Abstract:
A surgical tool such as a liposuction cannula is equipped with a sensor at the tool tip. The sensor continuously analyzes the type of tissue in contact with the tip based on the electrical properties of the tissue. When encountering a tissue type that should not be disturbed, the action of the surgical tool is stopped automatically. When used for liposuction, the cannula is mechanically decoupled from the handle when the wrong type of tissue is detected thus minimizing the inertia of the part that needs to be stopped. Besides electrical sensing, other sensors, such as mechanical or ultrasonic, can be used at the tip of the surgical tool or cannula to differentiate between tissue types

Description:
FIELD OF THE INVENTION  
       [0001]    The invention relates to the medical field and in particular to surgery and liposuction. 
       BACKGROUND OF THE INVENTION  
       [0002]    The efficiency of many surgical procedures could be improved if there was a safe way to automatically stop the action of the surgical tool when it encounters an organ or type of tissue that should not be disturbed. For example, during liposuction a cannula, inserted via a minimal incision, is moved rapidly across fatty tissue in order to remove fat by using suction. Aggressive use of the cannula may result is a perforated organ and even death. On the other hand, slow movements significantly extend the duration of the surgery. A similar situation exists when using en electro-surgical tool or even an ordinary scalpel to cut in the vicinity of an organ that should not be damaged. The action of an electro-surgical tool can be stopped instantly (by stopping the current flow) if the tool could detect that it is touching or approaching a type of tissue that should not be cut. It has been known that different body tissues have different electrical properties, and the type of tissue, and even the state of the tissue can be determined from such electrical measurements. By the way of example, studies demonstrated that cancerous tissue has different electrical properties from healthy tissue. The field of tissue discrimination by electrical methods is known as “Bioimpedance Spectroscopy” and a good survey article is: “The Dielectric Properties of Biological Tissues: Literature survey” by Gabriel and Corthhout (Phys. Med. Biol. 41, 1996, pp. 2231-2249). This article is hereby incorporated by reference. Also incorporated by reference is US patent application 2007/0270688 which shares common inventors with this application. It is known that much of the information is in the way the impedance, and in particular the dielectric constant (also known as permittivity) changes with frequency. By measuring the electrical properties at multiple frequencies, typically in the range of KHz to MHz, a “signature” is derived which uniquely identifies the tissue. In some procedures the ability to apply energy such as heat, ultrasound, microwaves, water jets and others energy sources is limited by the fear of directing the energy to the wrong organ or tissue. For example, it is known that heating up the fat during liposuction will liquefy it and make it easier to remove, but if a heated cannula is used it can cause burns to other tissues. It is desired to have a method of supplying the energy to the surgical tool as long as it is in contact with the correct tissue, instantly stopping the energy when the wrong type of tissue is touched. 
         [0003]    Because of the relative slowness of human response time, it is desired to have the corrective action taking place automatically rather than simply alarming the surgeon. For example, when moving a cannula it is desired to instantly stop the motion of the cannula when the wrong tissue is touched, rather than sounding an alarm and relying on the response time of the surgeon. 
       SUMMARY OF THE DISCLOSURE  
       [0004]    A surgical tool such as a liposuction cannula or an electrosurgical tool is equipped with a sensor at the tool tip. The sensor continuously analyzes the type of tissue in contact with the tip based on the electrical properties of the tissue. When encountering a tissue type that should not be disturbed, the action of the surgical tool is stopped. When used for liposuction, the cannula is mechanically decoupled from the handle when the wrong type of tissue is detected, thus minimizing the inertia of the part that needs to be stopped. Besides electrical sensing, other sensors can be used at the tip of the surgical tool or cannula to differentiate between tissue types. An ultrasonic transducer can discriminate tissue types based on acoustic impedance. A vibrating sensor can discriminate based on damping and other mechanical properties. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0005]      FIG. 1  is an isometric view of a liposuction tool according to the invention. 
           [0006]      FIG. 2  is a close-up view of the tool tip showing the sensing and heating elements. 
           [0007]      FIG. 3A  is schematic view of a spiral sensing element. 
           [0008]      FIG. 3B  is a schematic view of a radial sensing element. 
           [0009]      FIG. 3C  is a schematic view of a concentric sensing electrode based on the four wire principle, having separate current injection and sensing leads. 
           [0010]      FIG. 4  is a longitudinal cross section of the liposuction cannula and handle. 
           [0011]      FIG. 5A  is a cross section of the cannula in areas the electrical conductor is exposed. 
           [0012]      FIG. 5B  is a cross section of the cannula in areas the electrical conductor is insulated. 
           [0013]      FIG. 6  is a schematic diagram of the electrical circuit used to identify the type of tissue. 
           [0014]      FIG. 7  is a schematic view of an electro-surgical tool with an added sensing electrode according to the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0015]    Referring to  FIG. 1 , a liposuction tool comprises of a cannula  1  having an opening  4  internally connected to suction hose  5 . Cannula  1  is connected to handle  2  via a coupler  3  in the conventional manner. According to the invention the cannula tip is equipped with a tissue type sensor. While many sensing methods are possible, the disclosure will use, by the way of example, electrical sensing. Sensing electrode  7  is electrically insulated from cannula  1  which is typically metallic. The electrode connecting wire  8  is bonded to cannula  1  and fed through coupler  3  to conductor  9  bonded to tube  6  via insulating layer  10 . Tube  6  can slide into handle  2 . Electrical contact to conductor  9  is maintained by a sliding contact inside handle  2 . An optional energy source  35 , such as resistive heater, RF (Radio Frequency) heater or ultrasonic transducer is mounted near opening  4  and is electrically connected to handle  2  in a similar manner to electrode  7 . The RF frequency can be from hundreds of KHz to several GHz, i.e. from LF to Microwave. In some applications it may be desired to replace the energy source with a mechanical or chemical action such as a rotary cutter or a high pressure liquid jet. The words “energy source” in this disclosure should be interpreted broadly as anything beyond the motion induced by the surgeon&#39;s hands. The electrical connections are brought out of handle  2  via a flexible electrical cable  22 . 
         [0016]    The details of the sensing electrode are shown in  FIG. 2 . The metallic electrode  7  is insulated from metallic cannula  1  via a thin insulating layer  10 . Electrode  7  is connected to the rest of the system via conductor  8  which is covered by a second insulating layer  11 . Clearly other styles of conductors, like a miniature coaxial cable, can be used.  FIGS. 3A ,  3 B and  3 C show different styles of electrodes.  FIG. 3A  shows a spiral electrode similar to  FIG. 2 .  FIG. 3B  shows a radial electrode, with better performance at very high frequencies. In all three figures the return electrode is the body of the cannula, assuming it is made of metal or having a metal coated tip.  FIG. 3C  shows an electrode of the type known as “4 wire system”. In this system the voltage sensing electrodes  12  and  13  are separate from the current injection from electrode  7  to the metallic cannula. In  FIG. 3C  the insulating layer does not have a gap between electrode  12  to electrode  13 . The gap exposing the cannula is only on the inside of electrode  13 . A four wire system is less sensitive to electrode-tissue interface problems. This type of electrodes is also known as “Kelvin Electrodes” in electrical engineering. 
         [0017]    When energy source  35  is a heater it is desired to use a very thin foil (preferably 5 micron to 100 micron) on top of an electrically and thermally insulating layer  10 . This allows turning the heater off instantly when the sensing electrode  7  detects a tissue that should not be affected. Typical choices for insulating layer  10  and cover layer  11  are polyimide or ceramics. A ceramic coating such as alumina or zirconia can be applied by plasma spraying. 
         [0018]    In order to stop the action of the cannula instantly and prevent the perforation of an internal organ when detected by the sensing electrode  7 , the cannula can be instantly decoupled from the handle as shown in  FIG. 4 . The cannula  1  is connected to handle  2  via a coupler  3  in the conventional manner. The mating part  14  is mounted on tube  6  that can slide through handle  2  to the position shown by dotted line  27 . A ferromagnetic ring  25  is attached to tube  6  and is held in place by solenoid coil  23  having a ferromagnetic face  24 . It is important to have the mating faces of  24  and  25  highly accurate and smooth in order to minimize the power required by coil  23 . By the way of example, only about 2 W is needed to hold tube  6  with a force of 10 Nt when mating faces are accurate. When the system detects the presence of the wrong tissue the current through coil  23  is stopped, causing the handle to slide over tube  6  and preventing the cannula from perforating the organ touched by the tip. An extra feature of the design is that the cannula will decouple when an excessive force is used, as the holding force of coil  23  can be adjusted by the current through it. When the cannula is retracted slightly and no longer touches the wrong tissue, spring  26  will bring parts  24  and  25  together and the magnetic force will keep them together till the next instance of touching the wrong tissue. This mode of automatic reset is very compatible with the rapid forward and backward motion imparted to handle  2  by the surgeon during liposuction. Electrical conductor  8  is fed via coupler  3  via an insulated bore  15 . The mating part  14  is made of insulating material and has a conductive ring  16  that makes electrical contact between conductor  9  and conductor  8 . Conductor  9  is not covered by a top insulating layer, in order to have a sliding contact with carbon brush  17  and spring  18  (similar to electrical brushes in a DC motor). Wire  19  coming from external flexible cable  22  completes the circuit. Cable  22  also carries the return wire  21  (typically forming a coaxial cable with wire  19 ) as well as the leads  20  for powering coil  23 . 
         [0019]      FIG. 5A  shows the cross section of the cannula in part  6  and  FIG. 5B  shows the cross section of the main cannula body, where conductor  8  is insulated from cannula by insulator  10  and covered by insulator  11 . Both conductor  8  and insulating layers  10  and  11  can be very thin, preferably 5 to 100 microns. 
         [0020]    The principle of distinguishing between tissue types is based on bioimpedance spectroscopy (i.e. measuring tissue impedance at different frequency). A simple resistance measurement is not sufficient because of the large variability in resistance. By measuring both the real and imaginary (capacitive) component of the electrical impedance the tissue touched by sensing electrodes  7  can be identified. In liposuction, the tissue has to be matches to one of the following three categories: fat, blood (and saline solution which is electrically very similar to blood) and “other tissue” such as muscle, dermis, blood vessels, internal organs etc. Touching “other tissue” should cause the cannula to disengage or, if preferred, generate a warning signal such as a light coming on, or both. The saline solution is injected during the procedure and is very similar to blood. There is no need to distinguish it from blood. 
         [0021]    Fat is characterized by a high resistance and a high dielectric constant (also referred to as “permittivity”) dropping off rapidly above a few KHz. Blood and saline are characterized by a low resistance and a high dielectric constant staying high up to several MHz. All other tissues have a medium resistance and a dielectric constant gradually dropping off. Specific patterns can be stored in the system to recognize specific types of tissues. For example, cancerous tissue has a different bioimpedance signature than normal tissue and this can be used to remove it without damaging normal tissue. By using several frequencies, such as 1 KHz, 10 KHz, 100 KHz and 1 MHz a clear signature can be generated. There is little information in frequencies below 100 Hz and over 10 MHz. The art of measuring bioimpedance is well known and a typical circuit is shown in  FIG. 6 . A current source  28  can be switched by an electronic switch  30  to generate the required frequencies. This is typically implemented by a single IC digital frequency synthesizer. The synthesizer is connected as a current source to sensing electrode  7  in contact with tissue  29 . The impedances measured depend on the size of electrodes  7  but for a typical liposuction cannula were found to be in the range of 20 Ohms to 2000 Ohms. The impedance is separated into the resistive and capacitive components by multipliers (or mixers)  31 ,  32  multiplying the sensed signal by the sine and cosine outputs of the  28  (i.e. by two outputs shifted by 90 degrees). Amplifier  36  is used to buffer the signal and can be placed closer to electrode  7  for better performance (also known as “active probe”). Capacitors  33  filter out the high frequency component. Computer  34  compares the resistive and capacitive components to pre-programmed patterns to identify tissue type. The analog voltages are interfaced to computer  34  via analog to digital converters (not shown), unless the computer IC has a built-in analog to digital converter. After classification of tissue type computer  34  stops the current through coil  23  whenever it senses tissue other than blood or fat or by any other pre-programmed criterion. It can also control warning light  40  or any other form of warning as well as control energy source  35 . The details of such electronic circuits are well known in the art. A similar system can be used to control other surgical tools such as electro-surgical instruments. These well known tools use RF energy to cut tissue by ablation. They are preferred to scalpels for many procedures because of reduced bleeding.  FIG. 7  shows a tool  37  connected to an RF source (not shown) by wire  37 . The RF energy is fed to cutting tip  38  and forms a path between tip  38 , typically a U-shaped wire, and the body of the patient. A sensing electrode  7  is added to the standard tool. Since the RF is typically pulsed, the gaps between pulses can be used to ground tip  38  and sense type of tissue  29  by measuring the impedance between electrode  7  and temporarily grounded tip  38  in a similar manner to the system discussed earlier. When a tissue type that should not be cut is detected, RF energy to tip is stopped. Without RF energy the blunt tip  38  can not cut. For tools using continuous RF energy, the RF signal can be paused many times a second for a few milliseconds each time. This allows electrical measurements with very little affect on the operation of the tool. The same concept can be adapted to many other surgical tools. In this disclosure the term “surgical tool” should be interpreted broadly as any tool inserted into the body, regardless of shape or function. 
         [0022]    While the preferred embodiment describes the use of an electrical sensor at the tip of the surgical tool, it should be understood that the invention covers all methods of sensing. It is well known that there is a difference between the acoustic and mechanical properties of different tissues. If electrode  7  in  FIG. 2  is replaced by a thin ultrasonic transducer, the type of tissue can de established by the echo similar to the well known IVUS (Intravascular Ultrasound) system used inside arteries. Sometimes a mechanical transducer operating at a relatively low frequency (Hz to KHz range) can be used to measure stiffness and damping. Such a transducer can be piezoelectric or electromagnetic. In this disclosure the term “sensor” should be interpreted as any sensing method. 
         [0023]    Most sensors not only sense the immediate layer they contact but “see” some depth into the tissue. In the case of an electrical sensor this depth is approximately equal to the electrode spacing. Ultrasonic sensors can see a great depth into the tissue. This can be used to stop the action of the tool before it reaches the tissue that should not be disturbed. By setting the trip point of the system, a programmable thickness of removable tissue can be left in place. Similarly the response time of the system can be programmed in order to reduce nuisance tripping by a very brief contact or brief transition between tissue types. It is desired that these parameters can be set by the surgeon, as the preference may vary between surgeons.