A device of this kind is disclosed in the interna- tional patent application PCT/SE98/02446.

It is important that the site of the stent in the blood vessel can be accurately identified in a reliable manner so that the therapeutic treatment by means of therapeutic ultrasound energy supplied to the stent from an extra-corporeal position can be effected on the relevant part of the blood vessel.

An object of the invention is to provide a device of the kind referred to above which allows such accurate and reliable identification of the site of the stent in a non- surgical manner.

Another object of the invention is to provide a stent with improved absorption characteristics in order to increase the development of heat in the stent when exposed to a therapeutic ultrasound and to make the site of the stent more easily detectable.

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A further object of the invention is to provide means in a device of the kind referred to above, which allows identification of the stent in the blood vessel and can easily be included in the stent without interfering with the insertion of the stent into the blood vessel or the therapeutic function of the stent when located in the blood vessel.

A still further object of the invention is to include in a device of the kind referred to above means that allows the temperature of the stent to be determined when thera- peutic ultrasound energy is supplied to the stent.

According to the invention the objects mentioned above and such further objects as appear from the following description are obtained by the device of the kind referred to above which has the characterising features of claim 1.

In order to explain the invention in more detail illustrative embodiments thereof will be described with reference to the accompanying drawings in which FIG 1 is a diagrammatic view of a blood vessel with a stent therein illustrating the function of the device of the invention, FIG 2 is a partly broken away perspective view of the stent disclosed in the international patent application PCT/SE98/02446, FIGS 3 to 6 are perspective views of the stent illustrating different embodiments of the marker provided in the stent, FIG 7 is a fragmentary side view of a stent with temperature indicating bubbles, FIGS 8 and 9 are perspective views of the stent illustrating different embodiments of a temperature indicator.

FIG 1 shows a blood vessel 10 located in soft tissue 11 of a body covered by a skin 12. In the blood vessel a stent 13 has been inserted forming part of the device

described in the international patent application PCT/SE98/02446. Therapeutic ultrasound energy (0.5 to 15 MHz) is supplied to the stent 13 from an ultrasound transmitter 14 and is focused in the stent as indicated by dot-and-dash lines 15. In order that the ultrasound energy will be absorbed by the stent and will be converted to heat therein the therapeutic ultrasound should be reflected as little as possible at the outside surface of the stent device.

The site of the stent 13 in the blood vessel 10 is identified by means of an ultrasound camera (diagnostic ultrasound equipment) of any well-known construction emitting a pulsating diagnostic ultrasound field (2 to 25 MHz) indicated by dot-and-dash lines 16 from a transmitter/receiver 17 which receives reflected ultrasound from the body and transfers an ultrasound image to a display showing a picture of the view as"seen"by the ultrasound camera according to known technique in the art.

A stent as that described in the international patent application PCT/SE98/02446 may be difficult to identify on the picture delivered by the ultrasound camera due to small reflection of the diagnostic ultrasound at the outside surface thereof, which gives an unclear indication of the stent in the ultrasound picture on the display.

In one embodiment of the invention the stent 13 comprising according to FIG 2 an expandable socket 18 e. g. of metal netting with an outside cover 19 e. g. of plastic or silicon rubber as described in the international patent application PCT/SE98/02446, a material such as carbon or tungsten powder, or gas (e. g. air) or liquid bubbles or voids are included into cover 19 to provide an increased absorption of the diagnostic ultrasound energy therein so as to more clearly show the stent in the picture obtained on the display. The material or the bubbles or voids can be distributed in the cover in such a way that there is WO 01/05332

obtained an absorption gradient through the thickness of the cover material increasing from the outside to the inside surface of cover 19. The gradient may be created by forming cover 19 of a laminate the several layers of the laminate having different absorption degree increasing from the outside surface of the cover to the inside surface of the cover.

In another embodiment voids are provided on the surface of the stent in order to increase the absorption of ultrasound energy by providing as a cover on the socket a material having micropores such as the material marketed under the registered trade mark GORTEX FIG 3 discloses a further embodiment of the stent forming part of the device of the invention. In this case four (or more) axial ribs 20 are formed on the outside surface of cover 19 (but more ribs can be provided) and include a material or bubbles or voids increasing the absorption of ultrasound energy therein. The material of the ribs can be a porous material having closed cells. The ribs 20 can be formed directly on a socket 18 having no cover, or they may be formed integrally with a cover 19 e. g. when the cover is extruded onto the socket.

Ribs 20 not only facilitate identification of the stent in the picture produced on the display of the ultrasound camera but also absorb the therapeutic ultrasound energy for the heating of the stent.

According to FIG 4 ribs 21 extend helically around the socket, which provides the advantage that the heat developed in the ribs by the therapeutic ultrasonic energy, supplied to the stent will be distributed over the circumference of the stent. Ribs 21 can be constructed and produced in the same manner as ribs 20.

The thickness of the ribs preferably should be larger than the wavelength of the therapeutic ultrasound.

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In the embodiments described so far the material of the stent is modified in order to increase the absorption of ultrasound so as to improve heating of the stent by means of therapeutic ultrasound energy supplied by an external source and to facilitate identification of the stent in the picture obtained by an ultrasound camera.

The invention also provides a device of the kind referred to herein with a stent having markers arranged at each end thereof. The markers can be passive or active. The markers 22 in FIG 5 are passive markers and consist each of a bubble containing a gas or a liquid, or of a metal ball.

The marker reflects the ultrasound and is reproduced in the picture obtained on the display of the ultrasound camera as a dot which contrasts against the structure of the surrounding tissue shown on the display, and thus can be easily detected in the picture in determining the site of the stent. Gas bubbles extensively reflect the second and higher harmonics of the ultrasound, and by the use of one of said harmonics for the control of the display of the ultrasound camera a still more clear indication of the site of the stent can be obtained.

The active marker is a transponder that is activated by the diagnostic ultrasound field and transmits an ultrasound signal which is received by the trasmitter/receiver of the ultrasound camera. FIG 6 discloses a stent with transponders 23 at the ends thereof.

The transponder in one embodiment comprises a piezoelectric crystal which receives energy from the diagnostic ultrasound and then emits an ultrasound pulse to be received by the transmitter/receiver of the ultrasound camera.

The transponder 23 can also comprise a microchip to which energy is supplied by means of a coil 24, FIG 1, transmitting electromagnetic radiation starting from 150 Hz and upwards. The microchip is activated by the diagnostic WO 01/05332

ultrasound in order to produce and emit an ultrasound signal that is received by the transmitter/receiver of the ultrasound camera.

The stent can be provided with a temperature indicator of a passive or active type. Two or more bubbles are provided in the stent to function as passive markers as described above. However, the bubbles are filled with liquids of different kinds, which evaporate at different temperatures. FIG 7 shows three bubbles 25A, 25B, and 25C filled with liquid which evaporates at a lower temperature in bubble 25A, at an intermediate temperature in bubble 25B, and at a higher temperature in bubble 25C. The character of the indication obtained on the display of the ultrasound camera being different if the bubble is filled with a liquid or a vapour, it is possible to learn from the indication obtained if the stent is above or below a certain temperature. The difference between the indication of a bubble filled with liquid and a bubble filled with vapour will be particularly evident if the ultrasound camera operates with the second or a higher harmonic.

An active indication of the temperature of the stent can be effected by means of piezoelectric crystals mounted to the stent together with the transponder circuit. An electronic circuit (microchip) provided on the stent and connected to the crystals is energized by means of a coil transmitting electromagnetic energy to the electronic circuit (conf. FIG 1) which emits different electromagnetic frequencies related to temperature dependent oscillations of the piezoelectric crystals said signals being received by an extra-corporeal measuring system.

FIG 8 discloses a stent with a resistance wire 26 e. g. a platinum wire which is embedded into cover 12 of the stent or can be embedded into an axial rib provided on the stent. Also in this case an electronic circuit 27 (microchip) for emitting an electromagnetic signal is WO 01/05332

provided on the stent and is energized in the manner described above. The signal emitted by the circuit is dependent of the resistance of the resistance wire 26 which in turn is dependent of the temperature of the stent. The resistance wire can have zigzag shape as shown in FIG 9 in order not to interfere with the expansion of the stent.

Socket 18 when consisting of a metal netting can be connected to the electronic circuit to form an antenna thereof.

The helical rib 21 provided according to FIG 4 can form together with cover 19 a resonance circuit the resonance frequency of which varies with the temperature of the stent. An electronic circuit which is energized by means of a coil 24 as described above and forms a transmitter for electromagnetic signals is connected to the resonance circuit. Thus, the frequency of the electro- magnetic signals emitted by the electronic circuit and received by an extra-corporeal measuring system represents the temperature of the stent.

In a modification of this embodiment the resonance circuit has a fixed resonance frequency which does not change with the temperature. However, the resistance of the resonance circuit (the energy loss factor) will vary with the temperature of the stent and thus the magnitude of the emitted signal at the fixed resonance frequency will indicate the temperature of the stent.