Magnetic field gradient monitor apparatus and method
A method based on pure phase encode FIDs that permits high strength gradient measurement is disclosed. A small doped water phantom (1˜3 mm droplet, T 1 , T 2 , T 2 *<100 μs) within a microprobe is excited by a series of closely spaced broadband RF pulses each followed by single FID point acquisition. Two trial gradient waveforms illustrate the technique, neither of which could be measured by the conventional microprobe measurement. The first is an extended duration gradient waveform while the other illustrates this method's ability to measure gradient waveforms with large net area and/or high amplitude. This method is a point monitor with simple implementation and low cost hardware requirements.
1. A method of making a measurement of a magnetic field gradient within a local area of a magnetic field gradient in an NMR sample space, comprising the steps of:
(a) providing a reference sample in the local area;
(b) providing an RF probe suitable for holding the reference sample;
(c) exciting the reference sample by a broad band RF pulse;
(d) following the RF excitation, acquiring a free induction decay (FID) from the reference sample after a fixed duration phase encoding time;
(e) determining the phase for the measurement in the fixed duration phase encoding time; and
(f) determining the magnetic field gradient from the phase.
2. The method of claim 1 , wherein acquiring the FID comprises acquiring a single FID point or a cluster of individual FID points after the RF pulse, as the underlying magnetic field gradient changes in time.
3. The method of claim 2 , wherein the reference sample comprises doped water.
4. The method of claim 3 , wherein the water is doped with an MR contrast agent.
5. The method of claim 4 , wherein the contrast agent is gadolinium chloride (GdCl 3 ).
6. The method of claim 1 , wherein the RF probe comprises a single turn solenoid.
7. The method of claim 1 , wherein the RF probe comprises a multiple turn solenoid.
8. The method of claim 1 , wherein the RF pulses are low flip angle RF pulses, whereby RF pulses prior to the gradient pulse provide a reference phase.
9. The method of claim 1 wherein the RF probe is of a sufficiently small size relative to the sample space to permit placement of the probe at various locations within the sample space to enable measurement with the RF probe at various locations within the sample space.
10. The method of claim 1 , further comprising one or more additional RF probes of a sufficiently small size relative to the sample space to permit placement of the RF probes at various locations within the sample space to enable measurement with the RF probes at various locations within the sample space.
11. The method of claim 1 , wherein the RF probe comprises materials that are not magnetic susceptibility matched to the surroundings of the sample space.
12. The method of claim 1 , further comprising: repeating steps (c) to (f) while shifting the position in time of each subsequent pulse whereby a plurality of time points on the gradient waveform are acquired.
13. The method of claim 9 , further comprising providing an RF probe adapted for a magnetic resonance measurement, wherein the sample space is contained within the RF probe adapted for a magnetic resonance measurement.
14. The method of claim 9 , further comprising:
placing the probe at two locations in the sample space;
taking a single point pure phase encode measurement at each of the locations;
determining the phase for each of the measurements;
determining a magnetic shift B o (t) comprising determining the difference in phase between the two measurements.