1. Field of the Invention
The subject matter described herein relates generally to calibration and, more particularly, to calibrating movable transducers to avoid distortion.
2. Related Art
Current medical diagnostic demands require reducing an outer diameter of an invasive body such as a catheter to allow the catheter to travel through the narrow and tortuous regions of a patient's vascular system. Contained within the tubular body may be a mechanical rotation system that includes e.g., a motor, a driveshaft, and bearings. The motor may be used to rotate a transducer to acquire a real-time two-dimensional or three-dimensional image. The mechanical rotation system (motor, driveshaft, bearings, ultrasound cables, etc.) may not be “stiff” and may have significant dynamics, causing non-uniform motion and a non-linear relationship between the drive and the actual transducer motion. This, in turn, causes distortion of, and possibly instability in, the ultrasound image, which can make the clinical procedure more difficult, more time-consuming, less safe, or simply impossible. This distortion is sometimes referred to as non-uniform rotational distortion or “NURD”.
To date, no suitable device or method of calibrating an invasive probe to effectively reduce distortion, such as NURD, caused during use of movable transducers is available.
In accordance with an embodiment of the present invention, a method of mapping movement of a transducer within a probe that is configured for sensing within a body cavity, comprises supporting a transducer configured for at least one cycle of movement; providing a sensor; and using the sensor to identify a position of the transducer within the probe during the at least one cycle of movement.
In accordance with another aspect of the present invention, a test fixture for a probe that includes a transducer configured for at least one cycle of movement within the probe comprises a base with at least one probe support extending from the base which, in turn, comprises a probe mount. The test fixture also comprises a sensor that may be interconnected with the at least one probe support for identifying a position of the transducer during the at least one cycle of movement within the probe.
In accordance with a further aspect of the present invention, a catheter probe comprises a transducer configured for at least one cycle of movement within the probe and a sensor that is at least partially supported within the probe for identifying a position of the transducer during the at least one cycle of movement.
The following detailed description is made with reference to the accompanying drawings, in which:
One embodiment of the present invention concerns a device and a method for mapping movement of a transducer within a probe usable to reduce distortions and instability in an ultrasound image and to make the clinical procedure easier, less time-consuming and safer. In the present embodiment, the actual position, e.g. the rotation angle, of a transducer that undergoes at least one cycle of angular movement, is sensed and this information may be used to reduce the distortion, such as non-uniform rotational distortion (NURD). In one particular embodiment, an ultrasound probe is supported by test fixture and a sensor is provided for identifying a position of the transducer throughout the angular cycle of movement.
Referring now to
The probe support members 16, 18, 20 and 22 may comprise any suitably strong and durable material similar to that of the base 14 and each comprises a tube mount 32, 34, 36 and 38, respectively. Each tube mount 32, 34, 36 and 38 may comprise a similar material to that of the base 14 and may also comprise fasteners 40 and bushings or bearings 42. It will be appreciated that use of nested support surfaces obviate the use of separate bearings.
In this embodiment, the test fixture 10 also comprises a position indicator such as an optical encoder 44 that includes an encoder wheel 46 that is affixed to a probe support tube 48. The probe support tube 48 may be rotated by an actuator 50. A sensor 52 may be connected to and movable with the probe support tube 48. The sensor 52 may be any suitable sensor that senses an angular position of a transducer located in the probe 12 such as an electromagnetic, magnetic, optical, capacitive, acoustic and/or gravimetric sensor, described in more detail below.
Referring now to
One particular embodiment of the sensor 52 is shown in
It will be appreciated that the output from the sensor 52 may be included as part of a real-time position control feedback loop. Optionally, if the motion is repeatable, the sensor data could be pre-acquired, processed, and used to modify the drive signals sent to the motor 54 and gear box 56, without real-time feedback.
Referring again to
Other embodiments of a sensor 52 employable in the practice of the present invention include an electromagnetic coil located in the compartment 62, with the coil axis set nonparallel to the axis of transducer rotation. One or more coils or magnetic field sensors (or a magnet, if there is a coil or sensor on the transducer) are fixed, either internal or external to the probe. Motion of the transducer causes a varying mutual inductance between or induced current or voltage or resistance in the coils or sensors. The relationship between transducer position or motion and the induced electrical signals may be calculated, if the geometry of all components is known and well controlled.
In another embodiment, a magnet and a coil may be employed for sensor 52 which would provide a roughly sinusoidal variation of signal amplitude with rotation angle. To accurately sense the rotation angle, one must measure the relative signal amplitude with high accuracy, which can require calibration, filtering, and long averaging times. Optionally, as shown in
In a further embodiment of the sensor 52, a capacitive sensor may be employed. For example, a plate of a capacitor may be mounted to the compartment 62 or the transducer 58 itself and a parallel plate may be mounted to a nearby fixed non-rotating portion of the probe 12, such as inside the probe 12. As the transducer 58 is rotated by motor 54, the overlap and thus capacitance of the plates changes, and the position or motion of the transducer can be inferred from an AC measurement of the capacitance. Optionally, the capacitor can be made the frequency-determining element of a resonant circuit and the capacitance and motion derived from measurement of the resonant frequency. It will be appreciated that sensitivity to the motion will be increased and the effects of stray capacitance in the leads reduced, if the spacing between the sensor electrodes is minimized or the electrodes are made of interleaved plates.
In still a further embodiment, the sensor 52 comprises an optical sensor. In one example, a mirror may be located in the compartment 62 adjacent the moving transducer. A laser beam may be directed from a fixed source toward the mirror. A charge coupled device (CCD) or similar high-resolution multi-pixel detector may be positioned to intercept the reflected beam. As the transducer moves, the reflected beam will move across the detector. The relationship between transducer position and detected beam position may be calculated if the geometry is known and well controlled, or may be calibrated, e.g. by holding the transducer fixed within the probe housing and moving the probe relative to the optical system while measuring the probe motion with a linear or rotary encoder.
In another optical sensor embodiment one portion of an optical fiber is attached to the moving transducer, e.g., in the compartment 62, and another portion to a nearby non-moving part of the probe, in such a way that the transducer motion causes the fiber to twist and untwist. An optical signal (laser beam) may be sent through the fiber, either end-to-end or reflected and, it will be appreciated that as the fiber is twisted, the polarization of the transmitted beam will change. From measurements of the change in polarization, one can infer the position or motion of the transducer. Looping the fiber so that multiple strands are twisted in parallel and the optical beam passes through those strands in series could increase the sensitivity of the motion detection.
In still a further optional embodiment, the sensor 52 may comprise an acoustic sensor that may be employed along with a pattern of echogenic targets (high- or low-impedance acoustic reflectors) in the housing of the probe, outside the normal imaging field of view, or in a calibration device external to the probe. For example, if the transducer 58 is rotating within a metal tube catheter tip and imaging through a polymer-covered opening in the tube, the sensor 52 may comprise notches or barbs (projections) provided in the metal at the edges of the opening to create associated dark or bright artifacts in the ultrasound images. In this way, transducer 58 may be used to measure the position of itself. Where the sensor 52 comprises a separate ultrasound array, components external to the probe, e.g., an external housing and/or echogenic target(s) may be provided. Calibration of the sensor 52 may be carried out in a manner similar to that described above, where the external targets could be rotated about a fixed probe to determine a mapping transfer function. With the external target(s) located in a known position, the position of the moving acoustic sensor and thereby the transducer, could be determined using the transfer function applied to the senor output. Additionally, if the geometry of the targets is available, one could incorporate the targets in the non-rotating portion of the probe or, for example, use a non-rotating external target. The targets in the non-rotating portion of the probe may be provided as described above.
In yet a further embodiment of sensor 52, a rotary or linear optical encoder, an LVDT or resolver may be attached to the moving transducer and non-moving portion of the probe 12.
Referring now to
As shown at 90, the method may further comprise locating a sensor wherein an axis of rotation thereof is coincident with that of the transducer and wherein one step in sensing the position of the transducer comprises moving a sensor about the probe to calibrate the sensor. It will also be appreciated that in one aspect of this method, the sensor may be located within the probe.
While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to these herein disclosed embodiments. Rather, the present invention is intended to cover all of the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.