Abstract:
A microwave hyperthermia treatment system employs a set of antennas individually controllable to provide different phase and amplitude outputs and controlled to cycle through different sets of phases and amplitudes over time to minimize the effect of hotspots formed by any given set of phases and amplitudes and creating limiting high temperatures outside of a desired treatment area.

Description:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under 0625054 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     Background of the Invention 
     The present invention relates to medical hyperthermia treatment systems and in particular to an improved method for controlling microwave antennas for selective heating of tissue. 
     Hyperthermia treatment elevates the temperature of tissues for a variety of purposes including: (i) destroying tissues such as tumors by the application of heat, (ii) increasing the susceptibility of heated tissue to chemical or radiation therapy, and (iii) triggering heat activated or released drugs. It is generally known to use microwave electromagnetic radiation for hyperthermia treatment. Microwaves provide a number of advantages including an ability to pass though some body structures such as the skull for treatment of the brain, and an ability to be focused to permit, for example, localized treatment of a tumor surrounded by tissue with reduced damage to the surrounding tissue. 
     Microwave energy may be focused, for example, through a phased array of the antennas. In this technique, the phase and amplitude of the microwave radiation supplied to each antenna is adjusted to create constructive interference at the tumor site among radiated waves from each antenna and destructive interference outside of the tumor site among the waves from each antenna. By proper phasing of the emitted microwaves, the power deposited on the tumor may be increased over that deposited outside of the tumor region. The determination of the proper phase and amplitude of the microwave power at each antenna may take into account the electrical properties of the intervening tissue of the patient to correct for phase shifts and attenuation caused by this tissue. 
     As a practical matter, it is impossible to find a given set of phase and amplitude values that focus microwave radiation for an arbitrary treatment pattern while completely suppressing the deposition of microwave energy outside of the treatment pattern. Unwanted heated zones outside of the tumor region inevitably form to limit the amount of energy that can be applied to the tumor without damage to healthy tissue. Sometimes tissue in the hot zones can be cooled, for example, by chilled water or air applied to the skin. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that the unwanted heated zones can be minimized by cycling through different antenna array settings each having a common treatment zone but providing relative “cold spots” or suppression regions in different locations. Effective hyperthermia treatment schedules can be produced by offsetting potential hotspots in a given location in one antenna array setting with cold spots in that location in a different antenna array setting. 
     In one embodiment, the invention provides a treatment planning system including an electronic computer executing a stored program to receive data characterizing tissue in a tissue region of a patient and data defining a treatment region being a portion of the tissue region. The planning system may use the data characterizing the tissue to model a set of power deposition patterns and associated temperature distribution profiles from a known array of antennas emitting different given amplitudes and phases of radiofrequency power for each power deposition pattern. A treatment schedule may then be generated by selecting among this set of power deposition patterns, each having different hotspots and suppression regions of relative elevated and low temperatures, respectively, so that suppression regions of some selected power deposition patterns are matched to the potential hotspots of other power deposition patterns to control average power deposition to the tissue region outside the treatment region. 
     It is thus a feature of at least one embodiment of the invention to reduce hotspots that are created in focusing the signals transmitted by an antenna array through the use of a time-varying control of the antenna array, and to provide at least one system of identifying an effective time variation. 
     The treatment-planning program may further execute to vary the duration of each power deposition pattern in the treatment schedule to control average power deposition to the tissue region outside the treatment region. 
     It is thus a feature of at least one embodiment of the invention to better match potential hotspots and suppression regions by control of the relative duration of the antenna settings producing each. 
     The treatment planning program may further execute to select the order of each power deposition pattern in the treatment schedule according to a thermal model of tissue in the tissue region, the thermal model describing rates of heating and cooling of the tissue in response to changes in applied power. 
     It is thus a feature of at least one embodiment of the invention to permit control of time-consistency of the hyperthermia treatment. 
     The treatment-planning program may further accept physician input with respect to a desired minimum temperature in the treatment area and at least one desired maximum temperature in a portion of the tissue region outside of the treatment area. 
     It is thus a feature of at least one embodiment of the invention to permit sophisticated control of hyperthermia treatments including reduction of temperatures outside of the treatment area in susceptible zones. 
     The data characterizing the tissue in the tissue region of the patient may characterize the spatial distribution of the electrical properties of the tissue and the model may model microwave propagation throughout the tissue and absorption of microwave power by the tissue. 
     It is thus a feature of at least one embodiment of the invention to provide a treatment schedule that accurately reflects actual tissue of the patient with respect to microwave transmission. 
     Alternatively or in addition, the data characterizing the tissue of the tissue structure of the patient may characterize the thermal properties of the tissue and the selection of power deposition patterns may model heating and cooling of the tissue as a function of deposited power and time. 
     It is thus a feature of at least one embodiment of the invention to accommodate different absorption and thermal dissipation qualities of the actual tissue of the patient. 
     The selection of power deposition patterns to generate the treatment schedule may iteratively process different power deposition patterns or sequences to maximize an objective function accepting as an argument desired temperature in the treatment region and at least one temperature outside of the treatment region. 
     It is thus a feature of at least one embodiment of the invention to provide a general mechanism for reducing hotspots inherent in antenna-array focused microwaves. 
     Each power deposition pattern may be associated with a specific set of amplitude and phases of radiofrequency power for each antenna and may be held constant during a time of the power deposition pattern in the treatment schedule. 
     It is thus a feature of at least one embodiment of the invention to provide a tractable treatment schedule employing a finite set of time-sequenced antenna settings. 
     The patient data may be derived from MRI data acquired of the tissue region. 
     It is thus a feature of at least one embodiment of the invention to provide a source of data that may be matched to accurate thermal and electrical data about the tissue. 
     A microwave hyperthermia treatment apparatus per the present invention may include an array of antennas positionable about a patient and a radiofrequency power source connected to the antennas to provide radiofrequency power having independently controllable amplitude and phase for each antenna. A treatment controller may be connected to the radiofrequency power source controlling the phase and amplitude of the radiofrequency power for each antenna to change the frequency and amplitude at the antennas over a treatment time according to a treatment schedule, wherein the treatment schedule provides a series of at least three sequentially applied power deposition patterns producing different patterns of heating of tissue of the patient outside of a tumor site. 
     It is thus a feature of at least one embodiment of the invention to provide a hyperthermia system with improved multi-antenna control by switching between predefined power deposition patterns during the treatment. 
     The power deposition patterns and associated temperature distribution profiles may provide potential hotspots and suppression regions and the combination of power deposition patterns during the treatment schedule may match hotspots of one power deposition pattern with suppression regions of another power deposition pattern. 
     It is thus a feature of at least one embodiment of the invention to identify a set of predefined power deposition patterns based on a compensation principle between potential hotspots and suppression regions. 
     The treatment schedule may be in excess of one second. 
     It is thus a feature of at least one embodiment of the invention to provide a quasi-static treatment system adaptable to readily available hardware. 
     The treatment controller may provide discrete switching between the power deposition patterns. 
     It is thus a feature of at least one embodiment of the invention to permit a simple switch design for implementing the invention. 
     The radiofrequency power may be between 50 megahertz and 10 gigahertz. 
     It is thus a feature of at least one embodiment of the invention to provide a system for treatment of tissue within the body and, in particular, the brain. 
     The treatment schedule may be repeated multiple times in succession during treatment of the patient. 
     It is thus a feature of at least one embodiment of the invention to permit great flexibility in selecting the power deposition patterns while providing relatively constant treatment temperatures by cycling through a set of predefined power deposition patterns faster than the heating and cooling time of the tissue. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a simplified representation of a microwave hyperthermia system for use with the present invention showing an antenna array and a radiofrequency source for controlling phase and amplitude of signals to each antenna under computer control; 
         FIG. 2  is a simplified perspective view of the antenna array of  FIG. 1  showing the location of antennas about the head and the amplitude and phase of a radiofrequency signal at two example antennas; 
         FIG. 3  is a flow chart of the present invention describing the steps of developing a treatment schedule for controlling the antenna array of  FIG. 2 ; 
         FIG. 4  is a treatment map as may be defined by a physician for a particular treatment; 
         FIG. 5  is a two-dimensional representation of a power deposition pattern describing power applied to the treatment region including hotspots and a corresponding perspective phantom view of the same; 
         FIG. 6  is a figure similar to that of  FIG. 5  showing the identification of suppression regions compensating for the hotspots; 
         FIG. 7  is a diagrammatic representation of multiple power deposition patterns sequenced together in a treatment schedule; 
         FIG. 8  is a graph showing example durations of the application of each power deposition pattern; and 
         FIG. 9  is a block diagram of a general iterative optimization system that may be used with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a hyperthermia system  10  may provide an antenna fixture  12  supporting a plurality of antennas  14  about a treatment volume  15 . In one embodiment, the treatment volume may be defined by a substantially hemispherical shell  16  whose inner surface may contain a collar  18  receiving and supporting the top of the patient&#39;s head. The collar may be filled with de-ionized water that may be circulated through connecting hoses  20  with a cooler/pump  21  providing skin cooling of at approximately 15 degrees centigrade of the patient&#39;s head to minimize surface heating of the skin by microwave energy from the antennas  14  as will be described. 
     Referring also to  FIG. 2 , the antennas  14  may be arranged at multiple heights along an inferior superior axis of the patient and about an upper portion of the patient&#39;s head. The antennas  14  preferably direct microwave energy inward toward the treatment volume  15  and may, for example, be microwave horns or patch antennas or other antennas of a type known in the art and are spaced to provide for substantially uniform separation of less than six centimeters. 
     In one embodiment, 134 antennas  14  operating at 1 GHz are distributed evenly across five elliptical rings and three partial elliptical rings, each ring separated by two centimeter elevational spacing, the antenna-to-antenna spacing on each ring being approximately four centimeters. 
     Referring still to  FIGS. 1 and 2 , each antenna  14  may be connected to a radiofrequency power source  22  providing independent phase (phi) and amplitude (A) control of the radiofrequency power applied to the antenna. The radiofrequency power source  22  may provide a separate radiofrequency amplifier/synthesizer  24  for each antenna  14  or may use a single radiofrequency power source with separate amplitude and phase shifters. In one embodiment, a set of discrete phases and amplitudes may be implemented in a switching fashion. 
     The radiofrequency power source  22  may be controlled by a treatment controller  28  via an interface board  26 , for example, providing a multiplexed A/D converter outputting phase and amplitude values from the treatment controller  28 . The treatment controller  28  may include a processor  30  communicating with a memory  32  holding a stored program  34  and treatment plan data  36  describing a treatment schedule of changing phases and amplitudes of microwave frequency to be applied to the antennas  14  during treatment. 
     The treatment plan data  36  may be developed on the treatment controller  28  but preferably is developed off-line on a separate workstation  40  having a display  42  for displaying treatment maps for physician input, as will be described, generated by a communicating standard desktop computer  44  also having a processor  46 , a stored memory  48  holding a treatment planning program  51  and the treatment plan data  36 , the latter which may be transferred to treatment controller  28 . The desktop computer  44  may also communicate with input devices  50  by interface  52  according to well understood techniques for physician input as will be described. It will be appreciated that the processing and data storage required by the present invention may be freely distributed among one or more processors and different types of computers according to well-understood techniques. 
     Referring now to  FIG. 3 , in developing treatment plan data  36 , the treatment planning program  51  may first receive MRI input data, as indicated by process block  60 , of the particular patient to be treated. The treatment plan data may be registered to fiducial points such as bony prominences of the patient&#39;s skull or points on a stabilization fixture also alignable with the fixture  12 . This MRI data may be displayed on the display  42  for reference by a physician for development of a treatment map, to be described, and is preferably applied to a stored encyclopedia of tissue types to identify thermal and electrical characteristics of the tissue in different regions of the patient&#39;s brain. 
     At process block  62 , the treatment map  64  is prepared using the workstation  40 , for example, by displaying successive slices of a tissue region  66  of the patient as shown in  FIG. 4  which together describe volumetric data encompassing a tissue subject to microwave exposure. The slices may be reviewed by the physician to define a treatment region  68  within the tissue region  66  where elevated temperatures are desired for hyperthermia treatment. The boundaries of the treatment region  68  and desired minimum temperatures in one or more zones of this treatment region  68  may be entered by the physician or automatically set. 
     Optionally, the physician may also define one or more protection regions  72  to which maximum desired temperatures or other temperature constraints, such as measures of uniformity or the like, may be applied. Generally, the tissue region  66  outside the treatment region  68  will be limited to a temperature of 41 degree centigrade or less. 
     At process block  73 , the treatment-planning program  51  generates an initial “beamformer” being a set of independent amplitude and phases of microwave electromagnetic radiation to be applied to each of the antennas  14 . This initial beamformer is selected to maximize the fraction of deposited power density in the treatment region  68  with respect to the deposited power density outside the treatment region  68 , although other similar constraints may be used that promote a focusing of power deposition in the treatment region  68 . This beamformer may be generated, for example, by treating a set of linear equations providing phase and amplitude values for each antenna and incorporating microwave propagation characteristics of the patient tissue derived from the MRI data of process block  60  between each antenna and each voxel of tissue in the tissue region  66 . 
     Referring to  FIGS. 3 and 5 , at process block  74  tissue temperatures generated by the initial beamformer are then modeled using the thermal and electrical characteristics of the tissue of the patient to provide a power deposition pattern  76  (depicted two-dimensionally but preferably three-dimensional) that shows deposited power or tissue temperature at different portions of the tissue region  66  for predetermined duration of treatment, for example, the time required to raise the temperature within the treatment region  68  to 43-44 degrees centigrade. Typically, this power deposition pattern  76  will also show one or more hotspots  78   a - c  being areas of elevated power deposition or temperature outside of the treatment region  68 . For example, hotspots  78  may be defined as voxels in the head but outside of the treatment region  68  whose temperature exceeds 41 degrees centigrade or voxels exceeding a temperature of a susceptible region defined by the physician. 
     Referring now to  FIGS. 3 and 6 , at succeeding process block  77 , a suppression region  80  may be defined around each hotspot  78 . In one embodiment, for convenience, these suppression regions  80  are defined as spheres of approximately ten percent of the brain volume. Each suppression region  80  may be centered on a peak temperature of each hotspot  78 . For hotspots  78  larger than a suppression region  80 , such as hotspot  78   a  as depicted, an iterative process may be used where a first suppression region  80   a  is centered on a peak temperature of the hotspot  78   a  and then subsequent suppression regions  80   b  and  80   c  are applied to portions of the hotspot  78   a  not already circumscribed by a suppression region  80   a  centered on a peak temperature of those remaining regions, and so forth until all hotspots  78  are completely covered. 
     Referring now to  FIG. 7 , additional beamformers  84  are then generated, each defining the phase and amplitude of power at a respective antenna  14  using a process similar to that described with respect to process block  73  but with an optimization that maximizes power to the treatment region  68  and limits power to the suppression regions  80 . For example, the fraction described above with respect to the initial beamformer may be used with additional weighting provided to the desired suppression regions  80  of the tissue regions  66  outside of the treatment region  68 . 
     In one embodiment, different beamformers  84  may be generated each preserving a different suppression region  80  associated with each of the suppression regions identified in process block  77  to produce a number of beamformers  84  equal to the number of desired suppression regions  80 . In addition, one beamformers  84  may be developed providing suppression of power in the union or combinations of all suppression regions  80 , and one beamformer  84  may be developed without consideration of any of the suppression region  80  to emphasize treatment of the treatment region  68 . In a general case, some of these beamformers  84  preserving suppression regions  80  will generate power deposition patterns  76  having additional hotspots  78 ′. Typically these hotspots  78 ′ will be in different locations than the hotspots  78  of the initial beamformer. More typically, however, beamformers  84  emphasizing suppression regions  80  will have no new hotspots  78  or minor hotspots  78  beyond those identified at process block  74 . 
     Referring now to  FIGS. 3 and 7 , at process block  85 , the beamformers  84  may be arranged in order and given durations to create a treatment schedule  86  that will be used to control the antennas  14  during treatment of the patient. Typically the treatment schedule  86  will have a duration of greater than one second to permit simplified control of the radio frequency amplifier but shorter than a thermal time constant of the tissue. In one embodiment, the treatment schedule  86  may be cycled periodically for multiple times to provide the desired length of treatment but also to provide rapid switching among beamformers  84  for more consistent temperature generation. 
     The process of ordering and determining the length of time for each beamformer  84  in the treatment schedule  86  takes into account the thermal model of the tissue with respect to its cooling and heating under the application of power. This treatment schedule  86  may be determined automatically by an optimization process such as linear programming according to a goal of maximizing a uniformity of the heating of the tissue region  66  outside of the treatment region  68  or minimizing an error (for example, the least squared error) between the average temperature outside the treatment region  68  and the desired treatment map  64 . The determination of the treatment schedule  86  may also take into account the historical thermal state of the tissue implicit in the ordering to promote time uniformity in the produced temperatures, something that is also promoted by short time slices. 
     Referring now to  FIG. 8 , an example treatment schedule  86  will cycle through different beamformers  84  (indicated on the vertical axis) for different periods of time (indicated on the horizontal axis) according to this optimization. In this example, beamformer number  13  was optimized to cover all of the suppression regions  80  and accounts for more than half the duration of the treatment schedule  86 . Notably, some beamformers  84  are not used or given zero weighting. 
     Once the sequence of the treatment schedule  86  is complete, the treatment schedule  86  may be modeled and the resulting temperature distributions and other treatment statistics (e.g. maximum temperatures and durations) reviewed by the physician as indicated by process block  88 . If the treatment schedule  86  is approved, the treatment plan may be output as the treatment plan data  36  of  FIG. 1  as indicated by process block  94  to provide real-time control of the hyperthermia system  10 . 
     Referring again to  FIG. 3 , it will be recognized that at process block  88 , the process of process blocks  73 ,  74 ,  77 ,  85 , and  88  may be repeated, for example, with a different initial beamformer  84  at process block  73  or different linear programming criteria or the like to provide for an iterative process or alternative solutions meeting different goals. Alternatively, or in addition, this iteration may be used in a more general global optimization approach described below. 
     Referring now to  FIG. 9 , while the present invention has provided one method of choosing and ordering multiple beamformers  84 , using an initial definition of a beamformer and a matching of hotspots  78  to suppression regions  80 , it will be appreciated that other mathematical techniques may also be used for this purpose of creating an evolving sequence of beamformers. Such techniques include general global optimization techniques including stochastic optimization methods such as simulated annealing and the like. Generally each of these concepts will provide an initial seed set  96  of phases and amplitudes W i  for the antennas  14  for different times (t) and will apply them to an electromagnetic model of the tissue  98  and a thermal model of the tissue  100  to provide for a modeled temperature profile  102 . This temperature profile may be analyzed by an objective function  104  (for example considering any or all of peak temperature values, tissue damage, treatment speed, uniformity, accuracy and the like) typically incorporating input information  106  provided by the physician indicating desired treatment zones temperatures and metrics. The output of the objective function is used to modify the seed set  96  and its time sequence until the desired optimization is produced as measured by a threshold applied to the output of the objective function  104 . 
     Importantly, the present invention produces not a single set of phases and amplitudes but a schedule of different phases and amplitudes reflecting the fact that a single such set of phases and amplitudes necessarily results in the creation of limiting hotspots. While the application of the beamformers per  FIG. 8  is shown in discrete steps, it will be appreciated that the transitions in phase and amplitude may implemented as smooth transitions between states if desired. Nevertheless, the present invention anticipates a finite number of calculated beamformers. 
     It will further be appreciated that the invention is not limited to antenna arrays but that other means for producing spatially directive patterns, such as microwave lenses may be used. In addition, the invention may employ a spatially scanning (e.g., mechanically moved) set of antennas may be used to provide a time-varying spatial pattern of energy providing an effective antenna array of multiple emitters. That is, the set of microwave emitters may be implemented by as few as one emitter moved between locations and provided different amplitudes and phases of microwave power. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.