Step catheters 20141109backflow and prestress1 Backflow mitigation in stepped catheters Raghavan R.1, Brady M.L.1, Grabow B.3, and Block W.2,3,4 1Therataxis LLC, Baltimore, MD 21218, USA; Departments of 2Medical Physics, 3Bioengeneering , University of Wisconsin, Madison, WI 53715, USA; 4inseRT MRI, Madison, WI CORRESPONDING AUTHOR: Raghu Raghavan Therataxis, LLC 1101 E. 33rd Street, Ste. B305, Baltimore, MD 21218 (443) 451-7154 (phone); (443) 451-7157 (fax) [email protected] SHORT TITLE: Stepped catheters KEY WORDS: Convection-enhanced delivery, backflow, intraparenchymal infusion, magnetic resonance imaging, reflux-resistant, catheters, prestress.2 ABSTRACT Background: Backflow of infusate along the catheter shaft can lead to undesirable distributions of drug during convection-enhanced delivery into the brain parenchyma. Stepped catheter geometries have been suggested to limit backflow. Objective: The principal objective of this study was to compare two different-sized step catheter designs, SmartFlow™ 16- (SF16)and 14-gauge (SF14) catheters, for their backflow characteristics at various flow rates. Methods: The experiments were conducted in live porcine brains, with catheters were inserted into the thalamus. In-vivo infusions, monitored continuously, of diluted gadodiamide at flow rates increasing from 2.5 L/min to 10 L/min were conducted to observe the backflow in the SF catheters which were placed at the same location in either thalamus. Results: The smaller SF16 was far more likely to restrain backflow at the 3mm step than the larger SF14, particularly at the lower flow rates. This led to lower average backflow (3.9±2.0 mm for the 16-gauge vs. 9.0±5.0 mm for the 14-gauge at 2.5 μl/min). Conclusions: The SF16 stepped catheter proved more likely to restrict backflow at the step. A simple theoretical estimate, based on our previous study of backflow, suggests that prestress in the tissue is an additional effect on backflow –compared with the effects accounted for in the usual theories of backflow -- that may account for the superior performance. However, the SF16 has then two features which reduce backflow in comparison with the SF14: the smaller diameter of the tip, and the larger width of the step. The experiments could not disentangle the effects due to each, and unclear whether the smaller diameters of the step and shaft, or the larger step size are primarily responsible for the difference. Further studies would be valuable in clarifying the causes of backflow reduction.3 INTRODUCTION MATERIALS AND METHODS All experiments were performed at the University of Wisconsin, Madison. Animal model: All infusions were targeted in the thalamus of adolescent swine, as it is a large enough volume to contain multiple high-volume infusions during experiments at high flow rates. The swine was chosen over NHP models for its similarity of head size to humans, appropriate use of animal resources, lack of disease transmission risks, and substantial reduction in required animal care personnel during the procedure. The swine has also been extensively utilized elsewhere for investigations involving CED procedures in the brain [1-3]. Surgery and Targeting Method. We utilized a platform for MR-guided neurological surgery that was first implemented at UW-Madison [4-6] and is now being extended by a commercial entity (inseRT MRI, Madison, WI). The platform accesses the scanner hardware through a commercial portal termed RTHawk, (HeartVista, Palo Alto, CA) [4], that permits the design and execution of image-guided procedures using high level imaging plug-ins to define a procedural workflow. The platform allows the interventionalist to manipulate the brain ports used to orient catheters with real-time feedback similar to a stereotactic OR suite, rather than the iterative “shoot and scoot” manipulation provided commercially in MR image guidance today. Through a bidirectional link with RTHawk, image data acquired in real-time was displayed by Vurtigo (Visual Understanding of Real-Time Image Guided Operations, Sunnybrook Health Sciences Centre; Toronto, Canada), an open-source visualization platform that allows simultaneous display and interaction with multiple 3D and 2D datasets [7]. Changes to the desired acquisition geometry made in Vurtigo were sent to RTHawk and then on to the scanner, allowing visualization of dynamically acquired 2D planes overlaid on previously acquired 3D volumes. This link was implemented using a “geometry server” software program that communicates with RTHawk and Vurtigo. Real-time scan control and visualization was conducted on a high-performance external workstation with two quad-core Intel Xeon E5620 2.4 GHz CPUs, 12 GB of memory, an NVIDIA GF100 Quadro 4000 graphics card, and gigabit Ethernet controller, running 64-bit Linux. Scanner interface was via an internal Ethernet switch. Visualization display was available on a screen in the control room, which was placed in the scanner room window so that an operator could lean into the bore and reposition the MR-visible fluid-filled alignment guide to the optimal trajectory angle The device targeting capability is based on a real-time implementation of prospective stereotaxy [8]. The NavigusTM brain port consists of a MR-visible fluid-filled alignment guide seated in a ball-and-socket pivot joint with two degrees of rotational freedom. The visible tip of the guide was positioned at the center of the pivot joint, and the body of the guide extended away from the skull along the trajectory of device insertion. We identified the desired target point in the brain and the location of the pivot point by first acquiring a high resolution 3D T1-weighted “roadmap” volume with inversion recovery preparation. After identifying location markers of the alignment guide pivot point and the Ce target point, the trajectory alignment software tool calculated an “aiming point” outside the skull that was co-linear with the target and alignment guide pivot4 points. Real-time imaging of a plane perpendicular to and centered on the aiming point provided dynamic feedback during the alignment step, allowing the operator to incrementally move the alignment guide until the distal end of the alignment guide overlapped with the software-displayed aiming point. When the trajectory angle (antero-postrerior, medio-lateral direction) of the fluid-filled alignment guide was confirmed to be
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