NMR, MRI, and spectroscopic MRI in inhomogeneous fields
A method for locally creating effectively homogeneous or “clean” magnetic field gradients (of high uniformity) for imaging (with NMR, MRI, or spectroscopic MRI) both in in-situ and ex-situ systems with high degrees of inhomogeneous field strength. The method of imaging comprises: a) providing a functional approximation of an inhomogeneous static magnetic field strength B 0 ({right arrow over (r)}) at a spatial position {right arrow over (r)}; b) providing a temporal functional approximation of {right arrow over (G)} shim (t) with i basis functions and j variables for each basis function, resulting in v ij variables; c) providing a measured value Ω, which is an temporally accumulated dephasing due to the inhomogeneities of B 0 ({right arrow over (r)}); and d) minimizing a difference in the local dephasing angle φ({right arrow over (r)},t)=γ∫ 0 t √{square root over (|{right arrow over (B)} 1 ({right arrow over (r)},t′)| 2 +({right arrow over (r)}·{right arrow over (G)} shim G shim (t′)+∥{right arrow over (B)} 0 ({right arrow over (r)})∥Δω({right arrow over (r)},t′)/γ) 2 )}dt′−Ω by varying the v ij variables to form a set of minimized v ij variables. The method requires calibration of the static fields prior to minimization, but may thereafter be implemented without such calibration, may be used in open or closed systems, and potentially portable systems.
1. A method comprising:
(a) exposing a sample volume to a static magnetic field ({right arrow over (B)} 0 );
(b) modulating a RF field ({right arrow over (B)} 1 ) that the sample volume is exposed to;
(c) modulating a gradient field ({right arrow over (G)} shim ({right arrow over (r)},t)) that the sample volume is exposed to, the gradient field being a function of a position vector ({right arrow over (r)}) associated with the sample volume and time (t), the static magnetic field, the RF field, and the gradient field forming an effective field ({right arrow over (B)} EFF ({right arrow over (r)},t));
(d) detecting a nuclear magnetic resonance signal from the sample volume; and
(e) minimizing a dephasing of the sample volume exposed to the effective field, the dephasing being caused by inhomogeneity in the static magnetic field.
2. The method of claim 1 , wherein a degree of inhomogeneity in the static magnetic field is selected from the group consisting of greater than 10 parts per billion, greater than 100 parts per billion, greater than 1 part per million, greater than 10 parts per million, greater than 100 parts per million, greater than 0.001, and greater than 0.01.
3. The method of claim 1 , wherein the static magnetic field is generated external to a volume enclosed by a source of the static magnetic field.
4. The method of claim 1 , wherein the static magnetic field is generated internal to a volume enclosed by a source of the static magnetic field.
5. The method of claim 1 , wherein the static magnetic field is generated by a magnet selected from the group consisting of an electromagnet, a superconductive electromagnet, and a permanent magnet.
6. A method comprising:
(a) exposing a sample to a static magnetic field ({right arrow over (B)} 0 );
(b) modulating a RF field ({right arrow over (B)} 1 ) that the sample is exposed to;
(c) selecting a particular spatial location ({right arrow over (r)} p ) of the sample;
(d) modulating a gradient field ({right arrow over (G)} shim (t)) that the particular spatial location of the sample is exposed to, the static magnetic field, the RF field, and the gradient field forming an effective field ({right arrow over (B)} EFF ({right arrow over (r)} p ,t);
(e) detecting a nuclear magnetic resonance signal from excitation of a sample volume of the particular spatial location of the sample that is exposed to the effective field; and
(f) minimizing a dephasing of the sample volume that is exposed to the effective field, the dephasing being caused by inhomogeneity in the static magnetic field.
7. The method of claim 6 , further comprising:
varying the particular spatial location.
8. The method of claim 6 , wherein a degree of inhomogeneity of the static magnetic field is selected from the group consisting of greater than 10 parts per billion, greater than 100 parts per billion, greater than 1 part per million, greater than 10 parts per million, greater than 100 parts per million, greater than 0.001, and greater than 0.01.
9. The method of claim 6 , wherein the static magnetic field is generated external to a volume enclosed by a source of the static magnetic field.
10. The method of claim 6 , wherein the static magnetic field is generated internal to a volume enclosed by a source of the static magnetic field.
11. The method of claim 6 , wherein the static magnetic field is generated by a magnet selected from the group consisting of an electromagnet, a superconductive electromagnet, and a permanent magnet.
12. The method of claim 6 , wherein the magnet includes a pillow configuration magnet configured to at least partially surround one or more animal anatomical features selected from the group consisting of a head, a leg, an arm, an abdomen, a hand, and a body.
13. An apparatus comprising:
a magnet configured to generate a static magnetic field;
a first coil configured to generate an RF field;
a second coil configured to generate a gradient field; and
a processor, the processor configured to execute program instructions to:
(a) generate with the magnet the static magnetic field that a sample is exposed to;
(b) modulate with the first coil the RF field that the sample is exposed to;
(c) select a particular spatial location of the sample;
(d) modulate with the second coil the gradient field that the particular spatial location of the sample is exposed to, the static magnetic field, the RF field, and the gradient field forming an effective field;
(e) detect a nuclear magnetic resonance signal from excitation of a sample volume of the particular spatial location of the sample that is exposed to the effective field; and
(f) minimize a dephasing of the sample volume that is exposed to the effective field, the dephasing being caused by inhomogeneity in the static magnetic field.