Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-16
2001-10-02
Walberg, Teresa (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S437000, C607S122000
Reexamination Certificate
active
06298261
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to catheter tracking systems which serve to determine a position of catheters within the human or animal body. Furthermore, the present invention relates to methods for tracking catheters within the human or animal body.
The term catheter as used herein refers to any type of invasive surgical tool, used for insertion into a human or animal body for the purpose of providing remote access to a party of the body for performing some type of investigative and/or medical procedure.
With the increasing use of minimally invasive surgical techniques in medical diagnosis and therapy, there is a need for new methods of remotely locating and tracking catheters or other medical instruments inside a human or animal body. Currently, X-ray fluoroscopic imaging is the standard catheter tracking technique. However, excessive exposure to X-ray dosages by both the patient and clinician can be harmful. Thus, alternative catheter tracking methods are desirable.
Several alternative methods have been published including some which employ magnetic field measurements and others using ultrasonic measurements. One such ultrasonic catheter tracking technique is known as sonomicrometry. Sonomicrometry is based on finding distances between miniature omnidirectional ultrasound transducers by measuring a time taken for ultrasound signals to travel between the ultrasound transducers and then multiplying this by the speed of sound. It is assumed that the average speed of sound in the medium between the transducers is known and that the sound travels along a straight line. Both of these assumptions introduce errors into the distance calculations, ultimately leading to a level of uncertainty in the catheter location.
To locate the tip of a catheter using sonomicrometry, an ultrasound transducer is mounted proximate the catheter tip. A location of this transducer is then determined by measuring a time of flight of acoustic signals from the transducer on the tip to at least four other transducers acting as reference transducers disposed to detect the acoustic signals. The time of flight of the acoustic signals between the transducer on the tip and the reference transducers is representative of a distance of the tip of the catheter to the reference transducers. In combination, these distances serve to provide an indication of a position of the catheter in a three dimensional reference frame defined by the positions of the reference transducers.
A known catheter tracking system based on these sonomicrometric principles is described in U.S. Pat. No. 5,515,853 (Smith et al). This system measures the ultrasound travel times between pairs of transducers using short pulses of sound and clocked digital counters. The counters are started by the electrical pulse which drives the transmitting transducer, and are stopped by the detection of a pulse at the receiving transducer. Detection is accomplished by thresholding the received signal. Each transmitting transducer is activated in turn, after waiting for the last transmitted pulse to arrive at all receiving transducers, and for stray reflections from the various discontinuities inside the body to die away.
A disadvantage of this known catheter tracking system is that ultrasound signals do not travel in a straight line. Additionally, the speed of propagation of any ultrasound wave is dependent upon the material in which it is travelling. Ultrasound waves are subject to absorption, reflection, refraction, and scattering effects due to the material along its path, resulting in a loss of signal strength. An ultrasound wave travelling in the human body will suffer from all of the aforementioned effects, resulting in an error associated with each time of flight measurement, leading to uncertainty in determining the catheter location.
A technical problem of improving an accuracy with which a catheter tip is located is addressed by the catheter tracking system according to the present invention.
SUMMARY OF THE INVENTION
The invention proposed here employs a different method of measuring the time of flight of ultrasound signals, by recording and processing transmitted and received waveforms.
According to the present invention there is provided a catheter tracking system for locating and tracking a catheter head with reference to a part of the human or animal body, said catheter tracking system comprising of a plurality of spaced apart transducers at least one of which is disposed on a catheter and at least two others of which are disposed at predetermined reference points, and a signal processing unit coupled to said plurality of spaced apart transducers and arranged to selectively enable individual ones of said plurality of spaced apart transducers to operate as one of either a transmitter or a receiver, and to energise at least one of said transmitters such that at least one of said transmitters generates at least one transmitted signal which is detected by at least one of said receivers, wherein the time of flight of said at least one transmitted signal generated by said at least one transmitter and received by said at least one receiver is indicative of the distance between said at least one transmitter and said at least one receiver, whereby said times of flight between said at least one transducer disposed on said catheter and said at least two other transducers disposed at said predetermined reference points gives an indication of said location of said catheter with reference to said reference transducers, characterised in that said at least one transmitted signal has a predetermined waveform, and said signal processing unit further operates to compare said at least one transmitted signal generated by said at least one transmitter with a signal representative of said transmitted signal received by said at least one receiver, and consequent upon said comparison determines said time of flight of said at least one transmitted signal to a substantially greater accuracy.
One technique which could be used for comparison of the transmitted signal with the signal representative of the transmitted signal is cross-correlation of pairs of transmitted and received signals.
By cross-correlating the transmitted and received signals an increase in accuracy in the time of flight is effected from a peak produced from the cross-correlation at a temporal displacement corresponding to the time of flight.
The present invention offers several advantages over the prior art for a catheter tracking system. An improvement in the accuracy with which time of flight measurements are determined allows for distance measurements to be made at lower signal-to-noise ratios (SNR). This enables measurements to be made over longer propagation distances, thus overcoming the problem associated with low signal strength due to losses caused by absorption, reflection, refraction, and scattering of the ultrasound wave as it propagates through a medium.
Additionally, ultrasound transducers are generally very inefficient, and often need to be driven with high voltages to produce sufficient sound power. Comparing transmitted and received signals in this way significantly decreases the drive voltages carried on wires inside the catheters, effectively reducing both the electrical hazard to the patient, and the levels of potential electromagnetic interference. The output powers of small ultrasound transmitters are also limited by the need to avoid damage to neighbouring tissue.
Advantageously the predetermined waveform generated from each of said plurality of transducers are different with respect to each other, thereby facilitating contemporaneous detection of said signals.
The present invention allows for signals received contemporaneously from different transmitters to be separated and processed individually, provided the transmitted signals have known and distinguishable waveforms. For example, narrow band signals at different centre frequencies can be separated by bandpass filtering. This means that all the transmitting transducers could be activated contemporan
Crowell & Moring LLP
Dahbour Fadi H.
Roke Manor Research Limited
Walberg Teresa
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