Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1990-11-23
1992-07-28
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3320
Patent
active
051343720
DESCRIPTION:
BRIEF SUMMARY
Technical Field
The present invention relates to imaging method of separation of water and fat in MRI (Magnetic Resonance Imaging) so as to obtain a separation image of water and fat in a human body utilizing chemical shift, and more specifically to imaging method of separation of water and fat in MRI with quantitativeness where separation of water and fat is performed accurately.
BACKGROUND ART
In MRI, a separation imaging method is exemplified where chemical shift, i.e., deviation of the resonant frequency of the same type of nucleus due to difference in molecular structure of two components, is utilized, and the same tomography surface in a human body is indicated in separation into a proton image of water only and a proton image of fat only.
First, Dickson method in the prior art being one of the separation imaging method of water and fat will be described referring to FIGS. 4, 5A and 5B. FIG. 4 is a diagram representing pulse sequence of Dickson method in the prior art. In FIG. 4, t is time axis and the applying timing of 90.degree. pulse is made t=0. In the hereinafter description, the static magnetic field direction (being horizontal) is made Z axis, the vertical direction is made Y axis, and the horizontal direction (right-hand system) perpendicular to the Z axis is made X axis. RF designates a RF (Radio-Frequency) wave of Lamour frequency for rotating the magnetization vector of proton directed in the Z-axis direction by prescribed angle in the direction perpendicular thereto, which is called 90.degree. pulse or 180.degree. pulse depending on the rotational angle. SE designates a spin echo signal observed in that 90.degree. pulse in t=0 is applied and then phase of the magnetization vector dispersed in the XY plane due to unevenness of the static magnetic field is inverted by 180.degree. pulse and converged again. Time from 90.degree. pulse until obtaining the spin echo is made T.sub.E. In Dickson method, at first, S.sub.0 scan is performed in the applying timing of 180.degree. pulse mode t=T.sub.E /2 at intermediate time of T.sub.E after applying 90.degree. pulse. Next, S.sub.1 scan to apply 180.degree. pulse is performed at time t=T.sub.E /2-.epsilon. earlier than T.sub.E by .epsilon. after 90.degree. pulse. The time obtaining the spin echo is t=T.sub.E in both S.sub.0 scan and S.sub.1 scan.
In this case, above-mentioned satisfies following formula. fat with respect to water, i.e., 1/4 of period 1/(.sigma..multidot.f) of chemical shift. Difference of the resonant frequency of proton in water and fat, i.e., the chemical shift .sigma. is about 3.5 ppm. Consequently, when the static magnetic field intensity is 0.5 T (Lamour frequency f=21.3 MHz in .sup.1 H), .epsilon. is about 3.5 msec.
FIGS. 5A and 5B are diagrams each representing the phase relation of the magnetization vectors of water and fat in S.sub.0 scan and S.sub.1 scan. In FIGS. 5A and 5B, W designates magnetization vector of water and F designates magnetization vector of fat, and X'-Y' coordinates are system of coordinates rotating about the Z axis at rotational speed of magnetization vector of water. In S.sub.0 scan of FIG. 5A, at t=0, i.e., at application of 90.degree. pulse, phases of both vectors are coincident. At t=T.sub.E /2, phase deviation .psi. (-.pi.<.psi..ltoreq..pi.) of magnetization vectors of water and fat is produced due to chemical shift. Phases of both vectors are inverted about Y' axis by 180.degree. pulse from the Y'-axis direction. And then the magnetization vector of fat is rotated by the same amount as that before application of 180.degree.. pulse, and is coincident to the magnetization vector of water in phase at t=T.sub.E. In S.sub.1 scan of FIG. 5B, 180.degree. pulse is applied at t=T.sub.E /2-.epsilon.. This is earlier than the S.sub.0 scan by .epsilon. in time and by .pi./2 in angle of phase deviation. Time from the phase inversion by 180.degree. pulse until obtaining the spin echo is longer than time from 90.degree. pulse to 180.degree. pulse by 2.epsilon.. Since the magnetization vector of fat is rota
REFERENCES:
patent: 4628262 (1986-12-01), Maudsley
patent: 4950992 (1990-08-01), Nakabayashi et al.
patent: 4983918 (1991-01-01), Nakabayashi
patent: 5079505 (1992-01-01), Deimling et al.
Kojima Moonray
Tokar Michael J.
Yokogawa Medical Systems Limited
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