Techniques for fast stereoscopic MRI

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.

A Platform Integrating Acquisition, Reconstruction, Visualization, and Manipulator Control Modules for MRI-Guided Interventions

This work presents a platform that integrates a customized MRI data acquisition scheme with reconstruction and three-dimensional (3D) visualization modules along with a module for controlling an MRI-compatible robotic device to facilitate the performance of robot-assisted, MRI-guided interventional procedures. Using dynamically-acquired MRI data, the computational framework of the platform generates and updates a 3D model representing the area of the procedure (AoP). To image structures of interest in the AoP that do not reside inside the same or parallel slices, the MRI acquisition scheme was modified to collect a multi-slice set of intraoblique to each other slices; which are termed composing slices. Moreover, this approach interleaves the collection of the composing slices so the same k-space segments of all slices are collected during similar time instances. This time matching of the k-space segments results in spatial matching of the imaged objects in the individual composing slices. The composing slices were used to generate and update the 3D model of the AoP. The MRI acquisition scheme was evaluated with computer simulations and experimental studies. Computer simulations demonstrated that k-space segmentation and time-matched interleaved acquisition of these segments provide spatial matching of the structures imaged with composing slices. Experimental studies used the platform to image the maneuvering of an MRI-compatible manipulator that carried tubing filled with MRI contrast agent. In vivo experimental studies to image the abdomen and contrast enhanced heart on free-breathing subjects without cardiac triggering demonstrated spatial matching of imaged anatomies in the composing planes. The described interventional MRI framework could assist in performing real-time MRI-guided interventions.

An MR-compatible stereoscopic in-room 3D display for MR-guided interventions

BACKGROUND AND METHODS

A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured.

RESULTS AND DISCUSSION

Due to modifications, the center luminance of the 3D monitor was reduced by 14%, and the addition of a Faraday shield further reduced the remaining luminance by 31%. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6% was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically.

Two eyes see more than one: double echo stereoscopic MRA for rapid 3D visualization of vascular structures

OBJECT

A three-dimensional (3D) visualization of the target region during intravascular interventions in real-time is challenging since the acquisition of a time-consuming 3D dataset is required. In this work, a novel stereoscopic double echo sequence for achieving 3D depth perception by sampling only two oblique projection images is presented.

MATERIALS AND METHODS

A double echo (DE) FLASH pulse sequence was developed to acquire continuously stereoscopic image pairs of the vascular target anatomy. Stereo image data were displayed on a stereoscopic 3D LCD monitor in real time after image reconstruction. Phantom experiments followed by a depth perception test were performed to assess the usability of the stereo image pairs for 3D visualization. In an animal experiment the sequence was tested in vivo and was compared with a slower interleaved (IL) sequence variant.

RESULTS

In the phantom experiments an SNR difference of 6 % between left and right image was found which did not influence the depth perception. The DE acquisition was superior to the IL sequence (SNR(DE) = 10.3, 2.3 images/s over SNR(IL) = 7.1, 1.7 images/s), and during contrast enhancement the abdominal arterial vasculature was clearly perceived as a 3D structure.

CONCLUSION

A novel stereoscopic DE pulse sequence can be utilized for the fast 3D stereoscopic visualization of vascular structures in real-time.

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

PURPOSE

To demonstrate the ability of a unique interventional MR system to be used safely and effectively as the only imaging modality for all phases of MR-guided stent-supported angioplasty.

MATERIALS AND METHODS

An experimental disease model of renal stenosis was created in six pigs. An interventional MR system, which employed previously reported tools for real-time catheter tracking with automated scan-plane positioning, adaptive image parameters, and radial true-FISP imaging with steady-state precession (True-FISP) imaging coupled with a high-speed reconstruction technique, was then used to guide all phases of the intervention, including: guidewire and catheter insertion, stent deployment, and confirmation of therapeutic success. Pre- and postprocedural X-ray imaging was used as a gold standard to validate the experimental results.

RESULTS

All of the stent-supported angioplasty interventions were a technical success and were performed without complications. The average postoperative residual stenosis was 14.9%. The image guidance enabled the stents to be deployed with an accuracy of 0.98 +/- 0.69 mm. Additionally, using this interventional MRI system to guide renal artery stenting significantly reduces the procedure time, as compared to using X-ray fluoroscopy.

CONCLUSION

This study has clearly demonstrated the first successful treatment of renal artery stenting in an experimental animal model solely under MRI guidance and monitoring.

Advances in interventional cardiovascular MRI

Because of its superior soft tissue imaging, MRI has become a valuable diagnostic tool in cardiovascular disease. These strengths make MRI attractive to guide therapeutic catheter-based procedures, both conventional and novel. We review how to configure an interventional MRI suite, how MRI catheter devices differ from conventional radiographic catheters, and finally developments in preclinical and investigational clinical applications.

Real-time MRI guided atrial septal puncture and balloon septostomy in swine

Cardiac perforation during atrial septal puncture (ASP) might be avoided by improved image guidance. X-ray fluoroscopy (XRF), which guides ASP, visualizes tissue poorly and does not convey depth information. Ultrasound is limited by device shadows and constrained imaging windows. Alternatively, real-time MRI (rtMRI) provides excellent tissue contrast in any orientation and may enable ASP and balloon atrial septostomy (BAS) in swine. Custom MRI catheters incorporated "active" (receiver antenna) and "passive" (iron or gadolinium) elements. Wholly rtMRI-guided transfemoral ASP and BAS were performed in 10 swine in a 1.5T interventional suite. Hemodynamic results were measured with catheters and velocity encoded MRI. Successful ASP was performed in all 10 animals. Necropsy confirmed septostomy confined within the fossa ovalis in all. BAS was successful in 9/10 animals. Antenna failure in a re-used needle led to inadvertent vena cava tear prior to BAS in 1 animal. ASP in the same animal was easily performed using a new needle. rtMRI illustrated clear device-tissue-lumen relationships in multiple orientations, and facilitated simple ASP and BAS. The mean procedure time was 19 +/- 10 minutes. Septostomy achieved a mean left to right shunt ratio of 1.3:1 in these healthy animals. Interactive rtMRI permits rapid transcatheter ASP and BAS in swine. Further technical development may enable novel applications.

The future of real-time cardiac magnetic resonance imaging

Dynamic changes in cardiac structure and function are usually examined by real-time imaging techniques such as angiography or echocardiography. MRI has many advantages compared with these established cardiac imaging modalities. However, system hardware and software limitations have limited cardiac MRI to gated acquisitions that are lengthy and often result in failed acquisitions and examinations. Recently, MRI has evolved into a technique capable of imaging dynamic processes in real time. Improvements in hardware, pulse sequences, and image reconstruction algorithms have enabled real-time cardiac MRI with high spatial resolution, high temporal resolution, and various types of image contrast without requiring cardiac gating or breath-holding. This article provides an overview of current capability and highlights key technical and clinical advances. The future prospects of real-time cardiac MRI will depend on 1) the development of techniques that further improve signal to noise ratio, contrast, spatial resolution, and temporal resolution, without introducing artifacts; 2) the development of software infrastructure that facilitates rapid interactive examination; and 3) the development and validation of several new clinical assessments.

Real-time magnetic resonance imaging-guided stenting of aortic coarctation with commercially available catheter devices in Swine

BACKGROUND

Real-time MR imaging (rtMRI) is now technically capable of guiding catheter-based cardiovascular interventions. Compared with x-ray, rtMRI offers superior tissue imaging in any orientation without ionizing radiation. Translation to clinical trials has awaited the availability of clinical-grade catheter devices that are both MRI visible and safe. We report a preclinical safety and feasibility study of rtMRI-guided stenting in a porcine model of aortic coarctation using only commercially available catheter devices.

METHOD AND RESULTS

Coarctation stenting was performed wholly under rtMRI guidance in 13 swine. rtMRI permitted procedure planning, device tracking, and accurate stent deployment. "Active" guidewires, incorporating MRI antennas, improved device visualization compared with unmodified "passive" nitinol guidewires and shortened procedure time (26+/-11 versus 106+/-42 minutes; P=0.008). Follow-up catheterization and necropsy showed accurate stent deployment, durable gradient reduction, and appropriate neointimal formation. MRI immediately identified aortic rupture when oversized devices were tested.

CONCLUSIONS

This experience demonstrates preclinical safety and feasibility of rtMRI-guided aortic coarctation stenting using commercially available catheter devices. Patients may benefit from rtMRI in the future because of combined device and tissue imaging, freedom from ionizing radiation, and the ability to identify serious complications promptly.

Real-time magnetic resonance-guided endovascular repair of experimental abdominal aortic aneurysm in swine

OBJECTIVES

This study tested the hypotheses that endografts can be visualized and navigated in vivo solely under real-time magnetic resonance imaging (rtMRI) guidance to repair experimental abdominal aortic aneurysms (AAA) in swine, and that MRI can provide immediate assessment of endograft apposition and aneurysm exclusion.

BACKGROUND

Endovascular repair for AAA is limited by endoleak caused by inflow or outflow malapposition. The ability of rtMRI to image soft tissue and flow may improve on X-ray guidance of this procedure.

METHODS

Infrarenal AAA was created in swine by balloon overstretch. We used one passive commercial endograft, imaged based on metal-induced MRI artifacts, and several types of homemade active endografts, incorporating MRI receiver coils (antennae). Custom interactive rtMRI features included color coding the catheter-antenna signals individually, simultaneous multislice imaging, and real-time three-dimensional rendering.

RESULTS

Eleven repairs were performed solely using rtMRI, simultaneously depicting the device and soft-tissue pathology during endograft deployment. Active devices proved most useful. Intraprocedural MRI provided anatomic confirmation of stent strut apposition and functional corroboration of aneurysm exclusion and restoration of laminar flow in successful cases. In two cases, there was clear evidence of contrast accumulation in the aneurysm sac, denoting endoleak.

CONCLUSIONS

Endovascular AAA repair is feasible under rtMRI guidance. Active endografts facilitate device visualization and complement the soft tissue contrast afforded by MRI for precise positioning and deployment. Magnetic resonance imaging also permits immediate post-procedural anatomic and functional evaluation of successful aneurysm exclusion.

Real-time magnetic resonance imaging to guide pediatric endovascular procedures

Although x-ray fluoroscopy (XRF) has guided diagnostic and therapeutic transcatheter procedures for decades, certain limitations still exist. XRF still visualizes tissue poorly and relies on projection of shadows that do not convey depth information. Adjunctive echocardiography overcomes some of these limitations but still suffers suboptimal or unreliable imaging windows. Furthermore, ionizing radiation exposure in children imparts a cancer risk. An interventional platform using real-time magnetic resonance imaging (MRI) may offer superior image guidance without radiation. Although there are many remaining challenges, but real-time MRI has the potential to revolutionize transcatheter therapeutics.

MRI in guiding and assessing intramyocardial therapy

Cardiovascular intervention, using MRI guidance, is challenging for clinical applications. Real-time imaging sequences with high spatial resolution are needed for monitoring intramyocardial delivery of drug, gene, or stem cell therapies. New generation MR scanners make local intramyocardial and vascular wall therapies feasible. Contrast-enhanced MRI is used for assessing myocardial ischemia, infarction, and scar tissue. Active (microcoils) and passive (T1 and T2* mechanisms) tracking methods have been used for visualization of endovascular catheters. Safety issues related to potential heating of endovascular devices is still a major obstacle for MRI-guided interventions. Fabrication of MRI-compatible interventional devices is limited. Noninvasive imaging strategies will be critical in defining spatial and temporal characteristics of angiogenesis and myocardial repair as well as in assessing the efficacy of new therapies in ischemic heart disease. MRI contrast media improve the capability of MRI by delineating the target and vascular tree. Labeling stem cells enables MRI to trace distribution, differentiation, and survival in myocardium and vascular wall. In the long term, MRI in guiding and assessing intramyocardial therapy may circumvent the limitations of peripherally administered cell therapy, X-ray angiography, and nuclear imaging. MRI represents a highly attractive discipline whose systematic development will foster the implementation of new cardiac and vascular therapies.

Magnetic resonance fluoroscopy allows targeted delivery of mesenchymal stem cells to infarct borders in Swine

BACKGROUND

The local environment of delivered mesenchymal stem cells (MSCs) may affect their ultimate phenotype. MR fluoroscopy has the potential to guide intramyocardial MSC injection to desirable targets, such as the border between infarcted and normal tissue. We tested the ability to (1) identify infarcts, (2) navigate injection catheters to preselected targets, (3) inject safely even into fresh infarcts, and (4) confirm injection success immediately.

METHODS AND RESULTS

A 1.5-T MRI scanner was customized for interventional use, with rapid imaging, independent color highlighting of catheter channels, multiple-slice 3D rendering, catheter-only viewing mode, and infarct-enhanced imaging. MRI receiver coils were incorporated into guiding catheters and injection needles. These devices were tested for heating and used for targeted MSC delivery. In infarcted pigs, myocardium was targeted by MR fluoroscopy. Infarct-enhanced imaging included both saturation preparation MRI after intravenous gadolinium and wall motion. Porcine MSCs were MRI-labeled with iron-fluorescent particles. Catheter navigation and multiple cell injections were performed entirely with MR fluoroscopy at 8 frames/s with 1.7x3.3x8-mm voxels. Infarct-enhanced MR fluoroscopy permitted excellent delineation of infarct borders. All injections were safely and successfully delivered to their preselected targets, including infarct borders. Iron-fluorescent particle-labeled MSCs were readily visible on delivery in vivo and post mortem.

CONCLUSIONS

Precise targeted delivery of potentially regenerative cellular treatments to recent myocardial infarction borders is feasible with an MR catheter delivery system. MR fluoroscopy permits visualization of catheter navigation, myocardial function, infarct borders, and labeled cells after injection.

Real-time accelerated interactive MRI with adaptive TSENSE and UNFOLD

Reduced field-of-view (FOV) acceleration using time-adaptive sensitivity encoding (TSENSE) or unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) can improve the depiction of motion in real-time MRI. However, increased computational resources are required to maintain a high frame rate and low latency in image reconstruction and display. A high-performance software system has been implemented to perform TSENSE and UNFOLD reconstructions for real-time MRI with interactive, on-line display. Images were displayed in the scanner room to investigate image-guided procedures. Examples are shown for normal volunteers and cardiac interventional experiments in animals using a steady-state free precession (SSFP) sequence. In order to maintain adequate image quality for interventional procedures, the imaging rate was limited to seven frames per second after an acceleration factor of 2 with a voxel size of 1.8 x 3.5 x 8 mm. Initial experiences suggest that TSENSE and UNFOLD can each improve the compromise between spatial and temporal resolution in real-time imaging, and can function well in interactive imaging. UNFOLD places no additional constraints on receiver coils, and is therefore more flexible than SENSE methods; however, the temporal image filtering can blur motion and reduce the effective acceleration. Methods are proposed to overcome the challenges presented by the use of TSENSE in interactive imaging. TSENSE may be temporarily disabled after changing the imaging plane to avoid transient artifacts as the sensitivity coefficients adapt. For imaging with a combination of surface and interventional coils, a hybrid reconstruction approach is proposed whereby UNFOLD is used for the interventional coils, and TSENSE with or without UNFOLD is used for the surface coils.

Undersampled projection reconstruction for active catheter imaging with adaptable temporal resolution and catheter-only views

In this study undersampled projection reconstruction (PR) was used for rapid catheter imaging in the heart, employing steady-state free precession (SSFP) contrast. Active catheters and phased-array coils were used for combined imaging of anatomy and catheter position in swine. Real-time imaging of catheter position was performed with relatively high spatial and temporal resolution, providing 2 x 2 x 8 mm spatial resolution and four to eight frames per second. Two interactive features were introduced. The number of projections (Np) was adjusted interactively to trade off imaging speed and artifact reduction, allowing acquisition of high-quality or high-frame-rate images. Thin-slice imaging was performed, with interactive requests for thick-slab projection images of the signal received solely from the active catheter. Briefly toggling on catheter-only projection images was valuable for verifying that the catheter tip was contained within the selected slice, or for locating the catheter when part of it was outside the selected slice.

Catheter-based endomyocardial injection with real-time magnetic resonance imaging

BACKGROUND

We tested the feasibility of targeted left ventricular (LV) mural injection using real-time MRI (rtMRI).

METHODS AND RESULTS

A 1.5T MRI scanner was customized with a fast reconstruction engine, transfemoral guiding catheter-receiver coil (GCC), MRI-compatible needle, and tableside consoles. Commercial real-time imaging software was customized to facilitate catheter navigation and visualization of injections at 4 completely refreshed frames per second. The aorta was traversed and the left ventricular cavity was entered under direct rtMRI guidance. Pigs underwent multiple injections with dilute gadolinium-DTPA. All myocardial segments were readily accessed. The active GCC and the passive Stiletto needle injector were readily visualized. More than 50 endomyocardial injections were performed with the aid of rtMRI; 81% were successful with this first-generation prototype.

CONCLUSION

Percutaneous endomyocardial drug delivery is feasible with the aid of rtMRI, which permits precise 3-dimensional localization of injection within the LV wall.

Real-time volume rendered MRI for interventional guidance

Volume renderings from magnetic resonance imaging data can be created and displayed in real-time with user interactivity. This can provide continuous 3D feedback to assist in guiding an interventional procedure. A system is presented which can produce real-time volume renderings from 2D multi-slice or 3D MR pulse sequences. Imaging frame rates up to 30 per second have been demonstrated with a latency of approximately one-third of a second, depending on the image matrix size. Several interactive capabilities have been implemented to enhance visualization such as cut planes, individual channel scaling and color highlighting, view sharing, saturation preparation, complex subtraction, gating control, and choice of alpha blending or MIP rendering. The system is described and some interventional application examples are shown. To view movies of some of the examples, enter the following address into a web browser: http://nhlbi.nih.gov/labs/papers/lce/guttman/rtvolmri/index/htm.