GB2113099A - Apparatus for imaging and thermally treating tissue using ultrasound - Google Patents

Abstract

A selected portion of tissue to be treated is imaged using a pulse-echo ultrasound imaging system. This system comprises an ultrasound transducer which is driven during imaging at physioligically tolerable power levels. Once the imaging system is positioned to image the tissue portion to be treated, the power at which the transducer is driven is increased to a physioligically intolerable level which is sufficient to thermally treat that tissue portion. Following scarring, the tissue portion is again imaged to determine the effectiveness of the treatment. The preferred embodiment apparatus comprises a transducer-lens system for focusing ultrasound onto a preselected focal area, an imaging means for displaying images constructed from the echoes of said ultrasound, and a pulser means selectively operable at physioligically tolerable or intolerable power levels to selectively image or to cause thermal treatment of tissue in the focal area. <IMAGE>

Description

SPECIFICATION Apparatus for imaging and thermally treating tissue using ultrasound Field of the Invention The present invention relates to the field of ultrasonic thermal tissue treatment using ultrasound.

Background of the Invention The present invention relates to particular applications of body tissues scarring techniques, and, in particular, to techniques useful for varicose vein scarring. For general disclosures of ultrasonic tissue scarring, apparatus and techniques not relating to varicose veins, please refer to U.S.

Patent 3,735,755 and Hynyen et al. "Design of Ultrasonic Transducers for Local Hyperthermia", Ultrasound in Med. & Bio., 7:4, 397-402 (1981), which is hereby incorporated by reference.

Varicose veins are a significant problem to the medical community. Approximately 15% of the general population, and nearly 50% of the population over 50 suffer from varicose veins. In addition to the cosmetic defects caused by these veins, such veins are also associated with symptomatic pain, ulcers, edema, etc. Presently there are no simple, cheap, effective treatments of varicose veins despite the large need for such treatments. Such veins are normally 0.5 to 1.5 cm (.2 to .6 inch) in diameter, and only 0.6 to 1.3 cm (1/4 to 1/2 inch) beneath the skin.

For many years, it has been suggested that ultrasonic energy may be focused on to the vein to heat it so as to form a scar blocking the venous flow. Since there is not normally any intervening bone or gas that may impede ultrasound transmission, the principle behind this suggestion is exactly the same as a "burning glass" by which sunlight can be focused to an intense point. Except at focus, the energy density is not high enough to cause damage to the overlying and surrounding tissue, yet is great enough in the focal area to cause scarring and blockage of the venous vein flow. With the flow blocked, a vein shrivels up and disappears.

In principle, small hand-held applicators could be used in a rapid in-office procedure to replace present hospitalization for varicose veins. Since ultrasound has been suggested for use as a convenient method for destroying preselected structures or regions of body tissue since the 1 950's, it is not readily apparent why such techniques have not experienced any substantial degree of commercial success. As explained in "A Simple Method For Production of Trackless Focal Lesions With Focused Ultrasound: Physical Factors", by P. P, Lele, J. Physiol 160:494-512 (1 962), the theoretical basis behind such apparatus has long been well understood.In the intervening years, it has been suggested to use a variety of transducers of various shapes to produce lesions which would be suitable to block typical varicose veins. In recent years, Dr.

Rosenberg of Middlesex Hospital, New Brunswick, New Jersey and Dr. Welkowitz of the Rutgers Bio Engineering Department, New Brunswick, New Jersey, have investigated the use of focused ultrasound to destroy varicose veins. Drs.

Rosenberg and Welkowitz have suggested that a bowl shaped lead-zirconate titanate transducer of approximately 3 inch radius with a resonant frequency of approximately 1.4 MHz may be used with an appropriate electronic driver to produce a 500C rise in temperature in meat in 5 seconds.

Drs. Rosenberg and Welkowitz have calculated that the intensity of sound at the focus in such experiments was approximately 50 watts per cm2.

Drs. Rosenberg and Welkowitz have further reported that the focused sound wave is believed to be capable of destroying varicose veins by heating and denaturing the protein found in the endothelium thereof, and that this result is possible without undesirable effects of deep heating or surface heating. Using the abovedescribed transducer, the depth of the damaged region was reported to be 0.5 cm below the surface, and with appropriate timing, to have an area of 0.02 cm2. Since the depth of the focal region was found to be about the same as the depth of superficial veins, and since the damaged area was found to be small, Drs. Rosenberg and Welkowitz have concluded that a focused sound beam appears suitable to treat the veins.

Nonetheless, in preliminary studies with this system on some patients, erratic results were obtained when veins were excised and examined histologically. Such erratic results were attributed to poor coupling through the membrane at the front end of the transducer and transducer drift-off resonance as it heated, due to the high Q value of the subject transducer. Such erratic results are, of course, unacceptable in a therapeutic context.

Applicant has recognized that many of the shortcomings of prior art devices, such as the Rosenberg and Welkowitz device described above, may stem from the inability of such devices to accurately position and locate the transducer with respect to the tissue to be treated prior to the application of lesion producing energy to that tissue. Accordingly, a primary object of this invention is the provision of an apparatus capable of reliably producing lesions in a given target area.

Summary of the Invention The present invention provides a novel pulseecho ultrasound system for thermally treating tissue which is capable of sequentially functioning in imaging and lesion-producing modes, whereby the tissue to be treated is first located in the imaging mode, and then treated in the lesionproducing (therapeutic) mode by driving the ultrasound transducer at substantially elevated power levels. In accordance with the preferred embodiment, a high power, short ring down transducer having a cone angle of at least about 300, typically about 600, is employed which is focused on an ellipsoidal volume about 3 mm in diameter. A heat sink, such as a water bag is provided between the transducer and the skin to absorb heat from surface tissues. In one alternate embodiment both A-scan and B-scan displays are provided for use in the imaging mode.

The preferred method of this invention thus comprises imaging tissue in the vicinity of the specific tissue to be treated using a pulse-echo ultrasound imaging system comprising an ultrasound transducer driven at a physiologically tolerable power level. Once the portion of the tissue to be treated is located, an A-scan display is preferably used to confirm that the tissue is positioned directly in the focal area of the imaging system. The position of the imaging system relative to the portion of the tissue to be treated is then maintained while the transducer power is increased to levels at which treatment is effected.

In the preferred embodiment, such increases in transducer power are effected by driving the transducer at substantially higher amplitudes.

Following treatment, the transducer is again driven at a physiologically tolerable power level to image the treated portion of the tissue, preferably using the B-scan display for this purpose.

Accordingly, the system and methods of the present invention facilitate precise application of high-power ultrasound energy to that portion of body tissue to be treated. The use of a high-power, short ring down transducer for this purpose permits adequate resolution of the portion of tissue to be treated, while permitting the same transducer to be switched to its high-power, therapeutic mode to apply lesion producing energy.

Accordingly, a primary object of the present invention is a provision of an improved method and apparatus for thermally treating tissue using an external pulse-echo ultrasound system. These and other objects of the present invention will become apparent from the following more detailed description.

Description of the Drawing Figure 1 is a block diagram of the preferred embodiment apparatus of the present invention.

Description of the Best Mode of the Invention In view of the following description, one of ordinary skill in the art will recognize that various departures in the materials and methods described herein can be made without departing from the scope of the present invention, which is defined more particularly in the claims appended hereto.

The present invention provides a novel method for imaging and thermally treating selected body tissues using ultrasound, and more particularly, provides a novel method for treating varicose veins. Referring to Figure 1, tissue 100 in the vicinity of a varicose vein 102 disposed under the skin 103 is imaged using an ultrasound imaging system such as the ultrasound imaging system diagrammatically represented in Figure 1. In the preferred embodiment, the ultrasound imaging system is a pulse-echo imaging system comprising a high power, short ring down transducer 201, a low power pulser 203 and an imaging means such as an A-scan imaging system 207 and/or a B-scan imaging system 205. One of ordinary skill in the art will recognize that both A and B scans can be combined in single imaging system for simultaneous display at selected scan lines.In such instance, imaging systems 205 and 207 may be combined into a single unit. In order to focus ultrasound emitted from transducer 202 onto the portion of the vein to be treated, an ultrasound lens 209 should be interposed for focusing the ultrasound on the vein with a cone angle of at least about 300 preferably greater than 600. Interposed between the lens 209 and the skin 103 is a heat sink 211 which may be a water bag which draws heat from the surface of the tissue during lesion production in the high power mode.

One of ordinary skill in the art will recognize that good acoustic coupling between transducer 201, lens 209, heat sink 211, and the underlying body tissue 100 should be established during imaging and lesion production.

For varicose vein applications, it is preferred to provide a lens 209 which focuses the ultrasound on a focal area which is an ellipsoidal volume of approximately 3 mm in diameter, which is disposed at a depth of between about 1/4 to 1/2 inches (0.6 to 1 cm) below the skin surface. One of ordinary skill in the art will recognize that suitable ultrasound lens for this purpose will typically be coated to prevent multiple reflections which otherwise might interfere with the achievement of such a focal area.

In performing the.preferred varicose vein treatment, the imaging system is positioned to locate and image the portion 102a of the vein 102 to be treated. This portion of the procedure is conducted with the system switched through switch 213 into the imaging mode. Through switch 21 5, B-scan and/or A-scan imaging may be conducted during this phase of the procedure.

In either instance, these imaging systems include displays 219 or 221 having indicia 223a-b or 225a-b which represent the focal area of the transducer. It is presently preferred to use an A-scan type image in combination with a B-scan image to confirm that the transducer-lens assembly is properly focused so that focal area precisely coincides with the portion 102a of the vein to be treated. Once such imaging has been accomplished and the alignment of the transducer-lens assembly with respect to that vein portion assured, switch 213 may be thrown to place the apparatus into its lesion producing or high-power mode (as indicated by the dotted-line position for switch 213). In this mode, the high power pulser 227 will drive transducer 201 at a physiologically intolerable power level, such that thermal scarring of tissue in the focal area will occur. The amount of power necessary to damage tissue in a given area is dependent upon the volume of the focal area and the minimum temperature at which thermal scarring for the given tissue is likely to occur. In most instances, once the ellipsoidal volume of the focal area has been raised from 370C to about 600C, such scarring will definitely occur.

The bi-modal system of the present invention is made possible in part by the provision of a highpower, short-ring down transducer; For treating varicose veins, it is presently desired to provide at least a transducer having a ring down time to 20dB of no more than 3 microseconds and capable of providing at least 1 watt of acoustical power. It is presently anticipated that ring down times of greater than 3 microseconds will not be sufficient to achieve the degrees of resolution required in order to effectively image veins to be treated. Conversely, the provision of a transducer having a power rating of less than 1 watt of acoustic power should be avoided since such transducers are not believed to facilitate the rapid heating of tissue in the focal area without producing unacceptable degrees of consequential heating and damage of adjacent tissues.

Following lesion production, it is preferred to throw switch 213 into the imaging mode, and to examine the nature of a lesion produced during high-power operation. For this purpose the provision of a B-scan imaging system is presently preferred due to the inherent superiority of this system for this purpose.

One of ordinary skill in this art will recognize that various modifications may be made to the transducer-lens assembly described above, provided the performance criteria discussed above continue to be met. For example, instead of providing a fiat transducer which is coupled with a lens, a curved transducer may be provided (and the ultrasound lens eliminated) as long as such transducer meets the power and ring down requirements discussed above. Similarly, while cone angles of above about 600 are preferred, it is anticipated that adjustable cone angles may be used for treating tissue at various depths, provided in each instance the cone angle is sufficiently great to ensure that consequential damage of overlying tissues does not occur.

Those of ordinary skill in this art will recognize that various imaging systems may be utilized in accordance with the present invention, including sector scan or linear translation systems, and that such systems will be suitable, provided the degree of resolution of such systems is sufficient to facilitate the imaging of tissue portions which are no greater, and preferably somewhat smaller, than the effective lesion size to be produced during high-power operation.

As seen from the above, the present invention provides a superior method and apparatus for selectively scarring internal body tissues, such as varicose veins, The use of a dual mode system increases the likelihood that precise applications of lesion producing energy will produce therapeutic results.

Claims (9)

1. A pulse-echo ultrasound system for thermally treating selected tissue, comprising: (a) transducer means for producing and focusing ultrasound into a preselected focal area remote from said transducer means; (b) imaging means for displaying images corresponding to echoes of said ultrasound produced by said transducer means; and (c) pulser means for driving said transducer means selectively at a first physiologically tolerable imaging amplitude and a second physiologically intolerable amplitude sufficient to cause thermal treatment of tissue in said focal area.

2. An apparatus according to claim 1 wherein said transducer means comprises a transducer and a lens.

3. An apparatus according to claim 1 or 2 wherein the focal area is a generally ellipsoidal volume about 3 mm in diameter.

4. An apparatus according to any of claims 1 to 3 wherein the cone angle from said focal area to said transducer means is at least about 300.

5. An apparatus according to any of claims 1 to 4 wherein said transducer means comprises a high power, short ring down transducer.

6. An apparatus according to claim 5 wherein said transducer when driven by said pulser at said physiologically tolerable level has a ring down time of less than about 3 microseconds to 20dB.

7. An apparatus according to claim 5 or 6 wherein said transducer is driven to produce at least about 1 watt of acoustic power.

8. An apparatus according to any of claims 1 to 7 further comprising heat sink means disposed between said transducer means and said tissues for absorbing heat from the surface of said tissues.

9. An apparatus according to claim 8 wherein said heat sink means comprises a water bag.