Image-guided and -monitored renal artery stenting using only MRI

ERCP 2.0: Biliary 3D-reconstruction in patients with malignant hilar stricture

BACKGROUND

Endoscopic retrograde cholangiopancreatography (ERCP) for malignant hilar strictures is challenging. The correlation between Magnetic resonance cholangiopancreatography (MRCP) and per ERCP 2D fluoroscopic images is not obvious. The aim of this study was to evaluate the feasibility and potential usefulness of MRCP-based handmade biliary 3D reconstruction in this setting.

METHODS

Methods Patients who underwent MRCP followed by ERCP for biliary drainage of a malignant hilar stricture at our institution between 2018 and 2020 were reviewed. A handmade 3D segmentation using 3D slicer© (Kitware, France) was fashioned and reviewed with an expert radiologist. The primary outcome was the feasibility of biliary segmentation.

RESULTS

A total of 16 patients were included. The mean age was 70.1 (+/- 8.6) years-old and 68.8% had hilar cholangiocarcinoma. Handmade segmentation was successful in all cases. The agreement between the MRCP interpretation and the 3D reconstruction was 37.5%, as per the Bismuth classification. 3D reconstruction available prior to ERCP could have helped guide for better stent placement in 11 cases (68.8%).

CONCLUSIONS

MRCP-based biliary 3D segmentation-reconstruction, in patients with malignant hilar stricture is feasible and seems to provide a better anatomical understanding compared to MRCP and could help improve endoscopic management.

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.

MRI-guided endovascular intervention: current methods and future potential

INTRODUCTION

Image-guided endovascular interventions, performed using the insertion and navigation of catheters through the vasculature, have been increasing in number over the years, as minimally invasive procedures continue to replace invasive surgical procedures. Such endovascular interventions are almost exclusively performed under x-ray fluoroscopy, which has the best spatial and temporal resolution of all clinical imaging modalities. Magnetic resonance imaging (MRI) offers unique advantages and could be an attractive alternative to conventional x-ray guidance, but also brings with it distinctive challenges.

AREAS COVERED

In this review, the benefits and limitations of MRI-guided endovascular interventions are addressed, systems and devices for guiding such interventions are summarized, and clinical applications are discussed.

EXPERT OPINION

MRI-guided endovascular interventions are still relatively new to the interventional radiology field, since significant technical hurdles remain to justify significant costs and demonstrate safety, design, and robustness. Clinical applications of MRI-guided interventions are promising but their full potential may not be realized until proper tools designed to function in the MRI environment are available. Translational research and further preclinical studies are needed before MRI-guided interventions will be practical in a clinical interventional setting.

Magnetic resonance imaging for pathobiological assessment and interventional treatment of the coronary arteries

X-ray-based fluoroscopy is the standard tool for diagnostics and intervention in coronary artery disease. In recent years, computed tomography has emerged as a non-invasive alternative to coronary angiography offering detection of coronary calcification and imaging of the vessel lumen by the use of iodinated contrast agents. Even though currently available invasive or non-invasive techniques can show the degree of vessel stenosis, they are unable to provide information about biofunctional plaque properties, e.g. plaque inflammation. Furthermore, the use of radiation and the necessity of iodinated contrast agents remain unfavourable prerequisites. Magnetic resonance imaging (MRI) is a radiation-free alternative to X-ray which offers anatomical and functional imaging contrasts fostering the idea of non-invasive biofunctional assessment of the coronary vessel wall. In combination with molecular contrast agents that target-specific epitopes of the vessel wall, MRI might reveal unique plaque properties rendering it, for example, ‘vulnerable and prone to rupture’. Early detection of these lesions may allow for early or prophylactic treatment even before an adverse coronary event occurs. Besides diagnostic imaging, advances in real-time image acquisition and motion compensation now provide grounds for MRI-guided coronary interventions. In this article, we summarize our research on MRI-based molecular imaging in cardiovascular disease and feature our advances towards real-time MRI-based coronary interventions in a porcine model.

Interventional CMR: Clinical applications and future directions

Interventional cardiovascular magnetic resonance (iCMR) promises to enable radiation-free catheterization procedures and to enhance contemporary image guidance for structural heart and electrophysiological interventions. However, clinical translation of exciting pre-clinical interventions has been limited by availability of devices that are safe to use in the magnetic resonance (MR) environment. We discuss challenges and solutions for clinical translation, including MR-conditional and MR-safe device design, and how to configure an interventional suite. We review the recent advances that have already enabled diagnostic MR right heart catheterization and simple electrophysiologic ablation to be performed in humans and explore future clinical applications.

Magnetic Resonance-Guided Passive Catheter Tracking for Endovascular Therapy

The use of MR guidance for endovascular intervention is appealing because of its lack of ionizing radiation, high-contrast visualization of vessel walls and adjacent soft tissues, multiplanar capabilities, and potential to incorporate functional information such as flow, fluid dynamics, perfusion, and cardiac motion. This review highlights state-of-the-art imaging techniques and hardware used for passive tracking of endovascular devices in interventional MR imaging, including negative contrast, passive contrast, nonproton multispectral, and direct current techniques. The advantages and disadvantages of passive tracking relative to active tracking are also summarized.

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.

Intravascular Photoacoustic Imaging

Intravascular photoacoustic (IVPA) imaging is a catheter-based, minimally invasive, imaging modality capable of providing high-resolution optical absorption map of the arterial wall. Integrated with intravascular ultrasound (IVUS) imaging, combined IVPA and IVUS imaging can be used to detect and characterize atherosclerotic plaques building up in the inner lining of an artery. In this paper, we present and discuss various representative applications of combined IVPA/IVUS imaging of atherosclerosis, including assessment of the composition of atherosclerotic plaques, imaging of macrophages within the plaques, and molecular imaging of biomarkers associated with formation and development of plaques. In addition, imaging of coronary artery stents using IVPA and IVUS imaging is demonstrated. Furthermore, the design of an integrated IVUS/IVPA imaging catheter needed for in vivo clinical applications is discussed.

Multimodality cardiovascular molecular imaging technology

Cardiovascular molecular imaging is a new discipline that integrates scientific advances in both functional imaging and molecular probes to improve our understanding of the molecular basis of the cardiovascular system. These advances are driven by in vivo imaging of molecular processes in animals, usually small animals, and are rapidly moving toward clinical applications. Molecular imaging has the potential to revolutionize the diagnosis and treatment of cardiovascular disease. The 2 key components of all molecular imaging systems are the molecular contrast agents and the imaging system providing spatial and temporal localization of these agents within the body. They must deliver images with the appropriate sensitivity and specificity to drive clinical applications. As work in molecular contrast agents matures and highly sensitive and specific probes are developed, these systems will provide the imaging technologies required for translation into clinical tools. This is the promise of molecular medicine.

Improved in-stent lumen visualization using intravascular MRI and a balanced steady-state free-precession sequence

RATIONALE AND OBJECTIVES

To investigate the ability of an intravascular magnetic resonance (MR) loopless antenna to reduce the radiofrequency shielding of a vascular stent during signal reception as a way to improve the visualization of the in-stent lumen.

METHODS AND MATERIALS

Using a balanced steady-state free-precession (bSSFP) sequence and a dedicated vascular phantom, the signal-to-noise ratio (SNR) inside the lumen of a stent is evaluated as a function of the nominal flip angle and compared with the results obtained for a reference vessel without a stent. All experiments are performed using successively an intravascular loopless antenna and surface arrays coils. Using an optimized protocol, in vitro in-stent restenosis visualization and quantification experiments are performed to evaluate the validity of an approach using an intravascular antenna and cross-sectional images to depict a vascular lesion inside a stent.

RESULTS

The use of a loopless antenna effectively eliminates the radiofrequency shielding effect of the stent during signal reception. Furthermore, using a bSSFP sequence with a carefully chosen nominal flip angle, an equally good blood SNR can be obtained inside and outside the stent. Results of in vitro in-stent restenosis quantification measurements using the proposed method illustrate the benefits arising from the use of the intravascular antenna.

CONCLUSION

In the perspective of MR-guided vascular interventions, the presented results illustrate that the use of an intravascular antenna can significantly facilitate imaging inside a vascular stent. Potential applications include the monitoring of stent deployment as well as visualization and quantification of in-stent restenosis during an intervention.

Real-time reconstruction of sensitivity encoded radial magnetic resonance imaging using a graphics processing unit

A barrier to the adoption of non-Cartesian parallel magnetic resonance imaging for real-time applications has been the times required for the image reconstructions. These times have exceeded the underlying acquisition time thus preventing real-time display of the acquired images. We present a reconstruction algorithm for commodity graphics hardware (GPUs) to enable real time reconstruction of sensitivity encoded radial imaging (radial SENSE). We demonstrate that a radial profile order based on the golden ratio facilitates reconstruction from an arbitrary number of profiles. This allows the temporal resolution to be adjusted on the fly. A user adaptable regularization term is also included and, particularly for highly undersampled data, used to interactively improve the reconstruction quality. Each reconstruction is fully self-contained from the profile stream, i.e., the required coil sensitivity profiles, sampling density compensation weights, regularization terms, and noise estimates are computed in real-time from the acquisition data itself. The reconstruction implementation is verified using a steady state free precession (SSFP) pulse sequence and quantitatively evaluated. Three applications are demonstrated; real-time imaging with real-time SENSE 1) or k- t SENSE 2) reconstructions, and 3) offline reconstruction with interactive adjustment of reconstruction settings.

Interventional cardiovascular magnetic resonance imaging: a new opportunity for image-guided interventions

Cardiovascular magnetic resonance (CMR) combines excellent soft-tissue contrast, multiplanar views, and dynamic imaging of cardiac function without ionizing radiation exposure. Interventional cardiovascular magnetic resonance (iCMR) leverages these features to enhance conventional interventional procedures or to enable novel ones. Although still awaiting clinical deployment, this young field has tremendous potential. We survey promising clinical applications for iCMR. Next, we discuss the technologies that allow CMR-guided interventions and, finally, what still needs to be done to bring them to the clinic.

Photoacoustic imaging of coronary artery stents

Coronary stents are the most commonly used treatment in the United States to repair narrowed or weakened arteries. The ability to visualize the stent during the stenting procedure and post-surgery is crucial to correctly place the stent with respect to the vessel stenosis, and to identify its position within the vessel wall. Current imaging modalities suffer from low contrast, resolution and/or unfavorable artifacts that can inhibit correct visualization of the stent in the artery. We demonstrated the effectiveness of a combined intravascular photoacoustic and intravascular ultrasound imaging method for high resolution and sufficient contrast imaging of commercial stents with respect to the vessel wall.

Comparative evaluation of the geometrical accuracy of intravascular magnetic resonance imaging: a phantom study

RATIONALE AND OBJECTIVES

To evaluate and compare the accuracy of cross-sectional imaging using an intravascular antenna in the context of vascular morphological measurements performed during a magnetic resonance imaging (MRI)-guided vascular intervention.

MATERIALS AND METHODS

Cross-sectional imaging of a multimodality vascular phantom was performed using intravascular and surface MRI, multidetector computed tomography, and intravascular ultrasound (IVUS). Using a balanced steady-state free-precession sequence, 18 sequences parameters sets were investigated (12 for intravascular MRI and 6 for surface MRI). Vessel diameters for all images and modalities were computed using an automated vessel segmentation algorithm.

RESULTS

Using IVUS as a gold standard, imaging using an intravascular antenna leads to an increase in geometrical accuracy in comparison to traditional surface MRI. This level of accuracy appears to follow a significant inverse proportionality relation in respect to vessel wall signal-to-noise ratio (SNR). Taking into account the rapid decrease in SNR as a function of the distance to the intravascular antenna, these results imply that, for a given level of geometrical accuracy, faster sequences can be used for the imaging of smaller vessels.

CONCLUSION

Imaging using an intravascular antenna appears as a valuable assistance to increase the accuracy of vascular morphological measurements. This increase in geometrical accuracy would be beneficial during the realization of an MRI-guided intervention, either to perform pretreatment measurements or to assess the outcome of the procedure. Acquisition parameters should be tailored to vessel size and procedural time constraints.

Interventional cardiovascular magnetic resonance: still tantalizing

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."

Adapting the clinical MRI software environment for real-time navigation of an endovascular untethered ferromagnetic bead for future endovascular interventions

A dedicated software architecture for a novel interventional method allowing the navigation of ferromagnetic endovascular devices using a standard real-time clinical MRI system is shown. Through a specially developed software environment integrating a tracking method and a real-time controller algorithm, a clinical 1.5T Siemens Avanto MRI system is adapted to provide new functionality for potential automated interventional applications. The proposed software architecture was successfully validated through in vivo controlled navigation inside the carotid artery of a swine. Here we present how this MRI-upgraded software environment could also be used in more complex vasculature models through the real-time navigation of a 1.5 mm diameter chrome steel bead in two different MR-compatible phantoms with flowless and quiescent flow conditions. The developed platform and software modules needed for such navigation are also presented. Real-time tracking achieved through a dedicated positioning method based on an off-resonance excitation technique has also been successfully integrated in the software platform while maintaining adequate real-time performance. These preliminary feasibility experiments suggest that navigation of such devices can be achieved using a similar software architecture on other conventional clinical MRI systems at an operational closed-loop control frequency of 32 Hz.

New aspects of technical testing of intravascular stents

Maximum magnetic resonance artifacts from stents are often determined by the presence of ferromagnetic compounds in the delivery system. We propose a mathematic model describing artifacts caused by nitinol stents. A method for evaluation of critical radial rigidity of the stents was developed.

MR-guided intravascular interventions: techniques and applications

Magnetic resonance imaging (MRI) offers several advantages over other imaging modalities that make it an attractive imaging tool for diagnostic and therapeutic procedures. This tremendous potential of MRI has provided the rationale for increased attention toward MR-guided endovascular interventions. MR guidance has been used recently to navigate endovascular catheters and deliver stents, vena cava filters, embolization materials, and septum closure devices. However, its potential goes beyond just copying existing procedures toward the development of new minimally invasive techniques that cannot be performed with conventional guiding techniques. Because of technical limitations and safety issues associated with some of the currently available devices, a limited number of clinical studies have been performed so far. The overall success for this developing field requires considerable interdisciplinary research within both the interventional and the MR community. Only through a combined effort can this complex technology find its way into clinical practice. This review discusses the hardware and software improvements that have helped to advance endovascular interventions under MR imaging guidance from a pure research tool to become a clinical reality. In addition, technical and safety issues specific to endovascular MR image guidance will be described and practical applications will be shown that take advantage of the benefits of MR for endovascular interventions.

Interventional cardiovascular magnetic resonance imaging

Magnetic resonance imaging provides structural and functional cardiovascular information with excellent soft tissue contrast. Real-time magnetic resonance imaging can guide transcatheter cardiovascular interventions in large animal models and may prove superior to x-ray and adjunct modalities for peripheral vascular, structural heart, and cardiac electrophysiology applications. We describe technical considerations, preclinical work, and early clinical studies in this emerging field.

An overview on the advances in cardiovascular interventional MR imaging

Interventional cardiovascular magnetic resonance imaging (iCMR) represents a new discipline whose systematic development will foster minimally invasive interventional procedures without radiation exposure. New generations of open, wide and short bore MR scanners and real time sequences made cardiovascular intervention possible. MR compatible endovascular catheters and guide-wires are needed for delivery of devices such as stents or atrial septal defect (ASD) closures. Catheter tracking is based on active and passive approaches. Currently performed MR-guided procedures are used to monitor, navigate and track endovascular catheters and to deliver local therapeutic agents to targets, such as infarcted myocardium and vascular walls. Heating of endovascular MR catheters, guide-wires and devices during imaging still presents high safety risks. MR contrast media improve the capabilities of MR imaging by enhancing blood signal, pathologic targets (such as myocardial infarctions and atherosclerotic plaques), endovascular catheters and by tracking injected therapeutic agents. Labeling injected soluble therapeutic agents, genes or cells with MR contrast media enables interventionalists to ensure the administration of the drugs in the target and to trace their distribution in the targets. The future clinical use of this iCMR technique requires (1) high spatial and temporal resolution imaging, (2) special catheters and devices and (3) effective therapeutic agents, genes or cells. These conditions are available at a low scale at the present time and need to be developed in the near future. Such progress will lead to improved patient care and minimize invasiveness.