Ultrasonic tracking of ultrasound transducer(s) aboard an interventional tool

#The present invention is directed to one-way only beamforming of transmissive ultrasound and, more particularly, to using such beamforming for tracking an interventional tool during real-time imaging.

#Precise visualization of catheters and real-time knowledge of their localization with respect to the anatomy are needed for minimally invasive interventions. Intra-operative ultrasound is often used for these purposes. However, many surgical tools are difficult to image with conventional pulse-echo ultrasound. For instance, the usability of 3D Transoesophagial Echocardiography (3D-TEE) for guidance of catheter cardiac interventions is still limited because it is challenging to image catheters reliably with ultrasound. Catheters are specular reflectors that reflect the sound away from the imaging probe if the insonifying angles are not favorable. As a consequence, a catheter appears on and off on 3D-TEE images during its progression through the cardiac chambers. It also frequently happens that some parts of the catheter are visible and others not depending on the local angle between the catheter and the imaging beams, for instance the distal end of the catheter may be invisible and some point along its shaft may be mistaken as its tip. Also, due to weak reflection, signal from the catheter may be drowned in signal from the surrounding anatomy.

#Electromagnetic (EM) tracking sensors have been mounted aboard catheters for tracking their tip and other selected locations along their shaft. However, the positioning accuracy of such sensors can get really poor (of the order of 10 mm) in EM distorted operating environment. Additionally, an independent EM tracking system adds to the equipment cost and clutter in the catheter laboratory.

#FIG. 1 is a conceptual diagram offering a comparison between two-way beamforming and one-way only beamforming;

#FIG. 2 is a schematic diagram depicting a configuration, which uses synthetic aperture with virtual transducers, in which a receiving ultrasound transducer fixed to a catheter is disposed within a region of interest

#FIG. 3 is a conceptual diagram that portrays a synthetic aperture acquisition scheme in the top drawing, and the same scheme using virtual transducers in the bottom drawing;

#FIG. 1 offers, by way of illustrative and non-limitative example, a comparison between two-way beamforming and one-way only beamforming. The top figure, representative of two-way beamforming shows an imaging array 102 of N elements 104 issuing ultrasound that impinges on a reflector 106. Since the ultrasound waves go out and back (from the imaging array to the reflectors and back to the imaging array), we talk of “two-way” or “round-trip” beamforming. On receive (of the ultrasound that has reflected back), beamforming determines the reflectivity of the reflector 106 and the position of the reflector relative to the array 102. The array 102 send out a beam 108 that reflects off reflector 106 and returns to all elements 104 of the array 102. The flight of the pulse is over a distance r(P)+d(i,P) for element i. Each element 104 measures continually the amplitude of the return ultrasound. For each element 104, the time until a maximum of that measurement, i.e., the “round-trip time of flight,” is indicative of the total flight distance. Since the r(P) leg of the flight is constant, the return flight distance d(i,P) is determined. From these measurements, the relative position of the reflector 106 is computed geometrically. As to the reflectivity of the reflector 106, it can be indicated by summing the maxima over all i (i.e., over all elements 104).

#As seen from the bottom figure, one-way only (receive) beamforming, there is no echo. Instead, as in the case of transmitter 110, it emits a pulse 112 which is incident on each element 104 of the array 102. The flight here, in contrast to the two-way beamforming case, is over the distance d(i,P). The time from emission of the pulse 112 until the maximum amplitude reading at an element 104 determines the value d(i,P) for that element i. Thus, the position of the transmitter 110 can be derived geometrically, and the reflectivity calculated by summing the maximum amplitude readings.

#Although one-way beamforming is implementable in the time domain via delay logic, as discussed hereinabove, it can also be implemented in the frequency domain by well-known Fourier beamforming algorithms.