Richard Philip Mallozzi

The Alzheimer's Disease Neuroimaging Initiative (ADNI): MRI methods

The Alzheimers Disease Neuroimaging Initiative (ADNI) is a longitudinal multisite observational study of healthy elders, mild cognitive impairment (MCI), and Alzheimers disease. Magnetic resonance imaging (MRI), (18F)‐fluorodeoxyglucose positron emission tomography (FDG PET), urine serum, and cerebrospinal fluid (CSF) biomarkers, as well as clinical/psychometric assessments are acquiredat multiple time points. All data will be cross‐linked and made available to the general scientific community. The purpose of this report is to describe the MRI methods employed in ADNI. The ADNI MRI core established specifications thatguided protocol development. A major effort was devoted toevaluating 3D T1‐weighted sequences for morphometric analyses. Several options for this sequence were optimized for the relevant manufacturer platforms and then compared in a reduced‐scale clinical trial. The protocol selected for the ADNI study includes: back‐to‐back 3D magnetization prepared rapid gradient echo (MP‐RAGE) scans; B1‐calibration scans when applicable; and an axial proton density‐T2 dual contrast (i.e., echo) fast spin echo/turbo spin echo (FSE/TSE) for pathology detection. ADNI MRI methods seek to maximize scientific utility while minimizing the burden placed on participants. The approach taken in ADNI to standardization across sites and platforms of the MRI protocol, postacquisition corrections, and phantom‐based monitoring of all scanners could be used as a model for other multisite trials. J. Magn. Reson. Imaging 2008.

Making MRI quieter

We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems.

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.

Radiofrequency power deposition utilizing thermal imaging

Wavelength effects influence radiofrequency (RF) power deposition distributions and limit magnetic resonance (MR) medical applications at very high magnetic fields. The power depositions in spherical saline gel phantoms were deduced from proton resonance shift thermal maps at both 1.5 T and 3.0 T over a range of conductivities. Phase differences before and after RF heating were measured for both a quadrature head coil and a circular surface coil. A long echo time (TE) pulse sequence with a 3D phase unwrap algorithm provided increased thermal sensitivity. The measured thermal maps agreed with a model of eddy‐current heating by circularly polarized oscillating RF fields in a conducting dielectric sphere. At 3.0 T, thermal maps were acquired with a <0.32°C temperature rise at 4 W. Proton resonance shift thermal maps provided a measure of hot spots in very‐high‐field MR imaging (MRI), in which both the phase sensitivity and signal‐to‐noise ratio (SNR) were increased. The method provides a means of studying the heat distribution generated by RF coils excited by clinical pulse sequences. Magn Reson Med 51:1129–1137, 2004.

Accurate flip-angle calibration for 13C MRI.

13C imaging and spectroscopy in the presence of injected labeled compounds can vastly extend the capability of MRI to perform metabolic imaging. The details of imaging in the presence of injected compounds, however, pose new technological challenges. Pulse sequences, in general, rely on precise flip‐angle (FA) calibration to create high signal‐to‐noise ratio (SNR), artifact‐free images. Signal quantification also requires precise knowledge of the excitation FA. In MRI scans that rely on signal acquisitions from injected compounds, however, such FA calibration is challenged by low natural‐abundance 13C signal levels before injection, and by time‐varying signal following injection. A method to precisely set the FA at the 13C frequency based on FA calibration at the 23Na frequency is presented here. A practical implementation of a coil (a dual‐tuned, 23Na/13C low‐pass birdcage coil) suitable for this calibration in vivo is also documented. Accurate FA calibration is demonstrated at the 13C frequency for in vivo rat experiments using this approach. Magn Reson Med 58:128–133, 2007.