Integration and evaluation of a gradient-based needle navigation system for percutaneous MR-guided interventions.

The purpose of the present study was to integrate an interactive gradient-based needle navigation system and to evaluate the feasibility and accuracy of the system for real-time MR guided needle puncture in a multi-ring phantom and in vivo in a porcine model. The gradient-based navigation system was implemented in a 1.5T MRI. An interactive multi-slice real-time sequence was modified to provide the excitation gradients used by two sets of three orthogonal pick-up coils integrated into a needle holder. Position and orientation of the needle holder were determined and the trajectory was superimposed on pre-acquired MR images. A gel phantom with embedded ring targets was used to evaluate accuracy using 3D distance from needle tip to target. Six punctures were performed in animals to evaluate feasibility, time, overall error (target to needle tip) and system error (needle tip to the guidance needle trajectory) in vivo. In the phantom experiments, the overall error was 6.2±2.9 mm (mean±SD) and 4.4±1.3 mm, respectively. In the porcine model, the setup time ranged from 176 to 204 seconds, the average needle insertion time was 96.3±40.5 seconds (min: 42 seconds; max: 154 seconds). The overall error and the system error was 8.8±7.8 mm (min: 0.8 mm; max: 20.0 mm) and 3.3±1.4 mm (min: 1.8 mm; max: 5.2 mm), respectively.

Touchless scanner control to support MRI-guided interventions.

PURPOSE: MRI-guided interventions allow minimally invasive, radiation-free treatment but rely on real-time image data and free slice positioning. Interventional interaction with the data and the MRI scanner is cumbersome due to the diagnostic focus of current systems, confined space and sterile conditions. METHODS: We present a touchless, hand-gesture-based interaction concept to control functions of the MRI scanner typically used during MRI-guided interventions. The system consists of a hand gesture sensor customised for MRI compatibility and a specialised UI that was developed based on clinical needs. A user study with 10 radiologists was performed to compare the gesture interaction concept and its components to task delegation-the prevalent method in clinical practice. RESULTS: Both methods performed comparably in terms of task duration and subjective workload. Subjective performance with gesture input was perceived as worse compared to task delegation, but was rated acceptable in terms of usability while task delegation was not. CONCLUSION: This work contributes by (1) providing access to relevant functions on an MRI scanner during percutaneous interventions in a (2) suitable way for sterile human-computer interaction. The introduced concept removes indirect interaction with the scanner via an assistant, which leads to comparable subjective workload and task completion times while showing higher perceived usability.

Projector-based augmented reality system for interventional visualization inside MRI scanners.

BACKGROUND: Navigation support in interventional magnetic resonance imaging (MRI) is separated from the operating field, which makes it difficult to interpret positions and orientations and to coordinate the necessary hand movements. METHODS: We developed a projector-based augmented reality system to enable visual navigation of tracked instruments on pre-planned paths and visualization of risk structures directly on the patient inside the MRI bore. To assess the accuracy of the system, a user study was carried out with clinicians in a needle navigation test scenario. RESULTS: The targets were reached with an error of 1.7 ± 0.5 mm and the entry points with an error of 1.7 ± 0.8 mm. CONCLUSION: The accuracy results are similar to those reached by live image-guided interventions and related work and confirm that this projective augmented reality prototype for the interventional MRI can serve as a platform for current and future research in augmented reality visualization and dynamic registration.

Wireless video transmission into the MRI magnet room: implementation and evaluation at 1.5T, 3T and 7T.

Purpose To analyze the interference between a wireless high definition multimedia interface (WHDMI) and magnetic resonance imaging (MRI) image quality at 1.5T, 3T and 7T. Materials and methods A wireless video transmission system (WVTS) consisting of a WHDMI and a projector was used to transmit and display a video stream into the magnet room. MR image quality was analyzed at 1.5T, 3T and 7T. Signal-to-noise-ratio (SNR¯) $(\overline {{\rm{SNR}}} )$ and radio frequency (RF)-noise spectrum were measured at three transmitter positions (A: inside the cabin, B: in front of the waveguide and C: in the control room). WVTS system functionality tests included measurements of reliability, delay and image quality. Results With the WVTS mean SNR¯ $\overline {{\rm{SNR}}} $ values significantly decreased in comparison to the reference for all positions and fieldstrenghts, while the spectra's baseline is elevated at 1.5T and 3T. Peaks related to continuous wave interferences are apparent at all field strenghts. For WHDMI alone mean SNR¯ $\overline {{\rm{SNR}}} $ values were stable without significant differences to the reference. No elevation of the spectra's baseline could be observed. Functionality measurements confirmed high connection reliability with stable image quality and no delays for all field strengths. Conclusion We conclude that wireless transmission of video streams into the MRI magnet room is feasible at all field strengths without hampering image quality.

Percutaneous MR-guided interventions using an optical Moiré Phase tracking system: Initial results.

The aim of this study was the development and evaluation of a real-time guidance support using optical Moiré Phase Tracking (MPT) for magnetic resonance (MR) guided percutaneous interventions. A gradient echo sequence, capable of real-time position updates by the MPT system, was modified to enable needle guidance based on four rigidly attached MPT markers at the back of a needle. Two perpendicular imaging planes were automatically aligned along the calibrated needle and centered at its tip. For user guidance, additional information about the needle trajectory and the tip to target distance were added as image overlay. Both, images and guiding information were displayed on the in-room monitor to facilitate MR guided interventions. The guidance support was evaluated by four experienced interventional radiologists and four novices targeting rubber O-rings embedded in a custom-made phantom on a 3T wide-bore MRI system (80 punctures). The skin to target time, user error, system error and total error were analyzed. The mean skin to target time was 146s±68s with no statistically significant difference between experts and novices. A low mean user error (0.91mm±0.43mm), system error (0.53mm±0.27mm) and total error (0.99mm±0.47mm) was reached in all directions. No statistically significant difference in user error, system error and total error could be found between experts and novices. The presented tracking and image guidance system combined with the user interface offers continuous and interactive control of the imaging plane while puncturing in the magnet enabling accurate real-time feedback for both, experienced and non-experienced users.

Motion Correction in Proton Resonance Frequency-based Thermometry in the Liver.

The unique ability of magnetic resonance imaging to measure temperature noninvasively, in vivo, makes it an attractive tool for monitoring interventional procedures, such as radiofrequency or microwave ablation in real-time. The most frequently used approach for magnetic resonance-based temperature measurement is proton resonance frequency (PRF) thermometry. Although it has many advantages, including tissue-independence and real-time capability, the main drawback is its motion sensitivity. This is likely the reason PRF thermometry in moving organs, such as the liver, is not commonly used in the clinical arena. In recent years, however, several developments suggest that motion-corrected thermometry in the liver is achievable. The present article summarizes the diverse attempts to correct thermometry in the liver. Therefore, the physical principle of PRF is introduced, with additional references for necrosis zone estimation and how to deal with fat phase modulation, and main magnetic field drifts. The primary categories of motion correction are presented, including general methods for motion compensation and library-based approaches, and referenceless thermometry and hybrid methods. Practical validation of the described methods in larger patient groups will be necessary to establish accurate motion-corrected thermometry in the clinical arena, with the goal of complete liver tumor ablation.