Abstract:
An automatic atherectomy system uses a rotary burr at the tip of a catheter as a sensing device, in order to measure both electrical conductivity and permittivity of surrounding tissue at multiple frequencies. From these parameters it is determined which tissue lies in 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 burr can be replaced with any desired tool and the system can be used to automatically steer the catheter to the desired position. The steering may be done hydraulically, by pressurizing miniature bellows located near the catheter tip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/436,584 filed May 19, 2006, now pending, which is incorporated herein, by reference, in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This application 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 pulverizes 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 of these procedures are performed by a surgeon 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&#39;s time and operating room expenses. 
     SUMMARY OF THE INVENTION 
     The present embodiments of the invention provide an automated way to navigate within the body and remove undesired tissue without doing any harm to desired tissue, even in situations where the undesired tissue is adherent. The same approach 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 embodiments of the present invention provide 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 attempts to automate atherectomy relied on a guide wire (which cannot be used in case of complete occlusion) or on devices to help the surgical tool glide in a 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 approach described herein 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. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of according to one illustrated embodiment. 
         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 to  FIG. 1 , a blood vessel  1  having a wall  2  contains undesired plaque  3  as well as blood  4 . An atherectomy tool  5  is introduced using catheter tube  6 . The tool is driven by air motor  8  via flexible rotating cable  7 . No further details of the atherectomy system are given, as these are well known commercial systems such as the Rotablator™ system. An electrical contact  9  measures the electrical impedance between cable  7  and the return path which is ground (the patient is electrically grounded). The discriminator  10  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 by computer  12  and the catheter is automatically steered by hydraulic actuator  13  (via tubes  14 ) to remove the undesired plaque  3 . As it approaches the wall  2 , the electrical properties start changing allowing precise and gentle steering and removal up to the wall  2  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 burr  5  a sense wire  11  is used in conjunction of a conductive strip on burr  5 . 
     Referring now to  FIG. 2 , the rotary burr  5  has a standard diamond powder coating  15  and is rotated at high speed by cable  7 . It is made from electrically insulating material such as ceramic, with the exception of metallized strip  16 . Alternatively, it can be made of metal and coated with a hard ceramic coating except for strip  16 . A sense wire  11  terminates with tip  21  very close to burr  5 . Once per rotation strip  16  comes close to tip  21 . This point can be detected by measuring the electrical impedance between wire  11  and cable  7 . A sharp drop signifies this point, which is used as a rotational reference point. Catheter  6  contains five channels on top of the central channel used by cable  7 . Channel  18  is used for sense wire  11 . Channels  17  are used to steer burr  5  in the desired direction by inflating sections of bellows  20 . Referring now to  FIG. 3 , bellows  20  is divided into four separate sections  19  connected to channels  17 . Pressurizing a section will cause it to elongate, bending bellows  20  in the opposite direction. Fluid  22  is a saline solution or pure water. For lower electrical noise the outside of catheter  6  is metallized with a very thin coating  23 . 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 in  FIG. 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. 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 log 10 (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 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 by any one of the standard methods of AC impedance measurement. By the way of example, the I component is found by multiplying output f 1  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 f 1  shifted by 90 degrees by phase shifter  29 . The output of the multipliers are filtered by capacitors  31  and converted to digital by ND 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  FIG. 2 ) is nearest to tip  21 . By comparing the average voltage at wire  11  to the instantaneous voltage, the reference point is found independent of voltage. Signal f 1  is fed to sense wire  11  via resistor  32 . The sine wave envelope is detected by diode  33  and capacitor  34 . The average is derived by resistor  35  and capacitor  36 . Comparator  37  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 on wire  11  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 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  FIG. 5 . Graph  43  is the amplitude of the sinewave at sense wire  11 , used to generate the rotational reference. Graph  39  shows the conductivity, derived from the I component (the higher the conductivity the lower the I component will be). Graph  40  shows the permittivity, derived from the Q component (the higher the permittivity the lower the Q component will be). Graph  41  shows the permittivity at a much lower frequency. The horizontal scale is in degrees relative to the reference pulse, which is created when tip  21  is nearest to conductive strip  16 . In this example tip  21  is drawn close to the vessel wall. Using just the data at f 1 , it is difficult to tell the wall (0-90 degree range) from blood (above and below burr  5 , at 90-180 degree range and 270-360 degree range). At the second frequency f 2 , 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 f 3  increases the accuracy. Note that the rotational speed of the burr  5  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 to use a frequency of about 100 KHz as the lowest oscillator frequency. A second alternative, shown in  FIG. 7 , is to replace the rotary tissue sensing by four sense wires  11  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 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  FIG. 1  performs the discrimination between tissues based on the rules shown above and steers the burr  5  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. 
     The hydraulic actuators  13  are shown in  FIG. 6  in conjunction with  FIG. 1 . A motor, such as a stepper motor,  48  is driven from computer  12  via a standard interface. A piston  45  is moved in a cylinder  44  via the action of a thread  46  and a mating female thread  47 . The pressure is transmitted via hypodermic tubing  14  to channel  17  (not shown) in catheter  6 . Four identical units are used for +X, −X, +Y and −Y. 
     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 to 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  FIG. 7  can carry four temperature dependent resistors instead of sensing tips  21 . 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).