Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-12-17
2001-11-20
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S309000
Reexamination Certificate
active
06320381
ABSTRACT:
This application claims Paris convention priority of German patent application 199 00 578.8 filed Jan. 9, 1999, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention relates to a method of spatially resolved magnetic resonance spetroscopy according to the principle of chemical shift imaging (=CSI), wherein through application of an excitation pulse, spins in a measuring volume are excited and a metabolite signal thereof is read out, wherein between excitation pulse and acquisition of the signal, spatial encoding of the signal by means of a gradient pulse is effected in at least one spatial direction, wherein said spatial encoding gradient is varied successively from one recording step to the other, such that spatial allocation of the recorded signals in partial volumes of the measuring volume is obtained.
A method of this type is e.g. known from Brown, T R, NMR in Biomedicine 5, 238-243, 1992.
The spatially resolved magnetic resonance spectroscopy by means of so-called chemical-shift imaging (CSI) allows determination of the distribution of metabolites in materials, organisms, animals and humans. One problem in this connection is the optimization of the recorded signals with respect to further evaluation. The regional differences between relevant parameters like field homogeneity, susceptibility etc. result in corresponding significant differences in the recorded individual signals which complicate further evaluation of the signals etc., e.g. for representing the distributions of metabolites and quantifications of the signals, and considerably impede the application, mainly on the patient.
SUMMARY OF THE INVENTION
Therefore, it is an object of the method according to the invention to present a technique which allows correction of the local effects without essentially prolonging the acquisition time in order to achieve in this manner standardized and comparable results independently of occurring imperfections during the recording.
Magnetic resonance spectroscopy (MRS) is an examination method which has already been admitted in some countries to be used in clinical applications on patients, which enables important conclusions about pathological changes of metabolites. In particular, proton spectroscopy of a head and phosphorus-spectroscopic observation of the energy metabolism in muscles and other organs, have already met a wide range of application. In most cases, the recording of data is effected in so-called single-voxel technology, in which a spectrum is recorded in one single defined area of the body in each case. Methods of recording spatially distributed MR spectra are also known. The techniques most frequently used in this connection utilize the principle of the above-mentioned chemical-shift imaging (CSI), in which spatial encoding of the spatial signals (if required from a pre-determined limited range) is effected through corresponding so-called phase encoding gradients. Such technologies can be easily realized with respect to the technical side of the measurement, however, they require relatively long measuring times T
AQ
, which are defined by
T
AQ
=n·m·t
r
·n
aq
(1),
wherein
n and m represent the size of the spatial recording matrix, t
r
the repeating time between two recording steps and n
aq
the number of averaging for each recording step.
Since t
r
is, in almost all practical fields of application of CSI, in the range of between 1 and 3 seconds, due to the long T
1
relaxation periods of metabolites, it is possible even with moderate spatial resolution of 16×16 spectra, to obtain an acquisition time in the range of several minutes up to half an hour and more.
One problem in the post-processing of the spectra consists in that the spectra from different areas, obtained in this manner, cannot be compared directly due to the different local conditions like field inhomogeneity and susceptibility effects etc. The spatially different resonance frequencies, caused by field inhomogeneity and susceptibility effects do not only lead to a shift of the signals of the individual metabolites which must be corrected for creating the metabolite distributions.
Moreover, proton MRS leads to a local change of the efficiency of suppressing the undesired water signal which appears in a base line which is different for each spectrum and causes considerable disturbances. Additionally, these local changes of the magnetic field cause different line shapes of the individual signals which prevent calculation of the metabolite concentration from the spectra, at least by means of simple algorithms like amplitude measurement, integration etc.
Furthermore, even measurements carried out with the most modern gradient generation systems show different line shapes, depending on the location, due to the gradient-based eddy currents which can be corrected only partly even with complicated post-processing routines.
For recording individual spectra with single-voxel techniques, it is possible to eliminate all these imperfections by recording a reference signal. (See Klose U., Magnetic Resonance in Medicine 14, 26-30 1989). The data acquisition in this connection is carried out under otherwise identical conditions, however, without the suppression of water. Since the local effects influence the water signal in the same manner as metabolite signals, one can furthermore calculate a standardized spectrum through deconvolution of the metabolite signals with the reference signal. Due to the high intensity of the water signal, this reference signal can be achieved for individual voxel measurements in a few recording steps and thus within a measuring time of a few seconds.
Transfer of this method to CSI is principally possible, however, its practical application failed up to now due to the considerably increased acquisition time. Equation (1) also applies to the entire recording time of the reference signal irrespective of the intensity of the water resonance. The measuring time of a CSI experiment, which is long anyway, would thus additionally and considerably be increased even for choosing n
aq
=1.
For reducing this problem, various measures leading to a reduction of the acquisition of the reference data set are discussed in literature. These measures include, on the one hand, considerable reduction of t
r
for the reference recording, and on the other hand reduction of n and m and thus of the recording matrix of the reference signal. Both measures still include a significant increase of the total measuring time.
Due to the fact that the reference recording was not carried out under the same recording conditions as the actual CSI recording, the correction will be imperfect in addition. The method of recording the reference signal with short repeating period has furthermore the disadvantage that in this case, if t
r
is shorter than the relaxation time T
2
of the water, so-called steady-state signals are generated which can alter the useful signal in such a manner that it is no longer suitable as reference signal. For these reasons, these methods did not gain acceptance in practice up to now.
REFERENCES:
patent: H1218 (1993-08-01), Cory et al.
patent: 4646023 (1987-02-01), Young
patent: 5627469 (1997-05-01), Hong et al.
patent: 5879299 (1999-03-01), Posse et al.
patent: 0 429 295 (1991-05-01), None
NMR in Biomedicine; vol. 5, pp. 238-243, 1992 Truman R. Brown “Practical Application of Chemical Shift Imaging”.
Magnetic Resonance in Medicine 14, pp. 26-30, 1990 Uwe Klose “In Vivo Proton Spectroscopy in Presence of Eddy Currents”.
Functional Imaging, 1998 “Radio Waves:Magnetic Resonance”, Juergen Hennig Chapter 8 pp. 261-390.
Magnetic Resonance in Medicine 8, pp. 314-322, 1988 Xiaoping Hu et al. “SLIM: Spectral Localization by Imaging”.
“Spectral Localization with Optimal Pointspread Function”, Markus von Kienlin and Raymond Mejia, 1990/1991.
“In Vivo Measurement of Regional Brain Metabolic . . . ”, Stefan Posse et al., 1997, pp. 858-865.
“Magnetic resonance tomography of joint abnormalities; the use of chemica
Fetzner Tiffany A.
Klinikum Der Albert-Ludwigs-Universitat Freiburg
Patidar Jay
Vincent Paul
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