Surgery: kinesitherapy – Kinesitherapy – Ultrasonic
Reexamination Certificate
2000-11-28
2003-12-23
Bennett, Henry (Department: 3743)
Surgery: kinesitherapy
Kinesitherapy
Ultrasonic
C600S407000, C600S437000, C600S439000
Reexamination Certificate
active
06666833
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to thermal energy treatment systems, such as MRI-guided focused ultrasound systems and, more particularly, to systems and methods for focussing an acoustic energy beam transmitted through non-uniform tissue.
BACKGROUND
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby eliminating the need for invasive surgery. For this purpose, focused ultrasound systems having piezoelectric transducers driven by electric signals to produce ultrasonic energy have been employed.
In such systems, such as a MRI-guided focused ultrasound system, the transducer is positioned external to the patient, but in generally close proximity to a target tissue region within the patient to be ablated. The transducer may be geometrically shaped and positioned so that the ultrasonic energy is focused at a “focal zone” corresponding to the target tissue region, heating the region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. For example, this series of “sonications” may be used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or sectors, creating a tiling of the transducer face. The transducer elements are driven by radio frequency (RF) electrical signals at a single frequency offset in phase and/or amplitude. The phase and amplitude of the respective drive signals may be individually controlled such that a “focal zone” of the emitted ultrasonic energy has a desired distance, shape, orientation and energy level in the target tissue region.
For example, if all of the sectors are driven by drive signals that are in phase with one another, the ultrasonic energy will be focused substantially at a relatively narrow focal zone. Alternatively, the sectors may be driven with respective drive signals that are in a predetermined phase relationship with one another. The discrete nature of the phase differences among the sectors results in a number of zones that collectively define a wider area of focus and cause necrosis of a larger tissue region within a focal plane intersecting the focal zone. For example, these zones may collectively define an annulus surrounding a central zone.
More advanced techniques for obtaining specific focal distances, shapes and/orientations are disclosed in U.S. patent application Ser. No. 09/626,176, filed Jul. 27, 2000, entitled “Systems and Methods for Controlling Distribution of Acoustic Energy Around a Focal Point Using a Focused Ultrasound System;” U.S. patent application Ser. No. 09/556,095, filed Apr. 21, 2000, entitled “Systems and Methods for Reducing Secondary Hot Spots in a Phased Array Focused Ultrasound System;” and U.S. patent application Ser. No. 09/557,078, filed Apr. 21, 2000, entitled “Systems and Methods for Creating Longer Necrosed Volumes Using a Phased Array Focused Ultrasound System.” The foregoing applications, along with U.S. Pat. No. 4,865,042, are all hereby incorporated by reference for all they teach and disclose.
Such high frequency focused ultrasound systems may be employed in various parts of the body. Notably, when using high frequency focused ultrasound “energy beam” to thermally treat a certain area of the body, e.g., to ablate a tumor, the energy beam must be precisely focussed to the target location so as to avoid harming healthy tissue surrounding the target. As used herein, the terms “beam,” “energy beam,” or “acoustic energy beam” are used to refer generally to the wave sum of the waves emitted from the various transmitting elements of a focused ultrasound system. This focus is achieved by adjusting the phases and amplitudes of the individual waves to produce constructive interference at a particular location.
Towards this end, problems may be encountered when using a focussed ultrasound energy beam to treat a certain portion of the body in which the individual waves forming the beam must be transmitted through a non-uniform tissue medium, such as, e.g., the skull, the inner surface of which can be highly irregular in shape. This non-uniformity of the particular tissue medium introduces differential phase errors or phase aberrations in the respective ultrasound waves transmitted by transducer elements located at different places on the exterior of the body. For example, because the speed of sound is faster in bone than in tissue, the phase of a wave that passes through bone is advanced relative to one that passes through tissue. The relative amount of phase change depends on the thickness and consistency of the bone through which the individual waves are propagated. If the transmission paths of some transducer elements must travel through more or less bone medium than others, there is no total constructive interference of the waves at the intended focal zone, i.e., the beam is not well-focussed. Also, the different wave paths will usually cause different attenuation and, thus, the relative amplitudes of the individual waves will also not be optimal for creating the required focused interference pattern.
By way of another example, the speed of sound in fat tissue differs by about 7% from the speed of sound in muscle. If the target focal zone is underlying a non-uniform fat layer, this non-uniformity can cause phase aberrations in the individually transmitted waves, again depending on their respective transmission paths. Depending on the frequency of the waves, the aberrations. For example, at 1 MHz, a difference of one centimeter in the fat layer thickness in the transmission path of two different transmitting elements results in a phase difference of about 180°.
Notably, in a phased array ultrasound transmitting system, phase adjustments to individual transmitting elements can be made to compensate for the distortions caused- by transmission through a non-uniform tissue medium. Of course, this is only if the amount of phase shift needed for each respective transmitting element is known. A relatively accurate determination of the necessary phase corrections is possible in a closed loop fashion, by observing the beam intensity and adjusting the phases of the transmissions in order to maximize the beam intensity at a chosen point. To be able to focus the beam in the brain, however, one must be able to “see” inside the patient's skull to determine whether or not the beam is focussed.
One such “closed-loop” approach is to employ a strong point reflector in the target tissue region to reflect energy pulses from the beam back to fixed detection elements, so that the system can determine if the energy pulses were applied to the correct location. With this approach, focussing of the beam would be achieved by measuring the roundtrip time it takes energy pulses from each beam element to travel out of the beam element to the reflector and back to the system detector. Alternatively, the reflection of the entire transducer array from the reflector could be maximized. However, such a point reflector does not normally exist within the target tissue region and invasive surgery would be required for its placement. For example, one prior art approach for focussing in the brain requires that a reflector be inserted into the brain and is based on time-reversal concepts. See, e.g., U.S. Pat. Nos. 5,431,053; 5,428,999; 5,276,654; 5,092,336; and 5,010,885. Of course, the physical in
Friedman Zvi
Maor Dov
Bennett Henry
Bingham & McCutchen LLP
Dagostino Sabrina
Insightec-TxSonics Ltd
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