ACOUSTIC DIVERSION OF EMBOLI

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application 60/923,159, filed April 11, 2007. This application is a continuation-in-part of U.S. Patent Application 10/597,801, filed in the national phase of PCT Patent Application PCT/1L05/00163, filed

February 9, 2005 (and published as WO 2005/076729), which is a continuation-in-part of U.S.

Patent Application 10/162,824, filed June 4, 2002 (now U.S. Patent 6,953,438), which is a continuation of PCT Patent Application PCT/IBOO/01785, claiming the benefit of U.S.

Provisional Patent Applications 60/169,226, filed December 6, 1999, and 60/190,839, filed March 20, 2000. The disclosures of all these related applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical devices and procedures, and specifically to devices and methods for controlling embolic flow in the bloodstream. BACKGROUND OF THE INVENTION

The above-mentioned U.S. Patent 6,953,824 describes ultrasonic devices that make use of acoustic pressure in preventing emboli from reaching the brain during invasive cardiological procedures, such as cardiovascular surgery. (The term "embolus," as used in the context of the present patent application and in the claims, refers to any abnormal particle circulating in the blood. Such particles may include, inter alia, cholesterol, platelet clumps, blood clots, calcium flecks, air bubbles, fat, and combinations of these components.) This patent describes various different devices for diverting emboli, including invasive devices that are designed for placement in the chest cavity, esophagus or trachea.

The above-mentioned PCT publication WO 2005/076729 describes a device for controlling a flow of emboli in the aorta of a patient. The device includes an ultrasonic transducer, which transmits an ultrasonic beam into the aorta in the vicinity of the great origin of the neck vessels. The ultrasonic transducer is driven to generate the ultrasonic beam at a frequency and power level sufficient to divert at least a target fraction of the emboli of a given type and size into the descending aorta and away from the great origins of the neck vessels leading to the brain.

SUMMARY OF THE INVENTION

Embodiments of the present invention that are described hereinbelow provide methods and devices for ultrasonic diversion of embolic flows with enhanced control, deliverability and versatility. In these embodiments, an ultrasonic transducer is introduced into the chest cavity in proximity to the aortic arch (either through the patient's open chest during surgery, or through a body passage in proximity to the aorta). The transducer transmits ultrasonic waves into the aorta so as to divert emboli into the descending aorta and thus protect the brain from ischemic damage. Some of these embodiments provide means and methods for positioning, aiming and controlling the transducer so as to increase the effectiveness of diversion of emboli while reducing undesired exposure of the blood and other tissues in the vicinity of the aorta to ultrasonic energy.

The devices and methods described hereinbelow are useful particularly in preventing neurological damage that may occur due to release of emboli during cardiac surgery and other invasive cardiological procedures. The principles of the present invention may also be applied, however, in diversion of embolic flow in other locations, including, but not limited to, the carotid bifurcations.

There is therefore provided, in accordance with an embodiment of the present invention, a method for controlling flow, including: detecting an embolus in a stream of blood; and responsively to detecting the embolus, directing an ultrasonic beam into the blood so as to divert the embolus.

In some embodiments, detecting the embolus includes detecting ultrasonic energy reflected from the embolus. In one embodiment, detecting the ultrasonic energy includes directing a sequence of ultrasonic pulses into the blood, and processing the reflected ultrasonic energy due to the pulses in order to detect the embolus. Processing the reflected ultrasonic energy may include detecting a Doppler shift due to motion of the embolus in the blood.

Alternatively or additionally, processing the reflected ultrasonic energy includes defining a range of transit times of the ultrasonic pulses responsively to anatomical characteristics associated with a blood vessel, and detecting the embolus responsively to reflected pulses detected within the range.

In one embodiment, directing the beam includes operating a transducer in a diversion mode so as to generate the ultrasonic beam for diverting the embolus, and detecting the

ultrasonic energy includes operating the transducer in a low-power mode so as to direct the ultrasonic energy into the blood and detect the reflected ultrasonic energy. In another embodiment, directing the beam includes actuating a first transducer to generate the ultrasonic beam for diverting the embolus, and detecting the ultrasonic energy includes operating a second transducer to direct the ultrasonic energy into the blood and detect the reflected ultrasonic energy.

In disclosed embodiments, detecting the embolus includes detecting the embolus in a blood vessel, and directing the ultrasonic beam includes directing the ultrasonic beam into the blood vessel to divert the embolus. In some embodiments, the blood vessel includes an aortic arch of a patient, and directing the ultrasonic beam includes diverting the embolus into a descending aorta of the patient. Optionally, directing the ultrasonic beam includes generating the ultrasonic beam using an ultrasonic transducer in proximity to the aortic arch, and aligning the ultrasonic transducer with an axis of the aortic arch by sensing ultrasonic energy reflected from a vicinity of the aortic arch. Directing the ultrasonic beam may include deploying an ultrasonic transducer adjacent to the aortic arch in a chest cavity of the patient during surgery. Alternatively, directing the ultrasonic beam includes deploying an ultrasonic transducer in a body passage adjacent to the aortic arch.

In one embodiment, directing the ultrasonic beam includes generating an axially nonsymmetrical beam using an elongated transducer. Additionally or alternatively, directing the ultrasonic beam includes homogenizing the ultrasonic beam.

There is also provided, in accordance with an embodiment of the present invention, a method for controlling flow, including: directing an ultrasonic beam from a transducer into an aortic arch of a patient; sensing ultrasonic energy reflected from a vicinity of the aortic arch; and responsively to the reflected ultrasonic energy, aligning the transducer with an axis of the aortic arch.

In a disclosed embodiment, aligning the transducer includes aiming the transducer so as to maximize a Doppler shift in the reflected ultrasonic energy. Additionally or alternatively, aligning the transducer includes defining a range of transit times of the ultrasonic energy responsively to anatomical characteristics of the vicinity of the aortic arch, and aiming the transducer so as to reduce reflections of the ultrasonic energy that fall outside the range.

There is additionally provided, in accordance with an embodiment of the present invention, a method for controlling flow, including: driving an ultrasonic transducer to direct an ultrasonic beam into a stream of blood so as to divert emboli within the blood; and homogenizing the ultrasonic beam.

In one embodiment, homogenizing the ultrasonic beam includes driving the ultrasonic transducer with a multi-frequency driving current. In another embodiment, homogenizing the ultrasonic beam includes mechanically shifting the ultrasonic transducer while driving the ultrasonic transducer to generate the ultrasonic beam. . There is further provided, in accordance with an embodiment of the present invention, a method for controlling flow, including: introducing a probe into a body passage of a patient to a location that is adjacent to an aortic arch of the patient; and directing an ultrasonic beam from the probe into the aortic arch so as to divert emboli from the aortic arch into a descending aorta of the patient.

In some embodiments, the probe includes at least one ultrasonic transducer, and introducing the probe includes positioning the at least one ultrasonic transducer in the probe at the location. In one embodiment, the at least one ultrasonic transducer includes a plurality of ultrasonic transducers at a distal end of the probe, and positioning the at least one ultrasonic transducer includes passing the distal end in a collapsed state through the body passage to the location adjacent to the aortic arch, and expanding the distal end of the probe at the location so as to deploy the ultrasonic transducers for diversion of the emboli. The distal end of the probe may comprise a balloon, to which the ultrasonic transducers are coupled and which is configured to be inflated via the probe so as to expand at the location. In an alternative embodiment, the probe includes an ultrasonic waveguide, and directing the ultrasonic beam includes conveying the beam through the waveguide to the location.

Introducing the probe may include positioning the probe in an esophagus or a superior vena cava or neighboring veins of the patient. There is moreover provided, in accordance with an embodiment of the present invention, apparatus for controlling flow, including: a sensing circuit, which is configured to detect an embolus in a stream of blood; and

at least one ultrasonic transducer, which is configured to direct an ultrasonic beam into the blood responsively to detection of the embolus, so as to divert the embolus.

There is furthermore provided, in accordance with an embodiment of the present invention, apparatus for controlling flow, including: at least one ultrasonic transducer, which is configured to direct an ultrasonic beam into an aortic arch of a patient and to sense ultrasonic energy reflected from a vicinity of the aortic arch; and a control unit, which is ¹ configured to provide, responsively to the reflected ultrasonic energy, an indication of an optimal alignment of the transducer with an axis of the aortic arch. There is also provided, in accordance with an embodiment of the present invention, apparatus for controlling flow, including: at least one ultrasonic transducer, which is configured to direct an ultrasonic beam into a stream of blood so as to divert emboli within the blood; and means for homogenizing the ultrasonic beam. There is additionally provided, in accordance with an embodiment of the present invention, apparatus for controlling flow, including: a probe, which is configured to be introduced into a body passage of a patient to a location that is adjacent to an aortic arch of the patient; and at least one ultrasonic transducer, which is configured to direct an ultrasonic beam from the probe into the aortic arch so as to divert emboli from the aortic arch into a descending aorta of the patient.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic, pictorial illustration of a system for diversion of emboli during a cardiac surgical procedure, in accordance with an embodiment of the present invention;

Fig. 2 is a schematic side view of the chest cavity of a patient during cardiac surgery, showing details of the placement of an ultrasonic device adjacent to the aorta, in accordance with an embodiment of the present invention; Fig. 3 is a schematic sectional view of structures within the chest cavity of a patient, illustrating alignment of an ultrasonic device with the aorta, in accordance with an embodiment of the present invention;

Fig. 4 is a schematic, pictorial illustration showing an ultrasonic transducer, in accordance with an embodiment of the present invention;

Fig. 5 is a schematic, pictorial illustration showing an ultrasonic transducer array, in accordance with an embodiment of the present invention; Fig. 6A is a schematic sectional view of the upper body of a patient, showing placement of an ultrasonic transducer in a body passage, in accordance with an embodiment of the present invention;

Fig. 6B is a schematic frontal view of the heart of a patient, showing placement of an ultrasonic catheter in a body passage, in accordance with another embodiment of the present invention;

Fig. 7A is a schematic side view of an ultrasonic catheter in a collapsed state, in accordance with an embodiment of the present invention;

Fig. 7B is a schematic sectional view of the ultrasonic catheter of Fig. 7A, in accordance with an embodiment of the present invention; Fig. 7C is a schematic sectional view of an ultrasonic catheter in a collapsed state, in accordance with another embodiment of the present invention;

Fig. 8A is a schematic side view of the ultrasonic catheter of Fig. 7A in an expanded state;

Fig. 8B is a schematic sectional view of the expanded ultrasonic catheter of Fig. 8A; and

Fig. 9 is a schematic side view of an ultrasonic catheter, in accordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a schematic, pictorial illustration of a system 20 for diversion of emboli during an invasive procedure performed on a heart 22 of a patient 24, in accordance with an embodiment of the present invention. In this example, a surgeon 26 has opened the patient's chest by performing a median sternotomy, and has then attached a retractor 28 to spread the two parts of the sternum. The surgeon then cuts through the pericardium to expose the heart, as is known in the art. Before proceeding with the actual procedure on the heart, the surgeon places next to the aorta, near cranial end of the incision, an ultrasonic device 30 for diversion of emboli. Device 30 is deployed and operated to direct an ultrasonic beam into the aorta in such a way as to divert emboli in the aorta away from the great origins of the neck vessels.

Certain structural and functional characteristics of device 30 are shown in detail in the figures that follow. Other aspects of this device are described in the above-mentioned PCT publication WO 2005/076729.

Fig. 2 is a schematic side view of a chest cavity 32 of patient 24, showing details of the mounting and operation of device 30, in accordance with an embodiment of the present invention. Device 30 is placed against an aorta 36 of the patient, in proximity to the bifurcations at the great origins of neck vessels 38, which include the innominate artery, the left common carotid artery and the left subclavian artery.

Device 30 comprises an ultrasonic sensing transducer 42 and an ultrasonic diverting transducer 44, whose respective functions are described further hereinbelow. Although transducers 42 and 44 are shown, for the sake of visual clarity, as being located side-by-side within the same housing, other configurations of these transducers may also be used. For example, transducers 42 and 44 may be concentric. As another example, transducers 42 and 44 may be located in separate housings, with transducer 42 upstream so as to detect emboli in the ascending aorta near the aortic valve, and transducer 44 further downstream so as to divert the emboli in the aortic arch. Furthermore, although only a single sensing transducer and a single diverting transducer are shown in Fig. 2, system 20 may alternatively comprise multiple transducers of either or both types. Further alternatively, transducer 44 may carry out both the sensing function and the diversion function. Each transducer 42, 44 may comprise a piezoelectric element, for example, or an array of such elements. Transducers 42 and 44 may be coupled to aorta 36 through an acoustic coupler 46, in order to provide efficient energy transfer from the transducer to the blood vessel.

Fig. 2 also shows the trajectory of a stream of emboli 48 emitted through an aortic valve 50 (or possibly detached from the ascending aorta) into aorta 36. Actions of surgeon 26 during cardiac surgery, such as cannulation, de-cannulation and cross-clamping, are particularly likely to cause such emboli to be released into the bloodstream. Device 30, however, is aimed so that an acoustic beam generated by transducer 44 exerts pressure on emboli 48 toward the descending aorta and away from the origins of great vessels 38. Thus, the emboli are diverted away from the neck vessels, and the brain of patient 24 is protected from neurological damage that could result if emboli 48 were to pass through one of vessels 38 and lodge in smaller blood vessels in the brain.

Although the inventors have found the location and orientation shown in Fig. 2 to be optimal for diverting emboli into the descending aorta, other configurations can also be effective and are considered to be within the scope of the present invention. For example, ultrasonic transducers may be positioned at other locations and orientations along aorta 36 or in neighboring body passages, or in proximity to other blood vessels, in addition or alternatively to the location and orientation shown in Fig. 2. Furthermore, other methods may be used to deliver and place the ultrasonic transducer in the desired location. AU such methods are considered to be within the scope of the present invention. Some alternative methods, in which an ultrasonic probe is introduced via natural body passages, are described hereinbelow. Returning now to Fig. 1, it can be seen that device 30 is connected by cabling 54 to a console 56. The console comprises a power driver circuit 58, which generates radio frequency (RF) energy for driving device 30, typically at the appropriate optimal frequency for transducer 44. In some embodiments, the frequency generated by circuit 58 is in the range of 0.5 MHz or higher, with an electrical power output of at least 5 W for an unfocused beam. Alternatively, other beam configurations, frequencies and power levels may be used, in accordance with therapeutic needs and technical constraints, as explained in the above-mentioned PCT publication WO 2005/076729. The frequency and power level are typically chosen by balancing the target particle size and the desired diversion percentage against the possible side effects of excessive tissue heating. The operation of system 20 is controlled by a control unit 62, which typically

^■ comprises a microprocessor with suitable interface and logic circuits for interacting with the other components of the system. Typically, the control unit activates and de-activates driver circuit 58 in accordance with operational parameters that are input to the system via a user interface 64. When the driver circuit is activated, it may output either continuous wave (CW) or pulsed excitation (with a duty cycle less than 100%) to transducer 44.

In order to reduce tissue heating, it is desirable that transducer 44 be controlled to emit acoustic energy only when required, rather than operating continuously throughout the surgical procedure. In order to control transducer 44 in this manner, transducer 42 senses the presence of emboli 48 in aorta 36. Typically, for purppses of detection of emboli, transducer 42 operates continuously but at much lower power than transducer 44, which requires higher power to achieve effective diversion of emboli. Transducers 42 and 44 may operate at the same frequency or at different frequencies.

The output of transducer 42 is processed by a sensing circuit 60 in order to detect an embolic "signature" in the signals that are output by the transducer. The sensing circuit typically comprises an analog/digital converter and digital processing logic for digitizing and analyzing the signals. The logic may be realized, at least in part, in software running on a programmable processor, which may be the processor of control unit 62. Alternatively or additionally, for rapid detection of emboli, at least some of the logic in circuit 60 may be implemented in dedicated or programmable hardware signal processing components.

When sensing circuit 60 detects an embolic flow in this manner, it notifies control unit 62, which activates driver circuit 58 to drive transducer 44 to deflect the emboli. The driver circuit may be activated for a predetermined period, typically in the range of 0.01-10 sec, or for as long as the embolic flow continues in the aorta. Typically, transducer 44, when activated to divert emboli, emits ultrasonic energy at a frequency of 0.5 MHz or higher (although lower frequencies may be used in some cases), depending on the sizes of the emboli that are to be diverted, as explained in the above-mentioned PCT publication WO 2005/076729. The average ultrasonic intensity emitted by transducer 44 during activation is typically at least 0.3 W/cm ² , but higher or lower intensity may alternatively be used depending on beam geometry and application requirements. Once the appropriate period has elapsed, the control unit shuts off the power driver circuit until sensing circuit 60 detects another possible embolus. Alternatively, as noted earlier, transducer 44 may operate continuously at low power in order to perform the sensing function, switching to high-power operation for deflection of emboli when an embolic flow is detected.

The detection-driven approach that is described above permits the high-power diverting beam from transducer 44 to be inactive during most of the procedure, and to be activated for relatively short periods of time when embolic matter is detected in the aorta (typically in the ascending aorta or the aortic arch, as shown in Fig. 2). This approach minimizes the acoustic energy imparted to the body of the patient and minimizes heating of tissues that absorb the acoustic energy, as well as heating of device 30. When transducer 44 is activated, it may then operate at relatively high power for short periods of time, resulting in more efficient diversion of emboli without compromising the safety of the patient (since the overall average power that is transferred to the patient remains low) and without requiring active control by the operator of system 20. Furthermore, sensing circuit 60 may extract information regarding the size and composition of emboli from the signals output by the sensing transducer, and control unit 62

may use this information in controlling parameters of the diverting beam, such as power, frequency and/or duty cycle. For example, a gaseous embolus may require less diverting power than an embolus having a solid form.

In one embodiment, transducer 42 operates in Doppler mode, and sensing circuit 60 analyzes the Doppler signature of the signals received from the transducer. When an embolus passes through the vessel, the reflection of the sensing beam from the embolus causes a significant change in the Doppler signature. Sensing circuit 60 analyzes the Doppler signature in order to identify the embolic flow and discriminate (in real time) between signals due to emboli and artifacts. For the purpose of Doppler-based detection of emboli, it is desirable that transducer 42 emit a sequence of pulses of ultrasonic energy at a frequency in the range of 1-4 MHz, with a pulse repetition frequency (PRF) of at least 500 pulses/sec. Alternatively, other frequency ranges and repetition rates may be used. To avoid tissue heating, the intensity of this sonication should be low, generally with an average power no greater than about 0.72 W/cm ² (which is the maximum value allowed by safety standards for diagnostic ultrasound devices). Emboli typically travel through the aorta at a velocity of about 1 rnm/ms. To ensure timely activation of transducer 44, the emboli should be detected within roughly the first 10 mm of their travel through the aortic arch. To meet this criterion, sensing circuit 60 must detect the emboli within about 10 ms of their entry into the sensing region. This criterion, at the PRF of 500 pulses/sec, means that the circuit typically has a time span of five pulses in which to detect an embolus.

Alternatively, system 20 may incorporate other methods for detection of emboli, such as sonar, optical or electromagnetic methods, as are known in the art. Such methods may be used in place of or in addition to the ultrasonic detection methods that are described herein. All such detection methods, when coupled with activation of diverting transducer 44, are considered to be within the scope of the present invention.

In typical operation of system 20, as noted above, sensing circuit 60 receives and monitors the Doppler signatures of the reflected ultrasonic waves that are sensed by transducer 42. The sensing circuit continuously receives the background Doppler signature of the flowing blood, which is typically in the audible regime. When a moving object, such as an embolus, moves through the sensing field of transducer 42, the amplitude of the Doppler signal will typically increase and will exhibit a chirp. When sensing circuit 60 detects a change of this

sort in the Doppler signal that is over a certain preset threshold, control unit 62 actuates diverting transducer 44.

Typically, the threshold for embolic detection is set to a low value, so that the sensitivity of detection of emboli will be high, because false positive events (detecting an 5 embolus when no embolus is actually present) are generally , preferable to false negatives (which could allow an embolus to reach the brain). Other criteria for discriminating between true emboli and artifacts may include the duration of the change in the amplitude and/or frequency of the Doppler signature and the existence of reflections having similar characteristics in two (or more) successive pulses.

10. . . For example,, a change, in the Doppler signal may be considered to be indicative of an embolus if the peak Doppler frequency shift is higher than 500 Hz, whereas any signal with peak frequency shift lower than 250 Hz is considered to be an artifact. For peak frequencies between these two values, sensing circuit 60 may consider the transit time, i.e., the time delay between the ultrasound pulse transmitted by transducer 42 and the reflected pulse that 15 exhibited the change in the Doppler signal. The anatomical characteristics of the aorta define a range of transit times that may feasibly correspond to reflections from emboli in the aorta. Very short delays are thus considered to be artifacts. The Doppler signature may be considered to be due to an embolus, for example, only if the delay is longer than 4 ms. These criteria and numerical values are cited here by way of example only, and not limitation. Alternative 0 detection and artifact rejection algorithms will be apparent to those skilled in the art and are considered to be within the scope of the present invention.

Delay-based discrimination is useful because a simple reflection pulse from a location that is well within the lumen of the aorta (where nothing but blood should exist) is likely to be indicative of an embolus. The distance between transducer 42 and a reflecting object, such as 5 an embolus, that caused a certain Doppler signature in the reflected ultrasonic wave is given by the product of the time of flight of the wave by the sound velocity divided by two. (The velocity of sound in blood is roughly 1480 m/sec.) The range of delays that are considered to be indicative of possible passage of emboli through the field of view of the transducer may be preset, or it may alternatively be determined empirically for each patient upon placement of0 device 30.

In an embodiment of the present invention, sensing circuit 60 and control unit 62 perform a preliminary analysis of the reflected signals following deployment of device 30

during a calibration period before actual surgery begins on heart 22. Emboli are not expected during this period. Since blood does not generate substantial reflections, the signals received by transducer 42 during the calibration period can be assumed to be reflected pulses from the distal wall of aorta 36 or other remote tissues. The sensing circuit records these "baseline reflections" for later reference. Subsequently, during the surgical procedure, any reflected pulse that is received by transducer 42 at a delay that is shorter than (or otherwise different from) the baseline reflections indicates the existence of a foreign body somewhere in the lumen of the aortic arch, i.e., a suspected embolus.

In some cases, there may be stationary objects within the aortic lumen, such as an aortic cannula. In order to discriminate between suspected emboli and such stationary objects, sensing circuit 60 may compare the reflections from two or more successive pulses. If the a certain reflection signal arrives at approximately the same delay following each of two or more successive pulses, the sensing circuit will treat this signal as being due to reflections from a stationary object and will therefore disregard it. On the other hand, if the delays in the reflection signal differ by at least a threshold time difference, the reflections will be assumed to have originated from a moving embolus, and transducer 44 will be activated.

Fig. 3 is a schematic sectional view of the chest cavity of patient 24, taken along a line E[-iπ in Fig. 2, showing the placement of device 30 relative to aorta 36, in accordance with an embodiment of the present invention. This embodiment shows how the diagnostic detection capabilities of system 20 can be used in optimally positioning and orienting of device 30, so as to maximize the efficacy of diversion and minimize undesired exposure of other organs in the vicinity of the aorta (such as a spinal column 70) to acoustic energy. In this embodiment, it is assumed that transducer 44 is used for both the detection and the diversion functions that are described above. As illustrated in Fig. 3, it is desirable that device 30 be positioned so that an acoustic beam 68 emitted by transducer 44 is approximately parallel to the lumen axis of the aortic arch. In this way, the force of the acoustic beam will divert embolic matter efficiently downstream along the direction of the average flow vector in the aorta. Furthermore, when the acoustic beam is parallel to the axis of the aortic arch as shown in the figure, the residual energy of the beam that is not absorbed in the aorta will be deposited in the lung (not shown) without impinging on the esophagus, trachea or spine. The lung tissue has generally a more

favorable absorption capability and tolerance to heating and other possible consequences of ultrasonic energy than do these other organs.

To position the transducer so that beam 68 is parallel to the aortic arch, transducer 44 is operated in Doppler detection mode, as described above. Sensing circuit 60 analyzes the Doppler signature of the flowing blood within the aorta by causing transducer 44 to emit short ultrasound pulses into the aorta and to receive the echo pulses due to the flowing blood and stationary boundaries of the aorta and other organs. Reflections from these stationary boundaries do not cause any Doppler shift in the reflected pulses. The Doppler signature of the received echo pulse provides information about the component of blood velocity inside the aorta that is parallel to the direction of the beam.

Because the average velocity of the blood in the aortic arch is parallel to the lumen of the aortic arch, the maximum Doppler shift will be observed when beam 68 is approximately parallel to the lumen of the aortic arch, i.e., parallel to the average blood velocity in the aortic arch. Thus, sensing circuit 60 and control unit 62 may guide operator 26 in positioning the transducer, based on the Doppler signature, until the maximal Doppler shift is obtained. The resulting orientation will correspond to the situation shown in Fig. 3, in which the beam is roughly parallel to the aortic arch.

Optionally, sensing circuit 60 can be tuned to consider pulses reflected from specific locations in the aortic arch, by taking into account only pulses that are received within a certain range of time delays relative to the emitted pulse. This time range corresponds, as explained above, to a specific spatial range inside the aorta.

Additionally or alternatively, sensing circuit 60 can provide information about anatomical structures that may be on the way of beam 68, such as spinal column 70. The existence of a bone in the beam path is detected on the basis of reflected pulses that are received by transducer 44 at relatively long delays, corresponding to reflectors at distances greater than the length of the aortic arch. For example, in the example shown in Fig. 3, the length of the aortic arch is typically about 100 mm, meaning that an ultrasonic pulse reflected from a location within the aortic arch will return to the transducer with a delay no greater than about 133 μs relative to the emitted pulse. Any strong reflection that is received by the transducer at a delay significantly longer than 133 μs can be assumed to arise from a major reflector distal to the aortic arch, such as a bone. The transducer may then be realigned to eliminate this reflection. On the other hand, if beam 68 is fully absorbed in the lung (which is

an optimal absorber of ultrasonic energy), there will be no strong reflections at delays longer than about 133 μs. This principle may be applied to align the transducer so as to avoid unwanted ultrasound exposure not only to bony structures, but also to other solid tissues, such as the esophagus and trachea. Fig. 4 is a schematic, pictorial illustration showing an ultrasonic transducer 80, in accordance with an alternative embodiment of the present invention. This transducer can be used in place of transducer 44 in device 30 or in another ultrasonic device for diversion of emboli in the bloodstream. Transducer 80 is long and narrow, as shown in the figure. The narrow shape of transducer 80 can be advantageous in system 20 (and in other applications in which space is at a premium) in order to meet geometrical constraints that may be imposed by the patient's anatomy and/or other devices that are used in the procedure.

Transducer 80 has a cylindrical symmetry profile 82. Consequently, an ultrasonic beam 84 emitted by the transducer is axially non-symmetrical in both shape and divergence profile: The beam has relatively low divergence in the long (Y) dimension of the transducer and high divergence in the narrow (X) dimension. The low divergence in the long dimension means that the overall intensity of the beam drops relatively slowly with distance. Furthermore, despite the elongated shape of the transducer, the height and width of the beam will be roughly equal within a certain range of distances from the transducer. This range can be set by appropriate choice of the dimensions and profile of the transducer. The transducer may alternatively have an ellipsoidal or other profile that is not flat or circularly symmetrical.

Fig. 5 is a schematic, pictorial illustration showing an ultrasonic transducer array 90, in accordance with another embodiment of the present invention. Array 90 has a long, narrow overall shape, like transducer 80, and may be used in a similar manner to transducer 80. The array comprises multiple transducer elements 92. The relative phases of the driving currents that are applied to the transducer elements can be controlled in order to shape and steer the beam emitted by the array toward the appropriate region within the aorta.

Furthermore, transducer array 90 may be driven by power driver circuit 58 with a multi-frequency driving current, wherein each transducer element 92 is driven at a different, respective frequency. The emitted beam will then be a superposition of the individual beams that are emitted by transducer elements 92, resulting in a smoother average energy field. Homogenizing the beam in this manner reduces the possible creation of hot spots in the beam, which may otherwise reduce the efficacy of diversion and may cause tissue damage.

Even when only a single transducer is used, as in the preceding embodiments, the beam may be homogenized (so that hot spots are suppressed) by broadening the frequency range of the emitted ultrasonic energy. One technique that may be used for this purpose is pulsed excitation of the transducer, as described in the above-mentioned PCT publication WO 2005/076729. Another possibility is to modulate the frequency of the diverting beam, using a time-dependent power function of the form P(t)=P _o sin(f(t)*t), for example, wherein f(t) is the frequency. Any suitable time-dependency may be used, either continuous, such as a sinusoidal or linear dependence, or discrete. These embodiments require that power driver circuit 58 have the appropriate temporal and frequency capabilities and that the transducer itself have sufficient efficiency over the entire range of frequencies.

As another alternative, the ultrasonic beam may be homogenized by mechanical means. For example, as shown in Fig. 5, the ultrasonic transducer or transducer elements may be mounted on a moving base 94, which continuously shifts the transducers at a rate that is faster than the time-scale of temperature elevation in the tissue. For this purpose, base 94 may provide a vibrating surface, with vibrational amplitude on the order of the spatial scale of the inhomogeneity of the ultrasonic beam (typically a few millimeters). Other methods for moving the transducers will be apparent to those skilled in the art and are considered to be within the scope of the present invention.

Fig. 6A is a schematic section view of the upper body of a patient, showing placement of an ultrasonic transducer 104 in a body passage 102, in accordance with an alternative embodiment of the present invention, hi some applications, such as in minimally-invasive surgical procedures, placement of an ultrasonic device (such as device 30) in the chest cavity is not feasible. Even in open-chest procedures, there may not always be space to place a transducer in the chest cavity. It may therefore be advantageous, instead, to introduce a transducer into a body passage that is adjacent to the aorta, or to another blood vessel in which emboli are to be diverted.

In the example shown in Fig. 6 A, a flexible probe 100 is inserted through the patient's mouth into the esophagus. Because of the proximity between the esophagus and aorta 36, transducer 104 at the distal end of probe may be positioned adjacent to the aortic arch. The transducer may then be actuated (either in conjunction with detection of emboli, as described above, or independently of such detection) to divert emboli into the descending aorta.

Alternatively, probe 100 may be introduced into another body passage, such as the trachea or a suitable blood vessel. In this latter case, the probe may comprise a venous catheter, which is introduced, for example, through the jugular or femoral vein into the vena cava or neighboring veins and sonicates the aorta from a suitable location in the vena cava or neighboring veins adjacent to the aortic arch. In another embodiment (not shown in the figures), the ultrasonic probe may sonicate the aorta from a location in an artery, possibly from a location adjacent to the aorta arch within the aorta itself.

Fig. 6B is a schematic frontal view of heart 22, showing placement of an ultrasonic catheter 105 in a superior vena cava 106 of the patient, in accordance with another embodiment of the present invention. The catheter is positioned adjacent to aorta 36, so that transducer 104 may direct ultrasonic energy into the aortic arch and divert emboli toward the descending aorta. The methods of detection-driven actuation of transducer 104 that are described above may likewise be used in this embodiment and in the other alternative embodiments that are described herein. The use of detection-driven actuation in such embodiments is considered to be within the scope of the present invention.

Reference is now made to Figs. 7A, 7B, 7C, 8A and 8B, which schematically illustrate a catheter 110 for use in an intravascular application, in accordance with embodiments of the present invention. This catheter may be positioned, for example, in the superior vena cava, as shown in Fig. 6B or in other branching veins. Figs. 7A and 8A are side views of catheter 110, in collapsed and expanded states, respectively. Figs. 7B and 8B are sectional views through a distal end 112 of catheter 110, taken along respective lines VIIB-VIIB and VELA- VDIB in Figs. 7A and 8A. Fig. 7C is a sectional view of catheter 110 in the collapsed state in an alternative configuration.

Distal end 112 of catheter 110 contains an array of ultrasonic transducers 114. (Alternatively, the catheter may contain a single transducer, such as a shaped transducer of the type shown in Fig. 4.) In order to pass through the relatively narrow jugular or femoral vein, the distal end is initially collapsed, as shown in Figs. 7 A, 7B and 7C, so that transducers 114 are closely bunched together. The transducers may be coupled to the surface of an expandable balloon, which is deflated during insertion of the catheter, so that the transducers are folded together compactly. In Fig. 7B, the transducers are fixed to the interior surface of a balloon 116, whereas in Fig. 7C, the transducers are fixed on the exterior surface of a balloon 120. The structure of transducers mounted on the surface of the balloon can optionally be encapsulated

by additional external covering 122 (depicted in Figs. 7B and 7C as a circle) in which cooling liquid can flow in and out. Additionally or alternatively, cooling liquid can also through inner balloon 116 or 120 on which the transducers are mounted. Further alternatively, if the transducers are to be operated in a gas-backed mode, a gas, such as CO ₂ , can flow into and out of the inner balloon.

When distal end 112 reaches the desired location adjacent to the aortic arch in the superior vena cava or neighboring veins, distal end 112 is expanded, whereby transducers 114 are deployed side by side, as shown in Figs. 8 A and 8B. Such expansion may be achieved, for example, by inflating balloon 116 or 120, in a manner similar to the inflation of expanding balloon catheters that are known in the art. Covering 122 stretches over the transducers accordingly. Alternatively, as noted above, covering 122 may itself be inflated with a fluid, which fills the space between the covering and the balloon (so that the space will typically be wider than is shown in Fig. 8B). This fluid can be useful in cooling transducers 114 and in protecting the vein and other nearby tissues from the heat generated by the transducers. Further alternatively, distal end 112 may comprise a framework (not shown) made from an elastic, self-expanding material, such as Nitinol, which is constrained to remain collapsed during insertion of the catheter and is then released to expand in the superior vena cava (SVC).

Expansion of distal end 112, as shown in Figs. 8 A and 8B, causes transducers 114 to spread apart over a relatively large area along the wall of the vena cava that is adjacent to the aorta. Although the transducers are shown in Fig. 8A as a single row of elongated elements, the transducers may alternatively comprise multiple rows of elements (typically shorter elements), arranged at successive axial locations. This latter option may enhance the maneuverability of the catheter in the collapsed state of Fig. 7A. After distal end 112 has expanded, transducers 114 are then driven to emit a beam of ultrasonic energy into the aorta and thus to divert emboli into the descending aorta, as explained above. This beam may operate either continuously or intermittently, in response to detection of emboli, as explained above. (In this latter configuration, one or more of transducers 114 may perform the function of detecting emboli, in addition to or instead of diversion.) Spreading transducers 114 apart permits high overall ultrasonic power to be delivered to the aorta for effective diversion of emboli without excessive heating of the vessel walls at any one point, while permitting the catheter to be delivered via relatively narrow veins

(such as femoral or jugular veins). The catheters may be driven together by the same driving signal, or they may alternatively be driven by individual signals, as a phased array, to facilitate shaping and steering of the collective ultrasonic beam.

Fig. 9 is a schematic side view of an ultrasonic catheter 130, in accordance with an alternative embodiment of the present invention. In this embodiment, instead of deploying transducers at the distal end of the catheter, a transducer 134 is located at the proximal end of catheter 130, which comprises an ultrasonic waveguide 132 for conveying the ultrasonic energy from the proximal to the distal end. Waveguide 132 may, for instance, comprise a hollow shell, made of a suitable plastic, which is filled with a coupling material, such as a liquid, gel or polymer, having low acoustic attenuation and acoustical properties similar to the target tissue of patient, as described in the above-mentioned PCT publication WO 2005/076729. For example, the waveguide material in the catheter may comprise degassed water or acoustic gel.

Catheter 130 may comprise an acoustic mirror 138 at its distal end for the purpose of directing an ultrasonic beam 140 in the proper direction into the aorta. Optionally, the distal tip of the catheter containing the mirror may comprise a balloon 136, which is configured to expand when the distal end of the catheter is in place, in a manner similar to that illustrated in Figs. 8A and 8B.

Although the above embodiments refer to specific sorts of transducers that are inserted into certain body passages, the principles of these embodiments may likewise be applied in diversion of emboli using transducers of other types, in these and other passages that are in proximity to the aorta and other major arteries. Furthermore, the principles of detection-driven acoustic diversion of emboli may be applied to streams of blood not only in blood vessels, as in the embodiments described above, but also in extracorporeal acoustic filtration devices for the purpose of removing embolic matter from blood during cardiopulmonary bypass, for example. Some extracorporeal devices of this type, to which detection-driven diversion may be applied, are described in the above-mentioned U.S. Patent 6,953,438.

It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and

modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.