Stereotactic endovascular aortic navigation with a novel ultrasonic-based three-dimensional localization system.

BACKGROUND: Endovascular aortic procedures have been developed to treat many aortic diseases effectively. However, these procedures are also becoming increasingly complex given the development of branched or fenestrated endografts. Part of the difficulty lies in the limitations of current imaging paradigms. A more intuitive, three-dimensional (3D) mode of intraoperative imaging is desirable to accommodate the future progression of endovascular techniques. This article describes a novel endovascular catheter tracking device that uses ultrasonic signals, not ultrasound imaging. The tracking device displays real-time in vivo location on previously acquired 3D computed tomography (CT) images in an intuitive, endoluminal view. This system was tested in two swine and validated against fluoroscopy and by delivering stent grafts. METHODS: The ultrasonic-based localization system (ULS) provides real-time location information of a modified endovascular catheter and displays this location on preoperative 3D CT images. The 9F endovascular catheter has a small ultrasonic transmitter attached to its tip to signal its location to the ULS. Subsequent endovascular deployment of an aortic stent was carried out using only the ULS to target the stent placement position in the aorta of Yorkshire swine. System accuracy was measured against concurrent angiography as well as to deployed stents in situ. RESULTS: We successfully displayed the endovascular catheter tip location in real time along the registered CT aortic images, providing virtual endoluminal tracking. The relative accuracy of the ULS as compared with angiography for catheter movements in the abdominal aorta was found to have a mean error less than 1 mm. The ULS coordinates tracked within the lumen of the aortic image 98% of the time, as defined by the proportion of points within one radius distance of the aortic image centerline. Finally, three aortic stents were deployed using the ULS virtual image display to locate the target position in the aorta for stent deployment. Errors between target position and actual stent position ranged from -5.0 to +7.9 mm. CONCLUSIONS: This study demonstrates the feasibility of virtual image-guided endovascular aortic navigation using a ULS. This provides a 3D platform for virtual navigation on preoperative CT scan images during endovascular procedures that could assist in stent deployment as well as minimize or eliminate the need for procedural ionizing radiation and iodinated contrast. Future work will focus on miniaturization and refinements in accuracy that will be required to translate this technology into clinical application in endovascular procedures.

A non-disruptive technology for robust 3D tool tracking for ultrasound-guided interventions.

In the past decade ultrasound (US) has become the preferred modality for a number of interventional procedures, offering excellent soft tissue visualization. The main limitation however is limited visualization of surgical tools. A new method is proposed for robust 3D tracking and US image enhancement of surgical tools under US guidance. Small US sensors are mounted on existing surgical tools. As the imager emits acoustic energy, the electrical signal from the sensor is analyzed to reconstruct its 3D coordinates. These coordinates can then be used for 3D surgical navigation, similar to current day tracking systems. A system with real-time 3D tool tracking and image enhancement was implemented on a commercial ultrasound scanner and 3D probe. Extensive water tank experiments with a tracked 0.2mm sensor show robust performance in a wide range of imaging conditions and tool position/orientations. The 3D tracking accuracy was 0.36 +/- 0.16mm throughout the imaging volume of 55 degrees x 27 degrees x 150mm. Additionally, the tool was successfully tracked inside a beating heart phantom. This paper proposes an image enhancement and tool tracking technology with sub-mm accuracy for US-guided interventions. The technology is non-disruptive, both in terms of existing clinical workflow and commercial considerations, showing promise for large scale clinical impact.

Design and in vitro evaluation of a real-time catheter localization system using time of flight measurements from seven 3.5 MHz single element ultrasound transducers towards abdominal aortic aneurysm procedures.

Interventional surgical instrument localization is a crucial component of minimally invasive surgery. Image guided surgery researchers are investigating devices broadly categorized as surgical localizers to provide real-time information on the instrument's 3D location and orientation only. This paper describes the implementation and in vitro evaluation of a prototype real-time nonimaging ultrasound-based catheter localizer system towards use in abdominal aortic aneurysm procedures. The catheter-tip is equipped with a single element ultrasound transducer which is tracked with an array of seven external single element transducers. The performance of the system was evaluated in a water tank and additionally in the presence of pork belly tissue and also a nitinol-dacron stent graft. The mean root mean square errors were respectively 1.94±0.06, 2.54±0.31 and 3.33±0.06 mm. In addition, this paper illustrates errors induced by transducer aperture size and suggests a method for aperture error compensation. Aperture compensation applied to the same experimental data yielded mean root mean square errors of 1.05±0.07, 2.42±0.33 and 3.23±0.07mm respectively for water; water and pork; and water, pork and stent experiments. Lastly, this paper presents a video showing free-hand movement of the catheter within the water tank with data capture at 25 frames per second.

Fresnel-based beamforming for low-cost portable ultrasound.

In this paper, we propose a modified electronic Fresnel-based beamforming method for low-cost portable ultrasound systems. This method uses a unique combination of analog and digital beamforming methods. Two versions of Fresnel beamforming are presented in this paper: 4-phase (4 different time delays or phase shifts) and 8-phase (8 different time delays or phase shifts). The advantage of this method is that a system with 4 to 8 transmit channels and 2 receive channels with a network of switches can be used to focus an array with 64 to 128 elements. The simulation and experimental results show that Fresnel beamforming image quality is comparable to traditional delay-and-sum (DAS) beamforming in terms of spatial resolution and contrast-to-noise ratio (CNR) under certain system parameters. With an f-number of 2 and 50% signal bandwidth, the experimental lateral beamwidths are 0.54, 0.67, and 0.66 mm and the axial pulse lengths are 0.50, 0.51, and 0.50 mm for DAS, 8-phase, and 4-phase Fresnel beamforming, respectively. The experimental CNRs are 4.66, 4.42, and 3.98, respectively. These experimental results are in good agreement with simulation results.

The influence of stents on the performance of an ultrasonic navigation system for endovascular procedures.

OBJECTIVE: Image-guided surgery provides a mechanism to accurately and quickly assess the location of surgical tools relative to a preoperative image. Traditional image-guided surgery relies on infrared or radiofrequency triangulation to determine an instrument location relative to a preoperative image and has been primarily used in head and neck procedures. Advances in ultrasonic tracking devices, designed for tracking catheters within vessels, may provide an opportunity for image-guided endovascular procedures. This study evaluates the positional accuracy of an ultrasonic navigation system for tracking an endovascular catheter when different stents and graft materials have been deployed in an in vitro system. METHODS: Stent and graft materials commonly used in endovascular procedures were used for this study in combination with a custom three-head ultrasonic transducer navigation system. The stents evaluated were composed of Dacron/nitinol, polytetrafluoroethylene (PTFE)/nitinol, and bare nitinol. They were deployed into excised porcine tissue cannulized with a rotary drill, and a catheter with a custom microtransducer probe was inserted. The distance from each ultrasonic tracking module to a probe mounted on an endovascular catheter was measured using time of flight techniques, and the catheter position in three-dimensions was calculated using triangulation. RESULTS: The measured position was compared to the actual catheter position determined by a precision translation stage. The PTFE/nitinol, bare nitinol, and Dacron/nitinol stent materials were evaluated and resulted in a maximum error of 1.7, 3.0, and 3.6 mm and an SD of 0.7, 1.2, and 1.4 mm, respectively. A reduction in signal intensity of up to 6x was observed during passage of the endovascular probe through the stent materials, but no reduction in the accuracy of the ultrasonic navigation system was evident. CONCLUSION: The use of an ultrasonic-based navigation system is feasible in endovascular procedures, even in the presence of common stent materials. It may have promise as a navigational tool for endovascular procedures.

An ultrasonic navigation system for endovascular aortic repair.

The objective of our research is to develop an ultrasound, time-of-flight triangulation based system for real-time, intraoperative 3-dimensional tracking of minimally invasive surgical instruments. We have chosen to first apply this technology towards tracking catheters used in endovascular aortic stent-grafting. To demonstrate the feasibility of this concept, we have developed a system involving a custom catheter based transducer and performed an experiment in excised porcine tissue with a model aorta.