Method for coherent steady-state imaging of constant-velocity flowing fluids

#The present invention relates generally to magnetic resonance imaging, and more particularly, to an improved timing sequence of controlling gradient signals relative to rf pulses to provide a system for magnetic resonance imaging of moving fluids.

#NMR imaging as a medical diagnostic tool offers a number of important advantages over the various other means available for probing the human body. The most significant of these advantages result from the completely non-invasive nature of the technology, and the ability to obtain spatially encoded specimen data with a high degree of precision. Additionally, NMR has minimal, if any, hazards for either patients or operators of the apparatus; and perhaps most importantly, NMR image intensities are increasingly being found to be sensitive to various disease states. Clinical studies have shown that the relaxation time of malignant tissues are in general longer than those of the tissues of origin. This property is apparently not unique to cancerous tissue, but rather is indicative of the changes in molecular level structure of water associated with certain disease states. Other pathologies of NMR imaging include hydrocephalus; a carotid artery aneurysm; edema associated with kidney transplants; liver cirrhosis and several others.

#In general, known NMR techniques for imaging of body tissue have tended to be of somewhat limited image quality, spatial resolution, and have required comparatively long imaging times to complete. In view of these technical and medical considerations, it is clearly of importance to improve NMR imaging technology by all available means. In particular, signal to noise ratios need to be increased, imaging times need to be shortened, spatial resolution needs to be enhanced, and imaging of the transverse and/or longitudinal relaxation times need to be accomplished. Many of these factors are not mutually exclusive. As a result, there has been a tendency to trade an improvement in one factor to the detriment of others. These trade-offs are not always salutary, further pointing to the need for basic improvements in NMR methods and apparatus.

#FIG. 1 is a block diagrammatic view of a nuclear magnetic resonance system according to the present invention.

#FIG. 2 is a plot illustrating rf pulses showing free induction to k and echoes.

#FIG. 3 is a time plot illustrating the timing of various grading signals with respect to the echo signal and rf signal.

#In the following figures the same reference numerals will be used to identify the same components in the various views. While the present invention is illustrated with respect to nuclear magnetic resonance imaging, other forms of imaging may benefit from the teachings of the present invention.

#Referring now to FIG. 1, a simplified block diagram showing the major components of an NMR imaging apparatus 10 suitable for implementing the present invention. The overall system, generally designated, is made up of a general-purpose minicomputer 12 which is functionally coupled to a disc storage unit 14 and an interface unit 16. An rf transmitter 18, signal averager 20, and gradient power supplies 22, 24, 26 are coupled to the computer 12 through interface unit 16. The three gradient power supplies are used for energizing, respectively, a set of three gradient coils 28, 30, 32. These coils may be referred to by axis (X, Y or Z) or by various names such as slice, read and phase.

#The rf transmitter 18 is gated with pulse envelopes from computer 12 to generate rf pulses having the required modulation to excite resonance in the sample being imaged. The time between the pulses is referred to as the repetition time (TR). The rf pulses are amplified in an rf power amplifier 34 to levels varying from 100 watts to several kilowatts, depending on the imaging method, and are applied to a transmitter coil 36. The high power levels are necessary for large sample volumes such as are encountered in whole body imaging, and where short duration pulses are required to excite NMR frequency bandwidths.