FLEXIBLE DIRECTIVE ULTRASONICALLY MARKED CATHETER

Abstract: A directivity characteristic modification device, e.g. an acoustic lens (22), is mounted on a flexible ultrasonically marked catheter (1) provided with electrodes (24, 25) of the transducer (21). Electrical conductors (3 and 4), bonded to the electrodes (24, 25) of the transducer (21) are longitudinally positioned in the catheter and are linked at the proximal end with connectors (5 and 6). By changing the shape and building material of the lens (22), one can chose directivity characteritic of the whole assembly (21-22). This device is indwelled into the patient's body and connected to an outside transponder and serves for unambiguous localisation of the marked location on the catheter using an outside ultrasound scanner.

Description: FLEXIBLE DIRECTIVE ULTRASONICALLY MARKED CATHETER 1. Field of the Invention This invention relates to medical technology and in particular to the ultrasonically localizable catheter technology. 2. Background and Prior Art A particular problem in ultrasonic imaging of flexible catheters indwelled into human body is the fact that these devices are not always entirely within the scanning plane, so that without special solution one can not know which part of the catheter is seen. This is a particularly hard problem if the catheter is within the heart so that it moves and thus only occasionally enters the scanning plane. A similar problem arises in visualization of a catheter that is only partly visible when it passes a part of the body filled with gasses like lungs of the intestines. This problem is particularly complicated if the angle at which one images the catheter is essentially different from 90° to the catheter axis. The present solutions, US patents 4,697,595 and 4,706,681 (to Breyer et al.), and 5,076,278 (to Vilkomerson et al.) embrace the following basic principle: At the spot on the catheter that we want to localize (e.g. tip) a piezoelectric transducer is mounted. This transducer is connected with the proximal end of the catheter by conductors mounted within and along it. The conductors have connectors at their proximal ends. When this piezoelectric transducer comes within the scanning plane of an ultrasonic scanner, the pulses transmitted by it hit the said marker transducer so that there appear electrical signals at the said connectors that signal that the marked part of the catheter is within the scanning and imaging plane. Such signals can in different ways be used for generation of visible guiding marks on the scanner screen. A representative method of such application is the use of a transponder connected to the said connectors. The transponder responds to each incoming ultrasound pulse from the scanner with a series of impulses that generate ultrasonic pulses (signature) on the said marker transducer which, in turn, are visible on the scanner screen. There exist other, more complicated methods of the use of such a catheter, but they do not change the basic system philosophy. A particular subject of the technological compromise is the relation of sensitivity and the width of the response directivity function. Namely, if one uses a cylindrical of plate transducer, the increase of its surface increases the sensitivity, but makes the transducer more directive and consequently harder to detect from elevation angles nearer to the longitudinal catheter axis. The directivity can be broadened by reduction of the length of the ring (cylinder height) . Another method of broadening of the directivity has been described in the US patent 5,076,278 where one uses a transducer of curved outer surface for the purpose. This means that the thickness of the transducer is variable. Although this method improves both the sensitivity and broadens the directivity, new problems are created as follows: - application of more expensive planconvex transducers (outer surface curved, inner straight) , - variable frequency sensitivity for different elevation angles of incoming ultrasound pulses Our invention yields an increased sensitivity with broadened or narrowed directivity at will. This is achieved by mounting of an acoustic lens onto a standard (thus less expensive) piezoelectric transducer. This increases the flexibility of technological design and construction while reducing the manufacturing price. 3. Summary of the invention The invention consists in the mounting of an acoustic lens onto the outer side of the cylindrical or plate transducer. This lens modifies the directivity characteristic at will. One can, for the same purpose deform a (planparallel) piezoelectric plastic foil in such a way that it gets the same directivity properties as when the lens is applied. - The aim of the invention is an essential simplification of the change of the directivity characteristic (be it for its broadening or its narrowing) of the transducer assembly of the ultrasonically marked catheter. - Another aim of this invention is the reduction in manufacturing price of such a catheter by the use of less expensive quality materials and manufacturing methods. - Yet another aim of this invention is the augmentation of the flexibility-adaptability of manufacture since the directivity characteristic of the transducer assembly can be changed by changing the lens shape of material without - A - changing the form, type or dimensions of the piezoelectric transducer itself. Flexibly directive ultrasonically marked catheter according to the present invention consists of a piezoelectric transducer mounted onto a catheter connected to the catheter's proximal end by built in longitudinal conductors. An acoustic lens is mounted onto the said transducer. The said lens can be convergent or divergent with various angles depending on its curvature and the material it is made of. This essentially facilitates the manufacture of ultrasonically marked catheters with various directivity characteristics. For the general location and initial positioning a more isotropic directivity characteristic is favorable, while for the determination of elevation angle in electrosurgery a narrow directivity is needed. The described method is particularly applicable with solid state transducers (e.g. ceramic) . When piezoelectric foil is used (e.g. PVDF) , the change of directivity can be achieved by deforming its shape by pulling it over a forming member that results in the foil transducer having a more or less isotropic directivity characteristic depending on the said forming member. 4. Short description of figures Figure 1. is a perspective drawing of a flexibly directive ultrasonically marked catheter. For clarity of details, the catheter is shown much thicker compared to the length than it is in reality. Figure 2. is a cross sectional drawing of the axially symmetric directive ultrasonically marked catheter with the directivity axis approximately perpendicular to the catheter axis. Figure 3. is a cross sectional drawing of an ultrasonic lens or forming member. Figure 4. is a drawing in two projections of a flexibly directive ultrasonically marked catheter with the transducer in the form of a plate. Figure 5. is a cross sectional drawing of a flexibly directive ultrasonically marked catheter with the axis of its directivity characteristic tilted at an angle other than 90° to the catheter axis. Figure 6. is a drawing of a flexibly directive ultrasonically marked catheter in which a plastic piezoelectric foil is used and a barrel like forming member deforms its initial shape into a transducer with a more isotropic directivity. 5. Description of preferred embodiments According to figure 1, onto a place along the catheter 1 the piezoelectric transducer assembly (2) is mounted. This piezoelectric transducer is connected to electrical conductors 3 and 4 laid along the catheter 1 and with connectors 5 and 6 at the proximal end. The position of the piezoelectric transducer along the catheter is determined by the application needs and can be anywhere between the proximal and distal end of catheter 1. It is possible to use more than one transducer assembly, each of such assemblies connected to the outside circuits with its own conductors. Figure 2 shows the transducer assembly 2 from figure 1. The said assembly consists of transducer 21 and the lens 22. In the first embodiment (figure 2), the piezoelectric transducer 21 is of a cylindrical form mounted onto the catheter body 1. The transducer is built of piezoelectric ceramics or piezoelectric plastic. The most practical operating regimen is when the thickness or radial resonant frequency of the said transducers is near to central resonant frequency of the echoscope used in imaging of the catheter. The transducer itself has electrodes 24 and 25 deposited onto its sides, e.g. inner and outer side. Electrical conductors 3 and 4 are connected (by gluing, mechanical pressure or soldering) to these electrodes. The conductors can be connected to outside circuits via connectors 5 and 6. An acoustic lens 22 is mounted onto the transducer 21 (e.g. by glue 26). The lens 22 can be of any shape, for example convex as in figure 2. The lens is separately shown in figure 3. The lens 37 can be built of a multitude of segments divided by incisions 7. In this way the compliance of the whole lens can be increased. Such a convex lens that is symmetrical in the proximal-distal direction can be divergent (if the ultrasound speed in the lens is greater than in the surrounding blood) or convergent (if the speed of ultrasound in the lens is smaller than in the surrounding blood) . If one wants to localize the marked point from a broad viewing angle a divergent lens ought to be used (e.g. built of polysulphone or metylmetacrilate) . On the other hand if one wants to achieve a more directional field in the elevation angle, a convergent lens should be used (e.g. made of silicone rubber) . The basic shapes of both convergent and divergent lenses are equal. Changing the radius of the shape of lens 22 one can change the convergence or divergence angle. The cross sectional shape can have a circular, parabolic or some other curve of similar nature. This embodiment yields a circularly symmetric response around the catheter axis and has essentially a directivity characteristic perpendicular to the catheter axis. The whole transducer assembly is covered with insulating layer 27. This layer can, if needed, be made to be a quarter wavelength matching layer. If one uses a single layer the optimum material ought to have the characteristic acoustic impedance equal to the geometric mean between the characteristic acoustic impedance of the piezoelectric transducer 21 and the surrounding fluid (blood) . A second embodiment is shown in figure 4. In this case a piezoelectric plate 41 is used with the lens 42 glued onto it. Electrodes 44 and 45 are deposited onto the sides of the plate 41 and via connections 43 and 44 with the longitudinal electrical conductors 4 and 3 all the way to connectors 5 and 6. It is possible to use more than one plate like plate 41 and connect each of them with outside circuits via its own longitudinal conductors. The lens 42 can be of any shape, e.g. convex as in figure 3. Such a convex lens that is symmetrical in the proximal-distal direction can be convergent (if the speed of ultrasound in the lens is smaller than in the surrounding blood) . If one wants to localize the marked point from a broad viewing angle a divergent lens ought to be used (e.g. built of polysulphone or metylmetacrilate) . On the other hand if one wants to achieve a more directional field in the elevation angle, a convergent lens should be used (e.g. made of silicone rubber) . Changing the radius of the shape of lens 42 one can change the convergence or divergence angle. The cross sectional shape can have a circular, parabolic or some other curve of similar nature. This embodiment does not yield a circularly symmetric response around the catheter but has the response directivity perpendicular to the catheter axis. The whole transducer assembly is covered with insulating layer 47. The third embodiment is shown in figure 5. Here is the piezoelectric transducer 21 of cylindrical (ring) shape mounted onto the catheter body 1. This transducer is built of piezoelectric ceramic or plastic. The most practical is the use of thickness or radial resonance frequency near to the central frequency of the echoscope used for imaging of this catheter. The transducer has electrodes deposited onto its sides, e.g. inside and outside. Onto these electrodes electrical conductors 3 and 4 are connected (by soldering, conductive gluing or mechanical pressure) and these lead to connectors 5 and 6 for connection to outside electrical circuits. Onto the transducer 21 a lens 52 is mounted. The lens 52 is shaped in such a way that it is thicker on one side in the distal-proximal direction. In this way a preferred sensitivity in a chosen elevation angle 12 is achieved. The angle 12 is the tilt of the directivity characteristic axis 11 to the catheter axis. An additional convex curvature yields a divergent characteristic (if the speed of ultrasound is greater than in the surrounding medium - blood) or convergent characteristic (if the speed of ultrasound in the lens is smaller than in the surrounding medium - blood) . In this case too the lens can be built of segments as shown in figure 3 for a symmetrical lens. If one wants to localize the marked point from a broad viewing angle a divergent lens ought to be used (e.g. built of polysulphone or metylmetacrilate) . On the other hand if one wants to achieve a more directional field in the elevation angle, a convergent lens should be used (e.g. made of synthetic rubber) . Changing the radius of the shape of the said lens one can change the convergence or divergence angle. This embodiment is circularly symmetrical around the catheter axis, but tilted to the longitudinal catheter axis. The insulating layer equivalent to layer 27 in figure 2 has been omitted from the drawing, but it is clear that such a layer can be used in this case too. The fourth embodiment is shown in figure 6. In this case a planparallel foil transducer is used, e.g. PVDF foil. In order to achieve outside shape that yields a divergent directivity characteristic (approximating isotropic characteristic) , the piezoelectric foil 61 id pulled over the forming member 62 to acquire its form. In the concrete case the forming member is shown to have a barrel shape with the central hole appropriate for mounting onto the catheter 1. This forming member can have a shape that is essentially similar to the shape of the lens in figure 3. The said transducer itself has electrodes 64 and 65 deposited onto its sides (e.g. inner and outer side) . Electrical conductors 3 and 4 are connected onto these electrodes (e.g. by soldering, conductive gluing or mechanical pressure) and they lead to connectors 5 and 6 that are suitable for connection with outside electric circuits. The shape of the ultrasound transducer assembly is essentially defined by the shape of the forming member 62. This shape defines the directivity characteristic for reception and transmission of ultrasound waves. An insulating layer equivalent to layer 27 in figure 2 is not shown in this figure for clarity, but it is understood that such a layer can be deposited in this case too. The typical thickness of the said piezoelectric foil is of the order of magnitude of a few tens of micrometers. This invention represents an essential improvement of the flexibility of manufacture and properties of ultrasonically marked catheters that are used for ultrasonic guidance of catheter procedures in the body.

Claims: CLAIMS 1. Flexibly directive ultrasonically marked catheter characterized by comprising: - at least one piezoelectric transducer-lens assembly for use in conjunction with ultrasound scanners, - with the said transducer assembly of a shape that can be mounted onto the catheter and whose frequency response overlaps with the frequency spectrum of the said ultrasound scanner, - where the said transducer assembly consists of a piezoelectric transducer and an additional device for modification of its directivity characteristic, - where the said transducer assembly is connected to the outside electric circuits via electrical conductors built in along the said catheter. 2. Flexibly directive ultrasonically marked catheter according to claim 1 characterized by having the piezoelectric transducer assembly from claim 1 built of a cylindrical (ring) piezoelectric transducer onto whose outside an acoustic lens is mounted. 3. Flexibly directive ultrasonically marked catheter according to claim 1 and 2 characterized by the said piezoelectric transducer having electrodes on the outside and inside onto which electrical conductors are connected, said conductors running along the said catheter and terminated with connectors. 4. Flexibly directive ultrasonically marked catheter according to claims 1, 2 and 3 characterized by the said lens being convex and in terms of thickness profile longitudinally symmetric related to the axis of the said catheter. 5. Flexibly directive ultrasonically marked catheter according to claim 1, 2 and 3 characterized by the said lens being in terms of thickness profile longitudinally asymmetric related to the axis of the said catheter. 6. Flexibly directive ultrasonically marked catheter according to claim 1, 2 and 3 characterized by the speed of ultrasound within the said lens being different from the speed of ultrasound in the surrounding medium. 7. Flexibly directive ultrasonically marked catheter according to claim 1, 2 and 3 characterized by the said piezoelectric transducer being flexible and plastic. 8. Flexibly directive ultrasonically marked catheter according to claim 1, 2, 3 and 7 characterized by having a forming member mounted onto the said catheter over which it is possible to tightly pull over the piezoelectric transducer from claim 7. 9. Flexibly directive ultrasonically marked catheter according to claim 1, 2, 3, 7 and 8 characterized by the transducer according to claim 7 being pulled over the forming member from claim 8 so that it adapts to its form.

Inventors: BREYER BRANKO, FEREK-PETRIC BOZIDAR, BREYER, BRANKO, FEREK-PETRIC, BOZIDAR
Applicants: BREYER BRANKO, FEREK PETRIC BOZIDAR, SULZER OSYPKA GMBH, BREYER, BRANKO, FEREK-PETRIC, BOZIDAR
Application year: 1997
Main classification: A61B8
IPC codes: A61B8, A61B19, A61M25, H04R17