NMR and ESR probes for closed loop control of gradient fields

Details NMR and ESR probes for closed loop control of gradient fields NMR and ESR probes for closed loop control of gradient fields

1999-10-29

2001-09-25

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S322000

Reexamination Certificate

active

06294916

ABSTRACT:

This invention relates to magnetic resonance apparatus.
In magnetic resonance imaging, as is well known, magnetic resonance (MR) active nuclei in alignment with a main magnetic field are excited to resonance by the application of a pulse of r.f. energy at right angles to the main magnetic field, and r.f. signals generated after the pulse has ceased can be detected to give information on distribution of MR active nuclei e.g. H
2
in the body being imaged, which will give information about the type of tissue and its condition.
The spatial encoding of the resonance signals is achieved by applying magnetic field gradients in three orthogonal directions at the time the r.f. excitation pulse is applied and/or when r.f. (relaxation) signals are generated. To give an example of a magnetic field gradient, the patient might be lying on a couch with the main magnetic field in a direction from head to toe. The magnetic field gradient in this direction would have the effect that the magnetic field strength at the head of the patient, say, was greater than that at the centre of the patient, and the magnetic field strength at the toes would be less than that at the centre of the patient. The r.f. pulse would usually be of such a frequency as to excite only an axial slice of the patient, and the other two gradients would enable the distribution of MR active nuclei in orthogonal directions in the plane of the slice to be represented.
Unpaired electrons of atoms have also been used (as in electron spin resonance—ESR) to provide the same imaging information as MR active nuclei (nuclear magnetic resonance—NMR).
In the case of NMR, to obtain increased resolution in the MR image implies increasing gradients if the acquisition time is not to be increased, but this increases the mechanical stresses in the gradient windings and the thermal effects caused by the heating effects of the currents in the coils which are responsible for producing the magnetic gradients.
Typically, an MR image is built up from multiple acquisitions of data and, while the differences in the gradients from one acquisition to the next may be of the order of only 0.1%, the resulting artifacts in the image are several times as significant as those caused by noise.
It would be desirable to be able to measure MR gradients, not only to be able to control them between acquisitions, but also to be able to calibrate them in terms of position corresponding to a respective magnetic field strength.
The use of NMR probes for setting and checking the main field of magnet is well known (Eddy Currents and their Control, M Burl, I Young, Encyclopedia of NMR Edited by D M Grant and R K Harris, Wiley, 1996, pages 1845, 1846. Such probes consist of a MR active substance, usually doped water contained in a bulb, surrounded by a receiver coil. ESR probes have been used in the same way.
If used for calibrating gradients, the probe should be small in order to give as precise an indication as possible of the source of the r.f. signals. However, the signal from such a small probe would have a poor s
ratio, and so multiple measurements and averaging would be necessary.
If the probe was simply made larger, the signal from the probe would decay rapidly, since the spins are de-phased (dispersed) by the gradient, and a large object would result in a quick loss of signal.
The invention provides magnetic resonance apparatus comprising gradient coils for generating magnetic field gradients, a probe for monitoring a magnetic field gradient, the probe comprising an MR active substance and a coil for receiving r.f. signals from the substance, and a gradient coil surrounding the MR active substance and arranged to produce a magnetic field gradient which opposes that of the gradient being monitored.
The gradient coil surrounding the MR active substances enables the gradient being monitored to be reduced or cancelled, enabling a larger volume of MR active substance to be used without the disadvantages noted above.
The magnetic resonance apparatus may be for imaging. Advantageously, there is provided means for supplying the probe gradient coil with current derived from that supplied to the respective imaging gradient coil, for example, a fraction of the latter current can be tapped off using resistive means.
The opposing gradient produced by the gradient coil may be equal and opposite to that produced by the imaging coils to achieve cancellation of the imaging gradient over the region of the MR active substance.
Preferably, gradient coils surround the MR active substance to oppose three orthogonal imaging gradients.
Advantageously, the output of the probe provides an error signal for correcting for errors in the demanded imaging gradient.


REFERENCES:
patent: 3244876 (1966-04-01), Kanda et al.
patent: 3342991 (1967-09-01), Kroenbroger
patent: 3557777 (1971-01-01), Cohen
patent: 3597679 (1971-08-01), Habfast
patent: 5432449 (1995-07-01), Ferut et al.
M. Burl and I. Young; Eddy Currents and their Control; “Encyclopaedia of NMR” edited by D.M. Grant and R.K. Harris, Wiley, 1996; pp. 1841-1846.

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