Surgery: kinesitherapy – Kinesitherapy – Ultrasonic
Reexamination Certificate
2001-01-25
2003-02-04
Lateef, Marvin M. (Department: 3737)
Surgery: kinesitherapy
Kinesitherapy
Ultrasonic
C600S437000, C600S439000, C600S442000
Reexamination Certificate
active
06514220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the treatment of animal bodies by means of ultrasound, and comprises a method and apparatus for creating intense acoustic pressures interior to selected portions of an animal body without the need for focusing the incident acoustic radiation.
2. Background Information
Ultrasonic energy, applied to a selected region within a body from an extracorporeal transducer, is often used as a therapy means. Examples include ultrasonic ablation of tissue as shown, e.g., in U.S. Pat. No. 4,858,613, “Localization and Therapy System for treatment of spatially oriented focal disease”, issued Aug. 22, 1989, to Fry et al.; fracture of kidney stones, as shown, e.g., in U.S. Pat. No. 4,539,989, “Injury Free Coupling of Therapeutic Shock Waves”, issued Sep. 10, 1985 to Forssmann et al; heat therapy, as shown in, e.g., U.S. Pat. No. 4,586,512, “Device for Localized Heating of Biological Tissue, issued May 6, 1986 to Do-huu; and destruction of thrombi, as shown, e.g., U.S. Pat. No. 5,509,896 “Enhancement of Sonothrombolysis with External Ultrasound, issued Apr. 23, 1996 to Carter et al. The mechanisms of action for these applications require acoustic intensity levels sufficient to cause significant heating or mechanical disruption or destruction of tissue preferably only within a localized region.
The acoustic intensities used for treatment in the localized region of the body range from 0.5 to 100's of watts per cm
2
at the internal treatment site, at frequencies in the 100 kHz-2 MHz range. Prolonged exposure to intense acoustic fields causes tissue destruction through heating or mechanical action. Thus, it is important that the acoustic field be controlled so that only the target tissue receives prolonged exposure. Ultrasonic energy that passes through intervening layers of energy-absorbing tissue, like the skull, in order to reach the area targeted for treatment can cause heating in those intervening layers. Bone absorbs ultrasound at least thirty times more readily than brain tissue; thus, to avoid undue skull heating, acoustic intensities at the skull must be kept low.
The acoustic field at the skull can be shaped by geometric focusing, using either physical or electronic lenses to avoid undue skull heating.
FIG. 1
illustrates this type of system. Ultrasound transducer
100
contains a lens structure
110
that produces a concave shaped wave front
120
that converge along paths
130
to a point of intended focus
175
after transiting the skull
125
. The acoustic intensity at the convergence point
175
is many times higher than it is at the penetration area
150
of skull
125
.
Geometric focussing becomes impractical when low frequency ultrasound is used, because physically large, impractical transducers and lenses are required. In such cases, low frequency ultrasound, below 100 kHz, may be used in combination with a lytic agent such as tissue plasminogen activator (tPA); the low frequency ultrasound increases the thrombolytic rate of the tPA. See, e.g., Suchkova, V et al “Enhancement of Fibrinolysis with 40 kHz Ultrasound”,
Circulation:
1998, pp. 1030-1035. However, at this frequency, a geometrically focussed system requires transducers and lenses of 25 cm diameter or larger in order to provide a moderate degree of focussing. Such large transducers require liquid coupling media between the transducer/lens structure, and the subject tissue in order to efficiently couple acoustic waves into the tissue. Since bone is characterized by different sound velocities than either the coupling media or the target brain tissue, complex lenses that can correct for local variations in refractive index are used. Measurements of skull thickness and a map of refractive index over the entrance surface of the ultrasound are then required in order to compute the necessary shape of the lens. This leads to a complicated and large ultrasound system.
Another disadvantage of a lens based system is that there is no built in safety factor should the wrong lens or acoustic power levels be inadvertently used. Inertial cavitation, which arises in liquids in the presence of high acoustic intensity levels, is known to damage living tissue, and therefore should be avoided.
SUMMARY OF THE INVENTION
It is an therefore an object of this invention to provide an improved system that delivers the desired acoustic energy within a living being without employing cumbersome, large lenses with their unwieldy coupling components, while keeping the applied acoustic intensities at acceptably low levels at non target tissue sites.
It is further an object of this invention to provide a system that delivers the desired acoustic energy within a living being, but does not require a measurement of tissue thickness in order to do so.
Still a further an object of this invention is to provide a system that delivers the desired acoustic energy within a living being, but which does not require a measurement of refractive index in order to do so.
Yet another object of this invention is to provide a system that does not require that the shape of a lens, either physical or electronic, be varied in order to compensate for local refractive index variations within a living being.
Yet another object of this invention is to provide a system which includes a method to spatially position a region of therapeutic acoustic intensity within a desired region of a selected volume.
Yet another object of this invention is to provide a system which includes a method of placing a region of therapeutic acoustic intensity in a predetermined location within a selected volume.
We have observed that many body structures act as resonators. The brain vault, for example, is bounded by layers of differing acoustic impedance, thereby causing reflection of acoustic waves at these boundaries. At frequencies in the 0-500 kHz range, there is little attenuation of longitudinal acoustic waves in the brain or skull, and if reflection at a boundary layer is near total, then acoustic waves pass back and forth through tissue many times creating a trapped mode resonator.
FIG. 2
illustrates the acoustic field in such a resonator. Ultrasound transducer
200
emits wave fronts
250
that transit the skull
225
. Due to low acoustic loss in the tissue
235
in the cranial vault, the waves travel to the other side of the skull, where they are reflected as wave
255
due to the differing acoustic impedance of the bone and air which forms the skull and its outside boundary. These reflected waves
255
again travel across the cranial vault and again are reflected by the bone and air interfaces, and return across the cranial vault. This process is repeated many times, and builds up to the point where the internal acoustic energy losses in the cranial vault and the reflections at the skull balance the acoustic energy applied by transducer
200
. At points
280
where the acoustic waves intersect, pressure nodes and anti-nodes are formed, depending on whether the wave fronts interfere out of phase or in phase, respectively. A common measure of the resonant property of a system is its quality factor, defined as 2&pgr; times the ratio of stored energy to the lost energy per cycle. In practice, we have measured Q's of from 10 to more than 100 in isolated skull experiments, with node to anti-node pressure ratios of from 10 to more than 100.
We make use of this fact by treating a body part that is to be subjected to acoustic waves below 500 kHz, and preferably below 100 kHz, as a trapped mode resonator. Such a resonator can exhibit a high Q (e.g., a Q of 10 or more) at certain frequencies that cause wave front interference from multiple reflections to add up in phase. Examples of trapped mode resonators within a body include (a) the cranial vault bounded by air, bone and neck tissue; (b) arms; (c) legs; and (d) the thorax, all of which are bounded by air and other tissues. In high Q resonators, very high pressures can be achieved in the resonator cavity for very modest input power U. The differing impedance of
Fearnside James T.
Melton, Jr. Hewlett E.
Zanelli Claudio I.
Cesari and McKenna LLP
Lateef Marvin M.
Pass Barry
Walnut Technologies
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