Patent Publication Number: US-8989838-B2

Title: System for treatment of diseased tissue using controlled ultrasonic heating

Description:
RELATED APPLICATIONS 
     This application is a divisional of and claims the priority of application Ser. No. 11/076,669, entitled “Treatment of Diseased Tissue Using Controlled Ultrasonic Heating,” filed on Mar. 9, 2005, now U.S. Pat. No. 7,771,418, issued Aug. 10, 2010. 
    
    
     TECHNICAL FIELD 
     The present invention relates to treatment of various medical conditions using thermal therapy. Various aspects include interstitial treatment of tumors, benign prostatic hyperplasia, and in particular, prostate cancer. More specifically, embodiments of the present invention relate to treatment of diseased tissue using controlled ultrasound thermal therapy and medical imaging of the same. 
     BACKGROUND 
     Thermal coagulation therapy may be used for the treatment of localized diseased tissue, e.g., tumors, in a diseased organ or body. Generally, a target volume of tissue is sufficiently heated to achieve a therapeutic effect, such as thermal coagulation. Tissue thermal coagulation depends on a number of factors, and temperatures in the range of 55-60.degree. C. are generally considered sufficient to provide enough energy to cause such coagulation. Cell death results from heating to these temperatures, and a region of irreversible thermal damage can be observed with imaging following the treatment. In addition, heating can be produced from minimally-invasive applicators, eliminating the need for open surgery, and potentially reducing recovery time and morbidity for patients. This approach has been used with some success in the treatment of isolated primary liver cancers and colorectal metastases for patients otherwise ineligible for surgery. 
     Interstitial thermal therapy is currently practiced by inserting heating applicators directly into a target site within an organ. Several energy sources have been integrated into interstitial heating applicators, including lasers, ultrasound, microwave, and radiofrequency energy. Preferably, interstitial thermal therapy delivers sufficient thermal energy to coagulate an entire target volume, while avoiding undesirable thermal damage to normal tissue. This strategy is referred to as “conformal thermal therapy.” One limitation of present interstitial thermal therapy technology is the inability to control or adjust the three-dimensional pattern of energy deposition dynamically during a treatment. Most current applicators act as point or line sources of energy resulting in highly symmetric patterns of energy deposition in tissue. This makes it difficult to treat targets with complex geometry accurately, and does not take full advantage of the imaging information available with imaging technology such as magnetic resonance imaging (MRI). 
     One application of interstitial heating is transurethral prostate thermal therapy, which selectively destroys diseased prostate tissue using a device located within the prostatic urethra, and preserves adjacent normal tissues such as the rectal and bladder walls. Disease targets include prostate cancer and benign prostatic hyperplasia (BPH). Current transurethral thermal therapy technologies are incapable of producing a thermal treatment (cell death) pattern that conforms to the geometry of the prostate gland. 
     SUMMARY 
     The present invention includes a thermal therapy method using an ultrasound heating applicator to generate site-specific thermal lesions in diseased tissues which conform to a target volume and boundaries established by medical imaging, e.g., magnetic resonance imaging (MRI). Various embodiments of the invention can be used within an MR imager, permitting non-invasive real-time temperature measurement of the treatment volume and visualization of regions of thermal damage. 
     According to one or more aspects of the invention, tissue temperature measurements obtained with MRI during tissue heating provide feedback for generating site-specific thermal lesions that conform to the boundaries of the target diseased tissue volume while minimizing thermal damage to adjacent normal tissues. Control over the temporal and/or spatial pattern of energy deposition comes from the directional nature of the ultrasound beam produced by a multi-element heating applicator, wherein the depth of heating from each element can be controlled by adjusting its output ultrasound power and frequency. 
     In one or more embodiments of the invention, ultrasound energy is used to generate thermal lesions that conform to the target volume, e.g., a 3-D volume, of an abnormal prostate gland for the treatment of prostate diseases, including but not limited to prostate cancer and benign prostatic hyperplasia (BPH). 
     In one or more embodiments of the invention, temperature measurements made with MRI at or near the boundary of the target tissue volume control the output of the heating applicator or elements thereof. The applicator elements can take the form of transducer elements, individually assembled or made by segmenting a single original transducer element. In one or more embodiments of the invention, the measured boundary temperature is used to control any or all of: the scan rate, power, and frequency of each element of the heating applicator such that the entire target volume boundary reaches a target therapeutic temperature. In addition to absolute temperature metrics, other thermal metrics may be used to design, measure, and determine the appropriate treatment. For example a thermal dose, flux, or elevation above a fixed or variable thermal threshold can be used. The magnitude and frequency of the electrical signal delivered to each applicator element can be manipulated simultaneously providing control over the 3-dimensional shape of the thermal lesion created in tissue. 
     In some embodiments of the present invention, an ultrasound applicator includes one or more transducers provided with acoustic matching layers to enable operation at multiple frequencies for optimal control of the depth of thermal coagulation. In one or more exemplary embodiments, a plurality of frequencies may be delivered simultaneously from the same transducer, or different frequencies may be delivered from different elements of a transducer. 
     Additionally, in some embodiments, one or more transducer elements can be controlled independently to generate beams of varying radial depth as a function of the axial position along the applicator, and the entire device can be rotated to control the angular distribution of energy about the applicator&#39;s longitudinal axis. According to the invention, the control of all three variables (rotation, power, frequency) during treatment results in the flexibility to control the spatial deposition of energy, and ultimately the pattern of thermal damage in tissue. 
     In one or more embodiments, a temperature control system is also available wherein coolant is provided to the heating applicator, which can be used to heat or cool the immediately adjacent tissue, as well as remove heat from the transducers. For example, water or another fluid is passed over or near the surface of the transducers. The water or other fluid&#39;s temperature may be controlled at its source, and can be used as another factor in controlling the overall treatment process. 
     According to some aspects, the present invention provides an ultrasound-based thermal therapy method which can reliably produce site-specific thermal lesions in abnormal tissues based on thermal imaging of the tissues. The imaging can be MRI, and the thermal volumes can be controlled in a 3-D volume about an interstitial applicator. 
     According to some aspects, an MR thermometry-based algorithm is used to effectively control the rotation, power, and frequency of the heating applicator, or elements thereof, in order to achieve a thermal lesion that substantially conforms to the target volume defined by MRI. 
     Other aspects of the present invention include an ultrasound thermal therapy method which provides thermal ablation of diseased tissues with minimal or no incidental damage to normal tissues near the targeted treatment volume. 
     Some aspects of the invention provide an ultrasound-based method for site-specific thermal ablation of abnormal prostate tissues. 
     Also, aspects of the present invention provide an ultrasound thermal ablation method for site-specific treatment of tumors and other abnormalities of the brain, spinal cord, and other organs and anatomical parts of the body. The methods and apparatus provided herein can be used generally on any suitable patient or organ, regardless of gender, and even on human or animal subjects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustrating the basic components used to practice MR image-guided ultrasound thermal therapy according to the present invention; 
         FIG. 2  depicts the concept of conformal thermal therapy from a single directional ultrasound transducer, in the plane of rotation; 
         FIG. 3  depicts the nature of conformal thermal therapy using a multi-element ultrasound heating applicator, along the length of the applicator; 
         FIG. 4  shows the nature of the temperature distribution produced by an ultrasound transducer in tissue, and the relevant parameters that are used to control heating; 
         FIG. 5  illustrates the general method for performing MRI-guided thermal therapy with a multi-element ultrasound heating applicator; 
         FIG. 6  is a non-limiting example of a control strategy to select an appropriate rotation rate, ultrasound power and frequency based on temperature measurements at the boundary of the target volume and the maximum temperatures within the volume; 
         FIG. 7  shows four non-limiting examples from computer simulations demonstrating the capability to perform conformal thermal therapy of the prostate gland using a directional ultrasound heating applicator; 
         FIG. 8  shows the rotation rate, power and frequency (single element) as a function of angle to treat a target region around a prostate geometry; 
         FIG. 9  depicts a multi-element transducer design suitable for integration into the heating applicators of this invention; 
         FIG. 10  shows a non-limiting embodiment of a multi-element heating device designed for use in prostate thermal therapy; 
         FIG. 11  illustrates a rotation apparatus for angular scanning of a treatment volume; and 
         FIG. 12  illustrates the relationship between actual and target velocity of a rotating apparatus for scanning the treatment volume. 
     
    
    
     Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     DETAILED DESCRIPTION 
     Various embodiments of the present invention are directed to ultrasound heating applicators used to generate site-specific thermal lesions in diseased tissues, conforming to a predetermined three-dimensional (3-D) target volume and boundaries established by medical imaging. One imaging modality that may be used herewith is magnetic resonance imaging (MRI). Temperature measurements made with MRI techniques at or near the boundary of the target volume and/or the maximum temperature within the treatment volume are used to control the scan rate, power, and frequency of the heating applicator and/or of each element of the applicator, such that substantially the entire target volume reaches a predetermined therapeutic temperature or other thermal threshold. In one or more embodiments, the magnitude and frequency of the electrical power signal delivered to each element, and the rotation rate of the entire applicator can be controlled simultaneously, providing a 3-D profile of the thermal lesion created in the diseased tissue. In one or more embodiments, the method of the invention is used to generate a thermal lesion that substantially conforms to the shape of the prostate gland. 
     Magnetic resonance imaging (MRI) may be used to control the thermal therapy by providing 3-D real-time quantitative thermal imaging information during the treatment. In addition to the temperature distribution measurements, the MRI can be used to visualize the outer margin of cell death due to thermal damage at the time of treatment or shortly thereafter. These features provide “on-line dosimetry” and rapid evaluation of therapeutic outcome during a treatment. 
       FIG. 1  illustrates the main components of an exemplary thermal therapy and imaging system  100  according to one or more embodiments of the present invention. An imaging system  101 , employing an MRI device  102 , is used to acquire a 3-D image data set  116  prior to administration of thermal therapy in order to locate and define a tumor volume and the surrounding anatomy of interest. A target treatment volume  105 , which can be the same or similar to the tumor volume, is then prescribed from image set  116 , and can include anatomical and functional information important for treatment planning purposes. 
     The system includes treatment equipment located inside a MR magnet chamber  109 , such as a multi-element ultrasound heating applicator  106 , motor assembly  107  for controlling device rotation, and the MR imaging device  102  for monitoring and evaluating the progress of a treatment. The MR imaging device  102  is controlled by and sends MRI data  110  to the MRI console  113 , located outside the MR magnet chamber  109 . 
     A treatment control system  112  is used to operate motor assembly  107  and heating applicator  106  by passing electrical control and/or actuator signals to this equipment via cables  108  which pass through filtered panels to eliminate RF contamination of the MR imaging process. Treatment control system  112  includes hardware and/or software, including processors, program instructions, data storage devices, network communication devices, or any auxiliary control and actuation device, support algorithm, or system, as needed to achieve its function. Measurements and signals from applicator and motor equipment  106 ,  107  are sent back to the treatment control system  112  via any suitable conduit, generally referred to herein as cables  108 . Shielded wire, fiber-optic, or out-of-band wireless connections may be used as cables  108 . 
     Image data or information corresponding to the acquired MR images is sent to the treatment control system  112  through a connection  114  coupling the MRI console  113  and the treatment control system  112 . Alternatively, the MR imaging system  101  can be controlled by the treatment control system  112  through connection  114  coupling treatment control system  112  and MRI console  113 . 
     In one or more embodiments, treatment control system  112  employs treatment planning software to locate and/or define a target treatment volume  105  based on the acquired image data  116  that are displayed on the MRI console  113  and/or an independent display  111 . Image data  116  may include conventional MRI images, or other derivative images, data, or information that are adapted for the present purpose. For example, the image data used by system  100  may include filtered, post-processed, amplified, colorized, interpolated, transformed, mapped, or decimated data in raw or processed form, and in any suitable format. 
     Imaging data  116  can be used to guide the positioning of applicator  106 , including final position location, or to determine an insertion path of the heating applicator  106 . Imaging data  116  can also be used to obtain an initial set of scanning and operating parameters used to treat the tissue within target treatment volume  105 , while minimizing or avoiding thermal damage to surrounding normal tissue  103 . 
     According to one or more aspects of the invention, heating applicator  106  can be positioned within or adjacent to the target volume of tissue  105 . In some embodiments, heating applicator  106  is positioned within the urethra in order to perform thermal therapy of the prostate gland. In one or more embodiments, the positioning of heating applicator  106  is facilitated with rapid imaging, e.g., MRI, ultrasound (US), or computerized tomography (CT), to ensure the proper location of heating applicator  106  in relation to the target tissue volume  105 . In one or more embodiments, image-guided prostate treatment is performed with MRI using an transurethral applicator. 
     Once the heating applicator  106  has been inserted into the desired location in the target tissue volume  105 , ultrasound energy is delivered to tissue volume  105  to generate a localized spatial heating pattern while tissue temperature measurements are acquired to determine and control the spatial heating pattern. The spatial heating pattern may be controlled as a function of time to result in a spatio-temporally controlled thermal therapy regime applied to the target tissue volume  105 . 
     In one or more embodiments, the spatial heating pattern is determined with MR thermometry methods in multiple 2-D planes  104 , but a number of other thermographic techniques could be employed as well, including ultrasound imaging, point sensor measurements, or infrared thermography. Furthermore, given sufficient initial condition information, a thermal therapy plan could be designed and executed even in the absence of real-time imaging feedback. Such a treatment could rely on pre-computed treatment parameters generated on a suitable computer, e.g. treatment control system  112  or an off-line computer coupled to the system  100 . 
     The MR image data  116 , temperature maps corresponding to 2-D planes  104 , and relevant system parameters are displayed on the MRI console  113 , display  111 , and provided to treatment control system  112  during treatment. A closed-loop treatment may thus be achieved using the information from the acquired MRI data  110 ,  116  in a control system to determine or modify existing output parameters to the treatment equipment during the actual therapy. 
     Generally, heating applicator  106  is positioned by axial insertion of the applicator into an interstitial space within or near the target treatment volume  105 . Once inserted, heating applicator  106  is rotated about its axis using motor assembly  107  as desired to sonicate, heat, and treat the target volume  105 . The rotation of heating applicator  106  is carried out in a controlled fashion, sometimes referred to as a scan rate. In some or all instances, an entire therapeutic regimen can be accomplished using one heating applicator  106  positioned at one axial target location, and properly rotated in an angular dimension. Such a treatment can be adapted to cover a 3-D volume surrounding the heating applicator  106  and can provide control in the axial, radial, and angular dimensions, as will be described in more detail below. 
     Initial treatment planning parameters may vary according to the specific application at hand. Some parameters that can be used to design and execute a treatment according to aspects of the present invention include applicator scan rate, output power, and frequency parameters for all elements of the heating applicator, and derived from numerical simulations of the heating process, can be used to deliver a thermal treatment without active feedback in locations where dynamic changes in tissue properties or physiological response are not expected to be significant. 
     In tissue locations where dynamic changes in tissue absorption, blood perfusion, and other physiological parameters are anticipated, real-time medical imaging feedback through MRI data  110 ,  116  is provided to the treatment control system  112  to adjust the pre-calculated output parameters required to achieve the desired thermal lesion. In some or all embodiments of the invention, MR imaging data  110 ,  116  provides high temporal and spatial resolution that can be used for precise guidance and spatio-temporal control of the treatment. In some embodiments, such MR feedback information can be used to control the energy distribution from heating applicator  106 , without the need for a pre-determined treatment plan. 
     Effective imaging feedback may be provided by continuously acquiring MR images to quantitatively monitor the temperature distribution in the vicinity of heating applicator  106 , wherein such MR thermometry information is used to control the output parameters of heating applicator  106 . This control allows adequate heating of the target boundary without overheating the areas near the transducer surface. For example, heating may be carried out until the temperature along the entire target boundary volume  105  is elevated beyond a threshold temperature, or thermal dose, sufficient to achieve a desired therapeutic outcome. In one or more embodiments, the temperature required for thermal coagulation is chosen as a threshold temperature. Once this thermal threshold is reached, images sensitive to thermal damage, e.g., T2-weighted MR images or contrast-enhanced T1-weighted MR images, are acquired to evaluate the spatial pattern of thermal damage generated in the tissue, and compare the damage pattern to the desired target treatment volume  105 . Thus, MR imaging is used to provide independent measurement and/or corroboration of the tissue damage pattern predicted by MR thermometry, wherein tissue heating can be terminated or further heating can be performed as necessary. Real-time adjustments to the thermal therapy and the heating applicator  106  driving parameters could be carried out to optimize or correct the course of a treatment after it is underway. According to one aspect of the invention, if the thermal lesion substantially covers the entire target volume  105 , the treatment is considered complete, and the heating applicator  106  is de-energized and/or removed from the patient&#39;s body. Optionally, serial MR imaging of the target tissue volume  105  and/or the surrounding tissue  103  is performed following the thermal therapy to evaluate relevant physiological and metabolic parameters in or near the treatment zone. 
     Other attendant steps and devices may be employed in the process of providing the thermal therapy as described above. For example, an acoustically-transparent catheter can be inserted within or adjacent to the target tissue volume  105 . The catheter may be first inserted into the prostate gland via the urethra, and the heating applicator  106  is inserted into the catheter to deliver the thermal therapy. Also, other medical or surgical devices and/or pharmaceutical agents can also be inserted down the catheter into the target volume  105  to perform further diagnosis and treatment. An example of such other medical devices includes a miniature radio-frequency (RF) micro-coil, wherein the RF micro-coil can perform high-resolution MR imaging or spectroscopy of the target tissue volume before and after the delivered therapy to evaluate the outcome of the treatment. 
     An MR imaging coil may be integrated into the outer housing of the transurethral device, wherein the MR coil provides highly directional imaging with excellent signal to noise in the region of heating by the device. Combined with external or endorectal imaging coils as a phased array or in a parallel-imaging strategy, a large field-of-view with acceptable signal-to-noise ratio and a localized region of extremely high signal-to-noise ratio in the region of heating can be achieved. The wide field-of-view provides visualization of the target volume and surrounding anatomical structures, while the localized region of high signal-to-noise can produce temperature measurements of extremely high fidelity for feedback control of the thermal therapy. 
       FIG. 2  illustrates an example of conformal thermal therapy from a single directional ultrasound transducer, in the plane of rotation. The figure is a planar view with the page being in a plane perpendicular to the axial dimension of a heating applicator  204 . In other words, the heating applicator  204  is seen in transverse cross-section with a planar ultrasound transducer  205  integrated into it, which produces a collimated ultrasound beam  203  and resulting heating pattern in the surrounding tissue. The heating applicator can be rotated about its axis to a desired angular position  208 , or rotated at a desired angular scan rate  206  during treatment. The angular position  208  and angular scan rate  206  of the heating applicator  204  determine, at least in part, the spatial energy deposition. The target volume  201  can be identified in anatomical/functional imaging information acquired prior to treatment. A treatment volume boundary  202  is chosen so that it substantially encompasses the target volume  201  while generally sparing the surrounding or nearby tissue structures  207 . The surrounding or nearby tissue structures  207  may be sensitive or critical structures that cannot or should not be damaged by the thermal therapy. The target volume  201  in this plane is treated by rotating the heating applicator across the entire region. The extent or thickness of this plane of heating is determined by the axial length of a particular element in the heating applicator  204 . The degree to which the treatment volume boundary  202  can be controlled to conform to the target volume  201  is related to the rate of heating and the thermal conductivity of the tissue, profusion, and other acoustic and thermal factors. 
       FIG. 3  illustrates the nature of conformal thermal therapy using a multi-element ultrasound heating applicator, in a plane parallel to the (axial) length of the heating applicator  301 . Corresponding cross-sections of the target volume  306  and the treatment volume boundary  305  are shown in the figure. The heating applicator  301  is inserted into the target volume  306  such that the multi-element transducer  307  is able to treat one angular location of the entire target volume  306  simultaneously. In situations where the target volume  306  is axially larger in extent than the multi-element transducer  307 , the heating applicator  301  can be axially translated to cover this increased axial extent. Each element  309  in the multi-element transducer  307  produces a collimated ultrasound beam  303  with a frequency (f.sub.n), power (P.sub.n)  308  and dimensions determined by the size of the element and other acoustic and design factors. The acoustic energy passes through an acoustic window  304  made from a thin material with a low ultrasound reflection and absorption coefficient. The values of the frequency and power of each element  309  can be adjusted dynamically during treatment based on imaging information acquired during heating. The overall spatial pattern of energy deposition can be adjusted to conform to the geometry of the target volume  306 . The rotation  302  of the heating applicator  301  can be seen in the figure, and generally applies to all of the elements  309  in the device if they are coupled through a common structure. 
       FIG. 4  illustrates the nature of the spatial temperature distribution produced by an ultrasound transducer in tissue, and the relevant parameters that are used to control heating in an exemplary embodiment. The temperature profile  407  from a single element  402  reaches a maximum value T.sub.max  409  at some distance  401  from the surface of the heating applicator  400 , typically 6-8 mm. The temperature immediately adjacent to the heating applicator is closer to the temperature of the cooling water flowing through the applicator (T.sub.cool  412 ) during treatment. Further away than  401 , the temperature profile  407  decreases with distance from the heating applicator  400 . The target volume boundary  403  is partially shown being at about a distance r.sub.b(n)  404  for the nth element in the heating applicator. The temperature at this distance is denoted by T.sub.b(n)  411 . During treatment, it is desired to elevate the temperature  411  at the target volume boundary  404  to a given thermal threshold, T.sub.crit  410 . The difference between T.sub.b(n)  411  and T.sub.crit  410  is referred to as .DELTA.T.sub.b(n)  405 . It should be understood that a corresponding temperature profile would exist for each of the other elements of the heating applicator  400  so that an overall thermal output along the entire active transducer face and extending into the treatment volume is generated as a result of operating and driving the transducer elements. And it should be further appreciated that rotating heating applicator  400  about its axial axis would sweep out an angular spatially and temporally dynamic treatment zone as described above. 
     In some or all instances, the treatment plan may keep the maximum temperature along the ultrasound beam, T.sub.max  409 , below an upper limit T.sub.limit  408  where unwanted effects in tissue such as tissue boiling, vaporisation or charring may occur. Note that this maximum temperature may be monitored in real-time using MR imaging as described earlier, or using thermometry of any other suitable kind. The difference between T.sub.max(n)  409  and T.sub.limit  408  is referred to as .DELTA.T.sub.high(n)  406 . The values of .DELTA.T.sub.b(n)  405  and .DELTA.T.sub.high(n)  406  can be measured with MR thermometry in one or more embodiments of the invention. Both .DELTA.T.sub.b(n)  405  and .DELTA.T.sub.high(n)  406  for each element may be incorporated into a treatment algorithm used to perform conformal thermal therapy. 
       FIG. 5  illustrates some steps of an exemplary method for performing image-guided conformal thermal therapy with a multi-element ultrasound heating applicator. The temperature distribution within the target volume is measured at step  501  to evaluate whether the entire target region has reached a desired temperature at step  502 . The temperature measurements are made with MRI thermometry, and are either a volumetric or multi-slice acquisition. If the entire volume is heated to the desired level, the treatment is considered to be complete at step  508 . If areas remain to be heated, the system determines the temperature at a collection of control points, resulting in a set of input control temperatures {T.sub.c} at step  503  used in the control algorithm to determine the output parameters of the heating device at step  504 . These input temperatures include, but are not limited to the boundary temperatures for each element, the maximum temperature for each element, and the temperatures along the target boundary in advance and behind the current location of the ultrasound beam (refer to  FIG. 4 ). 
     In addition, the temperatures along critical anatomical structures can be included in the control algorithm to ensure these structures do not experience undesirable levels of heating. Once the output parameters  504  such as rotation rate of the heating applicator (w), and the power and frequency to each element (P.sub.n, F.sub.n) have been determined, the values are updated, and the temperature distribution is re-measured at some time later during the delivery of ultrasound energy  506 . It should be noted that numerous other steps and acts may be performed in addition to those illustrated. Also, equivalent or substituted acts and steps may be provided in place of those shown as will be appreciated by those skilled in the art and depending on the precise application at hand. For example, a thermal profile/image may be measured prior to beginning the thermal therapy and/or following the completion of the thermal therapy for reference. 
       FIG. 6  is a non-limiting example of a control algorithm that can be implemented to select an appropriate rotation rate (w), ultrasound power (P.sub.n) and frequency (F.sub.n) based on temperature measurements at the boundary and within the target volume (step  601 ). In one or more embodiments, a thermal treatment algorithm produces a heating pattern at an initial angular position (angle) .THETA.sub.o, with the device kept stationary at step  602 , and the ultrasound power and frequency to each element in the applicator chosen to reach the desired thermal threshold at the target boundary in a relatively short time. The initial frequency is chosen based on the target volume boundary radius of r.sub.b(n) at each element. The temperature at the target boundary for each element is measured and a proportional-difference control relationship is applied to adjust the power such that the boundary reaches a desired temperature. 
     Upon reaching the desired thermal threshold at angle .THETA.sub.o, the applicator is rotated with a scan rate selected that is inversely proportional to .DELTA.T.sub.b at the target boundary. According to one or more aspects of the invention, the applicator delivers power to each element at a power level that depends on the target boundary temperature at step  604 , and at a frequency that depends on the radius of the target boundary at step  603 . In the 3-D control algorithm of this invention, the scan rate is selected based on the element with the largest target radius (max(r.sub.b(n))) corresponding to the lowest rotation rate  602 . Once the rotation and heating is initiated, the algorithm prospectively evaluates points along the target boundary for any large transitions in radius. If a transition is found close to the current position, the applicator can be rotated quickly to that point in order to minimize heat conduction to tissues peripheral to the target tissue volume. The above process is repeated until the entire target volume has been heated to the desired temperature required for a particular therapeutic effect. 
     Frequency may be controlled based on the depth of the target boundary. In some embodiments, higher frequencies are used to heat targets at near the transducers, e.g., at distances less than 14 mm, while lower frequencies are used to heat deeper targets, in order to minimize the time required to heat to the desired depth and to enhance the range of heating available to the heating applicator. 
     The exact heating radius is generally dependent on tissue type and ultrasound frequency, and can be varied depending on location. In one or more embodiments of the present invention, the treatment control method also assesses the location of the target boundary ahead of the current heating location. This can compensate the treatment parameters to minimize the adverse effects of thermal conduction on the ability to shape the heating pattern sharply. In some embodiments of the invention, a 3-D control algorithm controls some or all of the elements simultaneously to form a coupled system, wherein power and frequency parameters are independently controlled for some or all transducers, but the rotation is fixed for the entire device. 
     Control of the output of the heating applicator can be accomplished in a number of ways in order to produce a conformal thermal lesion or damage pattern in the prostate gland. Quantitative MR temperature measurements can be used as feedback to control the output of the device until a targeted heating pattern is achieved. MRI can measure the temperature distribution in tissues with good spatial resolution (.about.1-2 mm), temporal resolution (.about.5-10 sec) and temperature resolution (.about.2.degree. C.) in multiple planes during a thermal therapy. Also, quantitative information can be used to assess the spatial heating pattern produced during a treatment. 
     The heating pattern from heating applicators is generally directional, and depending on the design, can even be highly directional. The temperature distribution along the beam direction is used to provide feedback to the heating applicator and/or the applicator translation-rotation apparatus to control transducer frequency, power and rotation rate. As the heating applicator rotates about its axis, the temperature control point is also moved, so that it is generally oriented along the beam direction, and such that the threshold for thermal coagulation is achieved along the entire target boundary by the end of the treatment. In some embodiments of the invention, a multi-element ultrasonic transducer is used, wherein the control algorithm reduces a 3-D problem to a collection of 1-D control algorithms by considering the temperature profiles along the beams of each element in the transducer. 
     The control relationships employed in the method of the invention can be determined empirically using a bio-heat transfer simulation of the heating to assess the accuracy and utility of the method to conform heating patterns to the boundary of the prostate. Prostate geometries can be obtained from clinical MR imaging data of patients with confirmed prostate cancer. In an exemplary case, the empirical relationships between the temperature at the target volume boundary and the output parameters of the heating applicator are given by: F={F high, rb&lt;13.5 .times. .times. mm F low, rb&gt;13.5 .times. .times. mm .times. .times. P={Kp .DELTA. .times. .times. Tb, T max&lt;T limit 0, T max&gt;T limit .times. .times. w=Kw .DELTA. .times. .times. Tb where P is the acoustic power, w is the rotation rate, F is the ultrasound frequency, and K.sub.p and K.sub.w are gain constants associated with the control algorithm. T.sub.b is the difference between the boundary temperature and a target critical temperature necessary for thermal coagulation. 
       FIG. 7  illustrates four exemplary outcomes using the above-described computer simulations demonstrating the capability to perform conformal thermal therapy of the prostate gland using a directional ultrasound heating applicator. Each panel in the figure shows the target prostate boundary  702  derived from a clinical axial MR image of the prostate gland. The boundary with solid circles  703  denotes the region that reached a thermal threshold of 55.degree. C. during the treatment. This level of heating results in rapid and irreversible cell death through thermal coagulation, and is referred to as 100% cell kill  703  in the figure to indicate that substantially all of the diseased cells within this area would be killed by the thermal treatment. The boundary with the open triangles  704  represents the region that was exposed to at least 30 equivalent minutes at 43.degree. C. of thermal isodose (T43=30 minutes). This is a measure of the combined time/temperature exposure standardized to a common temperature. This conservative level of thermal dose is typically considered to be the minimum thermal dose required to observe histological changes in prostate tissue, and represents the region outside which tissue would be unharmed by the heating. The contour is thus labeled as the 0% cell-kill boundary  704  in the figure to signify that substantially none of the cells in this region are killed by the thermal treatment. The results in the four panels indicate that the presently-described techniques can provide a variety of controllable conformal thermal therapies using quantitative temperature feedback and an appropriate control algorithm. These calculations involved controlling a single applicator element. The present methods and systems are also applied through multi-element transducer applicators such that a 3-D treatment volume can be scanned conformally with a pre-determined treatment volume using a pre-determined treatment plan. Such volumes and treatment plans can be modified “on the fly” using real-time thermal imaging as described herein. 
       FIG. 8  shows an example of the rotation rate, power, and frequency (single element) as a function of angle to treat a target region around a prostate geometry. The effect of rotation rate in this treatment is evident in the top left panel (a). The power (b) was maintained at the maximum allowed level in order to reduce the overall treatment time. The frequency (c) was increased once to improve the conformal heating at the posterior boundary of the prostate gland. The traced prostate boundary is shown as a plot of radius vs. angle in panel (d). In the 3-D implementation of this algorithm, the scan rate is the same for all elements due to the nature of the applicator device. 
     There are several ways to implement the thermal therapy algorithm of the present invention. In one embodiment of the invention, the treatment is performed without real-time imaging feedback, wherein information about the anatomy and functional status of the target volume is acquired with imaging and other physiological measurement techniques prior to treatment. In this embodiment of the invention, anatomic and physiologic information, including tissue composition, perfusion, and other relevant parameters provide input data to a treatment planning algorithm that models the ultrasound power deposition by the heating applicator and the resulting spatial heating pattern. The temperature feedback control is simulated or determined by measuring the temperature at one or more control points in a simulation, and the temperature measurements are used to control the output parameters of the heating applicator thereby yielding a specific treatment plan for a particular target volume. 
     In a second embodiment of the method of the invention, a treatment plan based on imaging and physiologic information acquired prior to thermal therapy is delivered to a target region of tissue under imaging guidance. MR thermometry is used to measure the temperature distribution throughout the heated volume during the therapy in order to evaluate the spatial heating pattern, wherein the measured temperatures are used to calculate the expected pattern of thermal damage for comparison with the actual thermal pattern, from which an error function for the treatment can be generated. Upon completion of the therapy, the error function is evaluated and heating re-applied to any regions of the target volume that were insufficiently heated. This process is repeated in order to minimize or reduce the error function. 
     In yet another embodiment, the heating applicator is positioned within the target volume, and measurements of the temperature distribution are used as inputs for a treatment algorithm to select the output parameters of the heating applicator during the therapy. MRI is used to measure the temperature at a locus of control points which can include, but are not limited to, the boundary of the target volume along the direction of the ultrasound beam, and the maximum temperature in this direction. The temperature measurements are evaluated with the control algorithm to select an appropriate scan rate for the device, and a power and frequency for each transducer element. These applicator device parameters are updated whenever new imaging information is available during the therapy. 
     Still according to other embodiments of the invention, a treatment plan is devised based on pre-operative imaging and physiologic information, wherein additional imaging information is acquired during treatment and used to update the treatment plan. In some instances, MRI measurements of the temperature distribution of the heated region are obtained during treatment and compared directly against the predictions of the treatment plan, with appropriate adjustments implemented as necessary. Accordingly, this approach takes advantage of the stability of a treatment plan, with an actual physical measurement made with imaging to account for discrepancies between the model and the behavior of the heated tissue. One algorithm of the present invention comprises a 3-D calculation of the heating pattern in tissue, wherein changes in tissue ultrasound attenuation and blood perfusion occurring during treatment are modeled dynamically. 
     Other aspects of the present invention provide a control system, which takes MR thermometry data from a MR scanner and processes the thermometry data in order to produce instructions for the heating applicator and/or driving apparatus with respect to its output variables. In a particular embodiment, data from the memory of the MR imager is used to calculate and display the temperature distribution from a phased-array coil configuration. The output power and frequency selected by the program, as well as the actual measured transmitted power and scan rate are displayed during treatment. The operator can interact with the system to override the existing output selected by the system, wherein the operator interface can provide the clinical team with predictions of the region of thermal damage, and could monitor important regions of underheating and/or overheating. 
       FIG. 9  illustrates an exemplary embodiment of a multi-element transducer  901  capable of producing independently-controllable ultrasound radial beams along its length for the generation of arbitrary patterns of thermal damage. The transducer  901  includes a single piece of piezoelectric material  902 , preferably a piezoceramic designed for use in high power ultrasonics such as PZT 4  or PZT 8 . A top  904  and bottom  903  electrode are deposited on the surfaces of the transducer to generate an electric field across the transducer. The entire transducer  901  is mounted on a sealed platform  909  with raised edges  910  resulting in a thin backing of air which enables efficient transmission of ultrasound from the front face of the transducer. The thickness of this air pocket is typically at least one acoustic wavelength. In one or more embodiments, the sealed platform and raised edges are made from an insulating ceramic such as alumina. The front surface of the transducer  901  is in contact with water  908  which enables coupling of acoustic energy into the target volume of tissue. If multiple frequency operation is desired, a matching layer of high acoustic impedance  905  can be bonded to the front or back surface of the PZT  902  in order to produce multiple discrete frequencies. In some embodiments, the high acoustic impedance layer is chosen to be one-half the thickness of the PZT  902 , resulting in a capability to transmit ultrasound at two discrete frequencies around the original resonant frequency of the transducer without layers. 
     In some embodiments, multiple independent transducer elements are created by cutting through the back electrode  903  into the ceramic to create multiple, joined elements, wherein the depth of cuts  906  determines the amount of interaction or coupling between adjacent segments. This aspect of the invention provides a practical approach for fabricating a multi-element transducer designed for conformal thermal therapy, wherein multiple elements of varying size can be created. 
     The performance of the transducer design disclosed in the present invention has been evaluated by obtaining measurements of the impedance spectrum of the elements and the output acoustic power to characterize the stability and efficiency. The effects of the depth of dicing cut  906 , the width of dicing cut  906 , and the dimensions of the diced elements (width, length) on the output power stability and efficiency have also been investigated. Finite-element calculations have also been conducted to investigate the optimal depth/width of dicing cuts  906 . 
     In one or more embodiments, the individual transducers are connected electrically to a printed circuit board under a platform that houses the transducers, which provides signal paths to a distal point where electrical cables can be connected. This practical design enables adjacent elements to be spaced within 70-100 um of each other (the width of the diced kerf) to minimize under-heated regions of tissue between elements. This approach also provides a simple method for producing arbitrary multi-element configurations capable of producing a three-dimensional pattern of thermal damage conformal to a specific anatomical target. For example, the arrangement and number of elements in the transducer are designed to produce a spatial heating pattern that conforms to the shape of the prostate gland. 
       FIG. 10  shows a non-limiting example of an ultrasonic heating applicator  1001  designed for transurethral ultrasound thermal therapy. The applicator consists of a multi-element transducer  1010  seated in a rigid tubular housing  1011 . This portion of the device is connected to a back end  1005  which includes an inlet  1003  and outlet  1004  port for water or other fluid to flow through the device. The water serves to remove thermal losses from the multi-element transducer  1010  as well as provide acoustic coupling between the transducer and surrounding tissue. The temperature of the water can be controlled in order to heat or cool the adjacent tissue. An additional port  1007  on the back end  1005  can be used to drain urine from the bladder, or flush water through the urethra. The drainage port is connected to an outlet port  1008  located near the tip of the applicator. The tip of the applicator may be flexible, to assist in positioning the device into the prostate gland. A thin tube  1013  connects the outlet port  1008  to the drainage port  1007  in the back end  1005 . Electrical signals are transmitted to the multi-element transducer  1010  via low-loss electrical cable which connects to the heating applicator via a multi-pin electrical connector  1006 . The heating applicator  1001  can deliver ultrasound energy to a targeted three-dimensional volume in tissue as described in the examples above. 
     In one or more embodiments of the invention, the heating applicator is a rigid tubular device designed for insertion into the urethra such that the ultrasound emitting portion is located in the prostatic urethra. The device incorporates a substantially planar radiating surface that includes a multi-element transducer  1010 . In one or more examples, the dimensions of the individual transducers are between 2 and 4 mm in height (along the diameter of the tubing), and their length is between 5 and 25 mm. The overall length of the multi-element transducer  1010  is designed to substantially cover the entire length of a prostate gland (3-4 cm). However, to treat a gland or organ longer than the transducer, the transducer may be translated along its axial dimension as mentioned earlier. 
     The frequency of ultrasound produced by these transducers is typically between 4 and 10 MHz, and the resulting ultrasound beam produced by each transducer element can be approximated by a plane wave that is relatively collimated as it passes into the tissue. The number of elements along the active area of the heating applicator is typically five, but the number of elements can be larger or smaller in accordance with the requirement to shape the heating pattern along the length of the device to the contour of the prostate gland or treatment volume. 
     The heating applicator  1001  may be used in conjunction with medical imaging technology, including but not limited to MRI. Rapid imaging, accomplished with MRI and/or ultrasound can be used to guide the device during insertion to ensure that correct placement is achieved. In one embodiment, during the delivery of ultrasound to tissue, MRI can be used to non-invasively measure the temperature distribution in the region of tissue around the heating applicator to ensure that excessive heating is avoided close to the applicator, and adequate heating occurs at the treatment boundary. For example, upon completion of treatment, MR images sensitive to thermal damage of tissue, such as T2-weighted and/or contrast-enhanced T1-weighted images can be acquired of the treatment volume to assess the extent of thermal coagulation. 
     In one or more embodiments, the invention includes an ultrasound device for thermal therapy of tissue consisting of a multi-element, multi-frequency transducer, wherein multiple planar ultrasound transducers incorporated in the device produce collimated acoustic fields that heat a localized region of tissue. Rotation of the heating applicator enables the acoustic energy to be delivered to a large volume of tissue, and control over the acoustic power and frequency, as well as the rate of rotation enables the adjustment of the depth to which therapeutic temperatures are achieved. Each transducer can be independently controlled to adjust the heating pattern along the length of the device, resulting in the ability to generate an arbitrary three-dimensional volume of thermal damage. 
     In some embodiments, the length of the device for transurethral prostate thermal therapy is extended to about 10-12 inches in order to access the prostate gland through the urethra, and the number and size of elements in the multi-element transducer varies specifically with the requirements for achieving conformal heating of the prostate gland. In addition, the device disclosed in the present invention incorporates water cooling through the applicator which serves to remove thermal losses in the transducers, as well as to couple ultrasound energy from the transducers into tissue. Water cooling (or more generally, heat exchange) may be incorporated into the heating applicator to produce convective heat transfer at the acoustic window where the ultrasound beam is emitted. Depending on the flow rate and temperature of the flowing water, this results in local cooling of the first 1-2 mm of tissue adjacent to the device, wherein the urethra and other normal tissues are protected during thermal ablation of anatomically contiguous diseased prostate tissue. 
     Further improvements in the device of the present invention include an acoustic window cut into the rigid tubing at the location of the multi-element transducer which enables ultrasound energy to pass into tissue. In one embodiment, the window is sealed with a thin polymer layer, which is preferably .about.12-25 micrometers thick. In another embodiment, the individual elements are connected with small coaxial cables to a multi-pin connector at the back of the applicator. In yet other embodiments, the 1-2 inch long tip of the device is flexible in order to help navigate the rigid device into the prostate, wherein the tip of this flexible portion is open and connected to a port at the back of the applicator through a thin long tube to allow urine to drain from the bladder during treatment. Yet another embodiment incorporates inflatable balloons at the tip of the device in order to anchor the device by the balloon which is positioned in the bladder. 
       FIG. 11  shows an embodiment of an MRI-compatible apparatus  1100  capable of rotating a heating applicator to deliver thermal therapy in a clinical MR imager. The apparatus consists of a bearing structure that can be attached to a heating applicator which is driven by MRI-compatible motors. Motor  1106  may be based on a piezoelectric principle, and operate using a sinusoidal driving signal. An encoder including a disk  1103 , sensor  1108  and mounting arm  1107  provides positional feedback to a motor controller on the delivery system in order to achieve stable and controlled motion. The bearing structure is made up of a top  1105  and bottom  1101  housing which encase an inner plate  1102  capable of rotation. A ceramic ring  1104  is attached to the inner plate, and is in contact with the piezoceramic motors. A cover  1109  is included to protect the components of the motor from damage. In some embodiments, the assembly is made of MRI-compatible materials including but not limited to plastics, glass, ceramics, and metals such as copper, brass and aluminum. 
       FIG. 12  illustrates a non-limiting example of the performance of a prototype MRI-compatible apparatus designed to rotate a multi-element directional heating applicator. The target and actual rotational speed is shown in two units (.degree./min, rev/min). A substantially linear relationship exists between the two variables, indicating the system is capable of accurate motion control. The range of motion is large enough to extend over three orders of magnitude, from 0.01 to 30 RPM (3 to 10000.degree./min). The range of motion is suitable for the required rotation rates to achieve conformal thermal therapy from the multi-element heating applicator described herein, and those skilled in the art will appreciate that many other uses can be realized for such a motion control system in an MRI environment. The described apparatus has been verified to function during the imaging process without producing any measurable interference in the MR images. 
     The present concepts may be embodied in systems or methods for practicing the concepts described herein and those which become clear to readers skilled in the present art, including but not limited to machinery, hardware, software and computer programs, imaging, medical technology, and others. 
     While a number of advantages and results may be achieved by practicing the inventions disclosed herein, it is not intended that experiments, treatments, or procedures including or relating to the present disclosure be carried out, especially on live patients, without sufficient knowledge, testing, and verification of the efficacy and safety thereof.