Abstract:
A medical probe assembly and method for ablating tissue using radio frequency energy is provided. Included in the medical probe assembly is an ablation probe and an alignment device. The alignment device comprises a surface and plurality of apertures through which the ablation probe can be guided into the target region of the patient. The apertures may be uniformly or non-uniformly spaced and parallel or non-parallel from each other. The apertures may be indexed from each other in a two dimensional plane. By adding one or more bosses or recesses to the apertures, the apertures may indexed from each other in a three dimensional space and provides an improved system and method for accurately creating compound lesions on tumors. Furthermore, by adding removable inserts to the recesses, the depth of the recess may be adjustable.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention relates generally to the use of ablation probes for the treatment of tissue, and more particularly, RF ablation probes for the treatment of tumors.  
         BACKGROUND OF THE INVENTION  
         [0002]    The delivery of radio frequency (RF) energy to target regions within tissue is known for a variety of purposes of particular interest to the present invention. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) in target tissue for the purpose of tissue necrosis.  
           [0003]    One method for RF ablation uses a single needle electrode, which when attached to a RF generator, emits RF energy from the exposed, uninsulated portion of the electrode. This energy translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. By varying the power output and the type of electrical waveform, it is possible to control the extent of heating, and thus, the resulting ablation. The diameter of tissue coagulation from a single electrode, however, has been limited by heat dispersion.  
           [0004]    Another method for ablation utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. PCT application WO 96/29946 and U.S. Pat. No. 6,379,353 disclose such probes. In U.S. Pat. No. 6,379,353, a probe system comprises a cannula having a needle electrode array reciprocably mounted therein. The individual electrodes within the array have spring memory, so that they assume a radially outward, arcuate configuration as they are advanced distally from the cannula. In general, a multiple electrode array creates a larger lesion than that created by a single needle electrode.  
           [0005]    When performing an ablation on a tumor, the general rule is to select an array that has a diameter that will produce a 1 cm margin of ablated tissue around the periphery of the actual tumor. For example, for a 1 cm tumor, the appropriate array diameter would be 3.0 cm. Unfortunately, many of the tumors currently treated are larger than 1 cm in diameter. Often, the tumor is larger than the largest available array device (4.0 cm) currently on the market, the LaVeen probe offered by Boston Scientific. In theory, the largest tumor size that the 4.0 cm device can treat on a single ablation is 2.0 cm (4.0 cm device−2.0 cm margin=2.0 cm tumor). When treating tumors that are larger than 2.0 cm, generally, an ablation is performed and then the array is repositioned around the initial ablation. This process is continued until the overlapping ablations create a 1 cm margin over the tumor.  
           [0006]    One difficulty experienced with creating a compound lesion is the reduced ultrasonic image visualization caused by an echogenic cloud from the initial ablation. Physicians must estimate the initial location and depth and then reposition the array for subsequent overlapping ablations. This process proves to be challenging because of poor imaging quality. Moreover, the individual ablation devices will generally not be steerable and capable of being redirected within the tissue, so there are few options for correcting the configuration after the needles have first penetrated into the tissue.  
           [0007]    Thus, there is a need to provide improved systems and methods for accurately creating compound lesions on tumors.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with a first aspect of the present inventions, a tissue ablation system is provided. The tissue ablation system comprises one or more ablation probes. In the preferred embodiment, the ablation probe(s) utilize radio frequency (RF) energy, but it can also utilize other types of energy, such as laser energy. The tissue ablation system further comprises an alignment device configured to be fixed relative to targeted tissue, e.g., a tumor. In the preferred embodiment, the alignment device can be conveniently adhered to the patient, but other types of suitable means can be used to affixed the alignment device relative to the targeted tissue. The alignment device can be any shape, including a customized shape, but in the preferred embodiment, it is disk-shaped.  
           [0009]    The alignment device comprises a surface and a plurality of apertures through which the ablation probe(s) can be guided. The apertures can be spaced apart along the surface in any of a variety of configurations. For example, the spacing between the apertures can either be fixed or adjustable. The spacing between the apertures can be uniform or non-uniform. The axes of the apertures can be parallel or non-parallel to each other. For example, if the apertures are parallel, the ablation probes(s) can be aligned in a Cartesian coordinate system. If the apertures are non-parallel, the ablation probe(s) can be aligned in an angular coordinate system. In one preferred embodiment, the apertures comprise a central aperture and remaining apertures that are placed in a plurality of concentric rings around the central aperture.  
           [0010]    Thus, it can be appreciated that the apertures can be indexed from each other in a two-dimensional plane. Optionally, the alignment device can comprise one or more bosses or recesses associated with a respective one or more of the plurality of apertures, wherein the boss(es) limits and recess(es) increase the distance that the ablation probe(s) can be guided through the aperture(s). If a plurality of boss(es) is provided, the bosses can have differing lengths. Likewise, if a plurality of recesses are provided, the recesses can have variable depths. The boss(es) can either be permanently mounted or removably mounted to the aperture(s). The recess(es) can also be “filled” with insert(s). Thus, it can be appreciated that the boss(es) and recess(es) allow the apertures to be indexed from each other in three-dimensional space.  
           [0011]    In accordance with a second aspect of the present inventions, a method for performing compound ablation in the body of a patient is provided. The method comprises affixing an alignment device relative to target tissue, such as, e.g., a tumor. The alignment device can be affixed using any suitable means, e.g., by adhering the alignment device to the skin of the patient. The method further comprises guiding an ablation probe within a first aperture in the alignment device to place the ablation probe adjacent the targeted tissue in a first region. For example, the ablation probe can be placed in contact with the targeted tissue (e.g., by embedding it) or placed a relative short distance from the targeted tissue. The ablation probe can be placed adjacent the targeted tissue using any suitable means. For example, the ablation probe can be introduced into the patient&#39;s body percutaneously, laparoscopically, or through a surgical opening.  
           [0012]    The method further comprises operating the ablation probe (e.g., using RF or laser energy) to create a first lesion in the first region. The method further comprises guiding the ablation probe within a second different aperture in the alignment device to place the ablation probe adjacent the targeted tissue in a second region, and operating the ablation probe again to create a second lesion in the second region. In addition, the ablation device can be guided to a different depth within the first aperture in the alignment device to place the ablation probe adjacent the targeted tissue in a second region, and operating the ablation probe to create a second lesion in the second region. The ablation probe may be removed completely from the first aperture prior to guiding it within the second aperture. Alternatively, the ablation probe may be moved from the first aperture to the second aperture without completely removing the ablation probe, e.g., by laterally guiding the ablation probe along a guiding slot between the first and second apertures. In any event, alternate guiding and operating of the ablation probe can be performed for a plurality of regions until the entire target tissue is ablated.  
           [0013]    The ablation probe can be guided within the first and second apertures in parallel directions, e.g., to align the ablation probe in a Cartesian coordinate system, or can be guided within the first and second apertures in non-parallel directions, e.g., to align the ablation probe in an angular coordinate system. The alignment device can optionally comprise a boss or a recess associated with the first aperture, in which case, the method can comprise limiting a distance that the ablation probe is guided within the first aperture by abutting a portion of the ablation probe against the boss or recess.  
           [0014]    In accordance with a third aspect of the present invention, another method of performing a compound ablation in the body of a patient is provided. The method comprises affixing an alignment device relative to target tissue, such as, e.g., a tumor. The alignment device can be affixed using any suitable means, e.g., by adhering the alignment device to the skin of the patient. The method further comprises guiding a plurality of ablation probes within a respective plurality of apertures in the alignment device to place the ablation probes adjacent the targeted tissue in a plurality of regions. For example, the ablation probes can be placed in contact with the targeted tissue (e.g., by embedding them) or placed a relative short distance from the targeted tissue. The ablation probes can be placed adjacent the targeted tissue using any suitable means. For example, the ablation probes can be introduced into the patient&#39;s body percutaneously, laparoscopically, or through a surgical opening.  
           [0015]    The ablation probes can be guided within the apertures in parallel directions, e.g., to align the ablation probes in a Cartesian coordinate system, or can be guided within the apertures in non-parallel directions, e.g., to align the ablation probes in an angular coordinate system. The alignment device can optionally comprise one or more bosses or recesses associated with one or more of the apertures, in which case, the method can comprise limiting a distance that one or more of the ablation probes is guided within the aperture(s) by abutting a portion of the ablation probe(s) against the boss(es) or recess(es). If a plurality of bosses or recesses are provided, the bosses or recesses can have differing lengths.  
           [0016]    The method further comprises operating the ablation probes (e.g., using RF or laser energy) to create a plurality of lesions within the plurality of regions. The ablation probes can either be operated in a unipolar mode or a bipolar mode (e.g., by conveying RF energy between two ablation probes).  
           [0017]    In accordance with a fourth aspect of the present inventions, an alignment device for one or more ablation probes is provided. In the preferred embodiment, the alignment device can be conveniently adhered to the patient, but other types of suitable means can be used to affixed the alignment device relative to the targeted tissue. The alignment device can be any shape, including a customized shape, but in the preferred embodiment, it is disk-shaped. The alignment device comprises a surface and a plurality of apertures through which the ablation probe(s) can be guided. The apertures can be spaced apart along the surface in any of a variety of configurations, as previously described.  
           [0018]    The alignment device further comprises one or more bosses and/or recesses associated with a respective one or more of the plurality of apertures, wherein the boss(es) or recess(es) limits the distance that the ablation probe(s) can be guided through the aperture(s). If a plurality of boss(es) or recess(es) is provided, the bosses or recesses can have differing lengths. If boss(es) are provided, the boss(es) can either be permanently mounted or removably mounted to the aperture(s). If recess(es) are provided, the recess may be associated with an insert that decreases the depth of the recess. Thus, it can be appreciated that the boss(es) and/or recess(es) allow the apertures to be indexed from each other in three-dimensional space.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The drawings illustrate the design and utility of a preferred embodiment of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate the advantages and objects of the present invention, reference should be made to the accompanying drawings that illustrate this preferred embodiment. However, the drawings depict only one embodiment of the invention, and should not be taken as limiting its scope. With this caveat, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0020]    [0020]FIG. 1 is a perspective view of a tissue ablation system constructed in accordance with one preferred embodiment of the present invention, wherein a single probe assembly is particularly shown used with the alignment device of FIG. 4;  
         [0021]    [0021]FIG. 2 is a perspective view of an ablation probe assembly used in the tissue ablation system of FIG. 1, wherein a needle electrode array is particularly shown retracted;  
         [0022]    [0022]FIG. 3 is a perspective view of the ablation probe assembly used in the tissue ablation system of FIG. 1, wherein a needle electrode array is particularly shown deployed;  
         [0023]    [0023]FIG. 4 is a perspective view of a first embodiment of an alignment device that can used in the tissue ablation system of FIG. 1;  
         [0024]    [0024]FIG. 5 is a cross-sectional view of the alignment device of FIG. 4;  
         [0025]    [0025]FIG. 6 is a cross-sectional view of a second embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0026]    [0026]FIG. 7 is a cross-sectional view of a third embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0027]    [0027]FIG. 8 is a perspective view of a tissue ablation system constructed in accordance with another preferred embodiment of the present invention, wherein multiple probe assemblies are particularly shown used with the alignment device of FIG. 7;  
         [0028]    [0028]FIG. 9 is a cross-sectional view of a fourth embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0029]    [0029]FIG. 10 is a cross-sectional view of a fifth embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0030]    [0030]FIG. 11 is a cross-sectional view of a sixth embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0031]    [0031]FIG. 12 is a cross-sectional view of a seventh embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0032]    [0032]FIG. 13 is a cross-sectional view of an eighth embodiment of an alignment device that can be used in the tissue ablation system of FIG. 1;  
         [0033]    [0033]FIGS. 14-17 are perspective views illustrating one preferred method of using the tissue ablation system of FIG. 1 to ablate a treatment region within tissue of a patient;  
         [0034]    [0034]FIG. 18 is a perspective view illustrating another preferred method of using the tissue ablation system of FIG. 1 to ablate the treatment region; and  
         [0035]    [0035]FIG. 19 is a perspective view illustrating a preferred method of using the tissue ablation system of FIG. 8 to ablate the treatment region. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    [0036]FIG. 1 illustrates a tissue ablation system  100  constructed in accordance with a preferred embodiment of the present invention. The tissue ablation system  100  generally comprises an ablation probe assembly  110 , which is configured for introduction into the body of a patient to ablate target tissue such as a tumor, a radio frequency (RF) generator  130  configured for supplying RF energy to the probe assembly  110  in a controlled manner, and an alignment device  140  configured for ensuring accurate positioning of the ablation probe assembly  110  relative to the target tissue. In the illustrated embodiment, only one probe assembly  110  is shown. As will be described in further detail below, however, multiple probe assemblies  110  can be connected to the RF generator  130  and simultaneously associated with the alignment device  140 , depending upon the specific ablation procedure that the physician selects.  
         [0037]    Referring further to FIGS. 2 and 3, the probe assembly  110  generally comprises a handle assembly  112 , an elongated cannula  114 , and an inner probe  118  (shown in phantom) slideably disposed within the cannula  114 . As will be described in further detail below, the cannula  114  serves to deliver the active portion of the inner probe  118  to the target tissue. The cannula  114  has a proximal end  120 , a distal end  122 , and a central lumen (not shown) extending through the cannula  114  between the proximal end  120  and the distal end  122 . The cannula  114  may be rigid, semi-rigid, or flexible depending upon the designed means for introducing the cannula  114  to the target tissue. The cannula  114  is composed of a suitable material, such as plastic or metal, and has a suitable length, typically in the range of 5 cm to 30 cm, preferably from 10 cm to 20 cm. The cannula  114  has an outside diameter consistent with its intended use, typically being from 1 mm to 5 mm, usually from 1.3 mm to 4 mm. The cannula  114  has an inner diameter in the range of 0.7 mm to 4 mm, preferably from 1 mm to 3.5 mm.  
         [0038]    The inner probe  118  comprises a reciprocating shaft  121  having a proximal end  123  and a distal end  124 , and an array  126  of tissue penetrating needle electrodes  128  extending from the distal end  124  of the shaft  121 . Like the cannula  114 , the shaft  121  is composed of a suitable material, such as plastic or metal. The electrode array  126  can be mounted anywhere on the shaft  121 . However, the electrodes  128  will typically be fastened to the shaft  121  at its distal end  124 , though the individual electrodes  128  can extend up to its proximal end  123 . Each of the needle electrodes  128  is a small diameter metal element, which can penetrate into tissue as it is advanced through tissue.  
         [0039]    As illustrated in FIG. 2, longitudinal translation of the shaft  121  in the proximal direction  129  relative to the cannula  114 , retracts the electrode array (not shown) into the distal end  122  of the cannula  114 . When retracted within the cannula  114 , the electrode array  126  (shown in FIG. 3) is placed in a radially collapsed configuration, and each needle electrode  128  is constrained and held in a generally axially aligned position within the cannula  114  to facilitate its introduction into tissue. The probe assembly  110  optionally includes a core member (not shown) mounted on the distal end  124  of the shaft  121  and disposed within the center of the needle electrode array  126 . In this manner, substantially equal circumferential spacing between adjacent needle electrodes  128  is maintained when the array is retracted within the central lumen.  
         [0040]    As shown in FIG. 3, longitudinal translation of the shaft  121  in the disial direction  131  relative to the cannula  114  deploys the electrode array  126  out of the distal end  122  of the cannula  114 . As will be described in further detail, manipulation of the handle assembly  112  will cause the shaft  121  to longitudinally translate to alternately retract and deploy the electrode array  126 .  
         [0041]    When deployed from the cannula  114 , the electrode array  126  is placed in a three-dimensional configuration that usually defines a generally spherical or ellipsoidal volume having a periphery with a maximum radius in the range of 0.5 cm to 4 cm. The needle electrodes  128  are resilient and pre-shaped to assume a desired configuration when advanced into tissue. In the illustrated embodiment, the needle electrodes  128  diverge radially outwardly from the cannula  114  in a uniform pattern, i.e., with the spacing between adjacent needle electrodes  128  diverging in a substantially uniform pattern or symmetric pattern or both. In the illustrated embodiment, the needle electrodes  128  evert proximally, so that they face partially or fully in the proximal direction  129  when fully deployed. In exemplary embodiments, pairs of adjacent needle electrodes  128  can be spaced from each other in similar or identical, repeated patterns that can be symmetrically positioned about an axis of the shaft  121 . It will be appreciated by one of ordinary skill in the art that a wide variety of patterns can be used to uniformly cover the region to be treated. It should be noted that a total of six needle electrodes  128  are illustrated in FIGS. 1 and 3. Additional needle electrodes  128  can be added in the spaces between the illustrated electrodes  128 , with the maximum number of needle electrodes  128  determined by the electrode width and total circumferential distance available. Thus, the needle electrodes  128  could be quite tightly packed.  
         [0042]    Each electrode  128  is preferably composed of a single wire that is formed from resilient conductive metals having a suitable shape memory. Many different metals such as stainless steel, nickel-titanium alloys, nickel-chromium alloys, and spring steel alloys can be used for this purpose. The wires may have circular or non-circular cross-sections, but preferably have rectilinear cross-sections. When constructed in this fashion, the needle electrodes  128  are generally stiffer in the transverse direction and more flexible in the radial direction. The circumferential alignment of the needle electrodes  128  within the cannula  114  can be enhanced by increasing transverse stiffness. Exemplary needle electrodes will have a width in the circumferential direction in the range of 0.2 mm to 0.6 mm, preferably from 0.35 mm to 0.40 mm, and a thickness, in the radial direction, in the range of 0.05 mm to 0.3 mm, preferably from 0.1 mm to 0.2 mm.  
         [0043]    The distal ends  127  of the needle electrodes  128  may be honed or sharpened to facilitate their ability to penetrate tissue. The distal ends  127  of these needle electrodes  128  may be hardened using conventional heat treatment or other metallurgical processes. The needle electrodes  128  may be partially covered with insulation, although they will be at least partially free from insulation over their distal portions  127 . The proximal ends  127  of the needle electrodes  128  may be directly coupled to the proximal end of the shaft  121 , or alternatively, may be indirectly coupled thereto via other intermediate conductors such as RF wires. Optionally, the shaft  121  and any component between the shaft  121  and the needle electrodes  128  are composed of an electrically conductive material, such as stainless steel, and may, therefore, conveniently serve as intermediate electrical conductors.  
         [0044]    Referring still to FIGS. 2 and 3, the steerable handle assembly  110  is mounted to the cannula  114  and inner probe shaft  121  and serves to conveniently allow the physician to alternately deploy and retract the electrode array  126 . Specifically, the handle assembly  110  comprises distal and proximal handle members  113  and  115  that are slidingly engaged with each other. The distal handle member  113  is mounted to the proximal end  120  of the cannula  114 , and the proximal handle member  115  is mounted to the proximal end  123  of the inner probe shaft  121 . The proximal handle member  115  also comprises an electrical connector  116 , which electrically couples the RF generator  130  to the proximal ends of the needle electrodes  128  (or alternatively, the intermediate conductors) extending through the inner probe shaft  121 . The handle assembly  110  can be composed of any suitable rigid material, such as e.g., metal, plastic, or the like.  
         [0045]    In the illustrated embodiment, the RF current is delivered to the electrode array  126  in a mono-polar fashion. Therefore, the current will pass through the electrode array  126  and into the target tissue, thus inducing necrosis in the tissue. To this end, the electrode array  126  is configured to concentrate the energy flux in order to have an injurious effect on tissue. However, there is a dispersive electrode (not shown) which is located remotely from the electrode array  126 , and has a sufficiently large area—typically 130 cm 2  for an adult—so that the current density is low and non-injurious to surrounding tissue. In the illustrated embodiment, the dispersive electrode may be attached externally to the patient, using a contact pad placed on the patient&#39;s skin. In a mono-polar arrangement, the needle electrodes  128  are bundled together with their proximal portions  127  having only a single layer of insulation over the entire bundle.  
         [0046]    Alternatively, the RF current is delivered to the electrode array  126  in a bipolar fashion, which means that current will pass between “positive” and “negative” electrodes  128  within the array  126 . In a bipolar arrangement, the positive and negative needle electrodes  128  will be insulated from each other in any regions where they would or could be in contact with each other during the power delivery phase. As will be described in further detail below, RF current can also pass between electrode arrays of two or more probe assemblies in a bipolar fashion.  
         [0047]    Further details regarding needle electrode array-type probe arrangements are disclosed in U.S. Pat. No. 6,379,353, entitled “Apparatus and Method for Treating Tissue with Multiple Electrodes,” which is expressly incorporated herein by reference.  
         [0048]    The probe assembly  110  may optionally have active cooling functionality, in which case, a heat sink (not shown) can be mounted within the distal end  125  of the shaft  121  in thermal communication with the electrode array  126 , and cooling and return lumens (not shown) can extend through the shaft  121  in fluid communication with the heat sink to draw thermal energy away back to the proximal end  124  of the shaft  121 . A pump assembly (not shown) can be provided to convey a cooling medium through the cooling lumen to the heat sink, and to pump the heated cooling medium away from the heat sink and back through the return lumen. Further details regarding active cooling of the electrode array  126  are disclosed in co-pending U.S. application Ser. No. ______ (Bingham McCutchen Docket No. 24728-7011), which is expressly incorporated herein by reference.  
         [0049]    Referring back to FIG. 1, as previously noted, the RF generator  130  is electrically connected, via the generator connector  116 , to the handle assembly  112 , which is directly or indirectly electrically coupled to the electrode array  126 . The RF generator  130  is a conventional RF power supply that operates at a frequency in the range of 200 KHz to 1.25 MHz, with a conventional sinusoidal or non-sinusoidal wave form. Such power supplies are available from many commercial suppliers, such as Valleylab, Aspen, and Bovie. Most general purpose electro-surgical power supplies, however, operate at higher voltages and powers than would normally be necessary or suitable for controlled tissue ablation.  
         [0050]    Thus, such power supplies would usually be operated at the lower ends of their voltage and power capabilities. More suitable power supplies will be capable of supplying an ablation current at a relatively low voltage, typically below 150V (peak-to-peak), usually being from 50V to 100V. The power will usually be from 20 W to 200 W, usually having a sine wave form, although other wave forms would also be acceptable. Power supplies capable of operating within these ranges are available from commercial vendors, such as RadioTherapeutics of San Jose, Calif., which markets these power supplies under the trademarks RF2000™ (100 W) and RF3000™ (200 W).  
         [0051]    Referring specifically now to FIGS. 4 and 5, the alignment device  140  generally comprises a rigid base  142  having flat top and bottom surfaces  143  and  144  that are separated by a thickness  146 . Although the rigid base  142  is disk-shaped in the illustrated embodiment, it can take on other shapes, such as rectangular, oval, triangular, or custom shaped, depending on the geometry of the tissue to be ablated. The size of the disk-shaped base  142  will ultimately depend at least in part on the volume of the tissue to be ablated.  
         [0052]    The alignment device  140  further comprises a plurality of apertures  152  spaced along the top surface  143  of the base  142 . The apertures  152  extend completely through the thickness  146  of the base  142 , such that the apertures  152  are likewise also spaced along the bottom surface  144  of the base  152 . In the illustrated embodiment, the apertures  152  are arranged in concentric rings around a center aperture. Depending upon the geometry of the tissue to be ablated and/or the geometry of the alignment structure, the apertures can be arranged in various other patterns.  
         [0053]    As shown in FIG. 1, each aperture  152  is large enough, such that the cannula  114  of the probe assembly  110  can be passed through the alignment device  140 , yet small enough, such that the distal handle member  113  of the handle assembly  112  cannot be passed through the alignment device  140 . That is, each aperture  152  allows the cannula  114  to be passed through the alignment device  140  until the distal handle member  113  abuts the aperture  152 , presumably when an interfering portion  111  of the distal handle member (i.e., the distal most portion of the handle member  113  having a diameter equal to the diameter of the aperture  152 ) coincides with the aperture  152 . Preferably, the diameters of the cannula  114  and apertures  152  are closely toleranced, and the structure  142  is relatively thick, so that the cannula  114  remains aligned with the longitudinal axis of the particular aperture  152  as it passes therethrough. In this embodiment, as shown in FIG. 5, the axes  153  of the aperture  152  are parallel to each other.  
         [0054]    Thus, it can be appreciated that the alignment device  140  can effectively align the distal end  122  of the cannula  114  within a two-dimensional Cartesian coordinate system, as it is passed through an aperture  152 , with the two dimensions (x and y coordinates) being provided by the spacing between the apertures  152  on the flat top and bottom surfaces  143  and  144 . To the extent that the cannula  114  can be inserted into the apertures  152  until the distal handle member  113  abuts the respective apertures  152 , the alignment device  140  can effectively align the distal end  122  of the cannula  114  within a three-dimensional Cartesian coordinate system, with the third dimension (z coordinate) being provided by the top surface  143  of the base  142 .  
         [0055]    To the extent that spacings between the apertures are known, the alignment device  140  indexes the distal end  122  of the cannula  114  within a two-dimensional plane. In this embodiment, the apertures  152  are equally spaced to provide a consistent and easily usable indexing scheme. In this manner, ablation of the entire tumor will be assured by properly spacing the centers of the lesions created on the tumor. It can be appreciated that, in alternative embodiments, some or all of the apertures  152  may not be uniformly spaced.  
         [0056]    In the preferred embodiment, the alignment device  140  is adhered directly to the patient although it is contemplated that other means for ensuring that the alignment device  140  remains fixed in relation to the target tissue can be utilized. For example, as shown in FIG. 5, the bottom surface  144  of the base  142  can be coated with a sticky substance  154  that is then covered with a substrate  156  that has a low affinity to the sticky substance  154 . Prior to the operation, the substrate  156  can then be peeled off of the base  142  to expose the adhesive  154  on the respective surface of the substrate  156 . As another example, the skin of the patient can be coated with a sticky substance. In either example, the alignment device  140  can then simply be adhered to the patient with very little pressure. Whichever method of adhesion is used, is preferable that it be temporary and not cause damage to the skin or other tissues while securing the alignment device  140  in a fixed position relative to the tumor.  
         [0057]    Referring now to FIG. 6, another alignment device  240  that can be used in the tissue ablation system  100  is described. The alignment device  240  is similar to the alignment device  140  illustrated in FIG. 5, with the exception that it comprises apertures  152  that have non-parallel axes  160 . In particular, the axes  160  of the apertures  152  are angled towards a longitudinal axis  162  of the alignment device  140 . Thus, it can be appreciated that the alignment device  240  can effectively align the distal end  122  of the cannula  114  within a three-dimensional angular coordinate system, with the two dimensions (angles φ and θ) being provided by the angles of the aperture axes  160 . To the extent that the cannula  114  can be inserted into the apertures  152  until the distal handle member  113  abuts the respective apertures  152 , the alignment device  140  can effectively align the distal end  122  of the cannula  114  within a three-dimensional spherical coordinate system, with the third dimension (radius p) being provided by the top surface  143  of the base  142 .  
         [0058]    The angles of the aperture axes  160  relative to the longitudinal axis  162  will depend upon the length of the cannula  114  (as dictated by depth of tumor) and the size of the tumor to be treated. For example, for a given tumor size, the angles of the axes  160  will be inversely proportional to the length of the cannula  114 , so that the locations of the distal end  122  of the cannula  114  will be distributed about the entire tumor as it is inserted through each of the apertures  152 .  
         [0059]    Referring now to FIG. 7, another alignment device  340  that can be used in the tissue ablation system  100  is shown. The alignment device  340  is similar to the previously described alignment device  140 , with the exception that it comprises a single boss  164  mounted to the center aperture  152  of the base  142 . The boss  164  prevents the distal end  122  of the cannula  114  to be guided to a lesser depth in the targeted tissue by offsetting the interfering portion  111  of the distal handle member  113  from the top surface  143  of the base  142 . Specifically, the boss  164  comprises a cylindrical bore  166  that is sized to pass the cannula  114  of the probe assembly  110 , yet causes the interfering portion  111  of the distal handle member  113  to abut against the boss  164 , thereby limiting the distal movement of the cannula  114 . In the preferred embodiment, the diameter of the bore  166  is equal to the diameter of the aperture  152 . Thus, it can be appreciated that the alignment device  340 , like the previously described alignment device  140 , can effectively align the distal end  122  of the cannula  114  within a three-dimensional Cartesian coordinate system. The difference is that, to the extent that the height of the boss  164  is known, the alignment device  140  allows the distal end  122  of the cannula  114  to be indexed in three-dimensional space, rather than just a two-dimensional plane.  
         [0060]    The boss  164  can be used with both monopolar and bipolar ablation techniques as described in more detail below, but are particularly useful in bipolar ablation to maintain the proper distance between two or more ablation probe assemblies  110 , as illustrated in FIG. 8.  
         [0061]    Referring again to FIG. 7, the boss  164  is permanently mounted to the center aperture  152 . In other embodiments, the boss  164  may be removably mounted to the center aperture  152  using suitable means, such as a threaded arrangement. In this manner, the physician can customize the alignment device  140 . For example, the physician can associate the boss  164  with another aperture  152 , or completely remove the boss  164  from the base  142 , so that the alignment device  140  indexes the distal end  122  of the cannula  114  within a two-dimensional plane, rather than three-dimensional space.  
         [0062]    Although the alignment device  340  has a single boss  164  to index the distal end  122  of the cannula  114  at a different depth when it is fully inserted into the center aperture  152 , a plurality of bosses  164  can be provided. For example, FIG. 9 illustrates an alignment device  440  that is similar to the previously described alignment device  340 , with the exception that it includes a plurality of bosses  164  that are associated with a respective plurality of the apertures  152 . In this manner, the alignment device  440  indexes the distal end  122  of the cannula  114  at a different depth when it is fully inserted into any one of apertures  152  with which a boss  164  is associated. As shown, the bosses  164  have different heights, so that the alignment device  140  can index the distal end  122  of the cannula  114  at a variety of depths.  
         [0063]    The use of bosses is not the only way to index the distal end  122  of the cannula  114  at different depths. For example, referring to FIG. 10, another alignment device  540  that can be used in the tissue ablation system  100  is shown. The alignment device  540  is similar to the previously described alignment device  340 , with the exception that it comprises a single recess  168  (rather than a boss) formed within the center aperture  152 . The recess  168  allows the distal end  122  of the cannula  114  to be guided to a greater depth in the targeted tissue by allowing the interfering portion  111  of the distal handle member  113  to extend below the top surface  143  of the base  142 . Specifically, the recess  168  is sized to pass the interfering portion  111  of the distal handle member  113 , so that it abuts against the center aperture  152  below the top surface  143  of the base  142 , thereby extending the distal movement of the cannula  114 . Thus, to the extent that the depth of the recess  168  is known, the alignment device  140 , like the previously described alignment device  140 , allows the distal end  122  of the cannula  114  to be indexed in three-dimensional space.  
         [0064]    Like the boss  164 , the recess  168  can be used with both monopolar and bipolar ablation techniques as described in more detail below, but is particularly useful in bipolar ablation to maintain the proper distance between two or more ablation probe assemblies  110 , as illustrated in FIG. 8.  
         [0065]    As illustrated in FIG. 11, an alignment device  640  similar to the alignment device  540  may optionally comprise an insert  170  that is removably mounted within the center aperture  152  using suitable means, such as a threaded arrangement. The insert  170  is cylindrical, although it is contemplated that it could be another shape such as square or rectangular, and has a bore  167  that is aligned with the central aperture  152 . The bore  166  is sized to pass the cannula  114  of the probe assembly  110 , yet cause the interfering portion  111  of the distal handle member  113  to abut against the insert  170 , thereby limiting the distal movement of the cannula  114 . In the preferred embodiment, the diameter of the bore  166  is equal to the diameter of the aperture  152 . Thus, the insert  170  functions to fill in the recess  168  of the center aperture  152 , such that the center aperture  152  functions as an aperture  152  with no recess.  
         [0066]    Although the alignment device  440  illustrated in FIG. 10 has a single recess  168  to index the distal end  122  of the cannula  114  at a different depth when it is fully inserted into the center aperture  152 , a plurality of recesses  168  can be provided. For example, FIG. 12 illustrates an alignment device  740  that is similar to the previously described alignment device  540 , with the exception that it includes a plurality of recesses  168  that are associated with a respective plurality of the apertures  152 . In this manner, the alignment device  740  indexes the distal end  122  of the cannula  114  at a different depth when it is fully inserted into any one of apertures  152  with which a recess  168  is associated. As illustrated in FIG. 12, the recesses  168  have different depths, so that the alignment device  740  can index the distal end  122  of the cannula  114  at a variety of depths. The alignment device  740  can be customized by providing inserts (shown in FIG. 11) that can be selectively inserted into the recesses  168 . The inserts can have different heights, so that the alignment device  140  can index the distal end  122  of the cannula  114  at a variety of depths.  
         [0067]    In further alternative embodiments, an alignment device  840  can have both bosses  164  and recesses  168 , as illustrated in FIG. 13, so that the interfering portion  111  of the distal handle member  113  can be offset from the top surface  143  of the base  142  or extend below the top surface  143  of the base  142 . In this manner, the distal end  122  of the cannula  114  can be indexed at a greater range of depths.  
         [0068]    Having described the structure of the tissue ablation system  100 , its operation in treated targeted tissue will now be described. The treatment region may be located anywhere in the body where hyperthermic exposure may be beneficial. Most commonly, the treatment region will comprise a solid tumor within an organ of the body, such as the liver, kidney, pancreas, breast, prostrate (not accessible via the urethra), and the like. The volume to be treated will depend on the size of the tumor or other lesion, typically having a total volume from 1 cm 3  to 150 cm 3 , and often from 2 cm 3  to 35 cm 3 . The peripheral dimensions of the treatment region may be regular, e.g., spherical or ellipsoidal, but will more usually be irregular. The treatment region may be identified using conventional imaging techniques capable of elucidating a target tissue, e.g., tumor tissue, such as ultrasonic scanning, magnetic resonance imaging (MRI), computer assisted tomography (CAT) fluoroscopy, nuclear scanning (using radiolabeled tumor-specific probes), and the like. Preferred is the use of high resolution ultrasound of the tumor or other lesion being treated, either intraoperatively or externally. The image of the tumor is used to determine where the alignment device  140  should be fixed in order to introduce the cannula  114  and inner probe  118  to the target site. It can also be appreciated that a plan for conducting multiple ablations on the tumor can be mapped out prior to the procedure using the image of the tumor and the alignment device  140 .  
         [0069]    Referring now to FIGS. 14-17, the operation of the tissue ablation system  100  is described in treating a treatment region TR, such as a tumor, located beneath the skin S of a patient. First, the alignment device  140  is affixed relative to the targeted tissue, as illustrated in FIG. 14. In the preferred embodiment, the alignment device  140  is adhered directly to the skin of the patient by, e.g., peeling the substrate  156  off of the bottom surface  144  of the base  142 , and pressing the base  142  against the body of the patient  172 . It is contemplated that other means of adhesion may be used.  
         [0070]    The cannula  114  of the probe assembly  110  is then guided within an aperture  152  of the alignment device  140 , as illustrated in FIG. 15. The cannula  114  passes through the aperture  152  of the alignment device  140  until its distal end  122  is adjacent a first target site TS 1  within the tumor T. The cannula  114  and inner probe  118  may be introduced into the first target site TS 1  percutaneously—i.e., directly through the patient&#39;s skin—or through an open surgical incision. If the cannula  114  is introduced through an open surgical incision, the incision will be made prior to fixing the alignment device  140  relative to the treatment region TR. In this case, the alignment device  140  will span the open incision. When the introduction is done percutaneously, the cannula  114  may have a sharpened tip like a needle, to facilitate introduction into the treatment region TR. In this case, it is desirable that the cannula  114  be sufficiently rigid, i.e., have a sufficient columnar strength, so that it can be accurately advanced through the surrounding volume of tissue. Alternatively, the cannula  114  may be introduced using an internal stylet that is subsequently exchanged for the shaft  121  and electrode array  126 . In this latter case, the cannula  114  can be relatively flexible, since the initial columnar strength will be provided by the stylet.  
         [0071]    After the cannula  114  is properly placed so that its distal end  122  is adjacent to the first target site TS  1 , the shaft  121  is distally advanced to deploy the electrode array  126  radially outward from the distal end  122  of the cannula  114 , as illustrated in FIG. 16. The shaft  121  is advanced sufficiently, so that the electrode array  126  fully everts in order to substantially penetrate the first treatment site TS  1 . If the probe assembly  110  has an optional core member (not shown) previously mentioned, then the sharpened end of the core member facilitates introduction of the electrode array  126  into the treatment region. The RF generator  130  is then connected to the ablation probe assembly  110  via the electrical connector  116 , and then operated to ablate the treatment region resulting in the formation of a lesion that is coincident with the first treatment site TS 1 .  
         [0072]    Referring to FIG. 17, the ablation probe assembly  110  is removed from the first aperture  152 , and then guided through a second different aperture  152  in the alignment device  140  to place the distal end  122  of the cannula  114  adjacent to the targeted tissue in a second target site TS 2  within the treatment region TR. The RF generator  130  is then operated a second time to create a second lesion that encompasses the second target site TS 2 . This process is performed using other apertures  152  until the entire treatment region TR is ablated. Thus, it can be appreciated that, by using the alignment device  140 , the distal end  122  of the cannula  114  is indexed with a two-dimensional plane that extends through the treatment region TR.  
         [0073]    In an optional method, lesions can be created within the treatment region TR at multiple depths, by retracting the electrode array  126  within the cannula  114  after performing an ablation, and adjusting the cannula  114  within the same aperture  152  so that its distal end  122  is adjacent another treatment site that is spaced from the first treatment site TS 1  along the axis  160  of the aperture  152 . The electrode array  126  is then deployed within the other treatment site, and the RF generator  130  is operated another time to create a second lesion that encompasses the other target site. This step can be repeated for the same aperture to generate lesions at various depths, and can be repeated for other apertures. This optional step is particularly useful if the depth of the treatment region TR is greater than the depth of a single lesion that can be created by the probe assembly  110 . If indexing of the various depths are desired, any one of the alignment devices  340 - 840  can be used to index the distal end  122  of the cannula  114  within the three-dimensional space occupied by the treatment region TR.  
         [0074]    In another preferred method, a plurality of ablation probe assemblies  110  may be guided through a respective plurality of apertures  152  in the alignment device  140  to place the distal ends  120  of the cannula  114  adjacent to multiple target sites TS of the tissue, and then the respective electrode arrays  126  can then be deployed from the distal ends  122  of the cannula  114 , as illustrated in FIG. 18. In a unipolar mode, the RF generator  130  can be operated to sequentially generate lesions from the probe assemblies  110  within the target region TR. In a bipolar mode, the RF generator  130  can operate pairs of probe assemblies  110  to generate lesions between the probe assemblies  110  by conveying RF energy between the respective electrode arrays  126 . For example, the probe assembly  110  associated with center aperture  152  can be sequentially paired with the remaining probe assemblies  110  to generate lesions between the center electrode array  126  and the remaining electrode arrays  126 .  
         [0075]    As illustrated in FIG. 19, the alignment device  240  can be used to offset the center electrode array  126  a predetermined distance from the remaining electrode arrays  126 . In this manner, the proper distance is maintained between the electrode arrays  126  to efficiently produce a lesion there between. One skilled in the art would appreciate that the needle electrodes  128  from the different ablation probe assemblies  110  should be insulated from touching the needle electrodes  128  from the other ablation probe assemblies  110 . This process may be repeated or a sufficient number of ablation probe assemblies may be used such that the entire target region is ablated.  
         [0076]    If indexing of the various depths are desired, any one of the alignment devices  340 - 840  can be used to index the distal ends  122  of the cannulae  114  within the three-dimensional space occupied by the treatment region TR.  
         [0077]    Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.