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

Current State of MRI-Guided Endovascular Arterial Interventions: A Systematic Review of Preclinical and Clinical Studies

BACKGROUND

MRI guidance of arterial endovascular interventions could be beneficial as it does not require radiation exposure, allows intrinsic blood-tissue contrast, and enables three-dimensional and functional imaging, however, clinical applications are still limited.

PURPOSE

To review the current state of MRI-guided arterial endovascular interventions and to identify the most commonly reported challenges.

STUDY TYPE

Systematic review.

POPULATION

Pubmed, Embase, Web of Science, and The Cochrane Library were systematically searched to find relevant articles. The search strategy combined synonyms for vascular pathology, endovascular therapy, and real-time MRI guidance.

FIELD STRENGTH/SEQUENCE

No field strength or sequence restrictions were applied.

ASSESSMENT

Two reviewers independently identified and reviewed the original articles and extracted relevant data.

STATISTICAL TESTS

Results of the included original articles are reported.

RESULTS

A total of 24,809 studies were identified for screening. Eighty-eight studies were assessed for eligibility, after which data were extracted from 43 articles (6 phantom, 33 animal, and 4 human studies). Reported technical success rates for animal and human studies ranged between 42% to 100%, and the average complication rate was 5.8% (animal studies) and 8.8% (human studies). Main identified challenges were related to spatial and temporal resolution as well as safety, design, and scarcity of current MRI-compatible endovascular devices.

DATA CONCLUSION

MRI guidance of endovascular arterial interventions seems feasible, however, included articles included mostly small single-center case series. Several hurdles remain to be overcome before larger trials can be undertaken. Main areas of research should focus on adequate imaging protocols with integrated tracking of dedicated endovascular devices.

Optimised passive marker device visibility and automatic marker detection for 3-T MRI-guided endovascular interventions: a pulsatile flow phantom study

BACKGROUND

Passive paramagnetic markers on magnetic resonance imaging (MRI)-compatible endovascular devices induce susceptibility artifacts, enabling MRI-visibility and real-time MRI-guidance. Optimised visibility is crucial for automatic detection and device tracking but depends on MRI technical parameters and marker characteristics. We assessed marker visibility and automatic detection robustness for varying MRI parameters and marker characteristics in a pulsatile flow phantom.

METHODS

Guidewires with varying iron(II,III) oxide nanoparticle (IONP) concentration markers were imaged using gradient-echo (GRE) and balanced steady-state free precession (bSSFP) sequences at 3 T. Furthermore, echo time (TE), slice thickness (ST) and phase encoding direction (PED) were varied. Artifact width was measured and contrast-to-noise ratios were calculated. Marker visibility and image quality were scored by two MRI interventional radiologists. Additionally, a deep learning model for automatic marker detection was trained and the effects of the parameters on detection performance were evaluated. Two-tailed Wilcoxon signed-rank tests were used (significance level, p < 0.05).

RESULTS

Medan artifact width (IQR) was larger in bSSFP compared to GRE images (12.7 mm (11.0-15.2) versus 8.4 mm (6.5-11.0)) (p < 0.001) and showed a positive relation with TE and IONP concentration. Switching PED and doubling ST had limited effect on artifact width. Image quality assessment scores were higher for GRE compared to bSSFP images. The deep learning model automatically detected the markers. However, the model performance was reduced after adjusting PED, TE, and IONP concentration.

CONCLUSION

Marker visibility was sufficient and a large range of artifact sizes was generated by adjusting TE and IONP concentration. Deep learning-based marker detection was feasible but performance decreased for altered MR parameters. These factors should be considered to optimise device visibility and ensure reliable automatic marker detectability in MRI-guided endovascular interventions.

A Holographic Augmented Reality Interface for Visualizing of MRI Data and Planning of Neurosurgical Procedures

The recent introduction of wireless head-mounted displays (HMD) promises to enhance 3D image visualization by immersing the user into 3D morphology. This work introduces a prototype holographic augmented reality (HAR) interface for the 3D visualization of magnetic resonance imaging (MRI) data for the purpose of planning neurosurgical procedures. The computational platform generates a HAR scene that fuses pre-operative MRI sets, segmented anatomical structures, and a tubular tool for planning an access path to the targeted pathology. The operator can manipulate the presented images and segmented structures and perform path-planning using voice and gestures. On-the-fly, the software uses defined forbidden-regions to prevent the operator from harming vital structures. In silico studies using the platform with a HoloLens HMD assessed its functionality and the computational load and memory for different tasks. A preliminary qualitative evaluation revealed that holographic visualization of high-resolution 3D MRI data offers an intuitive and interactive perspective of the complex brain vasculature and anatomical structures. This initial work suggests that immersive experiences may be an unparalleled tool for planning neurosurgical procedures.

Real-time device tracking under MRI using an acousto-optic active marker

PURPOSE

This work aims to demonstrate the use of an "active" acousto-optic marker with enhanced visibility and reduced radiofrequency (RF) -induced heating for interventional MRI.

METHODS

The acousto-optic marker was fabricated using bulk piezoelectric crystal and π-phase shifted fiber Bragg grating (FBGs) and coupled to a distal receiver coil on an 8F catheter. The received MR signal is transmitted over an optical fiber to mitigate RF-induced heating. A photodetector converts the optical signal into electrical signal, which is used as the input signal to the MRI receiver plug. Acousto-optic markers were characterized in phantom studies. RF-induced heating risk was evaluated according to ASTM 2182 standard. In vivo real-time tracking capability was tested in an animal model under a 0.55T scanner.

RESULTS

Signal-to-noise ratio (SNR) levels suitable for real-time tracking were obtained by using high sensitivity FBG and piezoelectric transducer with resonance matched to Larmor frequency. Single and multiple marker coils integrated to 8F catheters were readout for position and orientation tracking by a single acousto-optic sensor. RF-induced heating was significantly reduced compared to a coax cable connected reference marker. Real-time distal tip tracking of an active device was demonstrated in an animal model with a standard real-time cardiac MR sequence.

CONCLUSION

Acousto-optic markers provide sufficient SNR with a simple structure for real-time device tracking. RF-induced heating is significantly reduced compared to conventional active markers. Also, multiple RF receiver coils connected on an acousto-optic modulator can be used on a single catheter for determining catheter orientation and shape.

Safe guidewire visualization using the modes of a PTx transmit array MR system

Purpose

MRI‐guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents and hence allows safe tissue visualization. We propose that the maximum current modes, usually considered unsafe, be used at very low power levels to visualize conductive wires, and we investigate pulse sequences best suited for this application.

Methods

Spoiled gradient echo, balanced steady‐state free precession, and turbo spin echo sequences were evaluated for their ability to visualize a conductive guidewire embedded in a gel phantom when run in maximum current modes at very low power level. Temperature at the guidewire tip was monitored for safety assessment.

Results

Excellent guidewire visualization could be achieved using maximum current modes excitation, with the turbo spin echo sequence giving the best image quality. Although turbo spin echo is usually considered to be a high‐power sequence, our method reduced all pulses to 1% amplitude (0.01% power), and heating was not detected. In addition, visualization of background tissue can be achieved using null current mode, also with no recorded heating at the guidewire tip even when running at 100% (reported) specific absorption rate.

Conclusion

Parallel transmit is a promising approach for both guidewire and tissue visualization using maximum and null current modes, respectively, for interventional cardiac MRI. Such systems can switch excitation mode instantaneously, allowing for flexible integration into interactive sequences.

Positive contrast spiral imaging for visualization of commercial nitinol guidewires with reduced heating

BACKGROUND

CMR-guidance has the potential to improve tissue visualization during cardiovascular catheterization procedures and to reduce ionizing radiation exposure, but a lack of commercially available CMR guidewires limits widespread adoption. Standard metallic guidewires are considered to be unsafe in CMR due to risks of RF-induced heating. Here, we propose the use of RF-efficient gradient echo (GRE) spiral imaging for reduced guidewire heating (low flip angle, long readout), in combination with positive contrast for guidewire visualization.

METHODS

A GRE spiral sequence with 8 interleaves was used for imaging. Positive contrast was achieved using through-slice dephasing such that the guidewire appeared bright and the background signal suppressed. Positive contrast images were interleaved with anatomical images, and real-time image processing was used to produce a color overlay of the guidewire on the anatomy. Temperature was measured with a fiber-optic probe attached to the guidewire in an acrylic gel phantom and in vivo.

RESULTS

Left heart catheterization was performed on swine using the real-time color overlay for procedural guidance with a frame rate of 6.25 frames/second. Using our standard Cartesian real-time imaging (flip angle 60°), temperature increases up to 50 °C (phantom) and 4 °C (in vivo) were observed. In comparison, spiral GRE images (8 interleaves, flip angle 10°) generated negligible heating measuring 0.37 °C (phantom) and 0.06 °C (in vivo).

CONCLUSIONS

The ability to use commercial metallic guidewires safely during CMR-guided catheterization could potentially expedite clinical translation of these methods.

Magnetic Resonance Sequences and Rapid Acquisition for MR-Guided Interventions

Interventional MR uses rapid imaging to guide diagnostic and therapeutic procedures. One of the attractions of MR-guidance is the abundance of inherent contrast mechanisms available. Dynamic procedural guidance with real-time imaging has pushed the limits of MR technology, demanding rapid acquisition and reconstruction paired with interactive control and device visualization. This article reviews the technical aspects of real-time MR sequences that enable MR-guided interventions.

Magnetic Resonance-guided Active Catheter Tracking

Several advantages of MR imaging compared with other imaging modalities have provided the rationale for increased attention to MR-guided interventions, including its excellent soft tissue contrast, its capability to show both anatomic and functional information, and no use of ionizing radiation. An important aspect of MR-guided intervention is to provide visualization and navigation of interventional devices relative to the surrounding tissues. This article focuses on the methods for MR-guided active tracking in catheter-based interventions. Practical issues about implementation of active catheter tracking in a clinical setting are discussed and several current application examples are highlighted.

Dual echo positive contrast bSSFP for real-time visualization of passive devices during magnetic resonance guided cardiovascular catheterization

BACKGROUND

Cardiovascular magnetic resonance (CMR) guided cardiovascular catheterizations can potentially reduce ionizing radiation exposure and enable new interventions. Commercially available paramagnetic X-Ray devices create a small signal void in CMR images, which is ambiguous and insufficient to guide catheterization procedures. This work aims to improve real-time CMR of off-the-shelf X-Ray devices by developing a real-time positive contrast sequence with color overlay of the device onto anatomy.

METHODS

A dual-echo bSSFP sequence was used to generate both a dephased positive contrast image and bSSFP image simultaneously. A variable flip angle scheme was implemented to reduce the specific absorption rate (SAR) and hence device heating. Image processing was used to isolate the device from background signal, and the device was overlaid in color on the anatomy, mimicking active device visualization. Proof-of-concept experiments were performed using a commercially available nitinol guidewire for left heart catheterization in Yorkshire swine.

RESULTS

The dual echo pulse sequence generated a temporal resolution of 175 ms (5.7 frames/second) with GRAPPA acceleration factor 4. Image processing was performed in real-time and color overlay of the device on the anatomy was displayed to the operator with no latency. The color overlay accurately depicted the guidewire location, with minimal background contamination, during left heart catheterization.

CONCLUSIONS

The ability to effectively visualize commercially available X-Ray devices during CMR-guided cardiovascular catheterizations, combined with safe low-SAR pulse sequences, could potentially expedite the clinical translation of interventional CMR.

A controllable susceptibility marker for passive device tracking

PURPOSE

To design and demonstrate a new susceptibility-based tracking device with an artifact that can be mechanically turned on and off, thus permitting tracking and imaging at the device tip with limited artifact.

METHODS

The magnetic susceptibilities of readily obtainable grades of titanium and graphite were measured. Using numerical optimization, layer thicknesses for three concentric cylinders were found where the field from the graphite layer maximally cancelled the fields from titanium layers. The tracking elements were fabricated for an outer diameter of 3 mm and attached to a catheter to show feasibility of detection in phantoms and in vivo.

RESULTS

The device was successfully integrated into a 9F catheter, and its use with conventional guidewires under fluoroscopy was demonstrated by guiding the catheter through the bifurcation into the carotid artery. MR images including the catheter tip were acquired with the device in both the "on" and "off" positions.

CONCLUSION

A new passive tracking device with a susceptibility effect that can be enabled and disabled by sliding one of the components was designed, fabricated, and demonstrated in phantoms and in vivo. The device may also be integrated into many different interventional MR devices such as needles, ultrasound transducers for prostate biopsy, or any catheter-based devices.

MRI-guided vascular access with an active visualization needle

PURPOSE

To develop an approach to vascular access under magnetic resonance imaging (MRI), as a component of comprehensive MRI-guided cardiovascular catheterization and intervention.

MATERIALS AND METHODS

We attempted jugular vein access in healthy pigs as a model of "difficult" vascular access. Procedures were performed under real-time MRI guidance using reduced field of view imaging. We developed an "active" MRI antenna-needle having an open-lumen, distinct tip appearance and indicators of depth and trajectory in order to enhance MRI visibility during the procedure. We compared performance of the active needle against an unmodified commercial passively visualized needle, measured by procedure success among operators with different levels of experience.

RESULTS

MRI-guided central vein access was feasible using both the active needle and the unmodified passive needle. The active needle required less time (88 vs. 244 sec, P = 0.022) and fewer needle passes (4.5 vs. 9.1, P = 0.028), irrespective of operator experience.

CONCLUSION

MRI-guided access to central veins is feasible in our animal model. When image guidance is necessary for vascular access, performing this component under MRI will allow wholly MRI-guided catheterization procedures that do not require adjunctive imaging facilities such as x-ray or ultrasound. The active needle design showed enhanced visibility, as expected. These capabilities may permit more complex catheter-based cardiovascular interventional procedures enabled by enhanced image guidance.