Bruker Instruments, Inc.

Agilent Technologies, Inc.

Varian, Inc.

Subramaniam Sukumar

Wai Ha Wong

A low pass RF ladder coil, which is named as a millipede surface coil, comprises a first and a second conductor end strips running parallel to each other. A set of rung elements are placed between them, alternate rung elements are connected to the first and second strip respectively. The number and spacing of the rung elements are sufficient for resonating the coil at the desired imaging frequency. This millipede surface coil may have 100 or more rung elements. Fixed and variable capacitors are provided for separately tuning and matching the first and second mode resonant frequencies, and for coupling and impedance matching the two modes to external circuits. An active detuning is provided that detunes the ladder coil when a separate RF coil is transmitting.

Millipede surface coils

A birdcage-like coil with a pair of electrically conductive ring elements separated in a longitudinal direction and interconnected by three longitudinally extending electrically conductive elongated strips, two of which are diametrically oppositely disposed and the third is azimuthally at 90E from both of them, can create an RF magnetic field gradient when driven in a certain resonance mode. A similarly structured birdcage-like coil with a fourth strip to have two diametrically oppositely disposed strips can create two switchable orthogonal magnetic field gradient by switching off a selected one of the strips and driving the coil in a certain mode. A coil for generating alternative a homogeneous field and selectably one of two orthogonal gradient fields is formed by sandwiching a prior art birdcage long-pass coil with a pair of such coils and by switching on and off suitable ones of the switches in the strips.

B1 gradient coils

An method for determining the inhomogeneities of the magnetic field of the magnet of an NMR system makes use of unshielded coils while minimizing the effect of eddy currents induced in the magnet components by the gradient fields. A first dataset is acquired by ramping the level of a gradient magnetic field to a maximum value and generating an RF pulse to excite the spins. After a delay time t.sub.1, a single data point is sampled. While the spins are allowed to relax, the gradient field magnitude is changed to the next level. Another RF pulse is generated and, after time t.sub.1, the next data point is sampled. This process continues until a first complete dataset is acquired. The next gradient function is then applied in a similar manner, and the data points sampled after an RF pulse and a delay time of t.sub.2. During sampling of the second dataset, the gradient area for each spatial data point is equal to the gradient area for that spatial point during sampling of the first dataset. After the second dataset is acquired, a frequency distribution for the gradient direction is obtained using known techniques.

Shimming method for NMR magnet using unshielded gradient systems

A method of shimming an NMR magnet uses a plurality of 1D projections through a sample volume to determine the inhomogeneities of the field of the NMR magnet. The frequency distributions obtained are assembled from the phase signals of the various projections. In order to avoid frequency errors due to phase wrapping, the phase of each signal is monitored over time for discontinuities indicative of aliasing. For each phase wrap, the signal is 'unwrapped' by adjusting the value of the phase signal by 2.pi.. The same method is used to establish a shim field map for each of the shim coils being used. With one shim coil at a time being driven with a predetermined current, the detection method is repeated to acquire a shim-base frequency map for each shim coil. The base frequency map is then subtracted from the shim-base frequency maps to obtain shim field maps. The proper shim currents are then acquired through matrix operations on the shim field maps and the base frequency map.

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Subramaniam Sukumar