Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-03-20
2003-06-10
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06577126
ABSTRACT:
This application claims Paris convention priority of German Patent Application 10015068.3 filed on Mar. 25, 2000, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a method of producing magnetic resonance images, wherein a (n+1) dimensional k space is scanned, comprising an imaging pulse sequence with at least one RF excitation pulse followed by at least one RF refocusing pulse, wherein an incomplete, complex spin echo signal S
x
is measured and digitized in one part of the read-out interval [t
0
−½t
a
, t
0
+½t
a
] by means of a quadrature detector, which comprises a central part about the center (t=t
0
) of the spin echo signal in the time interval [t
0
−&egr;, t
0
+&egr;] with n(n=0, 1, 2, . . . ) phase encoding gradients before the read-out interval.
A method of this type is disclosed e.g. in U.S. Pat. No. 4,851,779 or U.S. Pat. No. 4,780,675.
In conventional methods of magnetic resonance for producing sectional images, the duration of scanning of the region of interest is that long that movements of the body region observed produce changes which impair the image quality. Due to the number of measuring sequences required for an examination, the total examination time could be between ½ and 1 hour. Long examination times are not desired, in particular not for patients.
To reduce the scanning time, it was tried to scan only incomplete k spaces. The incomplete k spaces are completed by complexly conjugated reflection of the detected data. To carry out this reflection, the entire (incomplete) data set must have been recorded before processing start. Reconstruction requires additional intermediate results in the form of phase images or images of a partial echo.
U.S. Pat. No. 4,851,779 discloses collecting an incomplete data set of three-dimensional magnetic resonance data and storing it in a memory. The incomplete data set is complete in a first and second direction, however, incomplete in a third direction. The detected data set includes data along the third direction between ±n central values and the half of the other values. One-dimensional inverse Fourier transformations are applied in the first and second direction to obtain an intermediate data set. A phase correction matrix or a plurality of phase correction vectors p(r) is produced from the intermediate data set and stored in a phase correction memory.
A symmetrical data set is produced as conjugated complex from the intermediate data set. The intermediate data set and symmetrical data set are inversely Fourier-transformed in the third direction (fa, fs), then the vectors of both image matrices are corrected with the corresponding phase vectors and combined into a line of a resulting three-dimensional image.
Disadvantageously, all data must be recorded in such a method before carrying out Fourier-transformation and intermediate results and phase matrices must be stored.
U.S. Pat. No. 4,780,675 discloses collecting an incomplete set of magnetic resonance image data and storing it in a memory. The incomplete data set comprises a central or first data set and an additional or second data set. A roll-off filter and Fourier transformation is applied to the central data set to obtain a normalized phase image. The first and second data set are Fourier transformed and phase corrected by multiplying with the conjugated complexes of the corresponding phase value. A third data set is produced by determining the conjugated complexes of the second or additional data set. The third data set is Fourier transformed and multiplied with a corresponding value of the phase image to produce a second phase-corrected image representation. The first and second corrected image representations are added and stored in an image memory.
Also in this method, all data must be recorded and stored before further processing. Additionally, the central data set is filtered before producing the phase diagram.
It is therefore the underlying purpose of the present invention to provide a method for faster imaging with substantially constant image quality without increased technical effort.
SUMMARY OF THE INVENTION
In accordance with the invention, this object is achieved in a simple but effective fashion in that the digitized, incomplete, complex spin echo signal S
x
is completed through adding zeros for the entire read-out interval and the central part is weighted with a function which is substantially anti-symmetrical about the point t=t
0
and has an amplitude of ½ at t
0
and subsequently is Fourier transformed for generating a Fourier-transformed signal.
In this method, only slightly more than half of the data of one dimension of a k space is detected. The remaining data is replaced by zeros. At least half of the k space must be detected in the reading direction to determine the position of the center of the spin echo signal and to prevent a loss in resolution.
The inventive method has the advantage that the image is continuously reconstructed and possibly corrected directly after recording each k space line. In particular for resonance images with more than one dimension, this proves to be advantageous since the directly following reconstruction saves considerable time.
After Fourier transformation, the phase can be corrected which produces real images with the same quality. The required phase corrections are known already before data recording. They are determined in a pre-scan. The data is reconstructed parallel to data recording.
By calculating the magnitude of the complex values and the omission of phase correction, image quality is considerably improved.
The article “Faster MR Imaging—Imaging with Half the Data” Society of Magnetic Resonance in Medicine, 1985, pages 1024-1025 describes using half of the phase encoding steps for image reconstruction, whereas the other half is empty and use the real part of the complex image formation for image representation. To prevent ringing artifacts, a roll-off filter must be applied which requires adding some additional phase encoding steps.
To improve the prior art approach, an alternative method variant which utilizes the same inventive basic idea, concerns a method of producing magnetic resonance images wherein a (n+1) dimensional k space is scanned, comprising an imaging pulse sequence with at least one RF excitation pulse in a first step, followed by at least one refocusing pulse, wherein in at least one part of a read-out interval [t
0
−½ t
a
, t
0
+½ t
a
) at least one part of a complex spin echo signal S
x
is measured and digitized, by means of a quadrature detector, which comprises a central part about the center t=t
0
of the spin echo signal in the time interval [t
0
−&egr;,t
0
+&egr;] having at least one phase encoding gradient before the read-out interval and wherein in subsequent steps, the phase encoding gradient is systematically incremented and the k space is incompletely scanned in the direction (k
y
) of the phase encoding gradient such that for each relative point in time in the read-out interval in the phase direction (k
y
) an incomplete signal S
y
is obtained having a portion central about k
y
=0 wherein for each relative point in time in the read-out interval, the digitized incomplete complex signal S
y
is completed through adding zeros along k
y
and the central part having a function which is substantially anti-symmetrical about the point k
y
=0 having an amplitude of ½ at k
y
=0 is weighted and subsequently Fourier-transformed for producing a Fourier-transformed signal.
Also in this case, only slightly more than half of the data must be read in the direction k
y
and data processing and simultaneous reading of the remaining data is possible. This permits on the one hand more rapid imaging and saves on the other hand memory capacity.
In an advantageous further development of the two above-described variants of the inve
Bruker BioSpin MRI GmbH
Fetzner Tiffany A.
Hackler Walter A.
Lefkowitz Edward
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