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This invention relates to medical diagnostic ultrasound systems and, in particular, to ultrasound systems and methods for sonothrombolysis stroke therapy .

Ischemic stroke is one of the most debilitating disorders known to medicine. The blockage of the flow of blood to the brain can rapidly result in paralysis or death. Attempts to achieve

recanalization through thrombolytic drug therapy such as treatment with tissue plasminogen activator (t-PA) has been reported to cause symptomatic intracerebral hemorrhage in a number of cases. Advances in the diagnosis and treatment of this crippling affliction are the subject of continuing medical research.

Sonothrombolysis is an emerging treatment modality for stroke that uses ultrasound targeting of the site of the occluding clot, microbubbles in systemic circulation, and sometimes t-PA, to break up the fibrin structures that make up a typical clot, so as to try to restore normal blood flow to the

occluded region in the brain. As used in this application, microbubbles are sometimes referred to as "vascular acoustic resonators," or VARs . Such treatments typically use head-mounted, single-element transducer ( s ) or array transducers to deliver the ultrasound through the temporal bone, and operate in continuous or pulsed mode. International patent publication WO 2008/017997 (Browning et al . )

describes a sonothrombolysis ultrasound system which provides microbubble-mediated therapy to a clot causing ischemic stroke. Microbubbles are infused, delivered in a bolus injection, or developed in the bloodstream and flow to the vicinity of a thrombus. Ultrasound energy is delivered to microbubbles at the site of the clot to disrupt or rupture the

microbubbles. This microbubble activity can in many instances aid in dissolving or breaking up the blood clot and return a nourishing flow of blood to the brain and other organs. Such microbubble activity can be used to deliver drugs encapsulated in

microbubble shells, and well as microbubble-mediated sonothrombolysis .

Clinical trials are ongoing in sonothrombolysis, using either a combination of ultrasound and t-PA, ultrasound and microbubbles, and/or ultrasound, t-PA, and microbubbles combined. In these trials a

continuous flow of microbubbles is infused into the subject's blood stream from a syringe pump while ultrasound is delivered for upwards of an hour to assure that blood clots are completely lysed. This treatment regimen constantly replenishes the flow of microbubbles to the therapy site and thus requires a continuous infusion of a constant amount of

microbubbles during the treatment period. A problem that has arisen during such lengthy periods of infusion is that the buoyancy of the microbubbles causes them to migrate to the top of the fluid in the syringe and stratify in levels of different

microbubble concentration. This leads to different concentrations being infused into the body over time, which is not desirable, as it introduces treatment uncertainties.

A system which has been developed to address microbubble stratification is illustrated in FIGURE 1. This illustration shows an ultrasound system which is delivering ultrasound to the brain of a patient through a headset 12 containing ultrasound transducers. A saline bag 14 would normally deliver a flow of fluid containing microbubbles to the vascular system of the patient through the tubing of an infusion set. Instead, the microbubble fluid is delivered from a syringe 16 which is operated by a specially modified syringe pump 18. The pump 18 includes a mechanism which gently rocks/rotates the syringe during the procedure, which alleviates the tendency of the microbubbles to stratify in the syringe. This is an ideal solution to prevent stratification during ultrasonic contrast imaging where the problem of microbubble migration in the syringe is not excessive, as these procedures tend to be short, often rely on a microbubble bolus

injection, and in-situ microbubble concentration is not critical. However, experiments with this

arrangement has shown that when infusion continues in excess of ten minutes, microbubbles still tend to settle at the top of the syringe, indicating that the rocking/rotation motion is not sufficient to

counteract microbubble buoyancy over extended periods of time. While this system potentially alleviates microbubble migration due to buoyancy for ultrasound contrast imaging applications, it does not solve the problem for constant microbubble concentration delivery over a longer period of time required for sonothrombolysis .

In accordance with the principles of the present invention, a sonothrombolysis infusion system and method deliver a constant supply of microbubbles from a syringe containing a microbubble solution and magnetic stirrer beads. Located in proximity to the syringe is a motorized magnetic stirrer. As the magnets of the stirrer move past the syringe they cause the magnetic beads to move in random or semi- random patterns of motion, continuously agitating the microbubble solution in the syringe and preventing microbubble stratification. In a preferred

implementation the magnetic stirrer has a rotating rod with magnets on it, which can be mounted with the syringe by a retention mechanism of a syringe pump. The rotational speed of the magnetic stirrer can be varied so that its speed is sufficient to prevent microbubble stratification while maintaining the structural integrity of the microbubbles .

In the drawings :

FIGURE 1 illustrates the setup of a

sonothrombolysis procedure including the delivery of infusing microbubbles with a syringe pump.

FIGURE 2 is a perspective view of a standard syringe pump.

FIGURE 3 illustrates a microbubble stirrer device constructed in accordance with the principles of the present invention.

FIGURE 4 is a partially cross-sectional end view of the stirrer device of FIGURE 3 when mounted in proximity to a syringe by a syringe pump.

FIGURE 5 is a side view of the stirrer device of FIGURE 3 mounted in proximity to a syringe by a syringe pump.

Referring to FIGURE 2, a conventional syringe pump is shown in a perspective view. The lower housing 20 contains a variable speed motor with gear train and a processor which controls the speed at which the motor rotates a worm gear 26 that advances a plate 28 mounted on two guide rods. The plate 28 advances as indicated by the arrow, pressing the plunger 32 of a syringe 16 into the barrel of the syringe (indicated at 16) and thereby expelling the fluid in the syringe at a controlled rate of flow.

The barrel of the syringe rests in a groove in a syringe mount 22 during operation and is retained in place by a spring-loaded bar 25 that is positioned onto the syringe barrel by a release knob 24. With the syringe barrel held in place by the bar 25, the plate 28 advances to the right, pressing the plunger 32 into the syringe and expelling its fluid out the distal end of the syringe.

FIGURE 3 illustrates in perspective a magnetic stirrer 40 constructed in accordance with the

principles of the present invention. The stirrer comprises a frame with an enclosure 44 at one end which houses a motor and its reduction gearbox. In a constructed embodiment the motor is a 6 volt motor with a drive shaft speed variable up to 200 rpm. The motor rotates an aluminum rod 42 having a bearing-mounted shaft 60. In a constructed embodiment the rod is about 5cm in length, which is sufficient to oppose the barrel of a 30cc syringe. Affixed around the outer circumference of the rod 42 are a number of disc magnets 50. A groove 48 is formed in the base 46 of the stirrer 40 and extends along the entire length of the frame.

The stirrer 40 is shown in use in a partially cross-sectional end view in FIGURE 4 when mounted on the barrel 34 of a syringe in a syringe pump. The barrel 34 of the syringe is seen to rest in a groove 36 in the syringe mount 22 of the syringe pump, and would normally be held in place by the spring-loaded bar 25. But in this implementation of the invention the stirrer 40 is positioned on top of the syringe barrel with the barrel extending partially into the groove 48 in the base 46 of the stirrer. The spring-loaded bar 25 extends through the stirrer, retaining stirrer base 46 in position on top of the syringe and the syringe in its proper position in the syringe pump. The rotating rod 42 and its magnets 50 are thus maintained in a substantially parallel alignment with the barrel of the syringe. When the stirrer is actuated, its motor rotates the rod 42 as indicated by the curved arrow, and the magnets 50 on the circumference of the rod 42 are continually rotated over the syringe, moving the magnetic beads inside the syringe barrel 34 through magnetic attraction and repulsion. While the groove 48 in this

implementation is seen to be a squared groove, the groove can also be a concave curved groove which contacts more of the surface of the syringe barrel when engaged. However a squared groove 48 as shown has been found to be sufficient to engage the syringe and be held in place by the bar 25.

FIGURE 5 is a side view of the assembly of

FIGURE 4. This illustration shows the magnetic beads 52 in the microbubble solution in the syringe barrel 34. As the rod 42 rotates as indicated by the arrow, the passage of the magnets 50 in close proximity to the syringe will cause the beads 52 to move in a semi-random pattern, agitating the microbubble solution and preventing stratification. In a

constructed embodiment the magnets 50 are quarter-inch diameter (0.64cm) Samarium cobalt disc magnets SC2512-500 available from Dura Magnetics, Inc. of Sylvania, OH, USA. The magnetic beads 52 are

approximately 5mm in length and 2mm in diameter.

Such beads are available from Cole-Parmer Instrument

Company, LLC of Vernon Hills, IL, USA as part number 08545-02. The magnets affixed to the stirrer rod 42 are powerful enough and pass close enough

(approximately 5mm) to the syringe barrel 34 and the magnetic beads 52 to move/spin the magnetic beads located inside the syringe. Thus, when the stirrer rod 42 is rotated by the stirrer motor, it rotates the magnets 50, which in turn move the magnetic beads 52 within the syringe, agitating the solution and thus preventing the microbubble solution from

stratifying. At a rotation speed of 60-200 RPM, and preferably in the range of 60-100 RPM, this assembly easily stirs the microbubble solution without causing microbubble destruction. Other rotation speeds may be used, depending on factors such as the

concentration of microbubbles , the number of beads used, and the size of the syringe. However, if the speed is too low, the microbubbles will still

stratify; if the rotation speed is too high, the magnetic coupling between the magnets 50 on the stirrer rod 42 and the magnetic beads 52 separates, again resulting in insufficient stirring of the microbubble solution. Selective adjustment of the rotational speed is often required.

Since magnetic beads are generally not used to stir liquids to be infused into the body, the beads should be sterilized before being placed in the syringe. This may be done in the normal manner as by use of an autoclave, ethylene oxide gas, etc.

Microbubble solutions are usually packaged in a vial with a rubber membrane as are other injectable liquids. The microbubble solution may be aspirated from such a vial by use of a 20 gauge or larger needle attached to the distal end of the syringe which pierces the membrane to withdraw the

microbubble solution.

It is currently estimated that approximately 50 ml of microbubble solution is required for continuous infusion during a one hour sonothrombolysis

treatment. This equates to a flow rate of 0.83 ml/min. This means that the amount of microbubble solution in the syringe should be 50 ml plus an additional amount to enable priming of the infusion tube set, plus a further amount to account for the inability to fully depress the plunger to the end of the syringe barrel due to the presence of the

magnetic beads in the barrel of the syringe.

Experience has shown that this additional amount of microbubble solution is about 10 ml, bringing the total amount of solution required to about 60 ml in many cases.

As microbubble stratification can occur to at least some degree in the tubing set after the

microbubble solution has been aspirated from the syringe, it is desirable to keep the length of the tubing as short as possible between the distal end of the syringe and the catheterized infusion site.

Preferably the tubing length is restricted to 30cm or less. Tubing length of 10cm or less has been found to result in no significant microbubble

stratification in the infusion tubing. A significant rate of flow will also help prevent stratification.

In use, several sterile magnetic beads, at least two or more, are placed in the barrel of a syringe and the plunger inserted as far as it can go into the barrel. A 12 gauge or larger needle is attached to the distal end of the syringe and used to pierce the membrane of a vial of microbubble solution. The solution is drawn into syringe by retracting the plunger until the required amount of solution, generally 50 ml or more, fills the syringe. The syringe needle is pointed upward and the plunger depressed slightly to expel any air in the barrel. The needle is removed and infusion tubing leading to a transcutaneous catheter is attached to the distal end of the syringe. The syringe is placed m the groove of the syringe mount of a syringe pump and the magnetic stirrer placed on the barrel of the syringe. The spring-loaded retention bar of the syringe pump is inserted through the stirrer and springs downward to hold the stirrer and syringe in place on the pump and the stirrer is turned on. If necessary, its speed is adjusted so that stratification is prevented without significant microbubble destruction. The syringe pump is actuated to prime the tubing, if necessary, by injecting microbubble solution (or saline) into the tubing. After priming the catheter is inserted into a vein of the patient. The syringe pump is actuated to begin the injection of a

continuous, controlled amount of microbubble solution into the patient while ultrasound is administered to the patient transcranially . An injection of t-PA can be added to the solution if desired via another infusion pump and connected tubing. At the

conclusion of the ultrasonic treatment the syringe pump is stopped, the stirrer turned off, and the catheter withdrawn from the vein of the patient.