Robert David Darrow

Tracking system to monitor the position and orientation of a device using magnetic resonance detection of a sample contained within the device

A tracking system employs magnetic resonance signals to monitor the position and orientation of a device, such as a catheter, within a subject. The device has an MR active sample and a receiver coil which is sensitive to magnetic resonance signals generated by the MR active sample. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to sequentially applied mutually orthogonal magnetic gradients to determine the devices position in several dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images. One or more devices can be simultaneously tracked.

Tracking system and pulse sequences to monitor the position of a device using magnetic resonance

A tracking system employs magnetic resonance signals to monitor the position of a device such as a catheter within a subject. The device has a receiver coil which is sensitive to magnetic resonance signals generated in the subject. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to sequentially applied mutually orthogonal magnetic gradients to determine the position of the device in several dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images.

Tracking system to monitor the position of a device using multiplexed magnetic resonance detection

A tracking system employs magnetic resonance signals to monitor the position of a device such as a catheter within a subject. The device has a receiver coil which is sensitive to magnetic resonance signals generated in the subject. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to applied magnetic gradients to determine the position of the device in several dimensions. Sensitivity of the measured position to resonance offset conditions such as transmitter frequency misadjustment, chemical shift and the like is minimized by repeating the process a plurality of times with selected amplitudes and polarities for the applied magnetic field gradient. Linear combinations of the data acquired responsive to the different applied magnetic field gradients are computed to determine the position of the device in three orthogonal dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images.

128-channel body MRI with a flexible high-density receiver-coil array.

To determine whether the promise of high‐density many‐coil MRI receiver arrays for enabling highly accelerated parallel imaging can be realized in practice.

Electroanatomic Mapping of the Left Ventricle in a Porcine Model of Chronic Myocardial Infarction With Magnetic Resonance–Based Catheter Tracking

Background— X-ray fluoroscopy constitutes the fundamental imaging modality for catheter visualization during interventional electrophysiology procedures. The minimal tissue discriminative capability of fluoroscopy is mitigated in part by the use of electroanatomic mapping systems and enhanced by the integration of preacquired 3-dimensional imaging of the heart with computed tomographic or magnetic resonance (MR) imaging. A more ideal paradigm might be to use intraprocedural MR imaging to directly image and guide catheter mapping procedures. Methods and Results— An MR imaging–based electroanatomic mapping system was designed to assess the feasibility of navigating catheters to the left ventricle in vivo using MR tracking of microcoils incorporated into the catheters, measuring intracardiac ventricular electrograms, and integrating this information with 3-dimensional MR angiography and myocardial delayed enhancement images to allow ventricular substrate mapping. In all animals (4 normal, and 10 chronically infarcted swine), after transseptal puncture under fluoroscopic guidance, catheters were successfully navigated to the left ventricle with MR tracking (13 to 15 frames per second) by both transseptal and retrograde aortic approaches. Electrogram artifacts related to the MR imaging gradient pulses were successfully removed with analog and digital signal processing. In all animals, it was possible to map the entire left ventricle and to project electrogram voltage amplitude maps to identify the scarred myocardium. Conclusions— It is possible to use MR tracking to navigate catheters to the left ventricle, to measure electrogram activity, and to render accurate 3-dimensional voltage maps in a porcine model of chronic myocardial infarction, completely in the MR imaging environment. Myocardial delayed enhancement guidance provided dense sampling of the proximity of the infarct and accurate localization of complex infarcts.

Electroanatomic Mapping and Radiofrequency Ablation of Porcine Left Atria and Atrioventricular Nodes Using Magnetic Resonance Catheter Tracking

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.

Large field-of-view real-time MRI with a 32-channel system.

The emergence of parallel MRI techniques and new applications for real‐time interactive MRI underscores the need to evaluate performance gained by increasing the capability of MRI phased‐array systems beyond the standard four to eight high‐bandwidth channels. Therefore, to explore the advantages of highly parallel MRI a 32‐channel 1.5 T MRI system and 32‐element torso phased arrays were designed and constructed for real‐time interactive MRI. The system was assembled from multiple synchronized scanner‐receiver subsystems. Software was developed to coordinate across subsystems the real‐time acquisition, reconstruction, and display of 32‐channel images. Real‐time, large field‐of‐view (FOV) body‐survey imaging was performed using interleaved echo‐planar and single‐shot fast‐spin‐echo pulse sequences. A new method is demonstrated for augmenting parallel image acquisition by independently offsetting the frequency of different array elements (FASSET) to variably shift their FOV. When combined with conventional parallel imaging techniques, image acceleration factors of up to 4 were investigated. The use of a large number of coils allowed the FOV to be doubled in two dimensions during rapid imaging, with no degradation of imaging time or spatial resolution. The system provides a platform for evaluating the applications of many‐channel real‐time MRI, and for understanding the factors that optimize the choice of array size. Magn Reson Med 52:878–884, 2004.

Coronary angiography by real-time MRI with adaptive averaging

Cardiac and respiratory motion present significant challenges for MR coronary angiography, which have not been completely resolved to date by either breath‐holding or respiratory navigation. Adaptive averaging during real‐time MRI may provide a useful alternative to these techniques. In this method, cross‐correlation is used to automatically identify those real‐time imaging frames in which the vessel is present, and to determine the location of the vessel within each frame. This information is then used for selective averaging of frames to increase the signal‐to‐noise ratio and to improve visualization of the vessel. The correlation theorem was employed to raise the speed of this algorithm by up to two orders of magnitude. Segmenting data collection and reconstruction into subimages allows the extension of this technique to higher spatial resolution. Adaptive averaging provides a robust method for coronary MRI which requires no breath‐holding, navigation, or ECG gating. Magn Reson Med 44:940–946, 2000.

Method for blood flow acceleration and velocity measurement using MR catheters

A method of magnetic resonance (MR) fluid flow measurement within a subject employs an invasive device with an RF transmit/receive coil and an RF transmit coil spaced a known distance apart. The subject is positioned in a static magnetic field. The invasive device is positioned in a vessel of a subject in which fluid flow is desired to be determined. A regular pattern of RF transmission pulses are radiated through the RF transmit/receive coil causing it to cause a steady-state MR response signal. Intermittently a second RF signal is transmitted from the RF coil positioned upstream which causes a change in the steady-state MR response signal sensed by the downstream transmit/receive coil. This is detected a short delay time later at the RF receive coil. The time delay and the distance between the RF coils leads directly to a fluid velocity. By exchanging the position of the RF transmit and transmit/receive coils, retrograde velocity may be measured. In another embodiment, more RF coils are employed. The changed MR response signal may be sensed at a number of locations at different times, leading to a measured change in velocity, or acceleration of the fluid.

Phase-field dithering for active catheter tracking.

A strategy to increase the robustness of active MR tracking of microcoils in low signal‐to‐noise ratio conditions was developed and tested. The method employs dephasing magnetic field gradient pulses that are applied orthogonal to the frequency‐encoding gradient pulse used in conventional point‐source MR tracking. In subsequent acquisitions, the orthogonal dephasing gradient pulse is rotated while maintaining a perpendicular orientation with respect to the frequency‐encoding gradient. The effect of the dephasing gradient is to apply a spatially dependent phase shift in directions perpendicular to the frequency‐encoding gradient. Since the desired MR signal for robust MR tracking comes from the small volume of nuclear spins near the small detection coil, the desired signal is not dramatically altered by the dephasing gradient. Undesired MR signals arising from larger volumes (e.g., due to coupling with the body coil or surface coils), on the other hand, are dephased and reduced in signal intensity. Since the approach requires no a priori knowledge of the microcoil orientation with respect to the main magnetic field, data from several orthogonal dephasing gradients are acquired and analyzed in real time. One of several selection algorithms is employed to identify the “best” data for use in the coil localization algorithm. This approach was tested in flow phantoms and animal models, with several multiplexing schemes, including the Hadamard and zero‐phase reference approaches. It was found to provide improved MR tracking of untuned microcoils. It also dramatically improved MR tracking robustness in low signal‐to‐noise‐ratio conditions and permitted tracking of microcoils that were inductively coupled to the body coil. Magn Reson Med 63:1398–1403, 2010.