An Integrated Robotic System for MRI-Guided Neuroablation: Preclinical Evaluation.

The goals of this study were to develop a method that combines cryoablation with real-time magnetic resonance imaging (MRI) guidance for pulmonary vein isolation (PVI) and to further quantify the lesion formation by imaging both acute and chronic cryolesions. Investigational MRI-compatible cryoablation devices were created by modifying cryoballoons and cryocatheters. These devices were used in canines (n = 8) and a complete series of lesions (PVI: n = 5, superior vena cava: n = 4, focal: n = 13) were made under real-time MRI guidance. Late gadolinium enhancement (LGE) magnetic resonance imaging was acquired at acute and chronic time points. Late gadolinium enhancement magnetic resonance imagings show a significant amount of acute tissue injury immediately following cryoablation which subsides over time. In the pulmonary veins, scar covered 100% of the perimeter of the ostium of the veins acutely, which subsided to 95.6 ± 4.3% after 3 months. Focal point lesions showed significantly larger acute enhancement volumes compared to the volumes estimated from gross pathology measurements (0.4392 ± 0.28 cm3 vs. 0.1657 ± 0.08 cm3, P = 0.0043). Additionally, our results with focal point ablations indicate that freeze-zone formation reached a maximum area after 120 s. This study reports on the development of an MRI-based cryoablation system and shows that with acute cryolesions there is a large area of reversible injury. Real-time MRI provides the ability to visualize the freeze-zone formation during the freeze cycle and for focal lesions reaches a maximum after 120 s suggesting that for maximizing lesion size 120 s might be the lower limit for dosing duration.

All examinations were performed with approval from the institutional animal care and use committee of Columbia University. To assess the feasibility of real-time magnetic resonance (MR) imaging-guided neurovascular intervention in a swine model, the authors placed stents in the carotid arteries of five domestic pigs. Seven-French vascular sheaths were placed in the target carotid arteries via femoral access by using active MR tracking. Ten nitinol stents (8-10 x 20-40 mm) were successfully deployed in the target segments of carotid arteries bilaterally. MR imaging and necropsy findings confirmed stent position. Necropsy revealed no gross vascular injury. Study results demonstrated the feasibility of performing real-time MR imaging-guided neurovascular intervention by using an active-tracking technique in an animal model.

To assess the feasibility of multiplanar vascular navigation with a new magnetically assisted remote-controlled (MARC) catheter with real-time magnetic resonance (MR) imaging at 1.5 T and 3 T and to compare it with standard x-ray guidance in simulated endovascular catheterization procedures. A 1.6-mm-diameter custom clinical-grade microcatheter prototype with lithographed double-saddle coils at the distal tip was deflected with real-time MR imaging. Two inexperienced operators and two experienced operators catheterized anteroposterior (celiac, superior mesenteric, and inferior mesenteric arteries) and mediolateral (renal arteries) branch vessels in a cryogel abdominal aortic phantom. This was repeated with conventional x-ray fluoroscopy by using clinical catheters and guidewires. Mean procedure times and percentage success data were analyzed with linear mixed-effects regression. The MARC catheter tip was visible at 1.5 T and 3 T. Among inexperienced operators, MARC MR imaging guidance was not statistically different from x-ray guidance at 1.5 T (67% successful vessel selection turns with MR imaging vs 76% with x-ray guidance, P = .157) and at 3 T (75% successful turns with MR imaging vs 76% with x-ray guidance, P = .869). Experienced operators were more successful in catheterizing vessels with x-ray guidance (98% success within 60 seconds) than with 1.5-T (65%, P < .001) or 3-T (75%) MR imaging. Among inexperienced operators, mean procedure time was nearly equivalent by using MR imaging (31 seconds) and x-ray guidance (34 seconds, P = .436). Among experienced operators, catheterization was faster with x-ray guidance (20 seconds) compared with 1.5-T MR imaging (42 seconds, P < .001), but MARC guidance improved at 3 T (31 seconds). MARC MR imaging guidance at 3 T was not significantly different from x-ray guidance for the celiac (P = .755), superior mesenteric (P = .358), and inferior mesenteric (P = .065) arteries. Multiplanar navigation with a new MARC catheter with real-time MR imaging at 1.5 T and 3 T is feasible and comparable to x-ray guidance for anteroposterior vessels at 3 T in a vascular phantom.

To evaluate a novel real-time phase-contrast magnetic resonance imaging (MRI) technique for the assessment of through-plane flow in the ascending aorta. Real-time MRI was based on a radial fast low-angle shot (FLASH) sequence with about 30-fold undersampling and image reconstruction by regularized nonlinear inversion. Phase-contrast maps were obtained from two (interleaved or sequential) acquisitions with and without a bipolar velocity-encoding gradient. Blood flow in the ascending aorta was studied in 10 healthy volunteers at 3 T by both real-time MRI (15 sec during free breathing) and electrocardiogram (ECG)-synchronized cine MRI (with and without breath holding). Flow velocities and stroke volumes were evaluated using standard postprocessing software. The total acquisition time for a pair of phase-contrast images was 40.0 msec (TR/TE = 2.86/1.93 msec, 10° flip angle, 7 spokes per image) for a nominal in-plane resolution of 1.3 mm and a section thickness of 6 mm. Quantitative evaluations of spatially averaged flow velocities and stroke volumes were comparable for real-time and cine methods when real-time MRI data were averaged across heartbeats. For individual heartbeats real-time phase-contrast MRI resulted in higher peak velocities for values above 120 cm s(-1). Real-time phase-contrast MRI of blood flow in the human aorta yields functional parameters for individual heartbeats. When averaged across heartbeats real-time flow velocities and stroke volumes are comparable to values obtained by conventional cine MRI.

Real-time magnetic resonance imaging–guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has considered the impact of this artifact on TDE accuracy. To determine the effect of intraoperative MRTI signal artifact on postoperative magnetic resonance imaging (MRI)-predicted ablative area. All ablations were performed using the Visualase MRI-Guided Laser Ablation System (Medtronic). Patients were grouped based on whether the intraoperative MRTI contained signal artifact that distorted the ablative region. Cross-sectional area of the ablative lesion from the MRI image was measured, and the difference between intraoperative TDE and postoperative MRI cross-sectional area was calculated and compared between groups with and without intraoperative MRTI artifact. A total of 91 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 43.9% of cases overall. The mean absolute difference between TDE and the postoperative MRI cross-sectional area was 94.8 mm2 (SEM=11.6) in the group with intraoperative MRTI artifact and 54.4 mm2 (SEM=5.5) in the nonartifact group. MRTI signal artifact is common during LITT. The presence of signal artifact during intraoperative MRTI results in higher variation between intraoperative TDE and postoperative MRI cross-sectional ablative area. In cases in which intraoperative MRTI artifact is observed, there may be a larger degree of variation between observed intraoperative TDE and measured postoperative MRTI ablative area.

In the past few years, radiotherapy (RT) has experienced a major technological innovation with the development of hybrid machines combining magnetic resonance (MR) imaging and linear accelerators. This new technology for MR-guided cancer treatment has the potential to revolutionize the field of adaptive RT due to the opportunity to provide high-resolution, real-time MR imaging before and during treatment application. However, from a technical point of view, several challenges remain which need to be tackled to ensure safe and robust real-time adaptive MR-guided RT delivery. In this manuscript, several technical challenges to MR-guided RT are discussed. Starting with magnetic field strength tradeoffs, the potential and limitations for purely MR-based RT workflows are discussed. Furthermore, the current status of real-time 3D MR imaging and its potential for real-time RT are summarized. Finally, the potential of quantitative MR imaging for future biological RT adaptation is highlighted.

The aims of this study were to evaluate in a canine model applicators designed for ablation of human benign prostatic hyperplasia (BPH) in vivo under magnetic resonance imaging (MRI) guidance, including magnetic resonance thermal imaging (MRTI), determine the ability of MRI techniques to visualize ablative changes in prostate, and evaluate the acute and longer term histologic appearances of prostate tissue ablated during these studies. An MRI-compatible transurethral device incorporating a tubular transducer array with dual 120° sectors was used to ablate canine prostate tissue in vivo, in zones similar to regions of human BPH (enlarged transition zones). Magnetic resonance thermal imaging was used for monitoring of ablation in a 3-T environment, and postablation MRIs were performed to determine the visibility of ablated regions. Three canine prostates were ablated in acute studies, and 2 animals were rescanned before killing at 31 days postablation. Acute and chronic appearances of ablated prostate tissue were evaluated histologically and were correlated with the MRTI and postablation MRI scans. It was possible to ablate regions similar in size to enlarged transition zone in human BPH in 6 to 18 minutes. Regions of acute ablation showed a central "heat-fixed" region surrounded by a region of more obvious necrosis with complete disruption of tissue architecture. After 31 days, ablated regions demonstrated complete apparent resorption of ablated tissue with formation of cystic regions containing fluid. The inherent cooling of the urethra using the technique resulted in complete urethral preservation in all cases. Prostatic ablation of zones of size and shape corresponding to human BPH is possible using appropriate transurethral applicators using MRTI, and ablated tissue may be depicted clearly in contrast-enhanced magnetic resonance images. The ability accurately to monitor prostate tissue heating, the apparent resorption of ablated regions over 1 month, and the inherent urethral preservation suggest that the magnetic resonance-guided techniques described are highly promising for the in vivo ablation of symptomatic human BPH.

We prospectively evaluated the feasibility and technical properties of a dedicated interventional magnetic resonance (MR) imaging protocol for near-real-time MR fluoroscopy-guided bilateral lumbar facet joint injections. A total of 44 facet joint injections were performed in 22 patients using a C-shaped open 0.2-T MR imaging system (Magnetom Concerto, Siemens Medical Solutions, Erlangen, Germany). A T1/T2-weighted fast-imaging-with-steady-precession (FISP) sequence with an end-to-end latency of 1.2 s facilitated sufficient near real-time MR imaging guidance in all cases. A T1-weighted two dimensional fast-low-angle-shot (FLASH2D) MR sequence identified final needle tip location. Different angles of the needle path had only minimal influence on the appearance of the needle artifact produced by both sequences, resulting in a symmetrical needle tip artifact. The joint cavity was successfully punctured in 79.5% (35/44) of joints, which was followed by intra-articular fluid accumulation in 75% (33/44). Inaccessible joints demonstrated a significantly (p=0.044) higher number of posterior osteophytes (66.7%, 6/9 joints) compared to accessible joints (26%, 9/35 joints). No complications occurred. Table time showed significant shortening over time with average table time of 33 (21-68.5) min. We conclude that MR fluoroscopy-navigated lumbar facet joint injections are feasible and safe.

Cleft of the secondary palate without cleft lip is difficult to visualize sonographically. This study was performed to assess the utility of sonography, standard magnetic resonance (MR) imaging, and real-time MR imaging in the diagnosis of isolated cleft palate. We prospectively assessed 5 fetuses at risk for isolated cleft palate on the basis of family history, micrognathia, or both, using sonography and standard and real-time single-shot fast spin echo MR sequences. Written informed consent was obtained under our Institutional Review Board-approved Health Insurance Portability and Accountability Act-compliant protocol. Images were assessed for confidence in a diagnosis of cleft or normal palate. Prenatal and postnatal diagnoses were compared. In 3 fetuses, micrognathia was visualized by sonography and MR imaging with standard and real-time sequences. One fetus at 19 weeks had a wide cleft of the entire secondary palate, and another fetus at 33 weeks had a cleft of the soft palate; these defects were seen only with real-time MR imaging. One 35-week gestational age fetus had a cleft soft palate that was visualized on standard and real-time MR imaging. Two fetuses with no abnormalities had the normal midline secondary palate seen only on real-time MR imaging. In all fetuses, real-time images were helpful in assessing the secondary palate because the entire midline naso-oropharynx could be visualized. Real-time MR imaging allows for rapid assessment of the midline structures, providing accurate diagnosis of isolated cleft palate.

The purpose of this study is to propose and evaluate a slice-to-volume registration (SVR) method integrating an advanced dual-slice real-time magnetic resonance image (MRI) and three-dimensional (3D) MRI volume of the tibiofemoral joint for determining their 3D kinematics. The real-time and 3D MRI of the knee were collected from 12 healthy adults at 5 static flexion positions and during dynamic flexion/extension movement. The 3D positions and orientations of the femur and tibia were obtained by registering their volumetric models constructed from the 3D MRI to dual-slice real-time MRI using an optimization process. The proposed method was quantitatively evaluated for its performance in terms of the robustness and measurement accuracy, and compared to those of a single-slice SVR method. Its repeatability in measuring knee kinematics during flexion/extension movement was also determined. In comparison to the single-slice SVR method, the dual-slice method was significantly superior, giving a successful registration rate>95%, a bias less than 0.5mm in translations and 0.6° in rotations and a precision <0.7mm in translations and 0.9° in rotations for determining the 3D tibiofemoral poses. For repeatability of the dual-slice SVR in measuring tibiofemoral kinematics during dynamic flexion/extension, the means of the time-averaged standard deviations were <0.9° for joint angles and 0.5mm for joint translations. A dual-slice SVR method in conjunction with real-time MRI has been developed and evaluated for its performance in measuring 3D kinematics of the tibiofemoral joint in 12 young adults in terms of the accuracy, robustness, and repeatability. The proposed MRI-based 3D measurement method provides a noninvasive and ionizing radiation-free approach for 3D kinematic measurement of the tibiofemoral joint, which will be helpful for future academic and clinical applications.

Based on a 3D pre-treatment magnetic resonance (MR) scan, we developed DREME-MR to jointly reconstruct the reference patient anatomy and solve a data-driven, patient-specific cardiorespiratory motion model. Via a motion encoder simultaneously learned during the reconstruction, DREME-MR further enables real-time volumetric MR imaging and cardiorespiratory motion tracking with minimal intra-treatment k-space data.&#xD;&#xD;Approach: DREME-MR integrates dynamic MRI reconstruction and real-time MR imaging into a unified, dual-task learning framework. From a 3D radial-spoke-based pre-treatment MR scan, DREME-MR uses spatiotemporal implicit-neural-representation (INR) to reconstruct pre-treatment dynamic volumetric MR images (learning task 1). The INR-based reconstruction takes a joint image reconstruction and deformable registration approach, yielding a reference anatomy and a corresponding cardiorespiratory motion model. The motion model adopts a low-rank, multi-resolution representation to approximate motion fields as products of motion coefficients and motion basis components (MBCs). Via a progressive, frequency-guided strategy, DREME-MR decouples cardiac MBCs from respiratory MBCs to resolve the two distinct motion modes. Simultaneously with the pre-treatment dynamic MRI reconstruction, DREME-MR also trains a multilayer perceptron (MLP)-based motion encoder to infer cardiorespiratory motion coefficients directly from the raw k-space data (learning task 2), allowing real-time, intra-treatment volumetric MR imaging and motion tracking with minimal k-space data (20-30 spokes) acquired after the pre-treatment MRI scan.&#xD;&#xD;Main results: Evaluated using data from a digital phantom (XCAT) and a human scan, DREME-MR solves real-time 3D cardiorespiratory motion with a latency of < 165 ms (= 150-ms data acquisition + 15-ms inference time), fulfilling the temporal constraint of real-time imaging. The XCAT study achieves mean(±S.D.) center-of-mass tracking errors of 0.73±0.38mm for a lung tumor and 1.69±1.12mm for the left ventricle. The human study shows good motion correlations (liver: 0.96; left ventricle: 0.65) between DREME-MR-solved motion and extracted surrogate signals.&#xD;&#xD;Significance: DREME-MR allows real-time 3D MRI and cardiorespiratory motion tracking with low latency, advancing intra-treatment MR-guided adaptive radiotherapy, including real-time multileaf collimator (MLC) tracking.

Comparison of Gadoxetic Acid–enhanced MR Imaging versus Four-phase Multi–detector Row Computed Tomography in Assessing Tumor Regression after Radiofrequency Ablation in Subjects with Hepatocellular Carcinomas

Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse. In this study, ten metacarpophalangeal joints were studied by inserting the finger of interest into a flexible tube tightened around a length of cable used to provide long-axis traction. Before and after traction, static 3D T1-weighted magnetic resonance images were acquired. During traction, rapid cine magnetic resonance images were obtained from the joint midline at a rate of 3.2 frames per second until the cracking event occurred. As traction forces increased, real-time cine magnetic resonance imaging demonstrated rapid cavity inception at the time of joint separation and sound production after which the resulting cavity remained visible. Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble. These observations are consistent with tribonucleation, a known process where opposing surfaces resist separation until a critical point where they then separate rapidly creating sustained gas cavities. Observed previously in vitro, this is the first in-vivo macroscopic demonstration of tribonucleation and as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking.

This paper investigates different statistical modeling frameworks for articulatory speech data obtained using real-time (RT) magnetic resonance imaging (MRI). To quantitatively capture the spatio-temporal shaping process of the human vocal tract during speech production a multi-dimensional stream of direct image features is extracted automatically from the MRI recordings. The features are closely related, though not identical, to the tract variables commonly defined in the articulatory phonology theory. The modeling of the shaping process aims at decomposing the articulatory data streams into primitives by segmentation. A variety of approaches are investigated for carrying out the segmentation task including vector quantizers, Gaussian Mixture Models, Hidden Markov Models, and a coupled Hidden Markov Model. We evaluate the performance of the different segmentation schemes qualitatively with the help of a well understood data set which was used in an earlier study of inter-articulatory timing phenomena of American English nasal sounds. Index Terms: speech production, articulatory modeling, realtime magnetic resonance imaging