1. Field of the Invention
The invention pertains to a method for deriving a magnetic resonance image of an object placed in a stationary and substantially homogeneous magnetic main field, comprising repeatedly exciting said object and sampling magnetic resonance signals in the presence of a read gradient superimposed on said main field thereby determining a first frequency image of said object on radius and angle coordinates by radially scanning through frequency-space. The invention also pertains to an apparatus to perform such method.
2. Description of the Related Art
A method as described in the preamble is known from the U.S. Pat. No. 4,070,611. In said patent is disclosed that magnetic resonance signals are induced in an object, notably a patient, present in a homogeneous magnetic field and are detected while exposing the region of the object to be imaged to a linear gradient superimposed on the homogeneous magnetic field. A resonance spectrum is obtained for each of a relatively large number of angularly displaced orientations of the linear gradient. The image is constructed by a mathematical process of back projection. This technique uses the fact that the Fourier transform of a one-dimensional projection of the spin density represents a one-dimensional cross-section of the three-dimensional Fourier transform of the spin density function.
In the article "NMR Imaging Methods Seen as Trajectories in the Reciprocal Space" in Bull. Magnetic Resonance, 6, No. 3, Nov. 1984, p. 140-141, is disclosed that the values of the magnetic resonance signal S(t) fundamentally correspond to the values of the Fourier transform of the spin density function in frequency-space at positions given by dk/dt=.gamma.G, k=(k.sub.x,k.sub.y,k.sub.z) determining the position in frequency-space (also called k-space), G=(G.sub.x,G.sub.y,G.sub.z) determining the magnetic gradient, and .gamma. being the gyromagnetic ratio. So, in a method as described in the preamble, the values of a frequency image of an image to be produced are found as values of the detected signal assigned to points along a radial line in frequency-space.
In U.S. Pat. No. 4,070,611 mentioned above is explained that radially scanning through frequency-space has the distinct disadvantage that information is collected in an in homogeneous way, i.e. because of the radial scanning the information is more dense near the origin of frequency-space, representing the lower frequencies, and there may be insufficient information of the higher image frequencies. In U.S. Pat. No. 4,070,611 a method is disclosed in which frequency-space is scanned along parallel lines. An improved version of this method is called spin warp and disclosed in U.S. Pat. No. 4,506,222.
However, especially under circumstances wherein the magnetic resonance signal has short decay times, which in case of nuclear (proton) magnetic resonance depends on the materials or tissues examined and which is almost always the case with electron spin resonance, it may be advantageous to start sampling of the magnetic resonance signal immediately after the end of the excitation. That is not possible with methods like spin warp and spin echo because in those methods an echo of the signal is used and time is needed to switch phase encoding gradients. So, especially in cases of short decay times, radial scanning may be very useful.