Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-07-10
2002-11-12
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S312000
Reexamination Certificate
active
06479996
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance imaging method comprising acquisition of sets of magnetic resonance signals from several scan-volumes of an object.
2. Description of Related Art
Such a magnetic resonance imaging method and such a magnetic resonance imaging system are known from the U.S. Pat. No. 5,928,148.
The known magnetic resonance imaging method in particular pertains to MR-angiography which forms magnetic resonance images of a patient's blood-vessel system. In order to improve contrast of notably the patient's arteries, a contrast agent is administered. According to the known magnetic resonance imaging method magnetic resonance signals are acquired from a large region of interest by translating the patient to successive stations at which the respective sets of magnetic resonance signals are acquired. To this end, the receiver system of the known magnetic resonance imaging system comprises a stationary local receiver coil which is supported adjacent the patient to acquire the magnetic resonance signals. In an other embodiment of the known magnetic resonance imaging system the receiver system comprises a multi-segment local coil which is moveable with the patient and its coil-segments are sequentially switched into operation. At the successive stations, respective portions of the patient to be examined are moved into the field-of-view of the magnetic resonance imaging system. The respective scan-volumes are defined by the respective portions of the patient from which the respective coil-segments acquire magnetic resonance signals at the successive stations. It is noted that the cited US-patent shows a patient's leg which is surrounded by several sets of surface coils. The separate sets of surface coils include different geometrical volumes, however, this does not cause different scan-volumes for the respective sets of surface coils. Notably, the scan-volume is determined by the smallest distance in k-space between scanned points in k-space, i.e. the smallest wavevector difference in the magnetic resonance signals. The scan volume is determined by the magnitude of the temporary read-gradients and phase-encoding-gradients. According to the known magnetic resonance imaging method, the respective scan-volumes must be aligned so that the sets of magnetic resonance signals can be concatenated in order to form a single MR-image of the entire region of interest.
SUMMARY OF THE INVENTION
An object of the invention is to provide a magnetic resonance imaging method involving acquisition several sets of magnetic resonance signals which provides magnetic resonance images having a better diagnostic quality.
This object is achieved by the magnetic resonance imaging method according to the invention wherein different spatial approaches are taken in the scanning of the respective scan-volumes.
As different spatial strategies are employed for scanning of the various portions of the object, differences between the shapes of the respective portions of the object are more accurately taken into account. For example, in MR-angiography of the lower extremities (the patient's legs), the invention takes account of the different shapes of notably the bloodvessel structures of the patient's abdomen, upper and lower legs. The respective scan volumes are accurately matched to the portions of patient to be examined. Consequently, the magnetic resonance images which are reconstructed from the magnetic resonance signals have a better diagnostic quality, in that small details having little contrast are nevertheless rendered well visible. Notably, the scan-volumes are adjusted in that for the individual scan-volumes the imaging-volume and/or the resolution of the magnetic resonance imaging system are adjusted. These are achieved by setting the smallest separation of points scanned in k-space and the number of points scanned in k-space by accordingly controlling the temporary read-gradients and/or phase encoding gradients and the sampling intervals employed in the acquisition of the magnetic resonance signals. That is the region in k-space which is scanned and the density of sample points are adjusted for separate scan-volumes.
These and other aspects of the invention will be further elaborated with respect to the preferred embodiments as defined in the dependent Claims.
Spatial approaches in the scanning of the scan-volumes may be achieved in various ways. Preferably, the spatial scanning may involve scanning of a number of slices in the respective scan-volumes. According to the invention, the number of slices employed in the individual scan-volumes is adjusted to the portion of the patient to be examined at issue. As portions of the patient to be examined are larger, larger scan-volumes with more slices are employed. Consequently, good coverage of the entire region of interest is achieved and superfluous scanning of empty, uninteresting or irrelevant space is avoided. Notably, the time required for acquisition of magnetic resonance signals from relatively small scan-volumes is reduced. As the time required for signal acquisition is shorter, the signal acquisition may be completed before the injected contrast agent reaches the patient's venous system. Hence, magnetic resonance signals from the patient's veins are avoided so that venous enhancement is avoided in the magnetic resonance image which is reconstructed from the magnetic resonance signals. Thus, it is avoided that the representation of the patient's arteries in the magnetic resonance image is obscured by magnetic resonance signals from veins that have become filled with contrast agent.
Alternatively, the slice thickness may be changed. This adjusts the spatial resolution in the direction transverse to the slices; this direction is usually indicated as the ‘slice direction’. Further, within the slices for respective scan-volumes the field-of-view and the scan matrix, i.e. the number of samples and the sample density in k-space may be varied. Thus, the size of the scan-volume and the resolution in parallel to the slices is adjusted for the individual scan-volumes. The field-of-view is the region in the slice at issue from which magnetic resonance signals are acquired and is determined by the temporary magnetic read-gradients and phase-encoding gradients. The scan matrix is a two-dimensional matrix of which the entries represent the sample points (in k-space) are determined by the read gradients and phase-encoding gradients. So that the spatial resolution in the direction parallel to the slices is varied
Hence, according to the invention, the size of the scan-volume and the resolution in all directions can be adjusted for the individual scan-volumes.
Further, contrast parameters that influence the contrast and/or signal-to-noise in the reconstructed magnetic resonance image may be adjusted for separate scan-volumes. Notably, these contrast parameters include the repetition rate, the echo-time, the flip-angle and the bandwidth employed in the pulse sequences of RF-excitations and temporary gradient used for scanning the scan-volumes for magnetic resonance signals.
In an other preferred implementation of the invention, the relative orientations of the scan-volumes are adjustable. Adjusting the relative orientations of the respective scan-volumes achieves accurate matching of the scan-volumes to the anatomy of the patient to be examined. The individual scan-volumes are oriented in the same way as the local orientation of the portion of the patient to be examined scanned in the relevant scan-volume. Individual scan-volumes have a longitudinal axis, for example the direction of the largest size of the scan-volume at issue. The orientations of the scan-volumes are for example obtained by adjusting the directions of the respective longitudinal axes of the respective scan-volumes to the portions of the patient to be examined. Thus, it is achieved that the scan-volumes adequately follow the shape of the portions of the patient of interest. Notably, in MR
Geraats Diana Maria Francisca
Ho Kai Yiu J. A. M.
Hoogeveen Romhild Martijn
Leiner Tim
Koninklijke Philips Electronics
Lefkowitz Edward
Shrivastav Brij B.
Vodopia John F.
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