Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-12-11
2003-01-21
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000, C324S318000, C324S320000, C324S322000
Reexamination Certificate
active
06509735
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance tomography apparatus of the type having a shim coil arrangement and a gradient coil arrangement, whereby, in order to generate an image dataset of a region of an examination subject, at least the region that must be imaged is borne in an imaging volume of the device, and an initial shim adjustment procedure is carried out.
2. Description of the Prior Art
In magnetic resonance tomography, the homogeneity of the base magnetic field is a decisive factor with respect to the quality of the magnetic resonance images. Field inhomogeneities of the base magnetic field within the imaging volume of a magnetic resonance tomography system cause geometric distortions of the magnetic resonance image that are proportional to the field inhomogeneities. Field homogeneity is particularly important in sequences known as rapid pulse sequences, such as in echo-planar methods.
Measures to improve the base magnetic field homogeneity are referred to as shimming. There are passive and active shim measures. In passive shimming, a number of iron plates are attached in the examination space of the magnetic resonance tomography system in an appropriate arrangement. The base magnetic field in the imaging volume is measured prior to attaching the iron plates. From the measured values, a computer program calculates the appropriate number and arrangement of the iron plates.
In active shimming, correction coils which homogenize the base magnetic field, known as shim coils, are provided in a shim coil arrangement. To operate the shim coil arrangement, a power pack that delivers highly constant and reproducibly adjustable direct currents is needed. The shim coil arrangement is used for fine correction whenever a very high homogeneity is needed, for instance to correct field distortions that are caused by the susceptibility of an examination subject.
As is known from German Patent 195 11 791, corresponding to U.S. Pat. No. 5,614,827, the base magnetic field in the imaging volume can be described by coefficients of spherical harmonic functions. This patent also teaches that linear base magnetic field deviations, i.e. first-order field disturbances, can be compensated by charging gradient coils with an offset current. The offset current is a constant current that is superimposed on a current that executes a gradient sequence. To compensate higher-order field inhomogeneities, respective shim coils are provided in correspondence to the order that must be compensated, each such coil essentially compensates the corresponding field disturbance, for which it must be charged with a suitable current. In magnetic resonance tomography, nine shim coils generally suffice even given high requirements, so that, together with the three gradient coils, twelve spherical coefficients that disturb the field homogeneity most severely can be cancelled.
Due to the field-distorting effect of the examination subject, a shim adjustment procedure is performed in the course of generating magnetic resonance images. In this process, the currents for the individual shim coils and the offset currents for the gradient coils are determined once subsequent to positioning the volume of a region of the examination subject that is to be imaged. The shim adjustment procedure is carried out according to the following steps in accordance with the abovementioned patent:
In a first step, two three-dimensional, spatially resolved magnetic resonance raw datasets are defined in the form of two three-dimensional raw data matrices whose phases have different sensitivities to inhomogeneities of the base magnetic field. The first raw data matrix is obtained using a first series of sequences with a first echo time. The same series of sequences is repeated with a second echo time which is larger than the first echo time. The second raw data matrix is obtained therefrom. In the second raw data matrix, base magnetic field inhomogeneities influence the phase of the measured signals more strongly, since the base magnetic field inhomogeneities have a longer effect longer due to the longer echo time.
In a second step, the two raw data matrices undergo a three-dimensional Fourier transformation.
In a third step, a three-dimensional phase difference matrix is calculated by determining phase differences between corresponding voxels of the two Fourier transformed matrices.
In a fourth step, phase differences between spatially adjacent voxels are calculated in the phase difference matrix PD. This is done for all three spatial directions, with a phase error dataset being produced for each direction, for example.
In a fifth step, the currents for each shim coil and for each gradient coil are computed based on the measured phase error dataset and on a predetermined matrix A. The matrix A characterizes the effect of one unit current on each voxel of the phase error dataset for each shim coil and for each gradient coil. This matrix A must be defined only once for each magnetic resonance tomography system and then remains constant as long as there are no modifications to the system.
In a sixth and final step, the computed shim currents in the shim coil arrangement and the offset currents in the gradient coils are correspondingly adjusted.
The computed shim setting remains unchanged for all magnetic resonance images of the examination subject that must be obtained. Apart from the one-time determination of the matrix A, the first step of the initial shim adjustment process described above is by far the most time consuming. Even when, instead of the first and second series of sequences, only one sequence is used, which delivers two signals with different echo times following a single excitation, and in addition a substantially smaller spatial resolution is selected compared to a diagnostic magnetic resonance image, the time frame for step one is always in the 30-second range.
Subsequent to a position change of the imaged region, for instance as a result of a movement of the examination subject, the shim setting that was calculated in the initial shim adjustment procedure is no longer optimal, and consequently the images that are created in the continued course of the magnetic resonance tomography procedure contain distortions dependent upon the position modification. Such distortions resulting from a change of position of the imaged region can be avoided by undertaking a new shim adjustment procedure as described above. For this purpose, the magnetic resonance data acquisition of the examination subject would have to be interrupted, so the time-consuming shim adjustment procedure can be performed again. This would lead to an unacceptable prolonging of the duration of the magnetic resonance tomography procedure, particularly given repeated position changes, and it is therefore not done in practice.
SUMMARY OF THE INVENTION
It is an object of the present invention to design a method of the abovementioned type with which it is possible to achieve an optimal shim adjustment at any time during a magnetic resonance tomography procedure in a time-efficient manner, even when the imaged region changes its position.
This object is inventively achieved in a method for operating a magnetic resonance tomography apparatus wherein a position change of the imaged region in relation to the imaging volume is detected, and a current in the shim coil arrangement is modified dependent on the detected position change.
For simplifying the description of the advantages of the inventive method, it is assumed, without limiting the method, that the base magnetic field in the imaging volume is sufficiently homogenous without an examination subject therein, and that a region of the examination subject that must be imaged is a head of a patient. Subsequent to positioning the head in the imaging volume of a magnetic resonance tomography device, an initial shim adjustment procedure is carried out as described above. With this procedure, the distorting effect of the head on the homogeneit
Mueller Edgar
Thesen Stefan
Lefkowitz Edward
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Vargas Dixomara
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