Abstract:
A method is provided of tissue ablation during a tissue ablation procedure. Ablation energy is applied by using a tissue ablation device to create an ablation at a tissue site. An ablation endpoint at the tissue site is detected by using an ablation endpoint device with one or more sensors that are positioned to monitor the ablation. The one or more sensors are selected from at least one of, a piezoelectric and a silicon MEMS sensor. Upon detecting the ablation endpoint, delivery of ablation energy to the tissue site ceases.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Ser. No. 61/022,681, which application is fully incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to detecting endpoints of ablation procedures, and more particularly to systems and methods that use one or more sensors that resonate at a frequency equal to the resonance frequency of the ablated tissue to determine endpoints of ablation procedures. 
         [0004]    2. Related Art 
         [0005]    During a medical ablation event, using electromagnetic energy including but not limited to RF, microwave and the like, the treating physician needs to know how far the ablation has proceeded in order to not over ablate. 
         [0006]    Imaging methods have been used, without much success, to determine the endpoint of a medical ablation procedure. 
         [0007]    In some medical applications, transvenous or intravenous ablation catheters with one or more electrodes are inserted into one or more heart cavities or put in contact with external areas of the heart to administer the ablation treatment to kill selected heart tissue. It is difficult to assess when to terminate the administration of the treatment in a manner that identifies when sufficient tissue has been destroyed to provide a clinically efficacious (transmural) linear ablation lesion. Particularly, “blind” or catheter-based ablation of cardiac tissue (such as to treat atrial fibrillation) can be more effective when patient-specific valid endpoints are used to recognize when a clinically efficacious lesion has been created. In the early ablation experience, acute termination followed by non-inducibility of the arrhythmia were used. Because these endpoints correlated poorly with long-term success, however, other parameters have been developed. Impedance and temperature measurements during the delivery of RF energy and the presence of conduction block after delivery of RF energy are the most common endpoints used in clinical practice. 
         [0008]    Accordingly, there is a need for improved endpoint determinations during medical ablation procedures. 
       SUMMARY 
       [0009]    An object of the present invention is to provide improved endpoint determinations devices, and their uses for medical ablation procedures. 
         [0010]    Another object of the present invention, acceleration or vibrations sensor devices are provided that are useful for determining the endpoints of medical ablation procedures. 
         [0011]    These and other objects of the present invention are provided in a method of tissue ablation during a tissue ablation procedure. Ablation energy is applied by using a tissue ablation device to create an ablation at a tissue site. An ablation endpoint at the tissue site is detected by using an ablation endpoint device with one or more sensors that are positioned to monitor the ablation. The one or more sensors are selected from at least one of, a piezoelectric and a silicon MEMS sensor. Upon detecting the ablation endpoint, delivery of ablation energy to the tissue site ceases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross sectional side view of a MEMS pressure sensor with selective encapsulation that can be used in one embodiment of the present invention. 
           [0013]      FIG. 2  is a top view of the MEMS device of  FIG. 1 . 
           [0014]      FIG. 3  is a cross sectional side view of a MEMS pressure sensor with selective encapsulation. 
           [0015]      FIG. 4  is a top view of the MEMS device of  FIG. 3 . 
           [0016]      FIG. 5  is a perspective view of a MEMS device in accordance with a third embodiment of a MEMS device that can be used with the present invention. 
           [0017]      FIG. 6  is a cross sectional view of the MEMS device shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In one embodiment of the present invention an endpoint detection device includes one or more sensors. Suitable sensors include but are not limited to an acceleration or vibration sensor and the like. The sensor can be of either piezoelectric or silicon MEMS technologies to determine an endpoint of an ablation process and can be used for other non-ablation applications. 
         [0019]    The endpoint detection device can be used in a variety of therapeutic applications including but not limited to, activity monitoring for implantable defibrillators and pace makers, positioning in an ear canal to measure brain trauma, to monitor tissue ablation progress, provide diagnostic information and therapeutic treatment in a variety of application including but not limited to neurology, and the like. In one embodiment the endpoint detection device can be used for long term implantable catheters. 
         [0020]    In one embodiment, the endpoint detection device is used to monitor tissue ablation. The ablation can be performed using an ablation source which is typically electromagnetic. Suitable electromagnetic energy sources include but are not limited to, RF, microwave and the like. 
         [0021]    With the present invention, one or more sensors are provided that resonate at a frequency equal to the resonance frequency of the ablated tissue. This resonance frequency is different from the frequency of the non-ablated tissue. The sensor can be coupled to an external detection device. The external device indicates when the sensor is excited to its resonance frequency. The sensor can be coupled to the external detection device by cable, wireless and the like. Achieving a specific resonance frequency is used to determine the endpoint of the tissue ablation procedure. 
         [0022]    As the ablation process proceeds tissue radiating from the ablation device is effected by the procedure. When the procedure reaches the tissue where the sensor or sensor array is positioned, the detector notifies the physician with an indication that the procedure should now be discontinued. 
         [0023]    In one embodiment, the endpoint detection device has an array of sensors that are mounted to or encapsulated in a biocompatible material, or are manufactured from a biocompatible material. The array of sensors is positioned at an ablation site and can fully or partially surround the ablation site. The array of sensors is tuned to a specific resonance frequency of the ablated tissue. This resonance frequency is different from the resonance frequency of the non-ablated tissue. When the ablation has reached the sensor array and thus the desired endpoint, the array of sensors produces an electrical signal. 
         [0024]    In one embodiment, the sensor is a piezoresistive sensor that has a substrate with two opposed surfaces. A dielectric insulative layer is on the a first surface of the substrate. A doped semiconductor layer is on top of the dielectric insulated layer. The semiconductor layer has a high resistivity. The doped semiconductor layer is annealed to one or more regions to lower resistivity of the semiconductor layer and define therein one or more sensor gauges of the annealed semiconductor material. Electrical contacts are adjacent to the annealed semiconductor material and overlay at least a portion of the annealed semiconductor material. 
         [0025]    One embodiment of a suitable MEMS sensor that can be used with the present invention includes a housing and a sensor die that can be attached to the housing by an epoxy or silicone adhesive. Wire bonds provide an electrical connection between wire bond pads of the sensor die and a lead frame. A protective dam and an encapsulation gel material can be included as disclosed in U.S. Pat. No. 6,401,545, incorporated herein by reference. 
         [0026]      FIG. 1  is a cross-sectional side view of a MEMS sensor  100  with selective encapsulation that can be used in one embodiment of the present invention. MEMS pressure sensor  100  comprises a housing  105  (partially shown) which is typically made of a plastic material. A sensor die  120  is attached to plastic housing  105  by an epoxy or silicone adhesive  110 . Wire bonds  140  provide an electrical connection between wire bond pads  122  of sensor die  120  and a lead frame  130 . Also shown are a protective dam  150  and an encapsulation gel material  160 , which serves as a protectant. 
         [0027]    The particulars of the various elements, as well as the technique for fabricating the improved MEMS sensor  100 , is as follows. The description of the various embodiments of the present invention is drawn primarily to a MEMS pressure sensor. However, the described embodiments of the present invention of selective encapsulation are applicable to a wide variety of MEMS sensors, including capacitive sensors which sense pressure, chemical, humidity, etc. 
         [0028]    The common denominator of these types of MEMS sensors with regard to the various embodiments of the present invention, is a transducer element such as a capacitive diaphragm or membrane which is sensitive to some ambient condition and which, for optimal performance, should be free of encapsulation gel. However, the remainder of the package, other than the transducer element, should be encapsulated for environmental protection. 
         [0029]    After a wafer containing numerous MEMS pressure sensor devices is diced into individual dies, each individual pressure sensor die  120  is attached to a housing  105 , which is typically made of plastic, by conventional means. Typically an epoxy or silicone adhesive  110  is used to attach pressure sensor die  120  to the base of plastic housing  105 . 
         [0030]    Subsequent to attaching pressure sensor die  120  to plastic housing  105 , wire bonds  140  are connected between die wire bond pads  122  and lead frame  130 . 
         [0031]    A subsequent step is to construct protective dam  150  between the outer perimeter of a pressure sensor diaphragm  121  and the inner perimeter formed by bond pads  122 . Pressure sensor diaphragm  121  is typically located in the center portion of pressure sensor die  120  as shown in  FIG. 3 . Protective dam  150  is then cured at high temperature. 
         [0032]    Following the curing of protective dam  150 , MEMS pressure sensor  100  is ready for encapsulation. During this step an encapsulation gel  160  is dispensed into the wire bond cavity region that is located between protective dam  150  and plastic housing  105 , thereby covering bond pads  122 , portions of lead frame  130  and wire bonds  140 . After selective encapsulation is completed, encapsulation gel  160  is cured. 
         [0033]    By way of example, protective dam  150  is constructed of a fluorocarbon based material to achieve the best media compatibility, i.e., to protect the integrity of the wire bonds  140  from contamination by foreign matter. However, fluorocarbon type material is also typically the most costly. Other cost effective materials include silicone and fluorosilicone base materials. Typically similar materials are used for both protective dam  150  and encapsulation gel  160 . 
         [0034]    Protective dam  150  is typically constructed by forming it as a unit using a device such as a dispensing collet. A dispensing collet is a nozzle type device where the design of the output opening of the nozzle corresponds the design of protective dam  150 . Thus, for rectangular shaped dams, the dispensing collet would have a rectangular shaped nozzle which would permit the formation of all four walls of the protective dam  150  simultaneously. The current design preferably uses a rectangular shaped protective dam  150  for consistency with the rectangular shaped pressure sensor diaphragm  121 . However, other shape diaphragms and protective dams are contemplated including, but not limited to, circular configurations, triangular configuration, pentagonal configurations, or the like. 
         [0035]    Alternatively, each of the four walls of protective dam  150  may be formed using a dispensing needle. In the dispensing needle method of protective dam construction, each of the dam walls is formed sequentially, as opposed to the dispensing collet method in which the dam walls are formed simultaneously. The dispensing needle essentially line draws each dam wall. Multiple passes for each wall can be made to control the height and width of protective dam  150 . 
         [0036]    The minimum height for protective dam  150  is preferably equal to approximately the loop height of wire bonds  140 , i.e. the apogee of wire bonds  140  above pressure sensor die  120 . The minimum height is driven by the requirement to insure complete encapsulation of wire˜bonds  140 . The maximum height of protective dam  150  is that of plastic housing  105 . However, in practice the height of protective dam  150  ranges between the apogee of wire bonds  140  and plastic housing  105  as shown in  FIG. 2 . 
         [0037]    For typical applications where the thickness of pressure sensor die  120  is approximately 645 microns (.mu.m), i.e., approximately 25 mils, and the total cavity height of the plastic housing  105  is approximately 135 mils, the nominal height of the protective dam  150  is in the range of 774-1,548 .mu.m, i.e., approximately 30-60 mils. Now referring to  FIGS. 3 and 4 , MEMS pressure sensor  101  with selective encapsulation  101  in accordance with another embodiment is depicted in which a vent cap  170  serves as a protectant. MEMS pressure sensor  101  includes vent cap  170  covering, sealing or otherwise encapsulating the wire bond cavity region instead using an encapsulation gel to fill the wire bond cavity. Vent cap  170  has a vent aperture  171  in the center which permits pressure sensor diaphragm  121  to receive unmolested ambient pressure. The prior art problem of gel over expansion is avoided by not having to fill the wire bond cavity with encapsulation gel. 
         [0038]    Formation MEMS pressure sensor  101  employs similar steps as described for the formation of MEMS pressure sensor  100  including attaching pressure sensor die  120  to plastic housing  105  (partially shown), wire bonds  140  which electrically connect pressure sensor bond pads  122  to lead frame  130 , and the construction of protective dam  150 . 
         [0039]    However, after protective dam  150  has been constructed on a top surface of pressure sensor die  120 , vent cap  170  is placed over the device. The outer edges of vent cap  170  mate with plastic housing  105 . The lower surface of the center portion of vent cap  170  is pressed down against protective dam  150 . Sealing vent cap  170  takes place by curing the device at high temperature. Alternatively, an adhesive material can be used to seal vent cap  170 . Also, various combinations of heat curing and adhesive may be employed to seal vent cap  170 . 
         [0040]    Preferably, vent cap  170  is formed from a plastic material which is compatible with plastic housing  105  and protective dam  150 . In alternative embodiments, vent cap  170  may be constructed from metal. However, for a metal embodiment, adequate clearance must be provided between vent cap  170  and wire bonds  140  so as to preclude electrical shorting of wire bonds  140  to vent cap  170 . The limitations of the height of protective dam  150  are similar to those described for MEMS sensor  100 . 
         [0041]    As shown in  FIG. 3 , vent cap  170  has an offset in the center portion where it contacts protective dam  150 . The purpose of the offset is to optimize the height of the protective dam  150  with respect to wire bonds  140  and plastic housing  105 . However, alternative embodiments may not need the offset. 
         [0042]    Now referring to  FIG. 5 , a MEMS pressure sensor  102  with selective encapsulation in accordance with yet another embodiment of the present invention is depicted. In this embodiment, protective dam  150  is formed at the wafer level by bonding a cap wafer  151  to a device wafer  125  by means of a glass frit  152  or other suitable adhesive. A preliminary step in the fabrication of MEMS sensor  102  is to form a plurality of sensor devices on a substrate such as device wafer  125 .  FIG. 5  illustrates diaphragms  121  and wire bond pads  122  of a typical sensor device. 
         [0043]    Independent of the sensor device formation on device wafer  125 , a second wafer sometimes referred to as a cap wafer  151  is patterned with a plurality of diaphragm apertures  153 , device channels  154  and cut lines  155 . A subsequent step is to form a bonding area by depositing a glass frit pattern by screen printing or other means on cap wafer  151 . Cap wafer  151  is then aligned and bonded to device wafer  125 . The cap/device wafer combination is then heat cured and diced into individual pressure sensor dies  120  having a protective dam  150  attached. 
         [0044]      FIG. 6  is a cross sectional view of encapsulated device  102  which further illustrates diaphragm aperture  153 . Each of pressure sensor dies  120  is attached to housing  105  as described in previous embodiments. Wire bonding is similarly accomplished by connecting wire bonds  140  between wire bond pads  122  and lead frame  130 . A wire bond cavity region is formed between the protective dam  150 , i.e., the combination of portions cap wafer  151  and glass frit pattern  152 , and housing  105 . The wire bond cavity is filled with lo encapsulation gel  160  similar to the previously described embodiments. The limitations of the height of the protective dam  150  are similar to those described with respect to MEMS sensor  100 . 
         [0045]    In one embodiment, the sensor die includes a transducer element, including but not limited to a capacitive diaphragm or membrane, that is sensitive to some ambient condition and which, for optimal performance, should be free of encapsulation gel material. 
         [0046]    In one embodiment, the sensor is a piezoelectric sensor with two metal plates to sandwich a crystal and make a capacitor. External force cause a deformation of the crystal and results in a charge which is a function of the applied force. In its operating region, a greater force results in more surface charge. This charge results in a voltage  v=   Q     1     /L , where  Q     1    is the charge resulting from a force f, and C is the capacitance of the device. 
         [0047]    The piezoelectric crystals act as transducers which turn force, or mechanical stress into electrical charge which in turn can be converted into a voltage. Alternatively, if a voltage is applied to the plates, the resultant electric field causes the internal electric dipoles to re-align which cause a deformation of the material. 
         [0048]    Although the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and detail may be made therein without departing from the spirit and scope of the invention.