Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-03-26
2003-06-10
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06577127
ABSTRACT:
PRIORITY CLAIM
This application claims priority from European Patent Application Serial No. 00201093.2, filed on Mar. 27, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance imaging method for imaging a time-dependent contrast with a section that increases in time and in which one or more k-planes in the k-space are scanned by the reception of magnetic resonance signals. The invention also relates to a magnetic resonance imaging system.
2. Description of the Related Art
A magnetic resonance imaging method and a magnetic resonance imaging system of this kind are known from United States patent specification U.S. Pat. No. 5,122,747.
During the execution of magnetic resonance imaging methods the object to be examined, for example the patient to be examined, is arranged in a steady magnetic field and (nuclear) spins are excited in the patient to be examined by means of RF excitation. Magnetic resonance signals are emitted upon relaxation of the (nuclear) spins. Spatial encoding of the magnetic resonance signals is realized by application of temporary magnetic gradients, also referred to as gradient fields, and a magnetic resonance image is reconstructed from the magnetic resonance signals on the basis of said spatial encoding.
In the case of MR angiography the patient to be examined is injected with a contrast medium that produces strong magnetic resonance signals, for example after excitation by means of a radio frequency (RF) pulse. The magnetic resonance signals thus generated are used to reconstruct a magnetic resonance image in which a part of the vascular system of the patient to be examined is reproduced.
It has been found in the practice of MR angiography that when the contrast medium reaches the arteries of the patient to be examined, the contrast of the arteries filled with the contrast medium increases relative to the surrounding tissue. This means that there is a time-dependent contrast with an increasing section. When a given period of time has elapsed after the contrast medium has reached the arteries, the variation of the concentration of contrast medium in the arteries is only slow as a function of time, and hence also the variation of the strength of the magnetic resonance signals from the arteries. It has notably been found that after the contrast medium has reached the arteries, the concentration of contrast medium in the arteries (the arterial contrast) first increases quickly and subsequently its increase is significantly slower until it reaches a maximum value, after which it decreases very slowly again. Approximately from 4 to 8 seconds after the maximum contrast has occurred in the arteries, however, considerable so-called venous enhancement starts to appear in the magnetic resonance images because the contrast medium has meanwhile reached the veins. The period of time of from 4 to 8 seconds between the maximum of the arterial contrast and the beginning of the considerable venous enhancement is also referred to as the arterial window.
According to the known method individual planes in the k-space are scanned along an essentially spiral-shaped trajectory. The spiral-shaped trajectory starts at the center in the k-space in the relevant plane and extends outwards therefrom in the form of a spiral. The individual plane is usually a (k
y
,k
z
,) plane, in which case the center of said plane is formed by the point (k
y
,k
z
)=(0,0). One or more magnetic resonance images are reconstructed from the magnetic resonance signals acquired during the scanning of the k-space; for example, a magnetic resonance image of a section along a plane through the patient to be examined is reconstructed for each of the individual planes in the k-space. The known method is used notably in magnetic resonance angiography. The magnetic resonance signals from the blood vessels are very weak prior to the injection with a contrast medium. When the injected contrast medium reaches the arteries, the strength of the magnetic resonance signals from the arteries increases very quickly and in a somewhat later stage the variations in time of the magnetic resonance signals from the arteries are much smaller. Finally, the contrast medium also reaches the veins of the patient to be examined and magnetic resonance signals of considerable strength emanate from the veins. The time-dependent contrast concerns the contrast in the magnetic resonance images that is due to the temporally varying concentration of contrast medium in the vascular system of the patient to be examined. The known magnetic resonance imaging method attempts to form a magnetic resonance image of high diagnostic quality of the arteries of the patient to be examined. Because the magnetic resonance signals are acquired along the spiral-shaped trajectory, the magnetic resonance signals can be acquired with a low magnitude of the wave number (low k values) before the contrast medium reaches the veins. The appearance of so-called venous enhancement in the magnetic resonance images is thus avoided.
For the known method it is notably necessary that the instant at which the acquisition of magnetic resonance signals commences at the center of the relevant k-plane corresponds very accurately to the instant at which the contrast medium reaches its maximum concentration in the arteries; in other words, the scanning along the spiral should commence at the instant at which the contrast medium reaches the arteries. Notably when the scanning along the spiral commences too early, that is, before the contrast medium has reached the arteries, so-called “ringing” artifacts occur. It has been found that when the known method is used in practice, the radiologist often commences the scanning along the spiral only well after the contrast medium has reached the arteries, thus avoiding said “ringing” artifacts. Because scanning is started only late, the magnetic resonance signals are then acquired while the concentration of the contrast medium in the arteries already decreases. Consequently, the contrast resolution of the reproduction of the arteries in the magnetic resonance image is lower or even a higher concentration of contrast medium may be required. Moreover, time is thus lost for the acquisition of magnetic resonance signals at low k values prior to the start of the venous enhancement.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic resonance imaging method which enables faithful reproduction of the time-dependent contrast in the magnetic resonance images and is much less sensitive to the accuracy of the “timing” of the acquisition of the magnetic resonance signals. The invention is used preferably for the imaging of a time-dependent contrast with a slowly varying section.
This object is achieved by means of a magnetic resonance imaging method in accordance with the invention wherein
a central sector of the k-space and a peripheral sector of the k-space are selected in advance in an individual k-plane, and
the peripheral sector is scanned separately from the scanning of the central sector,
the central sector contains the center of the individual k-plane,
and the center of the individual k-plane is situated outside the peripheral sector, where
the scanning of the central sector commences outside the center of the individual k-plane and before or during the increasing section of the time-dependent contrast.
In accordance with the invention the scanning of the central sector commences outside the center, that is, preferably before the increase of the arterial contrast. Consequently, more time is available for the scanning of the central sector prior to the start of the venous enhancement. The invention also utilizes the fact that in practical MR angiography the time-dependent contrast has a section which varies slowly in time as from an instant just before the contrast in the arteries becomes maximum and the start of the venous enhancement. Because the central sector is scanned separately in accordance with the invention, the center can be easily scanned during
Beck Gabriele Marianne
Harvey Paul Royston
Hoogeveen Romhild Martijn
Van Den Brink Johan Samuel
Van Yperen Gerrit Hendrik
Lefkowitz Edward
Shrivastav Brig B.
Vodopia John
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