Acousto-optic-based time domain electric field sensor for magnetic resonance imaging applications.

An acousto-optic (AO)-based electric field sensor is presented for time domain measurement under magnetic resonance imaging (MRI). A fully MR-compatible sensor is designed and fabricated using a phase-shifted fiber Bragg grating mechanically coupled to a piezoelectric transducer. Mechanical resonance of the piezoelectric transducer is matched to the operating frequencies of commonly used MRI systems to increase the sensitivity of the sensor. Sensitivity of the sensor is measured as 1.27 mV/V/m, with a minimum detectable electric field of 4.4 mV/m/√/Hz. Directivity of the sensor is measured with a 18 dB orthogonal component rejection. The dynamic range of the sensor is calculated as 117 dB/Hz, which allows the measurement of electric fields up to 3.2 kV/m. In MRI studies, the AO sensor was able detect local hot spots around a reference implant accurately with high signal-to-noise ratio. AO sensor exhibited similar or better performance when compared with commercially available MRI compatible electric field sensors. Furthermore, the small size of the sensor with the flexible fiber optic link could allow in situ measurements of electric fields during critical interventional procedures such as pacemaker lead or deep brain stimulator placement as an MRI dosimeter during diagnostic scans.

Interventional cardiovascular magnetic resonance: state-of-the-art.

Transcatheter cardiovascular interventions increasingly rely on advanced imaging. X-ray fluoroscopy provides excellent visualization of catheters and devices, but poor visualization of anatomy. In contrast, magnetic resonance imaging (MRI) provides excellent visualization of anatomy and can generate real-time imaging with frame rates similar to X-ray fluoroscopy. Realization of MRI as a primary imaging modality for cardiovascular interventions has been slow, largely because existing guidewires, catheters and other devices create imaging artifacts and can heat dangerously. Nonetheless, numerous clinical centers have started interventional cardiovascular magnetic resonance (iCMR) programs for invasive hemodynamic studies or electrophysiology procedures to leverage the clear advantages of MRI tissue characterization, to quantify cardiac chamber function and flow, and to avoid ionizing radiation exposure. Clinical implementation of more complex cardiovascular interventions has been challenging because catheters and other tools require re-engineering for safety and conspicuity in the iCMR environment. However, recent innovations in scanner and interventional device technology, in particular availability of high performance low-field MRI scanners could be the inflection point, enabling a new generation of iCMR procedures. In this review we review these technical considerations, summarize contemporary clinical iCMR experience, and consider potential future applications.

TABERNACL: Temporary Hemodynamic Stabilization In Vivo.

BACKGROUND: Acute aortic regurgitation is life-threatening with few nonsurgical options for immediate stabilization. We propose Trans-Aortic Balloon to Ease Regurgitation Applying Counter-Pulsation (TABERNACL), a simple, on-table temporary valve using commercially available equipment to temporize acute severe aortic regurgitation. METHODS: We hypothesize that an appropriately sized commercial balloon dilatation catheter-straddling the aortic annulus and connected to a counterpulsation console-can serve as a temporizing valve to restore hemodynamic stability in acute aortic regurgitation. We performed benchtop testing of valvuloplasty, angioplasty, and sizing balloons as counterpulsation balloons. TABERNACL was assessed in vivo in a porcine model of acute aortic regurgitation (n=8). We also tested a static undersized, continuously inflated transvalvular balloon as a spacer intended physically to obstruct the regurgitant orifice. RESULTS: Benchtop testing identified that Tyshak II and PTS sizing (NuMed Braun) balloon catheters performed adequately as temporary valves (ie, complete inflation and deflation with each cycle) and resisted fatigue, in contrast to others. When TABERNACL was used in the acute severe regurgitation animals, there was immediate hemodynamic improvement, with a significant 35% increase in diastolic aortic pressure by 16 mm Hg ([95% CI, 7-25] P=0.0056), 34% reduction in left ventricular end-diastolic pressure by -7 mm Hg ([95% CI, -10 to -5] P=0.0006), improvement in the aortic diastolic index by 0.28 ([95% CI, 0.18-0.39] P=0.0009), and reversal of electrocardiographic myocardial ischemia. As an alternative, static balloon inflation across the aortic valve stabilized regurgitation hemodynamics at the expense of a new aortic gradient and caused excessive ectopy from balloon movement in the left ventricular outflow tract. CONCLUSIONS: TABERNACL improves hemodynamics and reduces coronary ischemia by electrocardiography in animals with acute severe aortic regurgitation. TABERNACL valves obstruct the diastolic regurgitant orifice without systolic obstruction. This may prove a lifesaving bridge to definitive valve replacement therapy.

Ventricular Intramyocardial Navigation for Tachycardia Ablation Guided by Electrograms (VINTAGE): Deep Ablation in Inaccessible Targets.

BACKGROUND: Deep intramural ventricular tachycardia substrate targets are difficult to access, map, and ablate from endocardial and epicardial surfaces, resulting in high recurrence rates. OBJECTIVES: In this study, the authors introduce a novel approach called ventricular intramyocardial navigation for tachycardia ablation guided by electrograms (VINTAGE) to access and ablate anatomically challenging ventricular tachycardia from within the myocardium. METHODS: Guidewire/microcatheter combinations were navigated deep throughout the extravascular myocardium, accessed directly from the right ventricle cavity, in Yorkshire swine (6 naive, 1 infarcted). Devices were steered to various intramyocardial targets including the left ventricle summit, guided by fluoroscopy, unipolar electrograms, and/or electroanatomic mapping. Radiofrequency ablations were performed to characterize ablation parameters and reproducibility. Intramyocardial saline irrigation began 1 minute before ablation and continued throughout. Lesions were analyzed on cardiac magnetic resonance and necropsy. RESULTS: VINTAGE was feasible in all animals within naive and infarcted myocardium. Forty-three lesions were created, using various guidewires and power settings. Forty-one (95%) lesions were detected on cardiac magnetic resonance and 38 (88%) on necropsy; all undetected lesions resulted from intentionally subtherapeutic ablation energy (10 W). Larger-diameter guidewires yielded larger size lesions. Lesion volumes on necropsy were significantly larger at 20 W than 10 W (178 mm3 [Q1-Q3: 104-382 mm3] vs 49 mm3 [Q1-Q3: 35-93 mm3]; P = 0.02). Higher power (30 W) did not create larger lesions. Median impedance dropped with preablation irrigation by 12 Ω (Q1-Q3: 8-17 Ω), followed by a further 15-Ω (Q1-Q3: 11-19 Ω) drop during ablation. Intramyocardial navigation, ablation, and irrigation were not associated with any complications. CONCLUSIONS: VINTAGE was safe and effective at creating intramural ablation lesions in targets traditionally considered inaccessible from the endocardium and epicardium, both naive and infarcted. Intramyocardial guidewire irrigation and ablation at 20 W creates reproducibly large intramural lesions.

Transcatheter Electrosurgery: A Narrative Review.

Transcatheter electrosurgery describes the ability to cut and traverse tissue, at a distance, without an open surgical field and is possible using either purpose-built or off-the-shelf devices. Tissue traversal requires focused delivery of radiofrequency energy to a guidewire tip. Initially employed to cross atretic pulmonary valves, tissue traversal has enabled transcaval aortic access, recanalization of arterial and venous occlusions, transseptal access, and many other techniques. To cut tissue, the selectively denuded inner curvature of a kinked guidewire (the Flying-V) or a single-loop snare is energized during traction. Adjunctive techniques may complement or enable contemporary transcatheter procedures, whereas myocardial slicing or excision of ectopic masses may offer definitive therapy. In this contemporary review we discuss the principles of transcatheter electrosurgery, and through exemplary clinical applications highlight the range of therapeutic options offered by this versatile family of procedures.

EDEN (Electrocardiographic Radial Depth Navigation): A Novel Approach to Navigate Inside Heart Muscle.

BACKGROUND: Intramyocardial guidewire navigation is a novel technique that allows free transcatheter movement within ventricular muscle. Guidewire radial depth, between endocardial and epicardial surfaces, is ambiguous by x-ray and echocardiography. OBJECTIVES: The aim of this study was to develop a simple tool, EDEN (Electrocardiographic Radial Depth Navigation), to indicate radial depth during intramyocardial guidewire navigation. Combined with routine imaging, EDEN facilitates a new family of intramyocardial catheter procedures to slice, reshape, pace, and ablate the heart. METHODS: We mapped intramyocardial electrograms of left and right ventricular walls and septum during open- and closed-chest swine procedures (N = 53), including MIRTH (Myocardial Intramural Remodeling by Transvenous Tether) ventriculoplasty. We identified radial depth-dependent features on unipolar electrograms. We developed a machine learning-based classifier to indicate categorical position, and modeled the findings in silico to test understanding of the physiology. RESULTS: EDEN signatures distinguished 5 depth zones throughout left and right ventricular free walls and interventricular septum. Relative ST-segment elevation magnitude best discriminated position and was maximum (40.1 ± 6.5 mV) in the midmyocardium. Subendocardial positions exhibited dominant Q waves with lower-amplitude ST segments (16.8 ± 5.8 mV), whereas subepicardial positions exhibited dominant R waves with lower-amplitude ST segments (15.7 ± 4.8 mV). EDEN was unaffected by pacing-induced left bundle branch block. ST-segment elevation declined over minutes and reappeared after submillimeter guidewire manipulation. Modeling recapitulated EDEN features. The machine learning-based classifier was 97% accurate. EDEN successfully guided MIRTH ventriculoplasty. CONCLUSIONS: EDEN provides a simple and reproducible real-time reflection of categorical guidewire-tip radial depth during intramyocardial guidewire navigation. Used in tandem with x-ray, EDEN enables novel, transcatheter, intramyocardial therapies such as MIRTH, SESAME (Septal Surfing Along Midline Endocardium), and cerclage ventriculoplasty.

Reshaping the Ventricle From Within: MIRTH (Myocardial Intramural Remodeling by Transvenous Tether) Ventriculoplasty in Swine.

MIRTH (Myocardial Intramural Remodeling by Transvenous Tether) is a transcatheter ventricular remodeling procedure. A transvenous tension element is placed within the walls of the beating left ventricle and shortened to narrow chamber dimensions. MIRTH uses 2 new techniques: controlled intramyocardial guidewire navigation and EDEN (Electrocardiographic Radial Depth Navigation). MIRTH caused a sustained reduction in chamber dimensions in healthy swine. Midventricular implants approximated papillary muscles. MIRTH shortening improved myocardial contractility in cardiomyopathy in a dose-dependent manner up to a threshold beyond which additional shortening reduced performance. MIRTH may help treat dilated cardiomyopathy. Clinical investigation is warranted.