Ehud J. Schmidt

Background—: Subjects undergoing cardiac arrest within a magnetic resonance imaging (MRI) scanner are currently removed from the bore and then from the MRI suite, before the delivery of cardiopulmonary resuscitation and defibrillation, potentially increasing the risk of mortality. This precludes many higher-risk (acute ischemic and acute stroke) patients from undergoing MRI and MRI-guided intervention. An MRI-conditional cardiac defibrillator should enable scanning with defibrillation pads attached and the generator ON, enabling application of defibrillation within the seconds of MRI after a cardiac event. An MRI-conditional external defibrillator may improve patient acceptance for MRI procedures. Methods and Results—: A commercial external defibrillator was rendered 1.5 Tesla MRI-conditional by the addition of novel radiofrequency filters between the generator and commercial disposable surface pads. The radiofrequency filters reduced emission into the MRI scanner and prevented cable/surface pad heating during imaging, while preserving all the defibrillator monitoring and delivery functions. Human volunteers were imaged using high specific absorption rate sequences to validate MRI image quality and lack of heating. Swine were electrically fibrillated (n=4) and thereafter defibrillated both outside and inside the MRI bore. MRI image quality was reduced by 0.8 or 1.6 dB, with the generator in monitoring mode and operating on battery or AC power, respectively. Commercial surface pads did not create artifacts deeper than 6 mm below the skin surface. Radiofrequency heating was within US Food and Drug Administration guidelines. Defibrillation was completely successful inside and outside the MRI bore. Conclusions—: A prototype MRI-conditional defibrillation system successfully defibrillated in the MRI without degrading the image quality or increasing the time needed for defibrillation. It can increase patient acceptance for MRI procedures.

Active magnetic resonance (MR) tracking for gynecologic brachytherapy was made possible by attaching the microradiofrequency coils to the brachytherapy applicator. The rectangular planar microcoil was fabricated using flexible printed circuits with dimensions of 8 mm × 1.5 mm. A 5-Fr (1.6 mm) tungsten brachytherapy stylet was custom-machined to incorporate the microcoils. The finite element analysis and the phantom tissue studies show that the proposed device enables in situ real-time guidance of access routes to the target anatomy safely and accurately. The setup was tested in a Siemens 3T MR scanner. The microcoils can be localized rapidly (up to 40 Hz) and precisely (resolution: 0.6 mm × 0.6 mm × 0.6 mm) using an MR-tracking sequence.

Purpose Voltage‐based device‐tracking (VDT) systems are commonly used for tracking invasive devices in electrophysiological cardiac‐arrhythmia therapy. During electrophysiological procedures, electro‐anatomic mapping workstations provide guidance by integrating VDT location and intracardiac electrocardiogram information with X‐ray, computerized tomography, ultrasound, and MR images. MR assists navigation, mapping, and radiofrequency ablation. Multimodality interventions require multiple patient transfers between an MRI and the X‐ray/ultrasound electrophysiological suite, increasing the likelihood of patient‐motion and image misregistration. An MRI‐compatible VDT system may increase efficiency, as there is currently no single method to track devices both inside and outside the MRI scanner. Methods An MRI‐compatible VDT system was constructed by modifying a commercial system. Hardware was added to reduce MRI gradient‐ramp and radiofrequency unblanking pulse interference. VDT patches and cables were modified to reduce heating. Five swine cardiac VDT electro‐anatomic mapping interventions were performed, navigating inside and thereafter outside the MRI. Results Three‐catheter VDT interventions were performed at >12 frames per second both inside and outside the MRI scanner with 32 ms sequences with <0.5 mm errors, and <5% MRI signal‐to‐noise ratio (SNR) loss. At shorter repetition times, only intracardiac electrocardiogram was reliable. Radiofrequency heating was <1.5°C. Conclusion An MRI‐compatible VDT system is feasible. Magn Reson Med 71:1197–1209, 2014.

Wideband steady‐state free precession (WB‐SSFP) is a modification of balanced steady‐state free precession utilizing alternating repetition times to reduce susceptibility‐induced balanced steady‐state free precession limitations, allowing its use for high‐resolution myelographic‐contrast spinal imaging. Intertissue contrast and spatial resolution of complete‐spine‐coverage 3D WB‐SSFP were compared with those of 2D T₂‐weighted fast spin echo, currently the standard for spine T₂‐imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal‐to‐noise ratio efficiency (SNR per unit‐time and unit‐volume) of several tissues was measured, along with four intertissue contrast‐to‐noise ratios; nerve‐ganglia:fat, intradural‐nerves:cerebrospinal fluid, nerve‐ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB‐SSFP clinical advantages and disadvantages. At 3 T, WB‐SSFP provided spinal contrast‐to‐noise ratios 3.7–5.2 times that of fast spin echo. At 1.5 T, WB‐SSFP contrast‐to‐noise ratio was 3–3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural‐nerves:cerebrospinal fluid. WB‐SSFP signal‐to‐noise ratio efficiency was also higher. Three‐dimensional WB‐SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB‐SSFP's high resolution and contrast‐to‐noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal‐stenosis. WB‐SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology. Magn Reson Med, 2012.

Background— The MRI-compatible electrophysiology system previously used for MR-guided left ventricular electroanatomic mapping was enhanced with improved MR tracking, an MR-compatible radiofrequency ablation system and higher-resolution imaging sequences to enable mapping, ablation, and ablation monitoring in smaller cardiac structures. MR-tracked navigation was performed to the left atrium (LA) and atrioventricular (AV) node, followed by LA electroanatomic mapping and radiofrequency ablation of the pulmonary veins (PVs) and AV node. Methods and Results— One ventricular ablation, 7 PV ablations, 3 LA mappings, and 3 AV node ablations were conducted. Three MRI-compatible devices (ablation/mapping catheter, torqueable sheath, stimulation/pacing catheter) were used, each with 4 to 5 tracking microcoils. Transseptal puncture was performed under x-ray, with all other procedural steps performed in the MRI. Preacquired MRI roadmaps served for real-time catheter navigation. Simultaneous tracking of 3 devices was performed at 13 frames per second. LA mapping and PV radiofrequency ablation were performed using tracked ablation catheters and sheaths. Ablation points were registered and verified after ablation using 3D myocardial delayed enhancement and postmortem gross tissue examination. Complete LA electroanatomic mapping was achieved in 3 of 3 pigs, Right inferior PV circumferential ablation was achieved in 3 of 7 pigs, with incomplete isolation caused by limited catheter deflection. During AV node ablation, ventricular pacing was performed, 3 devices were simultaneously tracked, and intracardiac ECGs were displayed. 3D myocardial delayed enhancement visualized node injury 2 minutes after ablation. AV node block succeeded in 2 of 3 pigs, with 1 temporary block. Conclusions— LA mapping, PV radiofrequency ablation, and AV node ablation were demonstrated under MRI guidance. Intraprocedural 3D myocardial delayed enhancement assessed lesion positional accuracy and dimensions.

Purpose To develop MR‐tracked catheters to delineate the three‐dimensional motion of coronary arteries at high spatial and temporal resolution. Materials and Methods Catheters with three tracking microcoils were placed into nine swine. During breath‐holds, electrocardiographic (ECG)‐synchronized 3D motion was measured at varying vessel depths. 3D motion was measured in American Heart Association left anterior descending (LAD) segments 6–7, left circumflex (LCX) segments 11–15, and right coronary artery (RCA) segments 2–3, at 60–115 beats/min heart rates. Similar‐length cardiac cycles were averaged. Intercoil cross‐correlation identified early systolic phase (ES) and determined segment motion delay. Results Translational and rotational motion, as a function of cardiac phase, is shown, with directionality and amplitude varying along the vessel length. Rotation (peak‐to‐peak solid‐angle RCA ≈0.10, LAD ≈0.06, LCX ≈0.18 radian) occurs primarily during fast translational motion and increases distally. LCX displacement increases with heart rate by 18%. Phantom simulations of motion effects on high‐resolution images, using RCA results, show artifacts due to translation and rotation. Conclusion Magnetic resonance imaging (MRI) tracking catheters quantify motion at 20 fps and 1 mm³ resolution at multiple vessel depths, exceeding that available with other techniques. Imaging artifacts due to rotation are demonstrated. Motion‐tracking catheters may provide physiological information during interventions and improve imaging spatial resolution. J. Magn. Reson. Imaging 2009;29:86–98.