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United States Patent m
Date of Patent:
 METHOD AND APPARATUS FOR GENERATING LOCALIZED HYPERTHERMIA
 Inventor: Peter J. Klopotek, Framingham, Mass.
 Assignee: Summit Technology, Inc., Waltham, Mass.
 Appl. No.: 823,816
 FUed: Jan. 22,1992
 Int. CI.* A61N 5/00
 U.S. CI. 128/399; 128/24 AA
 Field of Search 128/660.03, 662.05,
128/662.06, 24 A, 399, 804; 606/1, 27, 28, 32
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S. E. P. Burgess et al., "Histologic Changes in Porcine Eyes Treated with High-Intensity Focused Ultrasound", Ann. Ophthalmol, 1987, 19:133-138. Fessenden, "Ultrasound Methods for Inducing Hyperthermia", Font. Radiat. Ther. One, vol. 18, pp. 62-69 (Karger, Basel 1984).
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Primary Examiner—Francis Jaworski
Attorney, Agent, or Firm—Thomas J. Engellenner
Ultrasound generating methods and apparatus are disclosed for producing controlled, localized hyperthermia in a selected heating zone of human tissue, utilizing at least one ultrasound transducer, preferably driven by sinusoidal excitation signals in a continuous wave or quasi-continuous wave mode to generate ultrasound. The temperature in the heated zone can be controlled by selecting the power, duration and frequency of the ultrasound. The penetration of the ultrasound, and thus the depth and volume of the target zone, can be controlled by selecting the excitation frequencies so to confine the absorption of the ultrasound beam in the target tissue. The invention is particularly useful in inducing controlled collagen shrinkage in corneal tissue to effective thermokeratoplasty (heat induced modification of the shape of the cornea).
16 Claims, 2 Drawing Sheets
METHOD AND APPARATUS FOR GENERATING
BACKGROUND OF THE INVENTION
The technical field of this invention is high frequency ultrasonic, or hypersonic, therapy and, in particular, the use of such therapies for thermokeratoplasty (heatinduced modifications of the shape of the cornea) and other surgical procedures based on controlled hyperthermia.
In recent years, researchers have developed a wide range of therapeutic and surgical procedures utilizing the application of localized hyperthermia at selected target sites within human patients. These techniques include destruction of tumors, ophthalmological procedures, such as eye thermokeratoplasty, and sealing of small blood vessels to reduce bleeding during surgery. Various types of apparatus have been employed to deliver this localized heat, including torches, heated needles, electric scalpel, microwave devices, lasers, and ultrasound generators.
The predictability, controllability and safety of thermokeratoplasty and other hyperthermia procedures is largely dependent upon accurate control of the location and temperature of the hyperthermia zone in the human tissue, as well as the duration of the heating. In particular, the success of many hyperthermia-based surgical and therapeutic procedures requires precise control of the space/time profile of the hyperthermia applied to the target tissue. An additional requirement is the avoidance of excessive overheating of, and damage to, surrounding tissue.
Appreciation of these requirements has led many researchers to the conclusion that virtually all conventional hyperthermia techniques have serious shortcomings. In particular, it is impracticable to utilize a thermal source to selectively deliver heat to target volumes of tissue through thermal diffusion, because when the distal portions of the volume are heated to the desired temperature, severe overheating of the tissue in direct contact with the source occurs.
Laser thermokeratoplasty methods offer alternatives to conventional, physical contact hyperthermia. Such techniques involve the application of a beam of infrared 45 radiation into the tissue volume. However, the application of infrared lasers to generate localized heating often relies upon the balancing of intensity enhancement in a focused laser beam and the optical attenuation presented by tissue material, factors that sometimes are not precisely controllable. In addition, the laser approach is expensive and requires a large supporting apparatus.
It is, accordingly, an object of the invention to provide improved methods and apparatus for generating 55 hyperthermia.
It is another object of the invention to provide hyperthermia methods and apparatus utilizing a source of radiation that deposits energy into tissue in a controllable manner.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.
thermia in human tissue. The term "ultrasound" as used herein is intended to encompass both conventional "ultrasound" as typically used to describe high-frequency acoustic waves up to about 100 megahertz and "hypersound" as typically used to describe very high-frequency acoustic waves greater than 100 megahertz. In general, "ultrasound" is used hereto to describe acoustic waves capable of inducing controlled hyperthermia in biological tissue, particularly the corneal tissue of the eye.
In accord with one aspect of the invention, the apparatus includes at least one ultrasound transducer that can be driven by electrical excitation signals to generate ultrasound. The apparatus can also include one or more propagation elements, for example, focusing elements for focusing the ultrasound beam generated by the transducer and elements that direct and control the size and shape of the ultrasound beam into the target tissue.
In one embodiment, the excitation elements drive the ultrasound transducer to generate ultrasound having a sinusoidal excitation and in a continuous wave or quasicontinuous mode. Ultrasound, as generated by the transducer, is focused by the focusing elements, or focusing surface, and transmitted through a ultrasound transmitting crystal and acoustical matching medium to generate heat in the target tissue.
In another aspect of the invention, a controller is provided for selecting a range of ultrasound frequencies, thereby controlling the spatial parameters of the heated volume. For example, the electronics control module can cause "the excitation elements to sweep through a selected range of ultrasound frequencies. Preferably, this range begins at higher frequencies, approximately 500 MHz, and ends at lower frequencies, at approximately 20 MHz.
In further aspects of the invention, the amplitude and/or duration of the ultrasound beam can be selected. The spatial dimensions and position of the ultrasound apparatus, such as the separation between the ultrasound generating source and the target tissue, or the focusing volume (f-number) of the ultrasound beam, can also be selected. The symmetry of the ultrasound beam, in addition, can be selected by applying a non-circular beam of ultrasound to the target tissue.
According to yet a further aspect of the invention, control elements are provided for selecting various hyperthermia parameters, such as the desired temperature of the hyperthermia zone, the volume of the heated zone within the target tissue and the penetration depth, as well as for selecting the frequency profile and combination necessary to deposit a dedicated portion of the beam energy at a desired location and depth within the hyperthermia zone.
Another aspect of the invention involves methods for performing thermokeratoplasty and similar surgical procedures. According to this aspect, ultrasound is generated and directed into the corneal target tissue. The ultrasound beam is controlled to deposit the energy into the stroma region of the cornea, at approximately 300 to 450 micrometers or less in depth, to cause the collagenous tissue to shrink, thereby changing the refractive power of the corneal surface.
The invention will next be described in connection with certain illustrated embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.