Tracking planar orientations of active MRI needles

Closed-loop asymmetric-tip needle steering under continuous intraoperative MRI guidance

Magnetic resonance imaging (MRI) provides excellent image contrast for various types of tissues, making it a suitable choice over other imaging modalities for various image-guided needle interventions. Furthermore, robot-assistance is maturing for surgical procedures such as percutaneous prostate and brain interventions. Although MRI-guided, robot-assisted needle interventions are approaching clinical usage, they are still typically open-loop in nature due to the lack of continuous intraoperative needle tracking. Closed-loop needle-based procedures can improve the accuracy of needle tip placement by correcting the needle trajectory during insertion. This paper proposes a system for robot-assisted, flexible asymmetric-tipped needle interventions under continuous intraoperative MRI guidance. A flexible needle's insertion depth and rotation angle are manipulated by an MRI-compatible robot in the bore of the MRI scanner during continuous multi-planar image acquisition to reach a desired target location. Experiments are performed on gelatin phantoms to assess the accuracy of needle placement into the target location. The system was able to successfully utilize live MR imaging to guide the path of the needle, and results show an average total targeting error of 2.5±0.47mm, with an average in-plane error of 2.09±0.33mm.

Visualization of active devices and automatic slice repositioning ("SnapTo") for MRI-guided interventions

The accurate visualization of interventional devices is crucial for the safety and effectiveness of MRI-guided interventional procedures. In this paper, we introduce an improvement to the visualization of active devices. The key component is a fast, robust method ("CurveFind") that reconstructs the three-dimensional trajectory of the device from projection images in a fraction of a second. CurveFind is an iterative prediction-correction algorithm that acts on a product of orthogonal projection images. By varying step size and search direction, it is robust to signal inhomogeneities. At the touch of a key, the imaged slice is repositioned to contain the relevant section of the device ("SnapTo"), the curve of the device is plotted in a three-dimensional display, and the point on a target slice, which the device will intersect, is displayed. These features have been incorporated into a real-time MRI system. Experiments in vitro and in vivo (in a pig) have produced successful results using a variety of single- and multichannel devices designed to produce both spatially continuous and discrete signals. CurveFind is typically able to reconstruct the device curve, with an average error of approximately 2 mm, even in the case of complex geometries.

Imaging in percutaneous nephrolithotomy

Image guidance is a critical factor for the performance of urologic interventions. Percutaneous minimally invasive procedures have been developed and are being used with constantly increasing frequency. Procedures such as percutaneous nephrolithotomy (PCNL) are not performed without any image guidance. Recent developments in medical imaging, such as three-dimensional radiographic fluoroscopy, CT, and magnetic resonance (MR) fluoroscopy, four-dimensional ultrasonography, and image fusion techniques, propose a new generation of image-guidance tools that promise to improve patient care. These developments have been used or have the potential to be used in PCNL and other urologic interventional procedures. Moreover, advanced needles and needle guidance systems provide a new perspective for the nephrolithotomy suite of the future. The current review presents existing imaging technology in PCNL and interventional urology as well as advanced imaging techniques that are being or are expected to be evaluated in PCNL practice.

Whole shaft visibility and mechanical performance for active MR catheters using copper-nitinol braided polymer tubes

BACKGROUND

Catheter visualization and tracking remains a challenge in interventional MR.Active guidewires can be made conspicuous in "profile" along their whole shaft exploiting metallic core wire and hypotube components that are intrinsic to their mechanical performance. Polymer-based catheters, on the other hand, offer no conductive medium to carry radio frequency waves. We developed a new "active" catheter design for interventional MR with mechanical performance resembling braided X-ray devices. Our 75 cm long hybrid catheter shaft incorporates a wire lattice in a polymer matrix, and contains three distal loop coils in a flexible and torquable 7Fr device. We explored the impact of braid material designs on radiofrequency and mechanical performance.

RESULTS

The incorporation of copper wire into in a superelastic nitinol braided loopless antenna allowed good visualization of the whole shaft (70 cm) in vitro and in vivo in swine during real-time MR with 1.5 T scanner. Additional distal tip coils enhanced tip visibility. Increasing the copper:nitinol ratio in braiding configurations improved flexibility at the expense of torquability. We found a 16-wire braid of 1:1 copper:nitinol to have the optimum balance of mechanical (trackability, flexibility, torquability) and antenna (signal attenuation) properties. With this configuration, the temperature increase remained less than 2 degrees C during real-time MR within 10 cm horizontal from the isocenter. The design was conspicuous in vitro and in vivo.

CONCLUSION

We have engineered a new loopless antenna configuration that imparts interventional MR catheters with satisfactory mechanical and imaging characteristics. This compact loopless antenna design can be generalized to visualize the whole shaft of any general-purpose polymer catheter to perform safe interventional procedures.

MRI endoscopy using intrinsically localized probes

Magnetic resonance imaging (MRI) is traditionally performed with fixed externally applied gradient magnetic fields and is hence intrinsically locked to the laboratory frame of reference (FoR). Here a method for high-resolution MRI that employs active, catheter-based, tiny internal probes that utilize the spatial properties of the probe itself for localization is proposed and demonstrated at 3 T. Because these properties are intrinsic to the probe, they move with it, transforming MRI from the laboratory FoR to the FoR of the device itself, analogous to an endoscope. The "MRI endoscope" can utilize loop coils and loopless antennas with modified sensitivity, in combination with adiabatic excitation by the device itself, to restrict the MRI sensitivity to a disk-shaped plane a few mm thick. Excitation with the MRI endoscope limits the eddy currents induced in the sample to an excited volume whose size is orders of magnitude below that excited by a conventional body MRI coil. Heat testing shows maximum local temperature increases of <1 degrees C during MRI, within regulatory guidelines. The method is demonstrated in a kiwifruit, in intact porcine and rabbit aortas, and in an atherosclerotic human iliac artery specimen, with in-plane resolution as small as 80 microm and 1.5-5 mm slice thickness.