Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-01-10
2003-09-23
Arana, Louis (Department: 2855)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S300000
Reexamination Certificate
active
06624632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques in the field of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). The present invention also relates to techniques for measuring the spatial distribution of the electrical properties of substances such as electrolyte solutions, the tissues of a living body and human tissues by the use of MRI or MRS. The present invention relates also to techniques for measuring the spatial distributions of the currents within these substances.
2. Description of the Prior Art
Magnetic resonance signals are high-frequency signals, typically on the order of microvolts, which have weak frequencies produced by the precession of atomic nuclei (spins) in a static magnetic field. The frequency of the precession is determined by the magnetic field strength and the type of nucleus in question. The spins are aligned by the homogenous static magnetic field, and are excited by the application of an RF field, with the resulting magnetic resonance signals being detected as a voltage with a resonant coil (antenna).
An MR image is abbreviated MRI, and the method for acquiring it is called MR imaging which is abbreviated MRI. Further, MR spectral curve is referred to as an MR spectrum which is abbreviated MRS, and the acquisition of it or the method for acquiring it is called MR spectroscopy, which is abbreviated MRS.
Since R. Damadian found in the early 1970s that the spin-lattice relaxation time T
1
and the spin-spin relaxation time T
2
vary with tissues significantly, and tumorous tissues have extremely longer relaxation times than normal tissues (R. Damadian, “Tissue detection by nuclear magnetic resonance.” Science vol. 171: pp. 1151-1153, published in 1971), the relaxation times T
1
and T
2
have been recognized as very important parameters in developing and designing magnetic resonance imaging systems and obtaining and evaluating magnetic resonance images and spectra.
The relaxation time T
1
is the time constant required for the spins excited in a static magnetic field to return to their initial state in which they can be excited again. Accordingly, if T
1
of the tissues of a living body or the like (examination subject) is particularly long, then a correspondingly longer time is needed for obtaining the magnetic resonance signals by repeating the excitation, returning the spins to the initial state, and for obtaining MRI or MRS by performing calculations such as two-dimensional Fourier transform or one-dimensional Fourier transform with a computer. In case of a clinical MRI apparatus, the patient, who is not allowed to move during the image pickup, is more burdened. Further, the number of patients who can be imaged over a given time is decreased. Accordingly, it is generally considered better for T
1
to be shorter.
The relaxation time T
2
is the time constant from a time when many spins are excited in resonance with the RF field (i.e., the field having the same frequency as the frequency of the precession relative to the magnetic flux density of the static magnetic field around the spins), so that the phases of the precessions are uniform and can be detected macroscopically as an induced electromotive force by an external resonance coil, to when the phases become irregular and non-uniform, so that the spins cannot detected. Accordingly, in order to obtain the signals, it is better in many cases for T
2
to be long, but there are also many cases where, even if T
2
is short, it suffices if the signals are obtained, by an appropriate signal obtaining technique, for instance immediately after the excitation thereof.
Typically, T
1
of the gray matter of the brain of the human body is 1.0 when it is measured in a magnetic field of 1T. Further, typically, T
2
of the gray matter of the human brain is 0.1. It has been conventionally believed that there is no method or means for changing these relaxation times T
1
and T
2
in a given static magnetic field (of 1.0 T, 1.5 T or the like) and at a given temperature (substantially 37° C. of the human body) unless some chemical substance or the like is introduced into the human body.
More specifically, it is known that the ions of a magnetic material such as a transition metal and lanthanide ions have unpaired electronic spins which have magnetic moments several hundreds of times as large as protons, and thus have strong relaxation effects. As an application example of such substances, the injection of a gadolinium compound, which is a paramagnetic material, into the circulatory system of an examination subject is widely practiced in the field of clinical MRI. If a gadolinium compound is introduced into the tissue of a living body, it has a relatively larger shortening effect on T
1
, which is originally long, than on T
2
which is originally short.
In other words, if the gadolinium compound is introduced into a vein, then it is absorbed into the blood or the brain tissue or the like if the cerebral blood vessel barrier has been destroyed by a cerebral infarction or the like. This selectively shortens the T
1
of the tissue, so that the site of disease or the like can be selectively imaged or depicted in a T
1
-weighted image (that is, an image which is generated, by obtaining successive sets of magnetic resonance signals by repeating the excitation after each return of the spins to the initial state.) In such an image a substance which has a short T
1
and is therefore apt to return to the initial state in which, even if previously excited, it can be excited again, produces a higher amplitude signal and thus appears brighter in the image.
A T
2
-weighted image is generated from a signal that is not obtained immediately after the excitation, but is obtained as a dark signal after waiting for a substance with a shorter T
2
to become irregular and non-uniform in phase by the T
2
relaxation and become undetectable.
Further, in the middle 1960's, E. O. Stejskal and J. E. Tanner developed a diffusion measurement method by nuclear magnetic resonance that uses a motion probing gradient (MPG) pulses. [E. O. Stejskal and J. E. Tanner, “Spin diffusion measurements: spin-echoes in the presence of time-dependent field gradient.” J. Chem. Phys. Vol. 42: pp. 288-292, published in 1965].
This is a method of measuring the magnitude of the movement of spins as a diffusion coefficient by utilizing the fact that, as long as the spins perform a precession at a stationary position, no influence is exerted even if two gradient magnetic fields which are identical in magnitude but opposite in direction are successively applied as MPG pulses, but, if the spins are moved by the diffusion, then the phases are made irregular and non-uniform eventually by the application of the MPG pulses. The MPG pulses may be applied by making them identical with each other in magnitude and direction and putting 180° RF pulses between them.
Further, D. Le Bihan, etc. introduced MRI techniques that incorporate MPG pulses into imaging sequences of MRI in mid-1980s. [D. Le Bihan, E. Breton, D. Lallemand, P. Granier, E. Cabanis and M. Laval-Jeantet, “MR imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurological disorders.” Radiology Vol. 161: pp. 401-407, published in 1986].
Since then, the diffusion-weighted MRI techniques have been widely used as a very important imaging methods because, for important lesions like acute cerebral infarctions which cannot be depicted, unless two or three days lapse after the beginning of the disease, by T
1
-weighted imaging or T
2
-weighted imaging. Using diffusion-weighted imaging, these important lesions are imaged 20 to 30 minutes after the development of the disease.
The diffusion of certain molecules in the same substance, such as water molecules diffusing in water, is called self-diffusion. Accordingly, the diffusion coefficient of a substance itself refers to the self-diffusion coefficient. Self-diffusion is originally isotropic. However, among the movements of spins in living
Iriguchi Norio
Sekino Masaki
Ueno Shoogo
Yamaguchi Kikuo
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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