Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-01-10
2002-04-30
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06380739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to medical magnetic resonance imaging (MRI), particularly, imaging to provide images of objects in motion, such as blood, using multi-echo sequences. Specifically, the present invention relates to imaging that uses an FSE (Fast Spin Echo) method or a FASE (Fast Asymmetric SE) method developed therefrom in order to not only image flows of such objects as blood so as to be depicted more steadily but also to depict a distribution of flow velocities with the help of a technique similar to a phase contrast method.
2. Description of Prior Art
Magnetic resonance imaging is based on a technique that magnetically excites nuclear spins of an object placed in a static magnetic field with an RF signal at a Larmor frequency thereof, acquires an MR signal emanated due to the excitation, and reconstructs an image on the basis of the MR signal.
In recent years, as one tomographic imaging technique that is frequently used in the field of MRI, an FSE method is known. The FSE method has a feature of being able to valid influences of the non-uniformity of a static field. Especially, a recent tendency is that echo train spacing (ETS) can be shortened thanks to the development of hardware techniques. Therefore, pulse sequences based on the FSE method that are shorter in echo train spacing or FSE-system pulse sequences developed therefrom are used to have depicted, without contrast mediums, targets in motion, such as blood, that were only tentatively imaged in the past. For example, papers concerning such study are shown by “M. Miyazaki et al., A novel MR angiography technique: SPEED acquisition using half Fourier RARE, JMRI 8: 505-507, 1998,” “D W Kaandorp, et al., Three-dimensional Flow Independent Angiography of Aortic Aneurysms using standard Fast Spin Echo, In “proceedings, ISMRM, 67
th
Annual Meeting” Sydney, Australia, p792, 1998,” “M. Miyazaki et al., Fresh Blood Imaging at 0.5-T: Natural Blood Contrast 3D MRA within Single Breathhold, In “Proceedings, ISMRM, 6
th
Annual Meeting” Sydney, Australia, p780, 1998,” and “Y. Kassai et al., 3D Half-Fourier RARE with MTC for Cardiac Imaging, In “Proceedings, ISMRM, 6
th
Annual Meeting” Sydney, Australia, p806, 1998.”
These imaging techniques adopt an ECG gating method by which a delay time from the R-wave is appropriately determined at a cardiac temporal phase representing slower velocities of blood flows passing vessels to be imaged, or a phase-encoding direction, which provides a higher depiction, is adjusted to agree with a conventional blood flow direction of interest.
As another approach for conventional blood flow imaging, a nulling technique for gradient moments has been known to suppress consequences of flows thereof.
However, the foregoing imaging methods are not enough for stable blood flow detection. For example, although the depiction is higher in blood flows along the phase encoding direction, it is reported that blood flows in the readout direction are unable to be depicted. Also reported are artifacts, referred to as “N/2 artifacts,” due to the oscillation of signals between the even and odd echoes belonging to multi-echoes. Furthermore, because excessive times for switching gradients are required, the nulling method of gradients is practiced only in cases where flows pass at slower velocities, resulting in a limited versatility.
SUMMARY OF THE INVENTION
The present invention has been performed in consideration of the drawbacks faced by the foregoing conventional MR imaging. One object of the present invention is to use pulse sequences for multi-echoes including FSE-system pulse trains so as to steadily depict objects in motion, such as blood flow, with no contrast medium, providing images of higher reliability in clinics.
Another object is to make it possible to image objects whose flow velocities range is wide regardless of magnitudes of flow velocities.
In order to realize the above objects, the present invention employs an imaging principle and a construction based on principles, which are as follows. In the following explanations, according to necessities, an exciting RF pulse is merely referred to as a “flip pulse” and a refocusing RF pulse as a “flop pulse.”
First, the imaging principle will be explained by comparison with conventional techniques.
In the FSE method, when an interval between flip and flop pulses is &tgr; (i.e., echo train spacing: ETS), it is essential that the first flip-flop interval &tgr;′ be precisely set to be &tgr;′=t/2 and an amount of a gradient pulse applied between flip and flop pulses be precisely half an amount A of gradient pulses to be applied thereafter. Therefore, in cases where a pulse sequence is designed based on the conventional FSE method, as a pulse train to be applied after the flip pulse, the fundamental pulse train of which &tgr;′ and A′ are sufficiently adjusted to &tgr;′=&tgr;/2 and A′=A/2 is repeated.
One example of the FSE-based pulse sequence thus-designed is shown in FIG.
1
(
b
) with its phase diagram shown in FIG.
1
(
a
). This pulse sequence represents only a flip pulse, a plurality of flop pulses and a readout gradient Gr, while slice and phase-encode gradients Gs and Ge are omitted from the drawing.
In the phase diagram shown in FIG.
1
(
a
), its longitudinal axis shows degrees of dephase of magnetic spins and its transverse axis shows time t. Additionally, solid lines extending in oblique directions show the states of transverse magnetization (transverse paths) in which the dephases advance. The transverse dotted lines show the states of longitudinal magnetization (longitudinal paths) which preserve dephase states as the longitudinal magnetization. For the FSE-based pulse sequence that satisfies the foregoing temporal condition &tgr;′=&tgr;/2 and area condition A′=A2, the states of the transverse and longitudinal magnetization are expressed in a neat condition as shown in FIG.
1
(
a
), resulting in MR images with no artifacts.
Conventionally, for imaging an object in motion using the FSE method, an imaging technique for practicing a flow compensation (FC) method or gradient moment nulling (GMN) method with the foregoing temporal and area conditions satisfied is proposed by “RS. Hinks et al., Gradient Moment Nulling in Fast Spin Echo, MRM 32: 698-706, 1994.”
However, in this FSE method cooperatively using the FC and GMN methods, additional gradient pulses are required for the ordinal FSE-basis pulses, resulting in, for example, a prolonged interval between the flip and flop pulses (i.e., the echo train spacing: ETS). Namely, as the time necessary for practicing the pulse sequence per excitation, data acquisition efficiency is degraded, prolonging the entire imaging time. Due to this drawback, it will be difficult to steadily catch an object to be depicted, even when the object, such as blood, flows faster.
Moreover, as described above, the echo train spacing ETS is shortened to, for example, 5 [msec] in order to try to obtain images of blood flow or the heart in motion. In this pulse sequence of which ETS is shortened, there is no room for cooperatively using the FC and GMN methods, because the gradient pulses and RF pulses are tightly arranged along the time axis. If this pulse sequence of which ETS is shortened is used, a higher depiction ability of blood flows along the phase encoding direction is obtained, as described before. However, the depiction ability along the readout direction is poor and “N/2 artifacts” occur because of the oscillation of signals on the even and odd echoes.
The behaviors of signals, which include “N/2 artifacts,” obtained on a conventional FSE-basis pulse sequence is now explained with reference to FIG.
2
. This example of the signal behaviors, which was simulated with an FSE-basis pulse sequence whose ETS=5 [msec], flop angle FA=150 degrees, and imaging region=35 [cm], show a flow velocity dependency when assuming
Kabushiki Kaisha Toshiba
Nixon & Vanderhye P.C.
Shrivastav Brij B.
Williams Hezron
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