Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-03-13
2004-09-21
Fulton, Christopher W. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S314000, C324S307000
Reexamination Certificate
active
06794870
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a method of NMR (=nuclear magnetic resonance) tomography (=MRT) for generating NMR gradient echo signals according to the principle of signal generation in the driven equilibrium (DE) or also called steady state free precession (SFP) wherein a periodic sequence of radio frequency pulses with a flip angle &agr; is applied with a time delay TR, wherein the phase of these radio frequency pulses is alternated in subsequent steps.
A SFP signal is generated by a continuous sequence of radio frequency pulses and was introduced by Carr as early as 1958 (Phys. Rev. 112, 1693 (1958)). Carr was able to show that implementation of the method with equidistant radio frequency pulses with constant amplitude and alternating phase produces a particularly high signal intensity of the SFP signal of on-resonance spins.
In 1986, this principle was converted by a FISP method (in the meantime called true FISP) into a method of MR imaging (A. Oppelt et al. electromedica 54, 15 (1986)). All gradients are switched such that their integral from the center of a pulse to the center of the subsequent pulse is zero. Subsequent pulses have flip angles &agr; and alternating phases: P
1
, P
3
, P
5
. . . =&agr;, P
2
, P
4
, P
6
. . . =−&agr;. The temporal separation between 2 pulses is called repetition time TR.
The problem with implementation is thereby the fact that the transition into the resulting signal steady state is effected only gradually within a time period determined by T
1
relaxation. Until this steady state has been reached, periodic signal fluctuations occur which produce strong image artefacts when using the sequence for MRT (see FIG.
2
).
Suppression of this initial signal fluctuation is achieved in that before the continuous sequence of radio frequency pulses, one single pulse with a flip angle &agr;/2 is applied with a time delay of TR/2 (Deimling, M. DE 44 27 497 A1). This suppresses the initial signal modulations and merely a monotonic signal change into the steady state takes place (FIG.
3
).
Suppression of signal modulation is explained on the basis of observation of the subsequent signals according to
FIG. 4
, wherein the radio frequency pulses are chosen to be applied each with a radio frequency field with a phase parallel to the y-axis of the transverse plane. The diagram of transverse magnetization Mx vs. Mz shows that the magnetization vector of the steady state magnetization Mss is tilted relative to the z-axis by an angle &agr;/2 such that Mss of subsequent radio frequency pulses is flipped between +−Mss. Initialization with &agr;/2 brings the original z-magnetization M0 to the correct tilting angle and the magnetization vector is transferred to Mss in subsequent radio frequency pulses corresponding to T
1
and T
2
relaxation wherein the signals (=absolute amount of the Mx-magnetization) decay monotonously towards Mss(x) and show no modulation.
This is true only for so-called on-resonance spins which experience no phase-change during TR. In MR tomography applications (=MRT) this condition is not met even for very small repetition times TR wherein TR is determined substantially by the switching speeds of the magnetic field gradients.
The magnetic field homogeneities dephase the spins by a phase angle of &Dgr;&phgr; between two excitations. With TR=4 ms, &Dgr;&phgr;=90° for an off-resonance frequency is e.g. &Dgr;&OHgr; of &Dgr;&phgr;/(TR* 360°)=66 Hz. This corresponds to an inhomogeneity of 1 ppm at a resonance frequency of 63 MHz at 1.5 tesla field strength. Such inhomogeneities cannot be avoided in applications on humans due to the occurring susceptibility effects.
FIG. 5
shows the signal development in a method optimised according to DE 44 27 497 A1 as a function of &Dgr;&phgr;. It can be clearly seen that spins with &Dgr;&OHgr; unequal 0 experience modulation over the first excitation periods.
The corresponding signal intensities for &Dgr;&phgr;=0°, 180° and 360° are shown in FIG.
6
. Transfer of the modulations into the steady state amplitude which is characteristic for &Dgr;&phgr; is very slow. These modulations produce image artefacts. The behavior differences of the spins which are characterized by &Dgr;&phgr;=0° and &Dgr;&phgr;=360° can be explained in that these spins are mutually dephased by 180° in the initial period TR/2 according to the method of DE 44 27 497 A1.
A further disadvantageous property of the method according to DE 44 27 497 A1 consists in that application of the small flip angle &agr;/2 for initialisation of the steady state sequence renders access to only a relatively small part of the originally present magnetization M0 corresponding to M0 sin &agr;/2).
In contrast thereto, it is the object of the present invention to further improve a method of the above-described type such that the above-discussed disadvantages can be eliminated.
SUMMARY OF THE INVENTION
In accordance with the invention, this object is achieved in an effective fashion, in that the periodic sequence of radio frequency pulses is preceded by a sequence of (n+1) radio frequency pulses with the following valid conditions:
a first excitation pulse with preferred flip angle &agr;
0
=90° precedes the subsequently equidistant sequence of radio frequency pulses at a preferred separation TR/2,
the flip angle &agr;
1
of the subsequent radio frequency pulse is larger than &agr; and equal or approximately equal to 180°,
the flip angle &agr;
i
of the i-th radio frequency pulse in the region i=2 . . . n is selected such that &agr;
i
is smaller or equal to &agr;
i-1
and larger or equal to &agr; and
the phases of these radio frequency pulses alternate.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail with reference to embodiments.
REFERENCES:
patent: 4973906 (1990-11-01), Bernstein
patent: 5347216 (1994-09-01), Foo
patent: 5541514 (1996-07-01), Heid et al.
patent: 5545992 (1996-08-01), Foo
patent: 6163153 (2000-12-01), Reiderman et al.
patent: 6339332 (2002-01-01), Deimling
patent: 44 27 497 (1996-02-01), None
patent: 196 30 758 (1997-02-01), None
patent: 198 36 612 (2000-02-01), None
patent: 199 31 292 (2001-02-01), None
patent: 2001029327 (2001-02-01), None
H.Y. Carr, “Steady-State Free Precession in Nuclear Magnetic Resonance”, Physical Review, Dec. 1, 1958, pp. 1693-1701, vol. 112, No. 5.
A. Oppelt et al., “FISP: Eine Neue Schnelle Puls-Sequenz fur Die Kernspintomographie”, Electromedia 54 (1986), Heft 1, pp. 15-18 Germany.
Hennig J. et al., “Optimization of Signal Behavior in the Transition to Driven Equilibrium in Steady-State Free Precession Sequences”, Magnetic Resonance in Medisine, Nov. 2002, Wiley, USA Bd. 48, No. 5, pp. 801-809, Not Prior Art, date no good.
J. Hennig et al., “Optimization of Steady State Free Precession Sequences by Continuous Transition Into Driven Equilibrium (Tide)” ISMRM Tenth Scientific Meeting, Honolulu, Hawaii, USA, May 16-24, 2002, p. 378, Not Prior Art, date no good.
D.G. Nishimura at al., “Analysis and Reduction of the Transient Response in SSFP Imaging” ISMRM Eighth Scientific Meeting, Denver, Colorado, USA, Apr. 1-7, 2000, p. 301.
Williams C F M et al., “Sources of Artifact and Systematic Error in Quantitative Snapshot Flash Imaging and Methods for Their Elimination” Magnetic Resonance in Medicine, Academic Press, Duluth, MN, US, Bd. 41, No. 1, 1999, pp. 63-71.
Fetzner Tiffany A.
Fulton Christopher W.
Hackler Walter A.
Universitätsklinikum Freiburg
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