Pressurizing Syringe

This invention relates to pressurizing syringes and, in particular, to the construction and use of a syringe-type device to controllably pressurize a fluid-filled enclosure.

In ultrasound diagnostic imaging systems an ultrasonic transducer is used to scan the tissue of a patient who is being imaged. The ultrasonic transducer is generally mounted in a fixture known as a scan head, and is contained in a fluid-filled chamber. The acoustic fluid in the chamber has a high sonic conductivity, and is used to couple sonic energy between the transducer and the patient over a path of a desired sonic impedance.

A problem frequently encountered in fluid-filled scan heads is the formation of bubbles. If air bubbles are allowed to develop in the fluid, they can adhere to the surface of the transducer or mirror in the chamber, if one is used. These bubbles will interfere with the flow of ultrasonic energy, since they act as low impedance points in the acoustic fluid which tend to scatter ultrasonic energy. The formation of bubbles can thus result in the production of a distorted ultrasonic image by the system.

The manner in which the air bubbles can enter the chamber are many. One common way for air bubbles to enter the chamber is through leaking seals during temperature cycling. If a seal in the fluid-filled chamber is not fluid-tight and the scan head is warmed, the fluid will expand, increase in pressure, and leak out through the seal. When the scan head cools, the fluid will contract, thereby producing a negative pressure in the chamber relative to atmospheric which can draw air into the chamber.

The problem of bubble formation due to temperature cycling may be greatly alleviated by pressurizing the fluid-filled chamber of the scan head. A scan head with a pressurized chamber is shown in U.S. patent application Ser. No. 352,910, filed Feb. 26, 1982 by Bayard G. Gardineer and entitled "LOW PRESSURE HEAD FLUID RESERVOIR." A scan head illustrated in this patent application contains a one-way valve in the side of the fluid-filled chamber through which the chamber can be initially filled and subsequently refilled. By initially pressurizing the chamber at a pressure greater than atmospheric, the chamber can go through temperature cycling and fluid can leak through the chamber seals as before. But if the pressure within the chamber is still greater than atmospheric pressure after the scan head has cooled, the pressure differential will prevent any drawing of air into the chamber. Air bubble formation due to temperature cycling is thereby prevented.

However, once the scan head is in the hands of a user, it is necessary to provide a means whereby the user can repressurize the scan head chamber from time to time without the need for a service call to the manufacturer. It would be possible, for instance, to supply the user with a pump and pressure gauge so that the scan head could be pumped with fluid until the gauge reaches the desired pressure. But such a system would be both complicated and expensive. Thus, it is desirable to provide a means for repeatedly pressurizing the scan head to a known pressure, including apparatus which is both simple and inexpensive.

In accordance with the principles of the present invention, a pressurizing syringe is provided which is suitable for pressurizing an enclosure to a predetermined pressure level. A syringe contains one or more snapdomes which "snap" at a predetermined pressure when the syringe plunger is depressed. The snapdomes thereby provide the user with both a tactile and audible signal when the syringe is delivering fluid at a desired pressure. The pressurizing syringe with snapdomes is simple, inexpensive, easy to use, and a familiar looking item to medical personnel.

in accordance with the principles of a further aspect of the present invention, a pressurizing syringe with a pressure-sensitive snapdome is used to pressurize the fluid-filled chamber of an ultrasound scan head. The syringe is first filled with acoustic fluid. The needle is then inserted into a fill port of the scan head, and is seated so as to be relatively fluid-tight. The plunger is then depressed until the syringe clicks. The plunger pressure is maintained at approximately the click pressure until the fluid pressure within the scan head stabilizes. If desired, the syringe can be clicked several times to ensure that the proper pressure is being maintained during the stabilizing period. After the fluid pressure has stabilized, the syringe is withdrawn from the fill port and the scan head is ready for use.

In the drawings:

FIG. 1 illustrates an exploded view of a pressurizing syringe constructed in accordance with the principles of the present invention; and

FIG. 2 illustrates the pressurizing of an ultrasound scan head by a syringe with a pressure-sensitive snapdome in accordance with the principles of the present invention.

Referring to FIG. 1, an exploded view of a pressurizing syringe constructed in accordance with the principles of the present invention is shown. In FIG. 1 the hollow outer or barrel section 10 of the pressurizing syringe has an opening at the proximal end 18 to admit the plunger assembly shown at the right side of the FIGURE. A hollow needle 12 is mounted at the distal end of the barrel section 10. Just above the needle 12 is an O-ring 14. A needle cover 16 is used to protect the needle 12 when the pressurizing syringe is not in use.

The plunger assembly of the pressurizing syringe is shown at the right of the FIGURE. A plunger 20 includes a generally pointed knob end 22. When assembled, the pointed knob is located inside a hollow snapdome enclosure 26, and is held there by an interior lip 27 of the enclosure. The assembly includes a number of dimpled snapdomes, shown at 24, which are stacked in the bottom of the enclosure 26 when the pressurizing syringe is fully assembled. Notches 25 are located on either side of the enclosure 26, one of which is shown in the FIGURE. The notches permit easy insertion of the stacked snapdomes into the enclosure, as well as flexing of the lip portion 27 when the knob 22 is engaged in the enclosure. Another generally pointed knob 29 is located at the bottom of the enclosure 26 and is used to engage a piston 28.

The pressurizing syringe of FIG. 1 is seen to contain conventional parts of a hypodermic syringe, except for the snapdomes 24 and the snapdome enclosure 26. The snapdomes in the enclosure, in combination with the downward pressure exerted on the snapdomes by the knob 22, enable the assembled pressurizing syringe to provide fluid at a known pressure. With the needle cover in place and a fluid located in the barrel section 10, the plunger assembly is inserted into the barrel section 10. As the plunger 20 is depressed, the knob 22 exerts a force against the stacked snapdomes, which force is in turn transferred to the fluid. As this force is increased, a point will be reached at which the snapdomes deflect, or "snap", emitting a popping sound and giving a tactile indication that a predetermined force level has been reached. Since the force needed to deflect the snapdomes is known, the force exerted on the fluid is predictable.

For instance, the pressure inside the barrel 10, neglecting friction, is given by the ratio of the force applied to the plunger divided by the cross-sectional area of the syringe. This is approximated by the area of the piston 28, which in an actual embodiment of the present invention was measured as 0.44 square inches. The force nominally exerted by each snapdome before it snapped was found to be 0.66 lb. Therefore, each snapdome should "snap" under a force of 0.66 lb. divided by 0.44 square inches, or 1.5 psi. However, it was found that the pointed knob, which concentrates the applied force over only a small area of the snapdomes, causes the snapdomes to deflect at a lower than predicted force of 1.37 psi.

Thus, if it is desired to pressurize the fluid in the syringe to a level of 1.37 psi, only one snapdome would be required in the enclosure 26. By stacking additional snapdomes in the enclosure, the forces required to deflect the snapdomes additively combine. For example, when six snapdomes are used, 4 lb. of pressure is required before the snapdomes snap. By dividing the 4 lb. force by the piston area of 0.44 square inches, a predicted fluid pressure of 9.1 psi is calculated. When the pointed knob is further taken into consideration, the predicted value becomes 8.2 psi. A six snapdome assembly was built and tested, and was found to produce a measured pressure of 7.8 psi, which is within five percent of the predicted value.

FIG. 2 shows a scan head 30 being pressurized by a pressurizing syringe 40 in accordance with the principles of the present invention. The scan head 30 is of the type shown and described in the aforementioned U.S. patent application Ser. No. 352,910, and in U.S. Pat. No. 4,330,874. The scan head 30 includes a first section 32, including an enclosed motor and drivetrain. The motor and drivetrain are connected to oscillate a mirror 36, which is contained in a fluid-filled chamber 37. As the mirror 36 oscillates, it translates ultrasonic energy between an ultrasonic transducer 38 and an aperture 39 in the bottom of the transducer section 34 of the scan head.

A duck billed check valve 50 is shown located at the top of the transducer section 34. The check valve 50 provides a fill port for the transducer section whereby acoustic fluid may be injected under pressure into the chamber 37.

In an actual embodiment of the scan head shown in FIG. 2, it has been found desirable to maintain the acoustic fluid pressure at around 10 psi to prevent air bubble formation in the chamber. Furthermore, it is recommended that the scan head be repressurized approximately once a week to ensure that small leaks do not lead in time to the development of negative fluid pressures. The pressurizing syringe of FIG. 1 satisfies these requirements. A hypodermic syringe is readily capable of delivering a pressure of 50 psi, which would rupture the seals in the scan head. But by using the pressurizing syringe with the snapdomes, the user is notified by the "snap" when the desired pressure has been reached. Moreover, the pressurizing syringe is easy to use, so that weekly repressurizations do not become overly inconvenient.

In order to deliver the desired fluid pressure of about 10 psi, seven of the above-described snapdomes, each snapping at approximately 0.67 lb., are required. The force required to overcome the cumulative resistance of the seven snapdomes is approximately 4.67 lb. Dividing this force by the piston area of 0.44 square inches yields a calculated pressure of 10.6 psi. As explained above, the relatively pointed knob 22 will cause the snapdomes to deflect at a pressure lower than the calculated pressure, about 9.6 psi. This has been found to be acceptable for the embodiment of the scan head of FIG. 2.

If the user has not used the pressurizing syringe previously, it may be desirable to test the syringe so as to acquire a sense of the required plunger pressure. This may be done by leaving the needle cover on the assembled syringe and pressing the plunger. The user can then note the force being exerted when the snapdomes click at about 10 psi of pressure.

Once the user has gained some familiarity with the pressurizing syringe, the pressurizing sequence may be initiated. Since the purpose of pressurizing is to keep air out of the chamber 37, it is advisable to first bleed air out of the syringe. This is done by removing the needle cover 16 and lightly pressing the plunger 20 until a small amount of fluid is emitted.

To pressurize the scan head, the needle 12 is inserted into the check valve 50. The plunger is then pressed slowly to fill the inside of the valve with fluid, thereby displacing all air from the valve. Once the valve has been filled, the syringe is pressed down firmly so as to seat the O-ring 14 at the top of the check valve 50, as shown in FIG. 2. The plunger is then depressed until the snapdomes click. The plunger may then be released slightly to feel the return click. The acoustic fluid should now be flowing through the check valve 50 and into the chamber 37.

The plunger pressure is maintained at approximately the click pressure for 45 to 60 seconds to allow the fluid pressure to stabilize. While the pressure is stabilizing the user may alternately press and relax the plunger slightly so as to hear the snapdome click and return click. This provides a positive indication that the syringe is maintaining the desired fluid pressure during stabilization.

After allowing sufficient time for pressure stabilization, the plunger pressure is released and the syringe is withdrawn from the check valve. Excess fluid may then be wiped from the scan head, readying it for use, and the needle cover put back in place on the syringe.