Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-10-20
2003-07-08
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S411000, C600S459000, C601S002000
Reexamination Certificate
active
06589174
ABSTRACT:
FIELD IF THE INVENTION
The present invention relates to the field of thermal therapy for treatment for various medical conditions, for example, tumors, and is concerned with ultrasound therapy, more particularly the devices and methods of use of such devices.
BACKGROUND OF THE INVENTION
Thermal therapy is a technique for the treatment of tumors in which heat is used to destroy cancerous tissue. It is a potential candidate for the treatment of solid, localized tumors in tissue. “Hyperthermia” refers to thermal therapies in which the target temperatures achieved in tissue are between 42 and 46° C. In this temperature region, the relationship between cell death, temperature and time is described by a thermal dose equation, and exposure times are typically between 30 and 60 minutes at 43-45° C. (Dewey, 1994). Thermal coagulation refers to thermal therapies in which target temperatures achieved in tissue are between 55 and 90° C. The application of temperatures in excess of 55° C. results in rapid destruction of tissue primarily through thermal coagulation. This higher temperature regime delivers sufficient energy to denature proteins and produces complete cell death in the treated region within a short time (seconds) (Thomsen, 1991).
The use of thermal coagulation for tissue destruction is predicated on effective guidance and monitoring of heat delivery. Real-time medical imaging plays an integral role, providing important information about anatomy, temperature, and tissue viability during and after the delivery of heat. This information can be used to target heat delivery to specific locations, monitor the amount of heat delivered, and assess the biological damage incurred, thereby eliminating the need to expose the treatment site to visual assessment. Monitoring the spatial delivery of heat with magnetic resonance imaging (MR) can avoid damage to critical structures and other normal tissue.
In interstitial thermal therapy, heat is produced by devices inserted directly into a target site within an organ. Potentially less invasive than conventional surgery, this approach can make possible the treatment of tumors in otherwise inaccessible locations. Several technologies have been employed for interstitial heating, including lasers, radio-frequency waves, and microwaves. These devices have been shown to be capable of generating temperature elevations sufficient for thermal coagulation of tissue. Some characteristics of these devices, however, limit their ability to treat large volumes or regions close to important anatomical structures. High temperatures (>90° C.) close to the device surface often leads to undesirable physical effects of charring or vaporization in tissue. Inadequate heating can occur at the target boundary due to rapid decreases in deposited power with increasing distance from the device. A common characteristic among existing interstitial devices is the shape of the spatial heating pattern, usually spherical or ellipsoidal. This property makes the treatment of asymmetrically shaped volumes of tissue difficult. The goal with interstitial thermal devices is to deliver a heating pattern which is as uniform as possible to the entire target volume of tissue, while avoiding excessive or inadequate heating.
The ability to generate rapid, localized temperature increases in tissue has led to the development of focused ultrasound as a method to treat tumors. Magnetic resonance (MR) imaging is well suited for use in conjunction with high intensity ultrasound as a means of treatment guidance and monitoring. MR-derived information can indicate beam position, tissue temperature, and can distinguish regions of thermal coagulation (McDannold et al., 1998; de Poorter et al., 1996; Chung et al., 1996). The feasibility of MRI-guided therapy with high intensity ultrasound has been demonstrated (Hynynen et al., 1996).
High intensity ultrasound treatment requires the coagulation of all the tissue within the tumor volume (Malcolm and ter Haar, 1996). In the case of a focused beam from an external transducer, multiple small lesions are placed throughout the target volume. For complete tumor coagulation, lesions must be closely spaced or overlapped, but gaps in coverage and unpredictable lesion formation can occur due to changes in the acoustic properties of heated tissue (Chen et al., 1997; Damianou et al., 1997).
A confounding factor, in the case of externally focused ultrasound is the heating of intervening tissue in the nearfield of the acoustic beam (Damianou and Hynynen, 1993). In the extreme case, this can result in burning of the skin (Rivens et al., 1996). To overcome this problem, sonications are separated by sufficient time for intervening areas to cool down, usually 1-2 minutes (Fan and Hynynen, 1996). This approach can reduce damage to intervening layers of tissue but treatment times become unacceptably long (1-2 hours). Transducer systems have, thus, been designed to coagulate larger volumes per sonication in an effort to reduce treatment times (Fjield et al., 1997; Ebbini and Cain, 1988; Lizzi et al., 1996, McGough et al., 1994).
A different approach is to use interstitial ultrasound heating applicators designed for insertion into tissue under image guidance, which deposit energy directly within a targeted region. The delivery of ultrasound is localized to the tumor, and the problem of heating intervening tissue layers is avoided. Interstitial transducers have been developed for a variety of applications including cardiac ablation (Zimmer et al., 1995), prostate cancer (Deardorff et al., 1998), and gastrointestinal coagulation (Lafon et al., 1998).
Scanning an acoustic beam permits the energy concentrated in the acoustic field to be distributed over a volume. This can result in more uniform heating of a larger region of tissue. The effects of scanning an acoustic beam for hyperthermia (Hynynen et al., 1986; Moros et al., 1988), and more recently for high intensity thermal coagulation (Chen et al., 1997) have been studied. At acoustic intensities sufficient for tissue coagulation, scanning generated continuous regions of thermal damage in excised liver specimens (Chen et al., 1997). This scanning technique is unsuitable for external ultrasound therapy due to excess nearfield heating, but is potentially well advantageous for interstitial ultrasound heating.
The main limitation with current interstitial devices is the output power of the transducers, due to their small size. High power is required to generate thermal coagulation in tissue within a reasonable time with a scanned acoustic beam. Effects of local blood flow could result in incomplete thermal coagulation if insufficient power is generated. With adequate power, however, the potential exists for the coagulation of large regions of tissue with interstitial ultrasound for treatment of tumors.
The theoretical heating patterns of single element and linear array transducers has been investigated in a previous study by Chopra et al. (2000). These calculations indicated the differences in the heating patterns from the two transducer designs, and highlighted the importance of achieving a high output acoustic power. However, there is a continuing need for a heating device which is able to deliver a uniform heating pattern to a target volume of tissue.
SUMMARY OF THE INVENTION
The present invention overcomes limitations of the prior art by providing an ultrasound heating applicator for thermal therapy of tissue. Preferably, an applicator according to the invention is compatible with imaging, more preferably MR imaging. Such an applicator is also preferably compatible with image-guided interstitial therapy, preferably of benign or malignant tissues. In its broad aspect the interstitial ultrasound applicator of the present invention is comprised of a transducer, preferably planar, with multiple acoustic matching layers enabling operation at a range of frequencies for optimal “control” of the depth of thermal coagulation.
In an embodiment of the present invention, an applicator has the capability for varying the frequency of
Bronskill Michael J.
Chopra Rajiv
Bereskin & Parr
Jaworski Francis J.
Pass Barry
Sunnybrook & Women's College Health Sciences Centre
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