Method of selecting specific region of sample

Details Method of selecting specific region of sample Method of selecting specific region of sample

1987-05-26

1989-03-21

Noland, Tom

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

G01R 3320

Patent

active

048147095

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. TECHNICAL FIELD
This invention relates to a method of selecting a specific region of a sample, and more particularly to a method of selecting a specific region of a sample which is suitable for NMR (nuclear magnetic resonance) imaging.
2. BACKGROUND ART
When a sample having atomic nuclei whose spin quantitization number is not zero is placed in a static magnetic field, the atomic nuclei exhibit a macro behaviour as an aggregate and can be regarded as a magnetization vector having magnetization in parallel with the direction of the static magnetic field and angular momentum. The relation between a magnetic moment .mu. representing the magnitude of magnetization and the angular momentum J is expressed as follows with .gamma. representing a gyromagnetic ratio:
The equation of motion can be expressed as follows with H.sub.0 representing the intensity of the magnetic field: ##EQU1##
When eq. (1) is differentiated and put into eq. (2), there is obtained the following equation: ##EQU2## Eq. (3) can be expressed as follows by a coordinate system rotating at an angular velocity .omega.: ##EQU3## The frequency .omega. at which H.sub.e =0 is referred to as a "resonance frequency .omega..sub.0 " (angular velocity expression) and is represented by the following equation:
If the direction of the static magnetic field H.sub.0 is called "Z" and directions orthogonal thereto are "X, Y" and a magnetic field H.sub.1 is applied from the X direction (this means a radio frequency magnetic field rotating at .omega..sub.0), the magnetization vector rotates on the Z - Y plane at an angular velocity expressed by .gamma..multidot.H.sub.1. A magnetic field H.sub.1 which is applied until the magnetization vector is 90.degree. relative to the Z axis (falls on the X - Y plane) is referred to as "90.degree. pulse", and "180.degree. pulse" is defined similarly. The component of the X - Y plane of the magnetization vector is induced as a signal in a coil disposed on the X - Y plane. The description given above explains briefly the principle of nuclear magnetic resonance.
One of the conventional methods which selectively obtains a signal from a specific region of a sample on the basis of the principle described above applies a radio frequency magnetic field having frequency components f.sub.1 (=.gamma.h.sub.1).about.f.sub.2 (=.gamma.h.sub.2) (where h.sub.1 and h.sub.2 represent the intensity of magnetic field in specific regions x.sub.1 and x.sub.2 of a sample in the x direction, respectively) to the sample in the presence of a gradient magnetic field in the x direction of the sample, and then nutates or tips by 90.degree. the magnetization vector in that region in order to selectively excite the magnetization vectors of the specific region x.sub.1 .about.x.sub.2 of the sample.
This method is excellent to selectively obtain a signal in a uni-dimensional direction. However, since the magnetization vector is brought down by 90.degree. for one selection, this method cannot be applied a plurality of times in order to make selections a plurality of times and to obtain spatial information on regions of a plurality of dimensions. In other words, in accordance with this method, selection for obtaining the spatial information of two or more dimensions cannot be made.
Another conventional method picks up selectively only the frequency components f.sub.1 .about.f.sub.2 corresponding to the specific region by bringing down by 90.degree. the magnetization vectors for a wide range including the specific region x.sub.1 .about.x.sub.2, then applying the gradient magnetic field and controlling the frequency band of the resulting signal. This method is a so-called "frequency filter system" and includes a system by use of an analog filter and a system using a digital filter in a narrow sense of the word. The analog filter has the disadvantage that the frequency band cannot be changed arbitrarily. Furthermore, its expansion to the spatial selection of two or more dimensions is not possible. The digital filter includes a digital filter in a narr

REFERENCES:
patent: 4563647 (1986-01-01), Young
patent: 4712066 (1987-12-01), Luyten et al.

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