No abstract

ABSTRACT:Shield currents or common mode currents affect coil tuning, coil-to-coil coupling in phased array coils, image inhomogeneity, and most importantly can cause serious patient burns. Traditionally in MRI, shield currents are reduced by cable traps; they consist of a wound coaxial cable inductor tuned to the desired resonance frequency by a capacitor between end turns of the coaxial cable ground shield. This method increases losses and effects the overall phase distance between the coil and the preamplifiers. We present a cable trap that does not connect or solder to the cable and is completely splitable, allowing easy fitting over any cable without affecting any coil parameters (tuning or phase length). Multiple cables can be placed inside the shield current trap to simultaneously attenuate the shield currents from up to eight cables, as currently designed. The shield current trap reduces shield currents at 64 MHz by ϳ30 dB.

There is a great advantage in signal to noise ratio (S/N) that can be obtained in nuclear magnetic resonance (NMR) experiments on very small samples (having spatial dimensions ∼100 μm or less) if one employs NMR “micro” receiver coils, “microcoils,” which are of similarly small dimensions. The gains in S/N could enable magnetic resonance imaging (MRI) microscopy with spatial resolution of ∼1–2 μm, much better than currently available. Such MRI microscopy however requires very strong (>10 T/m), rapidly switchable triaxial magnetic field gradients. Here, we report the design and construction of such a triaxial gradient system, producing gradients substantially greater than 15 T/m in all three directions, x, y, and z (and as high as 50 T/m for the x direction). The gradients are switchable within time ∼10 μs and adequately uniform (within 5% over a volume of [600μm3] for microcoil MRI of small samples.

We report the design and testing of a nuclear magnetic resonance (NMR) microcoil receiver apparatus, employing solenoidal microreceiver coils of dimensions of tens to hundreds of microns, using applied field of 9 T (proton resonance frequency 383 MHz). For the smallest receiver coils we attain sensitivity sufficient to observe proton NMR with signal to noise (S/N) one in a single scan applied to a ∼10 μm3 (10 fl) water sample, containing 7×1011 total proton spins. We also test the dependence of the S/N on important coil parameters, including coil composition and resistivity, turn spacing, and lead lengths.

Here we report further progress toward the goal of achieving proton magnetic resonance imaging microscopy with resolution approaching a few micrometers in all three dimensions. We obtain proton images of a phantom sample -a microcapillary containing water and 39 p.m diameter polymer microspheres -with a resolution of a few micrometers (perhaps about 5 p.m) in all three spatial dimensions.In recent publications [1-3] we presented solutions to some of the major obstacles to the goal of achieving proton magnetic resonance imaging (MRI) microscopy with resolution-approaching a few micrometers in all three dimensions. Such resolution should prove useful for imaging single biological cells, which have dimensions ranging typically from tens to hundreds o f micrometers. That work was preceded by advances achieved by many other groups working toward related goals [4][5][6][7][8][9][10][11][12][13][14][15][16]. Here we report further progress toward this goal, including specifically proton images with a resolution o f a few micrometers (perhaps about 5 gm) in all three spatial dimensions, obtained for a controlled "phantom" sample.MRI experiments were performed at room temperamre with a 9 Tesla (383 MHz proton resonance frequency) 101 mm bore-diameter Oxford superconducting magnet a n d a Tecmag Apollo console. The detection circuitry is the high-sensitivity microcoil setup of refs.