tip 52 having circular symmetry; distributing a 2-dimensional array of transducers mapped onto 3-dimensional circular symmetrical tip 52; a line of transducers 62 mapped along a tip 52 which is rotated around the central axis of the catheter 50; or scanning the ultrasound waterfront direction by both phasing or mechanical means. Various alterations and modifications may be made to the present

invention without departing from the scope of the invention. For example, although the particular embodiments refer to ablating occlusions in the blood vessels of a patient's vasculature, this is in no way intended to be limiting as, in use, the present invention may be implemented in a variety of applications where blockages or occlusions occur in body conduits, such as, for example, the lymphatic system, the alimentary canal, respiratory tracts, uterus or urethra etc.

Claims

CLAIMS

1. A system for crossing an occlusion in a body conduit, comprising an imaging catheter which, in use, can be advanced and orientated to the site of said occlusion in said conduit, the catheter being adapted to receive means for directing substantially forward-facing electromagnetic energy into said occlusion, said means for directing electromagnetic energy being electrically connected to a remote electromagnetic energy source, the distal tip of said catheter characterised by: a plurality of ultrasound transducers for imaging around and beyond said distal tip of said catheter, said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter.

2. A system for crossing an occlusion in a body conduit as claimed in claim 1, wherein said catheter is advanced and orientated through said body conduit by mechanical, electrical or other control means.

3. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said catheter further comprises at least three steering wires being symmetrically disposed around the body of said catheter.

4. A system for crossing an occlusion in a body conduit as claimed in claim 3, wherein each of said at least three steering wires orientates said distal tip of said catheter by compressing a soft material torus against a rigid backing plate located on said distal tip of said catheter.

5. A system for crossing an occlusion in a body conduit as claimed in claim 1, wherein said means for directing substantially forward-facing electromagnetic energy into said occlusion is a laser wire.

6. A system for crossing an occlusion in a body conduit as claimed in claim 1, wherein said remote electromagnetic energy source provides sufficient power and divergence to ablate said occlusion without unacceptable damage to said body conduit.

7. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said plurality of ultrasound transducers transmit ultrasound signals around and beyond said distal tip of said catheter and receive corresponding echo signals.

8. A system for crossing an occlusion in a body conduit as claimed in claim 7, wherein said corresponding echo signals are processed by an ultrasound processor to provide an image around and beyond said distal tip of said catheter on a monitor or other display or data capture means.

9. A system for crossing an occlusion in a body conduit as claimed in claim 8, wherein said ultrasound processor processes said corresponding echo signals in real-time and said image obtained around and beyond said distal tip of said catheter is used to position said catheter substantially in the centre of said body conduit to ablate said occlusion.

10. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said plurality of ultrasound transducers further comprises at least one array of low-resolution ultrasound transducers.

11. A system for crossing an occlusion in a body conduit as claimd in any preceding claim, wherein said plurality of ultrasound transducers further comprises at least three lines of low-resolution ultrasound transducers being symmetrically disposed around a conal-shaped distal tip of said catheter.

12. A system for crossing an occlusion in a body conduit as claimed in claim 11, wherein said ultrasound signals radiate orthogonally from the central axes of said lines of low-resolution ultrasound transducers being symmetrically disposed around a conal-shaped distal tip of said catheter.

13. A system for crossing an occlusion in a body conduit as claimed in any of claims 7 to 12, wherein the distance between said distal tip of said catheter and said occlusion is determined by sonar measurements of said corresponding echo signals.

14. A system for crossing an occlusion in a body conduit as claimed in claim 13, wherein the distance between said distal tip of said catheter and said occlusion is determined by phase measurements of said corresponding echo signals.

15. A system for crossing an occlusion in a body conduit as claimed in claim 13, wherein said sonar measurements of said corresponding echo signals are performed using a focusing device positioned between said plurality of ultrasound transducers and said occlusion.

16. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter reduces image distortion by distributing said plurality of ultrasound transducers spatially about the long axis of said catheter tip.

17. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter reduces image distortion by rotating at least one curvilinear array of ultrasound transducers temporally around said distal tip of said catheter.

18. A system for crossing an occlusion in a body conduit as claimed in claim 17, wherein rotating at least one curvilinear array of ultrasound transducers temporally around said distal tip of said catheter is achieved by mechanically or electrically rotating at least one curvilinear array of ultrasound transducers about said long axis of said catheter tip.

19. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said distal tip of said catheter is substantially formed as a cone, truncated cone or other pointed shape to assist advancement through said body conduit.

20. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said distal tip of said catheter is substantially annular.

21. A system for crossing an occlusion in a body conduit as claimed in any preceding claim, wherein said distal tip of said catheter has substantially hemispherical symmetry.

22. An imaging catheter which, in use, can be advanced and orientated to the site of an occlusion in a body conduit, the catheter being adapted to receive means for directing substantially forward-facing electromagnetic energy into said occlusion, the distal tip of said catheter characterised by: a plurality of ultrasound transducers for imaging around and beyond said distal tip of said catheter, said plurality of ultrasound transducers being symmetrically disposed around said distal tip of said catheter.

23. A system for crossing an occlusion in a body conduit as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.

24. An imaging catheter as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.