Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-01-28
2004-12-07
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06828787
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for magnetic resonance imaging, with which an imaging magnetic resonance measurement of a region of interest in an object under examination is carried out, wherein a magnetic resonance tomogram of the region is displayed that can be prescribed in position and orientation, and wherein a two- or three-dimensional magnetic resonance overview image of at least a part of the object under examination is produced before the imaging magnetic resonance measurement.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for obtaining images of the interior of the body of a living object under examination. In order to carry out magnetic resonance tomography, a basic field magnet generates a static, relatively homogenous basic magnetic field. It may be assumed that the basic magnetic field prescribes a direction z in space and thus defines a right-handed orthogonal coordinate system. During the recording of magnetic resonance images of prescribable slices rapidly switched gradient fields, that are generated by gradient coils, are superposed on this basic magnetic field. The spatial coding of the magnetic resonance signals that is required for the spatial resolution can be achieved by suitable selection of the gradient fields. A distinction is generally made between the slice direction, the readout direction and the phase coding direction, which are perpendicular to one another. High-frequency (radio-frequency) pulses for triggering magnetic resonance signals are radiated into the object under examination with high-frequency transmitting antennas during an imaging magnetic resonance sequence. The magnetic resonance signals triggered by these high-frequency pulses are picked up by high-frequency receiving antennas. The magnetic resonance images of one or more slices of the body region of interest, that can be prescribed in position and orientation, are produced on the basis of these magnetic resonance signals received with receiving antennas.
This reconstruction of the magnetic resonance images presupposes a unique spatial coding of the measured data. The size of the imaging or measuring field (FoV: Field of View) for detecting the region of interest must be prescribed for the spatial coding. If the receiving antenna covers the volume excited with the HF pulses, it is necessary to adapt this measuring field to the size of the object. If this adaptation is not undertaken, ambiguous signal codings occur that lead to folds in the reconstructed magnetic resonance images (aliasing artifacts, wrap-around artifacts).
Time-consuming additional measurements of data rows are frequently necessary to avoid folds in the phase coding direction. As a rule, the slice-selective excitations are used in the slice direction, while double the sampling rate is constantly used for measurement in many cases in the readout direction in order to avoid the folds, since there is no need to accept any temporal restrictions. However, it is desirable despite these measures to have the best possible adaptation of the measuring field to the region of interest, since an improvement in the spatial resolution is associated with a smaller measuring field. However, when a section is conducted obliquely, in the case of magnetic resonance imaging of slices whose normal direction does not coincide with an orthogonal spatial direction of the basic field or of a main body axis, it is difficult to estimate the required minimal size of the measuring field for which no folds, or only a prescribable level thereof, occur in the magnetic resonance image. Furthermore, in such cases, it is frequently impossible to estimate the relatively small measuring field available owing to the over-sampling. Consequently, for safety reasons, measurements are sometimes made with excessively large measuring or imaging fields, in order to avoid folds. However, an excessively large measuring or imaging field leads again to loss of resolution.
The prescription of the dimensions of the measuring field is currently performed by the user of the magnetic resonance installation by manually varying the image size and image position by means of a graphical slice positioning in a magnetic resonance overview image of the body region, or by entering the appropriate position parameters. Furthermore, the phase coding and readout directions can be interchanged manually in order to optimize the measuring field and to minimize the folds. The phase coding direction is selected in this case to be in the direction of the shortest axis of the two-dimensional measuring field. Occasionally, individual antenna or coil elements are also switched off, or additional saturation pulses are switched within the excitation pulse sequence in order to reduce the signal contribution of the folds in the magnetic resonance image. All of these measures are entered manually by the user of the installation and require a considerable degree of experience.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for magnetic resonance imaging that simplifies the selection of the optimal measuring field for the user of a magnetic resonance installation.
The object is achieved in accordance with the invention in a method for magnetic resonance imaging, an imaging magnetic resonance measurement of a region of interest in an object under examination is carried out in a known way, and a magnetic resonance tomogram of the region is displayed, that can be prescribed in position and orientation by a user wherein, before the imaging magnetic resonance measurement, a two- or three-dimensional magnetic resonance overview image of the object under examination, or a part of the object under examination, for example the thorax, is made, and can be used to localize, position and orientate slices for the subsequent magnetic resonance pictures. In accordance with the present invention, the envelope of the object under examination or of the part of the object under examination on which the overview image is based, is automatically calculated (i.e., calculated by computer without human intervention) from this magnetic resonance overview image. This envelope corresponds to the outer boundaries of the object. After the user has prescribed the position and orientation of the slice of the object under examination from which signals are to be obtained for the planned magnetic resonance picture, a cutting surface with the envelopes of the object under examination being formed from this position and orientation of the slice plane or cutting plane, and the dimension of the minimal measuring field for the planned imaging magnetic resonance measurement being calculated therefrom. This can be performed in conjunction with the stipulation that no folds occur in the magnetic resonance image subsequently to be obtained, or in conjunction with the stipulation that a degree or level (severity) of folds that is prescribed by the user, for example only in a narrow edge region of the magnetic resonance image. The determination of the minimal measuring field is performed in the inventive method in a fully automatic fashion without the need for the user to introduce any estimates or empirical values.
The present invention thus substantially simplifies the determination of the optimal measuring field during the magnetic resonance measurement, and so the minimal possible measuring field with the maximum resolution is used at any time. Far fewer estimates and settings by the user are required when planning the measurement owing to the automatic calculation of the cutting planes with the object, which are produced upon a change in the slice orientation for subsequent further magnetic resonance measurements. Use is made of the fact that the dimensions of any desired cutting surface with the object can be calculated automatically by means of the foregoing three-dimensional magnetic resonance overview image. This technique offers substantial advantages particularly when conducting a section
Gutierrez Diego
Schiff & Hardin LLP
Siemens Aktiengesellschaft
Vargas Dixomara
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