FIBRE OPTIC DEVICE FOR THE TRANSMISSION AND LATERAL IRRADIATION OF LASER ENERGY, PARTICULARLY FOR ANGIOPLASTY

Abstract: A fibre optic device for the transmission and lateral irradiation of laser energy, particularly for angioplasty, comprising at least one optical fibre (1), to the inlet end of which there is coupled a laser source and to the outlet end of which, after removing its coating (3), there is applied a microcapsule (5) which is transparent to laser radiation and has a substantially toroidal thickened portion (6) surrounding said end and arranged to give raise to a "corolla-shaped" output laser beam.

Description: This invention relates to a fibre optic device for the transmission and lateral irradiation of laser energy, particularly for angiosurgery. Angiosurgery is used in the medical field for removing arteriosclerotic plaques and similar obstructions which form in the arteries, in particulary the coronary arteries. Angiosurgery is currently effected by various methods, some of which involve operating from within the artery itself. One of these known methods comprises the use of a catheter provided at its end with a small inflatable ball. The catheter with the deflated ball is inserted into the artery and advanced until the ball reaches the obstacle to be removed. The ball is then inflated and as it expands it recanalizes the obstruction vessel. This method has serious drawbacks (see Nigel Sinclair et al. S.P.I.E. vol. 713 page 70, September 1986). In particular, rather than remove the obstruction, this method tends to widen the arthery at that particular location. This means that the beneficial effects are only temporary as the obstruction forms again after a short time. For these reasons particular attention has been given to laser techniques, which because of the high energy concerned are able to destroy and thus definitively eliminate arteriosclerotic plaques. A known technique is to insert into the artery an optical fibre with a sapphire tip, which emits a laser beam able to destroy the arteriosclerotic plaque facing it. Drawbacks of this known technique include on the one hand the large size of the sapphire tip, which means that the apparatus can be used only for peripheral large-diameter blood vessels, and on the other hand the risk of perforating the artery wall should this face the laser beam emitted from the sapphire tips as from any conventional fibre tip. To obviate these drawbacks it has been proposed to use a fibre optic probe with the laser outlet covered by a metal tip. The tip is heated by the laser beam and is moved forward so that it burns the arteriosclerotic plaque. The main drawback of this technique is the loss of the potential advantages of the laser source, which in this case is used only to heat the metal tip as in the case of a normal source of electricity, so losing the selective effect of the laser beam on the arteriosclerotic plaque relative to the surrounding healthy tissue. A further drawback is that the hot tip can adhere to the tissue with the risk of damage. From EP-A-195375 it is known an optical device comprising an optical fibre having an inlet end coupled to a laser and an outlet end, a rotationally symmmetric capsule terminating the outlet end and coaxial with that end, which has transparent walls. The aim of the invention is to eliminate all the mentioned drawbacks with a device of the above kind. This aim is reached according to the invention by a fibre optic device as described in claim 1. Some embodiments of the present invention are described in detail hereinafter with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a device according to the invention; Figure 2 is a longitudinal section through a first embodiment of the outlet end of the optical fibre covered with the microcapsule; Figure 3 shows the radiation diagram of the device of Figure 2; Figure 4 is a longitudinal section through a second embodiment of the outlet end of the optical fibre; Figure 5 shows the radiation diagram of the device of Figure 4; and Figure 6 is a longitudinal section through the outlet and of a multiple optical fibre. As can be seen from the figures the device according to the invention consists essentially of an optical fibre 1 which in the illustrated example comprises a quartz core 2 of 200-300 um diameter and a plastic coating 3. The inlet end of the fibre 1 is coupled in conventional manner to a laser source 4, whereas the outlet end, after the plastic coating has been removed, is housed in a microcapsule 5 constructed of a material transparent to laser radiation, such as quartz. The microcapsule 5, which consists preferably of a piece of capillary closed at one end, has at that end an annular thickened portion 6 formed when the end of the quartz capillary is fused for the purpose of closing it. The shape of this annular thickened portion and thus the optical characteristics of the system can be varied by varying the capillary heating time and the possible mechanical stressing exerted during heating, as will be clarified hereinafter. The microcapsule 5 is fixed to the plastic coating 3 by a collar 7 of sealing material which besides providing a fluid-tight seal between the two pieces also fixes the end of a capillary 8 which extends external to the microcapsule 5 parallel to the optical fibre 1 and is enclosed together with this latter within a single teflon sheath 9. The operation of the device according to the invention is as follows: the laser radiation, generated by the source 4 and transferred along the fibre, reaches the coating-free end 3 and emerges as a divergent beam. The angle of divergency depends essentially in known manner on the numerical aperture of the fibre and the fibre-source coupling. The emerging beam encounters the annular thickened portion 6 and as the thickness of this varies from its centre to its periphery, it behaves substantially as a lens of approximately toroidal shape, which has the effect of radiating a "corolla-shaped" laser beam that is of radiating the laser light approximately along the generators of a substantially conical surface, as illustrated diagrammatically in Figure 2. The width of the cone depends on the extent of the converging effect of the annular thickened portion 6 and on the optical characteristics of the fibre used. Figure 3 shows the radiation diagram for the optical fibre of Figure 2 (line a) in which the outlet end surface of the core 2 is cut orthogonally to the fibre axis. As can be seen from this radiation diagram, in which the horizontal axis represents the angle to the fibre axis and the vertical axis represents the emission intensity in the direction corresponding to these angles, the laser energy emitted by the fibre end collecting on a conical surface symmetrical about the fibre axis and having a cone angle of about 20°. The width of the two peaks (about 6°) gives an idea of the collimation effect obtained. For comparison purposes the line b shows the analogous radiation diagram for the same optical fibre as Figure 2 but without the microcapsule. In use, the fibre with the microcapsule is inserted into the artery to be treated and is advanced along it so that the collimated laser beam reaches the side wall of the artery to destroy the obstructions encountered during its advancement. It is apparent that the described device is particularly advantageous in angioplasty in that: the microcapsule 5 has a shape which combines considerable mechanical strength with the facility for easy movement along the artery without causing any damage to it; the optical system obtained ensures absolute absence of radiation in the direction of the fibre axis, so minimising the risk of damaging the artery especially at bends and bifurcations, at which the danger of piercing the facing wall is greatest; the shape of the radiated beam is particularly suited to destroying the arteriosclerotic plaques circularly associated with the artery wall. As stated, the shape of the thickened portion, the type of fibre, the shape of the outlet end of the core and its position in the microcapsule determine the optical characteristics of the system. For example the optical fibre core of the device shown in Figure 4 has a conical outlet end. This (see Chang and Aauth, J.O.S.A. vol. 68 No. 9,September 1978; Russo et al., S.P.I.E., vol. 492, 1986) determines a greater divergence of the radiation leaving the end and encountering the thickened portion 6, which then generates a ligth cone of greater width, as shown in the radiation diagram of Figure 5. In this, the full line represents the radiation diagram when the microcapsule 5 is present and the dashed line the radiation diagram without the microcapsule. Again in this case the collimation effect introduced by the microcapsule can be evaluated by noting the different length of the peaks on the two curves, these being about 16° for the conical end without microcapsule (line b) and about 6° for the conical end with microcapsule (line a). In both the embodiments shown, the optical fibre outlet end is coaxial to the microcapsule 5. If however the fibre outlet end is not aligned with or can be moved out of alignment with the microcapsule 5, the ligth cone leaving said microcapsule, although having the same cone angle determined essentially by the shape of the thickened portion 6, has a non-uniform ligth intensity distribution within the beam. The same non-uniform ligth distribution effect in the beam can be obtained by using instead of a single fibre a multiple fibre in which individual fibres are distribiuted in a uniform ring arrangement (Kapany - Fibre Optical: Principles and Applications, Academic Press, chapt. 5, 1967). In this way the conical light surface leaving the microcapsule 5 is obtained by the addition of several sectors, for each of which the energy contribution is given by one fibre. It is apparent that one varying the degree of coupling between the source and each fibre at the inlet end, the light distribution in the conical surface also varies. The methods for varying the source-fibre coupling efficiency are known and do not represent part of the invention. In practice, during the use of the device, the high energy available generates heat within the microcapsule 5, together with a dangerous pressure increase in the gas or air contained in the microcapsule. This pressure excess can be eliminated through the capillary 8. Alternatively, it is possible to create a vacuum in the microcapsule 5, although this requires a more laborious and costly construction. A further simpler method is to make a small hole in the microcapsule 5 through which the gas can vent to the outside. This method is however not valid in the case of angioplasty as the microcapsule generally operates surrounded by blood which could then enter its interior, resulting in inevitable blood damage at least if intense radiation is present.

Claims: A fibre optic device, particularly for laser angiosurgery, comprising a laser light source (4); at least one optical fibre (1) coupled to the laser source at one end and through which fibre the laser radiation is transmitted to an outlet end of the device from which the laser radiation is emitted at an angle to the fibre axis; a capsule (5) having a rotational symmetry with respect to a predetermined axis, an inner cavity and walls transparent to the laser radiation, which capsule is coaxially mounted to the other end of the fibre(s), so as to enclose the fibre(s), characterized in that the wall of the capsule has a rotationally symmetric thickened portion (6) projecting into the cavity and forming a convex inner surface arranged to transform the laser beam emanating from the fibre end into a diverging output beam having an annular-shaped cross-section. A device as claimed in claim 1, characterised in that the device comprises a single optical fibre (1). A device as claimed in claim 1, characterised in that the device comprises several individual optical fibres (1). A device as claimed in claim 1, characterised in that the outlet end of the optical fibre (1), is cut orthogonal to the fibre axis. A device as claimed in claim 1, characterised in that the outlet end of the optical fibre (1) is of conical shape. A device as claimed in claim 3, characterised in that the efficiency of the coupling between the laser source (4) and each individual fibre of the optical fibres (1) is adjustable. A device as claimed in claim 1, characterised in that the microcapsule (5) consists of a portion of capillary sealed at one end. A device as claimed in claim 1, characterised in that a collar of sealing material (7) is interposed between the outer peripheral surface of the optical fibre (1) and the inner lateral surface of the microcapsule (5). A device as claimed 1, characterised in that the interior of the microcapsule (5) is under vacuum conditions. A device as claimed in claim 1, characterised by comprising an aperture for communication between the inner cavity of the microcapsule (5) and the outside. A device as claimed in claim 10, characterised by comprising a capillary (8) which connects the interior cavity of the microcapsule (5) to the outside. A device as claimed in claim 11, characterised in that the optical fibre (1) and the capillary (8) are contained in a common single sheath (9). A device as claimed in claim 11, characterised in that that end of the fibre (1) within the microcapsule (5) is adjustable axially and/or transversely.

Inventors: CREA, FILIPPO, MARGHERI, GIANCARLO, RUSSO, VERA, SOTTINI, STEFANO
Applicants: CONSIGLIO NAZIONALE DELLE RICERCHE
Application year: 1988
Main classification: G02B6
IPC codes: G02B6, A61B18, H01S3, A61B17