Martin A. Rube

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.

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

Abstract 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.

Thermometry during MR-guided focused ultrasound in a preclinical model based on Thiel embalmed tissue

Abstract Background: The purpose of this work was to determine the accuracy of Proton Resonance Frequency (PRF) thermometry during MR-guided Focused Ultrasound (MRgFUS) ablation on explanted Thiel embalmed human and animal liver, fresh animal liver, and compared to gel phantom. Material and methods: PRF thermometry during MRgFUS was conducted using a 1.5T MRI system. The phantom and the organs were sonicated with the following energies:300J, 600J, 1000J and 1400J. The temperature increase which was measured using PRF thermometry during sonication was compared to actual temperature rise in the same conditions measured by fibre optic thermocouple. Results: Sonication of fresh animal liver showed temperature differences varying between 0.27°C and 0.40°C, whereas the phantom results showed temperature differences from 0.23°C to 0.40°C. For the Thiel embalmed organs, the temperature difference varied from 1.17 °C to 3.13°C for the ovine liver, and from 1.3°C to 3.10°C for the human liver. Conclusion: The temperature differences measured in the fresh liver were small and similar to those found for the gel phantom. However, the temperature differences calculated for the Thiel embalmed organs were higher compared to the fresh organ. This indicates that the PRF-based temperature calibration of the Focused Ultrasound machine for Thiel embalmed tissue is necessary.

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

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.

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

PurposeA 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.MethodsThree 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.ResultsSignificant 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.ConclusionsThis 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.

MR real-time tracking of hepatic motion during respiration in a Thiel Soft-fix cadaver

Thiel Soft-fix cadavers represent an alternative to more traditional formaldehyde cadaveric embalmment, preservatives sustain tissue flexibility, colouring, plus integrity therefore, Thiel cadavers simulate anatomically correct models for the purposes of human research, education and training. The purpose of this study was to track hepatic movement during artificial respiration with Magnetic Resonance Imaging (MRI).

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

PurposeInterventional 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.MethodsA 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.ResultsMRI-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.ConclusionsMRI-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.

Development of a fiber based Raman probe compatible with interventional magnetic resonance imaging

Raman spectroscopy has proven to be a powerful tool for discriminating between normal and abnormal tissue types. Fiber based Raman probes have demonstrated its potential for in vivo disease diagnostics. Combining Raman spectroscopy with Magnetic Resonance Imaging (MRI) opens up new avenues for MR guided minimally invasive optical biopsy. Although Raman probes are commercially available, they are not compatible with a MRI environment due to the metallic components which are used to align the micro-optic components such as filters and lenses at the probe head. Additionally they are not mechanically compatible with a typical surgical environment as factors such as sterility and length of the probe are not addressed in those designs. We have developed an MRI compatible fiber Raman probe with a disposable probe head hence maintaining sterility. The probe head was specially designed to avoid any material that would cause MR imaging artefacts. The probe head that goes into patient’s body had a diameter <1.5 mm so that it is compatible with biopsy needles and catheters. The probe has been tested in MR environment and has been proven to be capable of obtaining Raman signal while the probe is under real-time MR guidance.

Iron-Platinum Alloy Nanoparticles for Guidewire and Resonant Markers for Catheter Localization during Interventional MRI

MA Rube, Institute for Medical Science and Technology, University of Dundee, Dundee, UK, m.rube@dundee.ac.uk BF Cox, Institute for Medical Science and Technology, University of Dundee, Dundee, UK, b.cox@dundee.ac.uk M Gueorguieva, Institute for Medical Science and Technology, University of Dundee, Dundee, UK, m.gueorguieva@dundee.ac.uk D Kakchingtabam, Physics and Astronomy, University of St Andrews, St Andrews, UK, dk342@st-andrews.ac.uk P Andre Physics and Astronomy, University of St Andrews, St Andrews, UK, Pascal.Andre@st-andrews.ac.uk A Melzer, Institute for Medical Science and Technology, University of Dundee, Dundee, UK, a.melzer@dundee.ac.uk

PRF thermometry during MR-guided focused ultrasound ablation in a preclinical thiel model

Proton Resonance Frequency (PRF) MR Thermometry is a useful method for treatment planning with MR-guided Focused Ultrasound (MRgFUS), as it provides accurate, real-time temperature maps. Thiel is an embalming medium that retains physical properties and life-like characteristics of human and animal tissue. The aim of the present study was to determine the accuracy of PRF Thermometry during MRgFUS and to estimate the value of PRF coefficient of pre-clinical Thiel embalmed human and animal tissue, and compare to fresh tissue and gel phantom.