Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
2001-08-31
2004-02-24
Gutierrez, Diego (Department: 2859)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
Reexamination Certificate
active
06696889
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a magnetic resonance apparatus for acquiring pieces of chemical and physical information in relation to various types of substances by making use of a magnetic resonance phenomenon, and in particular, to magnetic resonance spectroscopy or magnetic resonance spectroscopic imaging based on a technique known as multiple-quantum-coherence transfer.
2. Related Art
A magnetic resonance apparatus has been widely used in applications such as chemical analysis and medical diagnosis in order to acquire chemical and physical information about various substances. Some representative techniques include magnetic resonance imaging (MRI; hereafter referred to as “MRI”), magnetic resonance spectroscopy (MRS; hereafter referred to as “MRS”), and magnetic resonance spectroscopic imaging (MRSI; hereafter referred to as “MRSI”).
The MRI has been mainly used in the medical field, in which a distribution of water can be imaged based on information about relaxation time or others of magnetic spins present in an organism. Thus, contour information and/or functional information of an object to be examined can be obtained in a non-invasive manner. For this reason, MRI systems have become indispensable modalities for the clinical purpose.
On the other hand, an MRS system is able to provide magnetic resonance spectroscopy of a substance, while an MRSI system has the capability of providing a spectroscopic distribution. Both systems detect a magnetic resonance signal of
1
H,
13
C,
31
P or others of a metabolite, so that they provide non-invasively information about metabolism in an object to be examined.
For magnetic resonance spectroscopy and magnetic resonance spectroscopic imaging, a difference between magnetic environments of
1
H or others (, which results from a difference between molecular structures of metabolites), that is, a difference between chimerical shifts causes a slight difference between their resonance frequencies. Such frequency differences produce separated peaks of resonance frequency curves of metabolites shown along the frequency axis. For example, “
1
H MRS” for the brain provides the peaks of various metabolites including N-acetyl-aspartate (NAA), creatine (Cr), choline (Cho), &ggr;-aminobutyric acid (GABA). Because these metabolites are substances produced due to chemical changes, that is, changes in the metabolism in the brain, it is expected that detecting the peaks enables diagnosis of metabolic errors.
As representatives for practical data acquisition sequences for the foregoing MRS and MRSI, there have been known various techniques including a PRESS (point resolved spectroscopy) technique and a STEAM (stimulated echo acquisition mode) technique.
FIG. 1
shows a PRESS sequence used as the data acquisition sequence for MRS, while
FIG. 2
a STEAM sequence used as the data acquisition sequence for MRSI. In both the sequences, the spatial three axes are depicted by references i, j and k, in which the three axes can arbitrarily be assigned to the physical x-, y- and z-axes.
In these sequences, a pulse for suppressing a water signal, which is for example a CHESS pulse, is first applied, thus the water signal being saturated in a pseudo manner. Localized excitation pulses consisting of appropriately combined radio-frequency magnetic pulses (RF pulses) and gradient pulses are then sequentially applied in the three-axis directions. In response to those applications, echo signals arising from a three-dimensional localized region are acquired. (Of the localized excitation pulses, the radio-frequency magnetic pulses are called slice selection pulses and the gradient pulses are called as slice gradient pulses, respectively, when necessary.) The PRESS sequence enables acquisition of spin echo signals, whereas the STEAM sequence enables acquisition of stimulated echo signals. Reconstructing the acquired signals provides a frequency spectrum at the localized region.
The PRESS and STEAM techniques are preferable for detecting peaks of NAA, Cr, Cho and others in spectroscopy. For instance, as shown in
FIG. 3A
, there are several hydrogen nuclei
1
H in the NAA molecule.
1
H-MRS is normally directed to the detection of
1
H present in CH
3
, which is a target to be detected. Carbon nuclei present in the NAA are numbered as illustrated in
FIG. 3A
, so the above CH
3
belongs to NAA C6. A hydrogen nucleus
1
H coupled with the carbon nuclei C6 is referred to as an NAA-6. The NAA-6 has a peak at a
1
H chemical shift of 2.02 ppm, and the peak is observable by the PRESS sequence or STEAM sequence.
In contrast, the other hydrogen nuclei
1
H of NAA, i.e., NAA-2 and NAA-3 are unobservable, because the NAA-2 and NAA-3 are subjected to a homonuclear spin-spin coupling (called J
HH
coupling) between their nuclei
1
H. The magnitude of this spin-spin coupling is expressed by a spin-spin coupling constant J
HH
(normally expressed with the unit “Hz”). The NAA-2 is individually coupled with two hydrogen nuclei
1
H of the NAA-3. As a result, the NAA-2 shows four split peaks, thus reducing the intensity of a signal. However, in the case of the NAA-6, it is magnetically equivalent to three hydrogen nuclei
1
H and there is no nucleus
1
H around the NAA-6, thus having no J
HH
coupling. Hence the peak of a higher intensity is provided, thus being observable. As described, when the NAA in an organism is observed, it is enough to detect the NAA-6. The fact that the NAA-2 and NAA-3 are difficult to observe has not become a problem, so that the foregoing PRESS sequence and others can be used to observe the NAA.
On the other hand, in the &ggr;-aminobutyric acid (GABA) that plays a significant role as a nerve transmission substance in the suppression system in the human brain, all the hydrogen nuclei 1H are connected to each other through the homonuclear spin-spin coupling. The GABA is one of the metabolites that are difficult to observe under the PRESS or STEAM sequence. The GABA has hydrogen nuclei
1
H belonging to GABA-2, -3 and -4, as shown by the molecular formula in
FIG. 3B
, all of which are J
HH
-coupled to each other. An abundance of the GABA is no less than about 1 mM, which is equivalent to about a tenth of that of NAA or Cr. This is one reason that makes it difficult to observe the GABA. To overcome this difficulty, several methods of editing a GABA peak, that is, GABA observation that makes use of the homonuclear spin-spin coupling, have been proposed.
One method is a difference spectrum method based on an inverted GABA-3. (Refer to “D. L. Rothmanetal., Proc. Natl. Acad. Sci. USA, vol.90, pp.5662-5666, 1993.”) In the GABA, a chemical shift of GABA-2 is 2.30 ppm, that of GABA-3 is 1.91 ppm, and that of GABA-4 is 3.01 ppm, and J
HH
is 7.3 Hz. Hence, in the case of the static field is 1.5 T in strength, which can be obtained by ordinary used clinical MR systems, a frequency difference &Dgr;&ohgr; between the GABA-2 and GABA-3 is &Dgr;&ohgr;=24.9 Hz and &Dgr;&ohgr;/J
HH
=3.4. Between the GABA-3 and GABA-4, &Dgr;&ohgr;=70.2 Hz and &Dgr;&ohgr;/J
HH
=9.6. Accordingly, the GABA-2 is strongly coupled to the GABA-3, while the GABA-3 is coupled to the GABA-4 with an intervening force slightly weaker than that between the GABA-2 and GABA-3. Thus, the difference spectrum method makes use of the coupling between the GABA-3 and GABA-4 in order to observe the GABA-4.
Pulse sequences used for this difference spectrum method are exemplified in
FIGS. 4A and 4B
. A pulse sequence of 90°-180° pulses shown in
FIG. 4A
enables a spin echo signal to be acquired. An echo time TE in this acquisition is set to ½J
HH
, i.e., 68 ms. In addition, jump and return pulses are used as the 180° pulse. The jump and return pulses are composed of two 90° pulses so as to form complex pulses radiated to GABAs other than the GABA-3 by setting a center frequency between the 90° pulses to the GABA-3, that is, 1.91 ppm. Additionally, the jump and return pulses can be composed so as to function as almo
Gutierrez Diego
Kabushiki Kaisha Toshiba
Nixon & Vanderhye P.C.
Vargas Dixomara
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