Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-05-17
2002-04-23
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06377045
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magnetic resonance imaging method utilizing a coil sensitivity profile.
The article “Coil Sensitivity Encoding for Fast MRI” by K. P. Pruesmann et al. in Proceedings ISMRM (1998), page 579, deals with a magnetic resonance imaging method involving sub-sampled acquisition of magnetic resonance signals.
The known magnetic resonance imaging method is used in the so-called SENSE technique. In order to form a magnetic resonance image of an object, for example a patient to be examined, the object is arranged in a steady, preferably as spatially uniform as possible magnetic field. Nuclear spins are excited in the object by one or more RF excitation pulses, thus generating a magnetic nuclear spin polarization. Due to precession and relaxation of the nuclear spin polarization, magnetic resonance signals are emitted. The magnetic resonance signals are received by the receiving coils with sub-sampled scanning of the k space for a given spatial resolution of the magnetic resonance image. Respective receiving coil images are reconstructed from the sub-sampled magnetic resonance signals acquired by the individual receiving coils. Due to the sub-sampling, such receiving coil images usually contain artefacts such as so-called aliasing effects. A final magnetic resonance image in which the artefacts due to sub-sampling, as they occur in the receiving coil images, have been significantly reduced or even completely eliminated is derived from the receiving coil images and on the basis of the spatial sensitivity profiles of the receiving coils.
The known magnetic resonance imaging method enables the formation of a magnetic resonance image which reproduces the local density of hydrogen nuclei (protons) in the body of the patient to be examined.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic resonance imaging method wherein spectroscopic information of the object to be examined, for example a patient to be examined, is reproduced in an image and wherein the magnetic resonance signals are acquired within a short period of time.
This object is achieved by means of a magnetic resonance imaging method according to the invention wherein
an RF excitation pulse is generated in an examination space,
magnetic resonance signals are generated, due to the RF excitation pulse, in an object to be examined in the examination space, the magnetic resonance signals being received in a sub-sampled fashion by a set of receiving coils having a coil sensitivity profile, wherein
the positions in a measuring plane in the examination space in which the magnetic resonance signals are generated are encoded on the basis of at least two linearly independent components of the wave vector of the magnetic resonance signals, and
the frequency distribution of the magnetic resonance signals represents spectroscopic information concerning the object to be examined, and
a spectroscopic magnetic resonance image is reconstructed from the sub-sampled magnetic resonance signals and the coil sensitivity profile.
A steady, as uniform as possible magnetic field is applied to the examination space in order to form the spectroscopic magnetic resonance image. As a result, the spins are oriented in the direction of the steady magnetic field in the object to be examined in the examination space. The object to be examined is, for example a patient to be examined who is arranged completely or partly in the examination space. The magnetic resonance signals are encoded on the basis of two linearly independent, for example mutually perpendicular, components of the wave vector. Such encoding in a two-dimensional plane in the k space is realized by superposing temporary gradient fields on the steady magnetic field. The magnetic resonance signals constitute a two-dimensional spatial Fourier transform of spatially varying spectral densities of the object to be examined. The spectral information is then represented by the temporal frequency components of the individual spatial frequency components of the magnetic resonance signals. Via inverse spatial Fourier transformation a set of spectra in separate positions in the object to be examined is derived from the magnetic resonance signals encoded in a plane in the k space. The set of spectra, i.e. local spectroscopic profiles form the spectroscopic magnetic resonance image. The local chemical composition of the object can be studied on the basis of such a set of spectra.
According to the invention the magnetic resonance signals are acquired by the receiving coils with sub-sampling of the scanning of the k space with a given spatial resolution of the spectral magnetic resonance image. As a result, the time required to receive the required number of magnetic resonance signals is substantially reduced. As the degree of such sub-sampling is higher, a smaller part of the k space will be scanned and less time will be required to acquire the magnetic resonance signals. The spectroscopic magnetic resonance image can be derived from the sub-sampled magnetic resonance signals in various manners without giving rise to serious disturbances due to the sub-sampling.
The time required for acquisition of the magnetic resonance (MR) signals is reduced by employing sub-sampling of the MR-signals. Such sub-sampling involves a reduction in k-space of the number of sampled points which can be achieved in various ways. Notably, the MR signals are picked-up through signal channels pertaining to several receiver antennae, such as receiver coils, preferably surface coils. Acquisition through several; signal channels enables parallel acquisition of signals so as to further reduce the signal acquisition time.
Owing to the sub-sampling, sampled data contain contributions from several positions in the object being imaged. The magnetic resonance image is reconstructed from the sub-sampled MR-signals with the use of a sensitivity profile associated with the signal channels. Notably, the sensitivity profile is for example the spatial sensitivity profile of the receiver antennae, such as receiver coils. Preferably, surface coils are employed as the receiver antennae. The reconstructed magnetic resonance image may be considered as being composed of a large number of spatial harmonic components which are associated with brightness/contrast variations at respective wavelengths. The resolution of the magnetic resonance image is determined by the smallest wavelength, that is by the highest wavenumber (k-value). The largest wavelength, i.e. the smallest wavenumber, involved, is the field-of-view (FOV) of the magnetic resonance image. The resolution is determined by the ratio of the field-of-view and the number of samples.
The sub sampling may be achieved in that respective receiver antennae acquire MR signals such that their resolution is coarser than required for the resolution of the magnetic resonance image. The smallest wavenumber sampled, i.e. the minimum step-size in k-space, is increased while the largest wavenumber sampled is maintained. Hence, the image resolution remains the same when applying sub-sampling, while the minimum k-space step increases, i.e. the FOV decreases. The sub-sampling may be achieved by reduction of the sample density in k-space, for instance by skipping lines in the scanning of k-space so that lines in k-space are scanned which are more widely separated than required for the resolution of the magnetic resonance image. The sub-sampling may be achieved by reducing the field-of-view while maintaining the largest k-value so that the number of sampled points is accordingly reduced. Owing to the reduced field-of-view sampled data contain contributions from several positions in the object being imaged.
Notably, when receiver coil images are reconstructed from sub-sampled MR-signals from respective receiver coils, such receiver coil images contain aliasing artefacts caused by the reduced field-of-view. From the receiver coil images and the sensitivity profiles the contributions in individual positions of the receiver coil images from dif
Dydak Ulrike
Folkers Paulus Johannes Maria
Lamerichs Rudolf Mathias Johannes Nicolaas
Pruessmann Klaas Paul
Van Den Brink Johan Samuel
Arana Louis
U.S. Philips Corporation
Vodopia John
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