The present invention relates to magnetic resonance imaging (MRI) techniques and, more particularly, to a technique for imaging at speeds faster than about 100 milliseconds.
For various reasons, it would be desirable to provide a technique for magnetic resonance imaging (MRI) in which images can be obtained in a very short period of time. Primary among the factors currently limiting the rate of growth of MRI in the diagnostic imaging marketplace are concerns relating to its cost-effectiveness, and its limited applicability to the study of organ systems subject to significant involuntary physiological motion, especially of the heart. Both of these problems (especially the latter) would be alleviated if the long data acquisition times (typically several minutes) could be decreased. This would reduce the total patient study time, increase patient throughput and hence optimize cost-effectiveness. If the total scan time is reduced to a small fraction of the cardiac period, motion artifacts due to all physiological motions including that of the heart, can be avoided.
Early high-speed whole body biological images suffered both from poor spatial resolution and from poor signal-to-noise ratio (SNR). (As used herein, "high-speed" signifies about 100 milliseconds for acquisition of a complete image). This low quality is, in part, a reflection of the relatively low magnetic field strength used. However, it is known that SNR increases approximately linearly with field strength, and thus higher quality images can, in principle, be obtained at higher field strengths.
Several problems are confronted in implementing high-speed imaging at high-field strengths, and the invention described herein provides solutions to these problems.
One major problem in using high magnetic fields for magnetic resonance imaging is that inhomogeneities in the magnetic field are, for practical purposes, unavoidable, and can introduce phase and geometrical distortion into magnetic resonance images of an object under investigation. Such magnetic inhomogeneities are introduced from two distinct sources. First, a static magnetic field has an inherent non-uniformity which typically varies in proportion to the magnetic field strength. As a practical matter, such inhomogeneity is considered unavoidable in the static magnetic field, since to eliminate it would be unduly difficult and unduly expensive. Secondly, the magnetic susceptibility of an object being imaged provides an object dependent additional contribution to the magnetic inhomogeneities in the system.
Additionally, at high frequencies the RF field is attenuated and subject to phase shifts when penetrating a weakly conductive object, such as a human body. This effect is certainly in evidence at 85 MHz, which is the proton resonance frequency at around 2 Tesla.
A further major problem in imaging at high field is the proton "chemical shift" effect. This problem arises because the signal from any given point in the image may, in general, contain essentially two closely-spaced frequencies; one from water protons and one from lipid protons. The imaging procedures used in MRI place signals deriving from particular volume elements into corresponding picture elements in the final image based on their unique frequencies; hence, the two distinct frequencies deriving from lipid and water in the same volume element of tissue will be incorrectly placed into different picture elements. This results in two superimposed images. This problem is usually solved at lower field strengths, or for slower imaging methods, simply by increasing the strength of the magnetic field gradients which provide the spatial discrimination in the image, to the extent that the bandwidth they impose per picture element is substantially greater than the frequency separation of the proton resonances of fat and water. In the case of high-speed imaging, the uses of such high bandwidths per picture element would be prohibitively difficult to implement. Furthermore, the bandwidth would necessarily have to increase with field strength, thereby reducing any SNR increases to a square root of field strength dependency.
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