Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1989-01-10
1990-07-10
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324308, G01R 3320
Patent
active
049409409
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of radio-frequency excitation in a nuclear magnetic resonance (NMR) experiment. The invention is more particularly applicable to the medical field in which NMR machines are employed for diagnosis in human beings. However, the invention can also find applications in the field of laboratory work or in industry whenever magnetic resonance phenomena are employed.
The magnetic resonance phenomenon results from the orientation acquired by the magnetic moments of particles of a body when said body is subjected to a constant orienting magnetic field. In order to exhibit this phenomenon, the body is excited with a radio-frequency excitation. It is possible to measure the signal representing de-excitation of the particles when they return to their state of equilibrium, when their magnetic moment is reoriented with the orienting field, at the end of angular shift of said orientation which depends on the excitation. The excitation is applied to the entire body which is subjected to examination and all the body particles emit a de-excitation signal upon completion. In the medical field in which it is sought to represent images of cross-sections of the body (of a patient) under examination, it is a customary practice to limit inception of the resonance phenomenon to selected cross-sections, the images of which are to be produced. To this end, the conditions of resonance are modified locally in such a manner as to ensure that only the selected slice is subjected to conditions of resonance corresponding to the frequency of the radio-frequency excitation. Outside the slice, the magnetic moments are not tilted and those particles which are external to the slice do not restitute any de-excitation signal.
Modification of local resonance conditions is obtained by increasing or reducing the intensity of the orienting field in the selected slice. For scientific and technological reasons, it is not possible to produce an abrupt change in intensity of a constant magnetic field on each side of a slice of space. Modification of the field then takes the form of a regular variation as a function of the abscissa of the loci of the space on an axis perpendicular to the slice to be selected The intensity of the orienting field is therefore subjected to a gradient along said axis and in practice the supplementary fields, added to the constant and homogeneous field throughout the space in order to produce this variation, are commonly referred-to as the field gradients. The resonance frequency of the resonance phenomenon is proportional to the intensity of the orienting field. In order to select a slice, it is therefore necessary, in radio-frequency excitation, to ensure that the spectrum of excitation contains only components which are included within a band limited by cutoff frequencies corresponding to the resonance frequencies of the particles of the edges of the slice.
In order to obtain a narrow-spectrum excitation signal, this signal cannot be instantaneous but lasts a certain time. In practice, powers of a few kilowatts applied during time intervals of a few tens of milliseconds are common. The fact that the excitation pulse is not instantaneous and that on the contrary it lasts a certain time accordingly gives rise to disturbances in the de-excitation signal. In fact, as a result of the field gradient, the particles of the edges of the slice do not resonate at the same frequencies. For example, if the center of the slice resonates at a frequency f.sub.O, the upstream and downstream edges of the slice with respect to the orientation of the field are capable of resonating at frequencies f.sub.O -.DELTA.f. or f.sub.O +.DELTA.f respectively. In consequence, the history of excitation in the edges of the slice is not the same as at the center even though the excitation time has been the same for all the particles of the slice. If the geometrical angular shift of all the magnetic moments is correctly obtained in respect of a given excitation energy (for example they are all located in a plane perp
REFERENCES:
patent: 4521732 (1985-06-01), Pegg et al.
Journal of Magnetic Resonance, vol. 61, 1985, B. Blumich, et al, "Quaternions as a Practical Tool for the Evaluation of Composite Rotations", pp. 356-362.
Journal of Magnetic Resonance, vol. 63, 1985, C. Counsell, et al, "Analytical Theory of Composite Pulses", pp. 133-141.
Physical Review B, vol. 25, Jun. 1, 1982, M. M. Maricq, "Application of Average Hamiltonian Theory to the NMR of Solids", pp. 6622-6632.
Journal of Magnetic Resonance, vol. 67, Mar. 1, 1986, R. Brandes, et al, "Generation of Tailored Radiofrequency Pulses by a Simple Audio Frequency Filter Method. II Analysis", pp. 14-27.
General Electric CGR SA
Tokar Michael J.
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