Investigation of active tracking for robotic arm assisted magnetic resonance guided focused ultrasound ablation.

BACKGROUND: Focused ultrasound surgery (FUS) is a technique that does not need invasive access to the patient while allowing precise targeted therapy. Magnetic resonance (MR) guided FUS provides capabilities for monitoring treatments. Because the targeted tumours are distributed at different positions, focus repositioning becomes necessary. METHODS: An MR compatible robot was used to increase the operational range of FUS application. Active tracking was developed to detect the robotic arm with regard to the MR coordinate system. The purpose of this study was to construct active tracking to allow a wide spatial range of repositioning the FUS transducer that is fast and accurate. The technique was characterised and validated by a series of positioning tests to prove its efficiency for guiding the robot. RESULTS: In the calibration range, tracking achieved an RMS accuracy of 0.63 mm. Results of phantom ablation showed a focal scanning precision Δx = 0.4 ± 0.37 mm, Δy = 0.4 ± 0.28 mm and Δz = 0.7 ± 0.66 mm. CONCLUSIONS: The active tracking localisation can be considered a feasible approach for the MR guided FUS system positioned by a robot. Copyright © 2016 John Wiley & Sons, Ltd.

Wireless mobile technology to improve workflow and feasibility of MR-guided percutaneous interventions.

PURPOSE: A wireless interactive display and control device combined with a platform-independent web-based user interface (UI) was developed to improve the workflow for interventional magnetic resonance imaging (iMRI). METHODS: The iMRI-UI enables image acquisition of up to three independent slices using various pulse sequences with different contrast weighting. Pulse sequence, scan geometry and related parameters can be changed on the fly via the iMRI-UI using a tablet computer for improved lesion detection and interventional device targeting. The iMRI-UI was validated for core biopsies with a liver phantom ([Formula: see text] [Formula: see text] 40) and Thiel soft-embalmed human cadavers ([Formula: see text] [Formula: see text] 24) in a clinical 1.5T MRI scanner. RESULTS: The iMRI-UI components and setup were tested and found conditionally MRI-safe to use according to current ASTM standards. Despite minor temporary touch screen interference at a close distance to the bore ([Formula: see text]20 cm), no other issues regarding quality or imaging artefacts were observed. The 3D root-mean-square distance error was [Formula: see text] (phantom)/[Formula: see text] mm (cadaver), and overall procedure times ranged between 12 and 22 (phantom)/20 and 55 min (cadaver). CONCLUSION: The wireless iMRI-UI control setup enabled fast and accurate interventional biopsy needle placements along complex trajectories and improved the workflow for percutaneous interventions under MRI guidance in a preclinical trial.

Preclinical feasibility of a technology framework for MRI-guided iliac angioplasty.

PURPOSE: Interventional MRI has significant potential for image guidance of iliac angioplasty and related vascular procedures. A technology framework with in-room image display, control, communication and MRI-guided intervention techniques was designed and tested for its potential to provide safe, fast and efficient MRI-guided angioplasty of the iliac arteries. METHODS: A 1.5-T MRI scanner was adapted for interactive imaging during endovascular procedures using new or modified interventional devices such as guidewires and catheters. A perfused vascular phantom was used for testing. Pre-, intra- and post-procedural visualization and measurement of vascular morphology and flow was implemented. A detailed analysis of X-ray fluoroscopic angiography workflow was conducted and applied. Two interventional radiologists and one physician in training performed 39 procedures. All procedures were timed and analyzed. RESULTS: MRI-guided iliac angioplasty procedures were successfully performed with progressive adaptation of techniques and workflow. The workflow, setup and protocol enabled a reduction in table time for a dedicated MRI-guided procedure to 6 min 33 s with a mean procedure time of 9 min 2 s, comparable to the mean procedure time of 8 min 42 s for the standard X-ray-guided procedure. CONCLUSIONS: MRI-guided iliac vascular interventions were found to be feasible and practical using this framework and optimized workflow. In particular, the real-time flow analysis was found to be helpful for pre- and post-interventional assessments. Design optimization of the catheters and in vivo experiments are required before clinical evaluation.

Resonant marker design and fabrication techniques for device visualization during interventional magnetic resonance imaging.

Magnetic resonance imaging (MRI) has great potential as an imaging modality for guiding minimally invasive interventions because of its superior soft tissue contrast and the possibility of arbitrary slice positioning while avoiding ionizing radiation and nephrotoxic iodine contrast agents. The major constraints are: limited patient access, the insufficient assortment of compatible instruments and the difficult device visualization compared to X-ray based techniques. For the latter, resonant MRI markers, fabricated by using the wire-winding technique, have been developed. This fabrication technique serves as a functional model but has no clinical use. Thus, the aim of this study is to illustrate a four-phase design process of resonant markers involving microsystems technologies. The planning phase comprises the definition of requirements and the simulation of electromagnetic performance of the MRI markers. The following technologies were considered for the realization phase: aerosol-deposition process, hot embossing technology and thin film technology. The subsequent evaluation phase involves several test methods regarding electrical and mechanical characterization as well as MRI visibility aspects. The degree of fulfillment of the predefined requirements is determined within the analysis phase. Furthermore, an exemplary evaluation of four realized MRI markers was conducted, focusing on the performance within the MRI environment.

Comparative ergonomic workflow and user experience analysis of MRI versus fluoroscopy-guided vascular interventions: an iliac angioplasty exemplar case study.

PURPOSE: A methodological framework is introduced to assess and compare a conventional fluoroscopy protocol for peripheral angioplasty with a new magnetic resonant imaging (MRI)-guided protocol. Different scenarios were considered during interventions on a perfused arterial phantom with regard to time-based and cognitive task analysis, user experience and ergonomics. METHODS: Three clinicians with different expertise performed a total of 43 simulated common iliac angioplasties (9 fluoroscopic, 34 MRI-guided) in two blocks of sessions. Six different configurations for MRI guidance were tested in the first block. Four of them were evaluated in the second block and compared to the fluoroscopy protocol. Relevant stages' durations were collected, and interventions were audio-visually recorded from different perspectives. A cued retrospective protocol analysis (CRPA) was undertaken, including personal interviews. In addition, ergonomic constraints in the MRI suite were evaluated. RESULTS: Significant differences were found when comparing the performance between MRI configurations versus fluoroscopy. Two configurations [with times of 8.56 (0.64) and 9.48 (1.13) min] led to reduce procedure time for MRI guidance, comparable to fluoroscopy [8.49 (0.75) min]. The CRPA pointed out the main influential factors for clinical procedure performance. The ergonomic analysis quantified musculoskeletal risks for interventional radiologists when utilising MRI. Several alternatives were suggested to prevent potential low-back injuries. CONCLUSIONS: This work presents a step towards the implementation of efficient operational protocols for MRI-guided procedures based on an integral and multidisciplinary framework, applicable to the assessment of current vascular protocols. The use of first-user perspective raises the possibility of establishing new forms of clinical training and education.

Heating and safety of a new MR-compatible guidewire prototype versus a standard nitinol guidewire.

Our purpose in this study was to examine heating of nitinol and polyetheretherketone (PEEK) guidewires during near-real-time MR imaging in an artificial vascular model an "aorta phantom". The first 100 cm of the nitinol- and PEEK-based guidewires both 145 × 0.08 cm were immersed in a saline-filled aorta phantom. The probes of a fiber-optic thermometer were positioned at the tips of both wires. Balanced steady-state free precession (bSSFP) [TE 1.6 ms; TR 3.5 ms; flip angle (FA) 60°; field of view (FOV) 40 cm; matrix 256 × 256; specific absorption rate (SAR); 1.15 Watt (W)/kg] and spoiled gradient-echo (SPGR) (TE 1.8 ms; TR 60 ms; FA 60°; FOV 40 cm; matrix 256 × 256; SAR 1.15 W/kg) pulse sequences were acquired in a 1.5-T MR scanner with use of an 8-channel array coil. Temperatures were recorded while the phantom was placed centrally in the bore of a MR scanner and in an off-center position (x = 24 cm, y = -5 cm, z = -10/10 cm). The temperature of the nitinol guidewire increased by 0.3 °C (center) and 1.1 °C (off-center position) with use of the bSSFP and by 9.6 and 13 °C (off-center position) with use of the SPGR sequence. Only minor temperature changes up to a maximum of 0.4 °C were observed with the MR-compatible PEEK guidewire when any position or sequence was applied. The PEEK guidewire showed substantially lower heating as compared to the nitinol guidewire in near-real-time imaging sequences in a phantom.

Wireless MR tracking of interventional devices using phase-field dithering and projection reconstruction.

PURPOSE: Device tracking is crucial for interventional MRI (iMRI) because conventional device materials do not contribute to the MR signal, may cause susceptibility artifacts and are generally invisible if moved out of the scan plane. A robust method for wireless tracking and dynamic guidance of interventional devices equipped with wirelessly connected resonant circuits (wRC) is presented. METHODS: The proposed method uses weak spatially-selective excitation pulses with very low flip angle (0.3°), a Hadamard multiplexed tracking scheme and employs phase-field dithering to obtain the 3D position of a wRC. RF induced heating experiments (ASTM protocol) and balloon angioplasties of the iliac artery were conducted in a perfused vascular phantom and three Thiel soft-embalmed human cadavers. RESULTS: Device tip tracking was interleaved with various user-selectable fast pulse sequences receiving a geometry update from the tracking kernel in less than 30ms. Integrating phase-field dithering significantly improved our tracking robustness for catheters with small diameters (4-6 French). The volume root mean square distance error was 2.81mm (standard deviation: 1.31mm). No significant RF induced heating (<0.6°C) was detected during heating experiments. CONCLUSION: This tip tracking approach provides flexible, fast and robust feedback loop, intuitive iMRI scanner interaction, does not constrain the physician and delivers very low specific absorption rates. Devices with wRC can be exchanged during a procedure without modifications to the iMRI setup or the pulse sequence. A drawback of our current implementation is that position information is available for a single tracking coil only. This was satisfactory for balloon angioplasties of the iliac artery, but further studies are required for complex navigation and catheter shapes before animal trials and clinical application.