We describe methods for simultaneously acquiring and subsequently combining data from a multitude of closely positioned NMR receiving coils. The approach is conceptually similar to phased array radar and ultrasound and hence we call our techniques the "NMR phased array." The NMR phased array offers the signal-to-noise ratio (SNR) and resolution of a small surface coil over fields-of-view (FOV) normally associated with body imaging with no increase in imaging time. The NMR phased array can be applied to both imaging and spectroscopy for all pulse sequences. The problematic interactions among nearby surface coils is eliminated (a) by overlapping adjacent coils to give zero mutual inductance, hence zero interaction, and (b) by attaching low input impedance preamplifiers to all coils, thus eliminating interference among next nearest and more distant neighbors. We derive an algorithm for combining the data from the phased array elements to yield an image with optimum SNR. Other techniques which are easier to implement at the cost of lower SNR are explored. Phased array imaging is demonstrated with high resolution (512 x 512, 48-cm FOV, and 32-cm FOV) spin-echo images of the thoracic and lumbar spine. Data were acquired from four-element linear spine arrays, the first made of 12-cm square coils and the second made of 8-cm square coils. When compared with images from a single 15 x 30-cm rectangular coil and identical imaging parameters, the phased array yields a 2X and 3X higher SNR at the depth of the spine (approximately 7 cm).

Techniques which can be used to follow the position of invasive devices in real-time using magnetic resonance (MR) are described. Tracking of an invasive device is made possible by incorporating one or more small RF coils into the device. These coils detect MR signals from only those spins near the coil. Pulse sequences which employ nonselective RF pulses to excite all nuclear spins within the field-of-view are used. Readout magnetic field gradient pulses, typically applied along one of the primary axes of the imaging system, are then used to frequency encode the position of the receive coil(s). Data are Fourier transformed and one or more peaks located to determine the position of each receiver coil in the direction of the applied field gradient. Subsequent data collected on orthogonal axes permits the localization of the receiver coil in three dimensions. The process can be repeated rapidly and the position of each coil can be displayed in real-time.

MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

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Motion artifact suppression technique (MAST) for MR imaging. J. Comput. Assist. Tomogr. 11, 369-377 (1987). 11. Q. S. Xiang, M. J. Bronskill, R. M. Henkelman, Two-point interference method for suppression of ghost artifacts due to motion. J. Magn. Reson. Imaging 3, 900-906 (1993). 12. Q. S. Xiang, R. M. Henkelman, Dynamic image reconstruction: MR movies from motion ghosts. J. Magn. Reson. h a g-ing 2 , 679-685 (1992). 13. P. R. Moran, A flow velocity zeugmatographic interlace for NMR imaging in humans. Projection angiograms of blood labeled by adiabatic fast passage. Magn. Reson. Med. 3, 454-462 (1986). 18. W. Zhang, A quantitative analysis of alternated line scanning in k space and its application in MRI of regional tissue perfusion by arterial spin labeling. J. M a p. Reson. Series B 19. G. H. Glover, Phase-offset multiplanar (POMP) volume imaging: a new technique.