Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1993-05-20
1994-05-31
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01N 3320
Patent
active
053172647
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for magnetic resonance spectroscopy or tomography in a preselectable region of a material. The process is especially suitable for investigating biological tissue by nuclear spin resonance tomography. The invention is also considered to include measurement processes (e.g. relaxometry, diffusion measurements, flow measurements) derived from the said process.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,743,850 (Sepponen) discloses a process for spatial determination of the relaxation time T.sub.1p in a rotating frame of reference. In this process, the sample is first irradiated with a .pi./2 pulse in order to generate a transverse magnetization, and then with a spin lock pulse at a 90.degree. angle to the .pi./2 pulse, both of the pulses being applied in the absence of gradients. The pulse application is followed by volume selection by means of applied gradients, and lastly by signal evaluation. The problem with this process is that strong radio-frequency and gradient fields are needed for layer- or volume-selective excitation. These strong fields are undesirable for in vivo applications.
The object of the present invention is to make available processes in which strong radio-frequency and gradient fields are not needed for layer- or volume-selective excitation of a sample for in vivo investigations, and in which it is possible to acquire a reliable and versatile signal from the region of interest.
SUMMARY OF THE INVENTION
According to the present invention, this object is achieved once a transverse magnetization has been excited (or already exists) in the material, by irradiating the preselectable region of the material with a radio-frequency pulse (spin lock pulse) and simultaneously applying a weak magnetic field gradient, The combination of the spin lock pulse and the applied gradient spin locks the transverse magnetization in the region of interest, while in the remaining regions of the material, the applied gradient destroys the spin coherences. The coherence in the region of interest, which is ultimately measured as free induction decay, is protected against phase decay by means of the spin lock pulse, in a manner discussed specifically below. All other spins are dephased by the weak field gradients.
The invention is compatible with many processes for editing (e.g. suppressing undesired signals) and processing the spin coherences. One emphasis of the invention is in the area of resonance spectroscopy, in which, in contrast to tomography, the main interest is not in generating images but in obtaining data.
In one embodiment of the invention, prior to application of a split lock pulse (SL), the transverse magnetization of the material is generated by irradiation with a hard (i.e. broad-band) radio-frequency pulse or a composite pulse or by means of another excitation process, with the angle between magnetic field and magnetization vector being preferably 90.degree.. This angle (flip angle) can also deviate from 90.degree..
In a further embodiment of the process according to the present invention, the spin lock spin coherences can be generated by various processes which are useful for other purposes. For example, homo- or hetero-, nuclear editing or line suppression methods can be used in preceding preparation and evolution intervals to generate the required coherences. Line suppression pulses can also be applied between or after the layer selection actions. Moreover, additional homo-, or hetero-, nuclear echo pulse sequences can be added (for example for editing or polarization transfer purposes) after a region has been selected.
In another embodiment of the process according to the present invention, a thin disc, whose thickness can be adjusted by varying the amplitude of the spin lock pulse and/or the strength of the field gradient, is selected as the region of interest of the material sample. It is especially advantageous to predefine the field gradient and keep it constant during the process, and to define the thickness of the disc simp
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Kimmich Rainer
Rommel Eberhard
Kohler Schmid & Partner
Mah Raymond Y.
Tokar Michael J.
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