US 20070270688 A1
An automatic atherectomy system uses the rotary burr at the tip of a catheter as a sensing device, in order to measure both the electrical conductivity and permittivity of the surrounding tissue at multiple frequencies. From these parameters it is determined which tissue lies in the different directions around the tip. A servo system steers the catheter tip in the direction of the tissue to be removed. In non-atherectomy applications the rotary tip can be replaced with any desired tool and the system can be used to automatically steer the catheter to the desired position. The steering is done hydraulically, by pressurizing miniature bellows located near the catheter tip.
1. A method of automatically controlling a tool for removing non-desired tissue from the body without harming desired tissue, said method comprising the steps of:
discriminating between desired and non desired tissue based on sensing the properties of the tissue adjacent to the tool tip;
automatically steering said tool in the direction of the non-desired tissue; and
eliminating the non-desired tissue.
2. A method for automated removal of plaque from blood vessels by the use of a rotary tool, said method comprising the steps of:
discriminating between blood vessel walls, blood and plaque based on sensing the properties of the tissue adjacent to the tool tip;
automatically steering said tool in the direction of the plaque; and
removing the plaque by rotary abrasion.
3. A method of automatically steering a catheter in a body lumen, said method comprising the steps of:
discriminating between the wall of said lumen and the inside of said lumen based on sensing the properties of the tissue adjacent to tip of said catheter;
automatically steering tip to avoid damaging said wall.
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The invasion relates to medical devices and in particular to procedures in which an undesired tissue has to be removed without harming an adjacent desired tissue, such as in atherectomy.
In many medical procedures an undesirable tissue is adherent or touching a desired tissue and the removal of the undesired tissue has to be done with extreme caution in order not to harm the desired tissue. A well known example is atherectomy, the process of removing plaque from blood vessels. The most common method of atherectomy is based on the use of a high speed rotary burr, mounted at the end of a very flexible catheter. The burr pulverized the plaque into such fine particles that they can be left in the blood stream. A well known system is manufactured by the Boston Scientific company (www.bostonscientific.com) under the name Rotablator™. No further data is given here about this system as it is a well known commercial system. Other potential uses of the invention are removal of tumors, such as prostate cancer, liposuction, dental work and more. Today most these procedures are performed by a surgeon is manipulating a surgical tool (directly or remotely) while observing the tool position using means such as fluoroscopy or ultrasound, or by tactile feel. In some procedures there is no need to remove tissue but there is still a need to navigate within the body, such as directing a catheter through the blood system. The present invention can save the majority of the surgeon's time and operating room expenses
The present invention provides an automated way to navigate within the body and remove undesired tissue without doing any harm to desired tissue, even in situations that the undesired tissue is adherent. The same invention can be used for just navigation, without tissue removal. The preferred embodiment shown is atherectomy. In atherectomy there is a need to differentiate between plaque and blood vessel wall. It is well known that plaque has different electrical properties than blood vessel wall; however the blood vessels are full of blood which has electrical properties similar to the vessel wall. In order to automate atherectomy a discriminator between vessel wall, plaque and blood is required. Also, it is desired to sense proximity to a vessel wall, not just contact. The present invention provides exactly this capability. A similar situation exists in some tumor removal procedures: some tumors have different electrical properties than healthy tissue but the in-situ measurement of these properties is complicated by the fact that the voids left in the process of tissue removal are being filled with fluids which affect the measurements. Prior at attempts to automate atherectomy relied on a guide wire (which can not be used in case of complete occlusion) or on devices to help the surgical tool glide in the correct trajectory within the blood vessel. Since the plaque can be softer or harder than the vessel wall, it is very difficult to rely on such “self steering” methods. The current invention identifies the different materials surrounding the rotary burr at the tip of the atherectomy catheter and automatically steers the burr to remove the undesired tissue, such as plaque.
The invention uses the tip of a catheter as a sensing device, in order to measure both the electrical conductivity and permittivity of the surrounding tissue at multiple frequencies. From these parameters it is determined which tissue lies in the different directions. A servo system steers the catheter tip in the direction of the tissue to be removed. In non-atherectomy applications the rotary tip can be replaced with any desired tool and the system can be used to automatically steer the catheter to the desired position. The steering is done hydraulically, by pressurizing miniature bellows located near the catheter tip.
In general, the invention can be used for a broad range of applications as the invention does not rely on the type of procedure used. It can be used with rotary burrs, stents, guide wires, suction, electro-surgery etc.
In atherectomy there is a need to differentiate between at least three types of tissue: vessel wall, plaque and blood. Both vessel wall and blood have high conductivity and high permittivity, while plaque has low conductivity and permittivity. The key for differentiating blood from vessel wall is the change in permittivity with frequency: the permittivity of the vessel wall falls much faster as the frequency increases.
Other features and advantages of the invention will become apparent by studying the description of the preferred embodiment in conjunction with the drawings.
Referring now to
The discrimination of tissue types is shown in
The impedance of the tissue to ground (the patient is grounded) is shown schematically as impedance 24. A current is sent from oscillator 26 via resistor 25, contact 7, cable 7 and burr 5 to the tissue impedance 24. The lower the impedance 24 the lower the voltage at contact 9 will be. Both the in-phase component I and the quadrature component Q is measured buy any one of the standard methods of AC impedance measurement. By the way of example, the I component is found by multiplying output f1 of oscillator 26 with the voltage senses at contact 9 using an analog multiplier 30. The Q component is found by multiplying the same voltage with the output of f1 shifted by 90 degrees by phase shifter 29. The output of the multipliers are filtered by capacitors 31 and converted to digital by A/D converters 38. This is the standard sine and cosine separation method for finding the conductivity and permittivity components of a complex impedance. For frequencies below a few MHz, the voltage at contact 9 can be digitized and the derivation of the conductivity and permittivity can be done completely via digital signal processing. In order to generate the rotational reference pulse, the point when the voltage of sense wire 11 drops each revolution has to be found. The actual voltage can vary over a wide range, depending on the surrounding tissue, but the dip is always when the conductive strip 16 (see
Since the rate of change of the permittivity with frequency is required, at least two frequencies have to be used, three would be even more accurate. These are generated by oscillators 26,27 and 28. For each frequency the circuit shown has to be replicated. It is also possible to use a single variable frequency source and single detection circuit and multiplex the detection process.
A typical discriminator output is shown in
3. Only low frequency processing is required, as processing can be done at the steering bandwidth instead of the rotation speed. Steering bandwidth is below 100 Hz.
The catheter has four actuation channels 17 and four sense wires 11 terminating in four tips 21. If a burr is used, tips can protrude to partially envelope burr.
The computer 12 in
There are similar data bases available on the internet for properties of malignant tumors versus healthy tissue.
The hydraulic actuators 13 are shown in
By the way of example, burr 5 is a standard burr with an external diameter of between 1.5 to 2.5 mm. Because the system is automated a single small burr can be used for all blood vessel sizes, as the computer will steer the bar in all radial directions to clean a large vessel. Catheter 6 is slightly smaller than burr 5. Diameter of piston 45 is 1-2 mm and stroke is about 10 mm. Piston 45 and cylinder 44 are made of very hard material, such as alumina, ruby or tungsten carbide, with a lapped fit. The pressure of the fluid is fairly high, typically 50-100 Kg/cm2. Typical component values for the discriminator 10 are: frequencies in the 1 KHz to 1 GHz range, time constants of filter 31 of about 10-100 uS, time constant of capacitor 34 of 10-100 uS, time constant of capacitor 36 and resistor 35 of 10-100 mS.
An alternate way of steering is by using push-wires in channels instead of a liquid. The actuators and catheter are very similar to the ones discussed earlier.
Still another way of steering is use catheter tips made of ferromagnetic material and have a controlled external magnetic field. A variation is a system having a fixed external field and a catheter tip carrying three orthogonal coils to generate a force in any desired direction. This is available as a commercial system under the trademark Niobe. It is sold by the Stereotaxis corporation (vwww.stereotaxis.com).
While the preferred embodiment relates to atherectomy and used electrical impedance sensing other applications and other sensing methods are part of this invention. By the way of example, different tissues can be discriminated by their mechanical properties such as stiffness, hardness and damping. This can be sensed by a vibrating tip. Tissues can also be discriminated by thermal properties. A tip similar to
Also, the word “automatically” in this disclosure and claims should be broadly interpreted, from a simple assist to the surgeon in operating surgical systems to fully unattended operation of such a system. In the minimal version the surgeon fully controls the system; the tissue discriminator just assists the surgeon in the decision and operation of the atherectomy or other system. In a fully unattended operation the catheter can also be automatically advanced into the body and can be programmed to enter the correct blood vessel when coming to a junction point where there are multiple choices of routes. In the same manner, the “tool” or “catheter tip” should be broadly interpreted to include both contact tools (burrs, rotary wires, blades, suction, electro-surgery etc) as well as non contact tools (lasers, water-jet, gas jet etc).