Patent Application
20070066901
The present embodiments relate to mechanically scanned ultrasound transducers. In particular, drive mechanisms for mechanically scanned ultrasound transducers are provided.
Ultrasonic transducers are wobbled with motors. The motors mechanically scan the transducer from side-to-side. The motor is located close to the ultrasound transducer. For hand-held probes, wobbler arrays are provided for three-dimensional imaging. For small probes, such as catheters, the motor is located in the tip of the probe catheter or outside the patient in a control box. If the motor is located outside the catheter, a drive and cable transfers the motor's rotational motion to the ultrasound transducer for mechanical scanning.
Electric motors emit electromagnetic interference (EMI), which can corrupt the received ultrasound signals from the transducer, particularly if the signals from the transducer are not amplified at the transducer. Effective shielding may be difficult in space-constrained applications such as catheter ultrasound imaging. In addition, miniature electric motors may be prohibitively expensive to include in a disposable catheter ultrasonic probe. By locating the motor away from the transducer, transferring the rotational energy of the motor is challenging since the catheter diameter is narrow and most of the cross-sectional area of the catheter is used for electrical connections to the acoustic array. In addition, the cable for transferring rotational energy may limit flexibility.
By way of introduction, the preferred embodiments described below include methods, systems, and wobbler drive mechanisms for mechanically scanning an ultrasound transducer array. A fluid drive moves the transducer array. A pump causes fluid flow. The fluid flow transfers energy to the transducer array for moving the transducer array.
In a first aspect, an ultrasound transducer system is provided for mechanical scanning. A fluid channel has fluid operable to move a moveable transducer array.
In a second aspect, a method is provided for mechanically scanning an ultrasound transducer array. The ultrasound transducer array is driven with a fluid. The ultrasound transducer array moves in response to the driving.
In a third aspect, an ultrasound transducer system is provided for mechanical scanning. A transducer array is rotatable about an axis spaced from an emitting face of the transducer array. Alternatively, the mechanical scanning can be translational along a designated path. Combinations of rotational and translational motion are also possible. A fluid is operable to rotate and/or translate the transducer array in response to a pump.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments and may be later claimed independently or in combination.
The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.
A fluid-driven system actuates an ultrasound array for volume imaging. The pressure or flow of a fluid powers movement of the ultrasound array. Using an extended fluid channel, a wobbler may be used in probes constrained by space, such as intravascular catheters, or in probes susceptible to electromagnetic interference from electric motor-driven systems. The fluid-driven system allows the freedom to locate the fluid pump away from the ultrasonic array, reducing electrical interference.
For a cardiac or intravascular catheter implementation, the fluid may be circulated through the flexible catheter. The fluid pump may be located away from the acoustic array and processing electronics, allowing for better shielding or even amplification of the ultrasound signals before passing near the fluid pump. This location could reduce the cost of catheter-based ultrasound since no motor or pump is required in the disposable or limited-use part, such as the catheter. The power to actuate the ultrasonic array at the end of the cable or catheter is provided by the moving fluid through the catheter. The fluid channel(s) are simple, limiting the need for lengthy mechanical drive shafts. The volume for the fluid drive might be smaller than a comparable mechanical drive shaft. The fluid filled catheter or cable may remain flexible.
The pump 12 is a fluid pump, such as a suction or reciprocating pump. The fluid pump provides constant or variable pressure or flow in one direction or reciprocating between two directions. Fluid pumps are used in many medical systems and are capable of imparting large linear forces of many pounds or more. As an example, syringe pumps (e.g., the Harvard PHD2000 syringe pump) may impart 15 PSI to 1000 PSI with flow rates ranging from 0.003 μl/min to 220 ml/min. Higher or lower pressures or flow rates may be provided. The pressure used is sufficient to move the transducer array 16 but avoid causing leaks in the fluid channel 14.
The pump 12 connects with the fluid channel 14. For example,
The pump 12 is positioned to remain outside of a patient while the transducer array 16 is positioned to be within the patient. The pump 12 is within a probe control box or housing, within an ultrasound imaging system or within a separate pump housing. The fluid channel 14 interconnects the pump 12 with the transducer array 16. For example, the pump 12 drives fluid through the fluid channel 14 in a catheter, and the fluid motion is converted into mechanical movement of the ultrasonic array 12 by a mechanical fixture or drive located at the transducer array 16. In alternative embodiments, the pump 12 is positioned adjacent to the transducer array 16, such as being positioned in a hand-held transducer array housing of a probe used externally to the patient.
The fluid channel 14 is a tube, a cavity, a container, a chamber, a reservoir, or combinations thereof. The fluid channel 14 is plastic, rubber, ceramic, wood, metal, semiconductor, PVC, combinations thereof or other now known or later developed materials. The fluid channel 14 is entirely a same material or structure or includes different materials or structures at different locations. For example, the fluid channel 14 extends through a plastic pump 12, along a rubber hose, and into a metal transducer array housing. The fluid channel 14 is generally straight, but may include bends, elbows or other curves. The fluid channel 14 is flexible, but may have rigid portions or be entirely rigid.
The fluid channel 14 extends from the pump 12 to adjacent to the transducer array 16 in one or more path ways, such as a single channel or a loop. In alternative embodiments, the fluid channel 14 extends part way from the pump 12 to the transducer array 16, and fluid flow or pressure is converted to mechanical rotation and/or translational motion for transferring energy to the transducer array 16.
The fluid channel 14 contains fluid. The fluid is a gas or liquid, such as water or saline. The fluid channel 14 is sealed to prevent leakage of the fluid. Alternatively, the fluid channel 14 is unsealed. A relief valve may be provided, such as adjacent the pump 12.
The fluid moves the transducer array 16. For example, flow or pressure from the fluid within the fluid channel 14 is converted to mechanical energy, causing the transducer array 16 to rotate or translate. The pump 12 causes the fluid to move the transducer array 16.
The fluid moves within the fluid channel 14 in a substantially continuous flow. The flow or pressure of the fluid is maintained substantially constant during mechanical scanning. By providing a looping fluid channel 14 (e.g., a source path and a return path), the fluid flows in one direction through the loop. The moving fluid also removes some of the heat dissipated from the array 16. To maintain an about constant wobble or transducer movement rate, the flow or pressure is kept substantially constant. The flow or pressure decreases when stopping the mechanical scan and increases when starting the mechanical scan, but provides substantially continuous flow after starting and before stopping. In alternative embodiments, the flow or pressure is varied, such as increasing or decreasing a flow rate as a function of transducer location. For example, the transducer array 16 is moved more quickly or slowly at edge regions or the ends of the mechanical scan. As another example, the speed of the wobbling ultrasonic transducer array 16 is varied by the rate of fluid flow or different gear ratios in the mechanical conversion. The flow rate may vary to account for changes in rate from the mechanical conversion, providing substantially constant motion of the transducer array 16.
In another embodiment, the fluid moves within the fluid channel 14 in a substantially reciprocating flow. The fluid moves back and forth or periodically changes direction of pressure or flow in the fluid channel 14. The fluid channel 14 is a single path or a looping path between the pump 12 and the transducer array 16. Depending on the frequency, fluid type, fluid mass, length of the fluid channel 14, and strength of the fluid channel 14, reciprocating motion moves the transducer array 16. The pressure of the fluid is varied at the desired frequency of mechanical movement. Depending on the frequency of operation and characteristics of the fluid and fluid channel 14, the transducer array 16 is actuated by small fluid displacements under high pressure or with larger movements under low pressure.
Referring to
Other drives may be used to convert fluid motion to mechanical motion. Combinations of different drives may be used.
Referring to
The transducer array 16 is moveable. For example, the transducer array 16 is mounted adjacent an axis to rotate about the axis. As another example, the transducer array 16 is mounted on a lever arm (e.g. a block, housing or other structure) to rotate about an axis spaced away from the transducer array 16. As yet another example, the transducer array 16 includes guides, bearings, sliders or other structures to translate with or without rotation. Any now known or later developed wobbler array structures may be used.
The transducer array 16 is allowed to rotate or move without limit. Alternatively, the transducer array 16 is limited by the drive 20 or stops 76. For example, pegs, plates, blocks or other structures prevent movement of the transducer array 16 past a particular position. One or more stops are provided to limit movement in one direction or to a range of motion. For example, the fluid moves the moveable transducer array 16 between opposing stops 76 in response to different flow directions.
An encoder, sensor or networks of sensors may provide position feedback to the fluid pump 12 and the ultrasound system. For example, a rotary or linear encoder for digital feedback, a rotary or slide potentiometer for analog feedback, an optical sensor, a capacitive sensor, or any other currently known or later developed sensor indicates position for image reconstruction.
The system 10 or part of the system 10 is provided within a probe housing. The transducer array 16 is within or on the probe housing. Any probe housing may be used, such as a hand held probe. Probes operable to ultrasonically scan from within a patient may be used, such as a catheter, intraoperative probe, transesophageal probe, or endocavity probe.
In act 80, the ultrasound transducer array is driven with a fluid. Driven includes applying force with fluid, such as through flow and/or pressure. Fluid in a fluid channel is pumped. Substantially continuous flow of the fluid in a first direction is generated, or substantially reciprocating flow of the fluid in at least two directions is generated. The reciprocating flow may be in a single fluid path or through a fluid loop. The fluid path may be short, such as associated with a hand held probe, or may extend 10 or more centimeters, such as associated with a tube within a catheter, intraoperative probe, transesophageal probe, or endocavity probe. The fluid flow or pressure rotates a turbine, rotates a paddle, moves a piston, combinations thereof or operates another drive for converting to mechanical motion.
In act 82, the ultrasound transducer array moves in response to the driving. The movement is back and forth, such as wobbling, or continuous. Rotational, translation or other motion is generated.
Referring again to
Since the power transfer from the fluid to the hypothetical transducer array may be inefficient, about 10 mW of power flow in the fluid is used as an order of magnitude estimate to determine the necessary fluid pressure and flow rates. A flexible Viton microtube with inner diameter of D=1.6 mm (cross-sectional area A=2e−6 m2) may be capable of withstanding about 15 psi (about 1e5 Pa) of pressure continuously. For a power flow of 10 mW at a pressure of 1e5 Pa, the average flow velocity (V) is about 0.05 m/s since the power flow is the product of the pressure, cross-sectional area, and average fluid velocity across the cross-section. This corresponds to a volume flow rate of 1e−7 m3, or about 6 mL/min, which is a typical flow rate for fluid syringe pumps used in medical applications. If the fluid is water (ρ=1000 kg/m3, μ (dyn. viscosity)=8.9e−4 Pa·s), the Reynolds number (Re) for this tube diameter and velocity is Re=ρ*V*D/μ=90, which corresponds to laminar flow. For laminar flow, the pressure drop over L=1 meter of horizontal tubing is calculated as Δp=32*L*u*V/D2=553 Pa, which is roughly half a percent of the input pressure. In general, the pressure drop and corresponding energy loss may be ignored at these low flow rates. In case the tubing is oriented vertically so that the tube input is 1 meter below the transducer, then the pressure drop increases to about 1e4 Pa, or about 10% of the input pressure. The effects of bends and fittings are not included in the calculations.
The 1.6-mm inner-diameter tube may be small enough for transesophageal probe applications, but may be too large for catheter applications. For the same power delivery of 10 mW and a same 15 PSI of input pressure, the velocity of the flow increases as the tube diameter shrinks. Although the flow remains laminar, the pressure drop becomes a significant fraction of the input pressure:
At 0.5 mm, more than half of the power or energy is lost in transport through the fluid channel. However, the amount of power necessary to wobble the transducer array may also shrink as the array and tube size shrink. For enough flow for 5 mW with the same input pressure, the pressure drop is only half as large. The pressure drops scale linearly with tube length, so a 2-m tube may have twice the pressure drop and energy/power loss.
While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.
