Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-05-17
2003-07-15
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06593740
ABSTRACT:
BACKGROUND OF THE INVENTION
A magnetic resonance imaging method utilizes a coil sensitivity profile.
The article “Coil Sensitivity Encoding for Fast MRI” by K. P. Pruessmann 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, so that magnetic nuclear spin polarization is generated. Nuclear spins are excited in the object by one or more RF excitation pulses. 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 of wave vectors of the magnetic resonance signals 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.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic resonance imaging method enabling the formation of one or more successive magnetic resonance images whereby dynamic processes taking place in the object to be examined can be accurately followed. It is another object of the invention to provide a magnetic resonance imaging method enabling the formation of magnetic resonance images which contain hardly any disturbances due to physical or physiological processes or, for example, motion in or of the object to be examined.
This object is achieved by means of a magnetic resonance imaging method according to the invention wherein
an RF excitation pulse is generated,
a series of magnetic resonance signals which are due mainly to the RF excitation pulse is acquired in a sub-sampled fashion by means of a set of one or more receiving coils during an acquisition time,
a magnetic resonance image is derived from the sub-sampled magnetic resonance signals and on the basis of the spatial coil sensitivity profile of the set of receiving coils, and wherein
the degree of sub-sampling is chosen to be such that the acquisition time remains below an upper limit,
which upper limit is a predetermined function of the decay time in which a significant decay of the magnetic resonance signals occurs.
As the degree of sub-sampling applied is higher, the time required for the acquisition of the magnetic resonance signals will be shorter. The sub-sampling is then always related to the scanning of the k space in order to form the magnetic resonance image with a given spatial resolution. As the degree of sub-sampling applied is higher, fewer magnetic resonance signals will have to be acquired so as to reconstruct the magnetic resonance image with a high spatial resolution. According to the invention the degree of sub-sampling is chosen to be such that the sub-sampled magnetic resonance signals can be acquired within such a short acquisition time that hardly any or no decay of the magnetic resonance signals occurs, notably due to the dephasing of the excited nuclear spins. The decay of the magnetic resonance signals becomes manifest as a decrease of the signal levels of successive magnetic resonance signals in a series. If no steps were taken, such a decrease would cause disturbances in the magnetic resonance image. The decay of the magnetic resonance signals is usually caused by a disturbance of the phase relation of the local precessional magnetization. An important cause is T
2
or T
2
* dephasing of the excited spins. Dephasing may also occur due to diffusion of the excited spins. Moreover, dephasing is caused by eddy currents and/or by chemically induced frequency shift, notably the so-called water-fat shift. Inhomogeneities in the steady magnetic field and inhomogeneities in the composition of the object to be examined cause dephasing of the magnetic moments of the excited spins. Such dephasing occurs notably due to the so-called T
2
relaxation of the excited spins or as a loss of phase coherence due to diffusion of the excited spins. Due to such dephasing, the amplitude of successive magnetic resonance signals decreases. Notably successive MR echo signals, generated due to an individual RF excitation pulse and a subsequent RF refocusing pulse and/or read-out gradients, have an amplitude which becomes lower as the MR echo signals are emitted later after the RF excitation pulse.
When a magnetic resonance image is formed while a contrast agent is administered to the patient to be examined, in principle decay of the magnetic resonance signals occurs because after some time the contrast agent disappears from the part of the patient to be examined, notably from a part of the blood vessels. Because the concentration of the contrast medium decreases in, for example the blood vessels, the signal level of the magnetic resonance signals decreases. According to the invention the degree of sub-sampling can be chosen to be so high that the acquisition time is significantly shorter than the time in which the magnetic resonance signals decay significantly due to a decrease of the presence of the concentration of the administered contrast agent in, for example the blood vessels. The shorter the acquisition time with respect to the relevant decay time, the less decay of the magnetic resonance signals will occur and hence the higher the diagnostic quality of the magnetic resonance image will be. The preferred duration of the acquisition time used is dependent on the circumstances in which the magnetic resonance image is formed and on the desired diagnostic quality.
Furthermore, decay of the magnetic resonance signals is also caused by motion of or within the patient to be examined. Such motion occurs, for example at the end of a period during which the patient has held his or her breath. During exhaling motions occur in the body of the patient to be examined which cause dephasing of the excited spins.
According to the invention the acquisition time can be kept so short that hardly any signal decay of the magnetic resonance signals occurs. It is thus achieved that the degree of disturbance due to signal decay, notably due to the dephasing, is significantly reduced in the magnetic resonance image reconstructed from the magnetic resonance signals and on the basis of the coil sensitivity profiles. The decay time is dependent on the main causes of the signal decay. For example, when the signal decay is caused mainly by dephasing with the dephasing time T
2
or T
2
*, good results are obtained by taking 2T
2
and 2T
2
*, respectively, as the upper limit for the acquisition time. A high diagnostic quality is thus achieved for the magnetic resonance image, i.e. small details are reproduced in the magnetic resonance image in such a manner that they can still be suitably distinguished in space.
The 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. For example, receiving coil images are reconstructed from the magnetic resonance signals from individual receiving coils.
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 per
Folkers Paulus Johannes Maria
Hoogeveen Romhild Martijn
Pruessmann Klaas Paul
Van Den Brink Johan
Weiger Markus
Koninklijke Philips Electronics , N.V.
Lefkowitz Edward
Vargas Dixomara
Vodopia John
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