US20070270688A1 - Automatic atherectomy system - Google Patents

Abstract

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.

Description

FIELD OF THE INVENTION
• 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.
BACKGROUND OF THE INVENTION
• 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.
SUMMARY OF THE INVENTION
• 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.
DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic view of the invention.
• FIG. 2 is an isometric close-up view of the catheter tip.
• FIG. 3 is a cross section of the catheter tip showing the steering method.
• FIG. 4 is a schematic diagram of the tissue discriminator.
• FIG. 5 is a graph of the different waveforms produced by the discriminator.
• FIG. 6 is an isometric view of the actuation mechanism.
• FIG. 7 is an isometric view of an alternate sensing method.
DETAILED DESCRIPTION OF THE INVENTION
• Referring toFIG. 1, ablood vessel1 having awall2 containsundesired plaque3 as well as blood4. A.atherectomy tool5 is introduced usingcatheter tube6. The tool is driven by air motor8 viaflexible rotating cable7. No further details of the atherectomy system are given, as these are well known commercial systems such as the Rotablator™ system. Anelectrical contact9 measures the electrical impedance betweencable7 and the return path which is ground (the patient is electrically grounded). Thediscriminator10 measures the complex impedance to ground by measuring the In-Phase current (I) and the Quadrature, or 90 deg out of phase current (Q). From these measurements the conductivity and permittivity of the tissue can be computed, based on the well known methods of electrical impedance measurements. A full explanation is given later. Based on the measured value, the type of tissue is determined bycomputer12 and the catheter is automatically steered by hydraulic actuator13 (via tubes14) to remove theundesired plaque3. As it approaches thewall2, the electrical properties start changing allowing precise and gentle steering and removal up to thewall2 but without actually touching the wall. This is possible as the measured properties are also a function of tissue thickness, so when the plaque becomes very thin the properties of the underlying layer are showing through. In order to determine the rotational orientation of burr5 asense wire11 is used in conjunction of a conductive strip onburr5.
• Referring now toFIG. 2, therotary burr5 has a standarddiamond powder coating15 and is rotated at high speed bycable7. It is made from electrically insulating material such as ceramic, with the exception ofmetallized strip16. Alternatively, it can be made of metal and coated with a hard ceramic coating except forstrip16. Asense wire11 terminates withtip21 very close toburr5. Once perrotation strip16 comes close totip21. This point can be detected by measuring the electrical impedance betweenwire11 andcable7. A sharp drop signifies this point, which is used as a rotational reference point.Catheter6 contains five channels on top of the central channel used bycable7.Channel18 is used forsense wire11.Channels17 are used to steerburr5 in the desired direction by inflating sections ofbellows20. Referring now toFIG. 3, bellows20 is divided into fourseparate sections19 connected tochannels17. Pressurizing a section will cause it to elongate, bending bellows20 in the opposite direction.Fluid22 is a saline solution or pure water. For lower electrical noise the outside ofcatheter6 is metallized with a verythin coating23. A sub-micron thickness, deposited by sputtering or evaporation, is sufficient. Such a thin coat does not affect flexibility.
• The discrimination of tissue types is shown inFIG. 4. To discriminate plaque from the wall of a blood vessel by electrical properties is relatively easy and well known in the medical literature, as plaque has a higher electrical impedance (both lower conductivity and lower permittivity). It is more difficult to differentiate the wall from the blood filling the vessel, as both have high conductivity and high permittivity. However, the permittivity of the wall falls much faster (by about a factor of 1000 faster) with frequency. This can be seen from the following table. While the cited values differ between studies, all studies show that permittivity of blood falls much slower than permittivity of the vessel wall as frequency increases.

• 	log10(Freq)

  	3	5	6	7	8

  Conductivity (S/m)

  Blood	0.70	0.70	0.70	1.00	1.49
  Fat	0.025	0.025	0.030	0.040	0.060
  Muscle	0.40	0.40	0.40	0.40	0.75
  Fibrous	0.24	0.24	0.24	0.29	0.33
  Material
  Calcium	0.08	0.08	0.10	0.12	0.17
  Vessel Wall	0.58	0.58	0.58	0.67	0.83

  Relative Permittivity

  Blood	4100	4000	2000	300	75
  Fat	20000	100	50	30	12
  Muscle	400000	10000	8000	200	70
  Fibrous	2000	500	50	5	3
  Material
  Calcium	10500	500	250	70	30
  Vessel Wall	100000	5000	4000	100	30

• The impedance of the tissue to ground (the patient is grounded) is shown schematically asimpedance24. A current is sent fromoscillator26 viaresistor25,contact7,cable7 andburr5 to thetissue impedance24. The lower theimpedance24 the lower the voltage atcontact9 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 ofoscillator26 with the voltage senses atcontact9 using ananalog multiplier30. The Q component is found by multiplying the same voltage with the output of f1 shifted by 90 degrees byphase shifter29. The output of the multipliers are filtered bycapacitors31 and converted to digital by A/D converters38. 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 atcontact9 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 ofsense wire11 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 strip16 (seeFIG. 2) is nearest to tip21. By comparing the average voltage atwire11 to the instantaneous voltage, the reference point is found independent of voltage. Signal f1 is fed to sensewire11 viaresistor32. The sine wave envelope is detected bydiode33 andcapacitor34. The average is derived byresistor35 and capacitor36.Comparator37 generates a positive output when the instantaneous value is below the average value. Again, the reference pulse generation can also be digital if the signal onwire11 is digitized from the start.
• 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 byoscillators26,27 and28. 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 inFIG. 5.Graph43 is the amplitude of the sinewave atsense wire11, used to generate the rotational reference.Graph39 shows the conductivity, derived from the I component (the higher the conductivity the lower the I component will be).Graph40 shows the permittivity, derived from the Q component (the higher the permittivity the lower the Q component will be).Graph41 shows the permittivity at a much lower frequency. The horizontal scale is in degrees relative to the reference pulse, which is created whentip21 is nearest toconductive strip16. In thisexample tip21 is drawn close to the vessel wall. Using just the data at f1, it is difficult to tell the wall (0-90 degree range) from blood (above and belowburr5, at 90-180 degree range and 270-360 degree range). At the second frequency f2, the permittivity in the wall area increases much faster than in the blood area. The plaque is easy to spot as it has much lower conductance and permittivity. Using the values of table 1 for frequencies of 1 KHz and 10 MHz, the conductivity and permittivity of plaque (fat+calcium+fibrous material) are below 0.1 S/m and 50, while blood is above 0.7 S/m at both frequencies and wall is above 0.58 at both frequencies. The permittivity of the wall is much higher than blood at 1 KHz (100,000 vs. 4100) but falls much faster at 10 MHz, dropping a factor of 1000 for the wall but only a factor of 14 for blood. This example shows that by using just three factors: conductivity, permittivity and ratio of permittivity at 10 MHz to 1 KHz the three tissues can be discriminated with a large margin. Adding a third frequency f3 increases the accuracy. Note that the rotational speed of theburr5 is about 1-3 KHz. For oscillator frequencies below that, the results will have to be sampled and integrated over many rotations. This is not a problem, as the steering is done at a much lower bandwidth than the measuring. An alternative is use a frequency of about 100 KHz as the lowest oscillator frequency. A second alternative, shown inFIG. 7, is to replace the rotary tissue sensing by foursense wires11 instead of a single one, and have each one connect to a discriminator. Each one of the wires corresponds to one actuator direction. The advantages are:
1. A completely standard burr can be used, however sensing does not extend tip of burr.2. System can be used for applications not requiring rotary burrs.
• 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 fouractuation channels17 and foursense wires11 terminating in fourtips21. If a burr is used, tips can protrude to partially envelope burr.
• Thecomputer12 inFIG. 1 performs the discrimination between tissues based on the rules shown above and steers theburr5 into the undesired tissue, in this case plaque. Clearly the decision rules and parameters will change with the application and the tissue used. A large data base of impedance data for a large number of tissues is posted on the Italian National Research Council website at: http://niremf.ifac.cnr.it/cgi-bin/tissprop/htmlclie/uniquery
• There are similar data bases available on the internet for properties of malignant tumors versus healthy tissue.
• Thehydraulic actuators13 are shown inFIG. 6 in conjunction withFIG. 1. A motor, such as a stepper motor,48 is driven fromcomputer12 via a standard interface. Apiston45 is moved in acylinder44 via the action of athread46 and a matingfemale thread47. The pressure is transmitted viahypodermic tubing14 to channel17 (not shown) incatheter6. Four identical units are used for +X, −X, +Y and −Y.
• By the way of example,burr5 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.Catheter6 is slightly smaller thanburr5. Diameter ofpiston45 is 1-2 mm and stroke is about 10 mm.Piston45 andcylinder44 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 thediscriminator10 are: frequencies in the 1 KHz to 1 GHz range, time constants offilter31 of about 10-100 uS, time constant ofcapacitor34 of 10-100 uS, time constant of capacitor36 andresistor35 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 toFIG. 7 can carry four temperature dependent resistors instead of sensingtips21. A constant current is passed through resistors and their temperature is measured by the voltage drop across them. Different tissues have different heat conductivities: plaque will conduct less than blood vessel wall while blood will conduct heat rapidly, as convection exists.
• 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).

Claims (20)

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.

4. A method as inclaim 1 wherein said discrimination is based on electrical properties.

5. A method as inclaim 2 wherein said discrimination is based on electrical properties.

6. A method as inclaim 3 wherein said discrimination is based on electrical properties.

7. A method as inclaim 1 wherein said discrimination is based on mechanical properties.

8. A method as inclaim 2 wherein said discrimination is based on mechanical properties.

9. A method as inclaim 3 wherein said discrimination is based on mechanical properties.

10. A method as inclaim 1 wherein said discrimination is based on thermal properties.

11. A method as inclaim 1 wherein said discrimination is based on the electrical properties of conductivity, permittivity and changes of permittivity with frequency.

12. A method as inclaim 2 wherein said discrimination is based on the electrical properties of conductivity, permittivity and changes of permittivity with frequency.

13. A method as inclaim 1 wherein said steering is done hydraulically.

14. A method as inclaim 2 wherein said steering is done hydraulically.

15. A method as inclaim 3 wherein said steering is done hydraulically.

16. A method as inclaim 1 wherein said steering is done by using a magnetic field external to the body.

17. A method as inclaim 2 wherein said steering is done by using a magnetic field external to the body.

18. A method as inclaim 3 wherein said steering is done by using a magnetic field external to the body.

19. A method as inclaim 2 wherein said discrimination is based on the fact that plaque has lower electrical conductivity than blood or blood vessel wall, and the electrical permittivity of blood vessel walls falls faster with frequency than the permittivity of blood.

20. A method as inclaim 2 wherein the rotary action of the tool is used to scan the different tissues surrounding the tool.

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