Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-11-06
2002-05-21
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S319000
Reexamination Certificate
active
06392412
ABSTRACT:
This application claims Paris Convention priority of DE 199 54 925.7 filed Nov. 16, 1999 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention is related to the invention “Method of Correcting Linear Field Inhomogeneities in a Magnetic Resonance Apparatus” which was filed claiming prority of DE 199 54 926.5 filed Nov. 16, 1999 the full disclosure of which is hereby incorporated by reference. The two methods can complement one another in an advantageous fashion. The cited method can precede the present method to eliminate linear field inhomogeneities in advance.
The present invention concerns a method of determining and correcting higher order magnetic field inhomogeneities of a nearly homogeneous magnetic field B
0
in the investigation volume of a nuclear magnetic resonance apparatus, wherein magnetic resonance is excited in a sample located in the investigation volume through a radio frequency pulse, at least one additional linear magnetic gradient field is applied, and a magnetic resonance signal is measured.
A method of this type is e.g. known from U.S. Pat. No. 5,345,178 for a nuclear magnetic tomography apparatus.
In the known method, a gradient or spin echo sequence is measured after radio frequency excitation of a sample. The NMR signal is then Fourier transformed and a phase profile is determined within a predetermined area. This procedure is repeated for a plurality of projection directions and the phase curves obtained are analyzed with a fit method. Coefficients of a series expansion of the magnetic field dependence in spherical harmonic functions are determined therefrom and correction currents are, in turn, calculated for individual shim or gradient coils associated with the coefficients.
The known method has i.a. the disadvantages that the chemical shift between water and fatty constituents of the signal produces undesired geometric shifts and it cannot be applied or only disadvantageously with samples with short T
1
/T*
2
times.
For this reason, there is a need for a rapid, uncomplicated, direct shimming method which can be made insensitive to the influences of chemical shifts and which can also be successfully applied with samples having short relaxation times.
SUMMARY OF THE INVENTION
This object is achieved by a method of the above-mentioned type comprising the following steps:
A) a first radio frequency excitation pulse is irradiated onto the sample;
B) a first phase gradient G
ix
is applied in a predetermined direction x;
C) at a fixed time t
dx1
after the first radio frequency excitation pulse, a value S
ix1
of the magnetic resonance signal from the sample is measured, digitized and stored;
D) a second radio frequency excitation pulse is irradiated onto the sample;
E) a second phase gradient G
ix2
is applied in the predetermined direction x;
F) at a fixed time t
dx2
after the second radio frequency excitation pulse, a value S
ix2
of the magnetic resonance signal from the sample is measured, digitized and stored, wherein G
ix2
and t
dx2
are selected such that the integrals
∫
0
tδx
1
Gx
1
ⅆ
t
=
∫
0
tδx
2
Gx
2
ⅆ
t
are identical;
G) the steps A) to F) are repeated several times with systematically altered strength of the phase gradient G
ix
;
H) the values of the measured resonance signals S
ix1
are combined, in dependence on the associated gradient strength G
ix
, into a quasi-spin echo data set S
x1
;
I) the values of the measured resonance signals S
ix2
are combined, in dependence on the associated gradient strength G
ix
, into a quasi-spin echo data set S
x2
;
J) the data set S
x1
is Fourier transformed and optionally phase-corrected such that the phases &phgr;
ix1
of the phase-corrected resonance signals S′
ix1
all have essentially the same value;
K) the data set S
x2
is also Fourier transformed and phase-corrected with the same parameters as the data set S
x1
in step J), wherein the difference &phgr;
′ix1
-&phgr;
′ix2
of the phase-corrected phases of the signals S′
ix1
and S′
ix2
represent a measurement for a profile of the magnetic field inhomogeneity along the direction x;
L) in the following measurements of magnetic resonance in the apparatus, a correction magnetic field B(x) is applied in the investigation volume for homogenizing the magnetic field B. which compensates for the magnetic field inhomogeneity determined in step K).
One single measuring point is recorded at each of two defined points in time following each excitation rather than a complete signal echo or FID. The times t
d1
and t
d2
are always the same as is therefore the dephasing due to inhomogeneities of the magnetic field B0 . Through application of a phase gradient in the interval between t=0 and t=t
d1
or between t=0 and t=t
d2
, additional dephasing is produced which can be controlled in a defined manner. At the times of data recording, the two effects overlap. The gradient strengths are selected such that each measuring value at t
d1
has a corresponding one at t
d2
which has the same dephasing due to the applied phase gradient but not due to the field inhomogeneity of B
0
. Evaluation according to the above-mentioned steps produces a phase difference profile in the direction of the applied phase gradient which, except for the T
2
relaxation effects, depends only on the effect of the B
0
inhomogeneity in this direction. The phase correction mentioned in step J) is optional: it is only important that the same phase correction is applied for both data sets.
The number of required excitations can be reduced if the times t
d1
and t
d2
and the gradient strengths are in rational relationship to one another. It is then possible to use one measuring point for at least some gradient strengths of the phase gradient with both t
d1
and t
d2,
wherein the one at t
d2
is associated with a measurement at t
d1
with stronger gradient such that, for this pair, the condition of step F) is met.
The influence of chemical shift is eliminated in that a “quasi spin echo” is generated and evaluated in the phase direction rather than in the reading direction, i.e. the measuring points are, in each case, at the same relative point in time (t
d1
, t
d2
) after excitation and do not differ with respect to dephasing through chemical shift. It is recommended to select the times t
d1
, t
d2
such that the fatty and water contributions of the signals in the B
0
field are at least approximately in phase.
The time t
d
can be selected largely freely and can, in particular, be very short for samples with short relaxation times T
1
/T*
2
.
The method has been described above with reference to one direction but can be easily applied for several directions. Compensation in two dimensions is advantageous, in particular, for investigations of slices of an object. In multiple slice investigations, the field can be homogenized separately for each slice.
It is of course possible to extend the method in a corresponding manner to three-dimensional volumes by carrying it out analogously for a further dimension to permit homogenization of the field in the entire sample volume or, in connection with volume-selective measures, for selected partial volumes and optionally for many different volumes within an object.
In a particularly advantageous fashion, the method is carried out analogously for a plurality of predetermined directions which are selected such that, according to methods known per se, the principal expansion coefficients of a series expansion of the magnetic field B
0
in spherical harmonic functions can be determined, in particular, those of second order. An article by R. Gruetter in Magnetic Resonance in Medicine 29:804-811 (1993) describes e.g. a shimming method with the title “Fastmap”. In a preferred variant, measurement is carried out along only six projection directions for determining the coefficients u
Bruker Medical GmbH
Vincent Paul
LandOfFree
Method of correcting higher order field inhomogeneities in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of correcting higher order field inhomogeneities in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of correcting higher order field inhomogeneities in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2840080