Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1996-09-30
2000-03-07
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01V 300
Patent
active
060345284
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method in the form of a pulse sequence for operating a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
A pulse sequence is known from Journal of Magnetic Resonance, Series B 101 (1993), February, No. 1, pp. 106-109 wherein, during a first phase, a sequence of hard radio-frequency pulses is emitted and a gradient in a first direction is simultaneously activated and, during a third phase, the gradient in the first direction is inverted and the nuclear magnetic resonance signals arising in a sequence of sampling intervals are sampled under the influence of the inverse gradient. The first and third phases are repeated n times in order to obtain a complete image.
Short image exposure times are thereby of special significance. This is true not only in view of an optimally high patient throughput but, for example, in order to avoid motion artifacts. Short image exposure times are unavoidable for certain exposure techniques, for example cine mode (registering moving images).
Fast pulse sequences have special significance, for example, for determining the spread of contrast agents, for observing heart motion, for the brain function and for the kinematics of joints since a plurality of images must be registered in fast succession.
Of the methods that have been hitherto disclosed, the shortest image exposure times (40 through 100 ms) can be achieved with the EPI method. The EPI method is disclosed, for example, by European Letters Patent 0 076 054.
At the beginning of the pulse sequence, an RF excitation pulse is thereby beamed onto an examination subject under the influence of a slice selection gradient in a first direction. Nuclear spins are thereby excited in a slice of the examination subject. After the excitation, a phase-encoding gradient is activated in a second direction and a readout gradient is activated in a third direction. First, second and third direction reside perpendicular to one another. The readout gradient is composed of a pre-phasing pulse as well as of sub-pulses of alternating polarity. Due to this alternating polarity of the readout gradient, the nuclear spins are dephased and rephased in alternation. After a single excitation, so many signals are thereby acquired that the entire k-space is scanned, i.e. that the existing information suffice for the reconstruction of a complete tomogram.
The phase-encoding gradient is briefly activated with every change in the polarity of the readout gradient. The phase position of the nuclear spins is thus advanced by one step each time. The arising nuclear magnetic resonance signals are sampled in the time domain, digitized, and the numerical values acquired in this way are entered into a raw data matrix. An image of the examination subject is then reconstructed from this raw data matrix on the basis of a two-dimensional Fourier transformation. The speed advantage of the EPI method is essentially based thereon that a plurality of signals are acquired after a single excitation, these signals sufficing for the reconstruction of a complete tomogram. All signals that ultimately represent gradient echoes must be acquired within the T2* decay. The readout gradient must therefore be very rapidly bipolarly switched, so that considerably technological demands are made of the system.
Further, gradient echoes as generated in the EPI method have the disadvantage compared to spin echoes that they are sensitive to local field inhomogeneities.
U.S. Pat. No. 4,818,940 discloses a pulse sequence wherein a plurality of spin echoes are acquired due to a plurality of successive 180.degree. radiofrequency pulses after a 90.degree. radiofrequency pulse. The exposure times in this method, however, are longer than in the EPI method and the permitted radiofrequency stress on the patient is soon reached given fast repetition.
U.S. Pat. No. 5,126,673 discloses a pulse sequence wherein a sequence of many equidistant radiofrequency pulses, what is referred to as
REFERENCES:
patent: 4818940 (1989-04-01), Hennig et al.
patent: 5125407 (1992-06-01), Harms et al.
patent: 5126673 (1992-06-01), Hennig
patent: 5212448 (1993-05-01), LeRoux et al.
patent: 5229717 (1993-07-01), Hinks
patent: 5432448 (1995-07-01), Heid
patent: 5545990 (1996-08-01), Keifer et al.
patent: 5952827 (1999-09-01), Feinberg
"Burst Imaging," Hennig et al., MAGMA, vol. 1, No. 1, Mar. 1993, pp. 39-48.
"Quest--A Quick Echo Split NMR Imaging Technique," Heid et al., Magnetic Resonance in Medicine, vol. 29, No. 2, Feb. 1, 1993, pp. 280-283.
"Ultra-Rapid Gradient Echo Imaging," Magnetic Resonance in Medicine, vol. 33, No. 1, Jan. 1, 1995, pp. 143-149.
"Quest--A Quick Echo Split Imaging Technique," Heid et al., Book of Abstracts, p. 433, Annual Meeting of the Society of Magnetic Resonance in Medicine, 1992.
"Elimination of Transverse Coherences in FLASH MRI," Crawley et al., Magnetic Resonance in Medicine, vol. 8, (1988), pp. 248-260.
"Dante Ultrafast Imaging Sequence (DUFIS)," J. Mag. Res., Series B 101 (1993), pp. 106-109.
"parameter Relations for the Shinnar-Le Roux Selective Excitation Pulse Design Algorithm," Pauly et al., IEEE Trans. on Med. Imaging, vol. 10, No. 1, Mar. 1991, pp. 53-65.
Arana Louis
Siemens Aktiengesellschaft
LandOfFree
Method for operating a nuclear magnetic resonance tomography app does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for operating a nuclear magnetic resonance tomography app, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for operating a nuclear magnetic resonance tomography app will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-366485