The present invention relates to a Fourier transformation imaging method in which a nuclear magnetic resonance (NMR) tomographic image is formed so that an object of interest imaged is located at a central portion of a display screen or image area.
A gradient magnetic field is applied in a manner that the intensity of the magnetic field linearly varies from the minus region to the plus region in response to the space position shown in FIG. 12. The intensity H.sub.0 of the magnetic field is that of a static magnetic field. The minus and plus regions of the intensity of the magnetic field is separated at line H.sub.0. There is a point at which the intensity of the magnetic field does not vary, even though the gradient magnetic field is applied. The point is called a gradient magnetic field center. In a general apparatus for imaging magnetic resonance signals, the gradient magnetic field center is located at the center of a display screen. Accordingly, it is necessary that the object of interest be brought to the gradient magnetic field center, when the center of the object of interest is desired to be imaged at the center of the display screen. In the case where the center of the object of interest is apart from the gradient magnetic field center, the Fourier transformation of a measured resonance signal provides an image area, as defined by a framework 53 shown by solid lines in FIG. 1A or 1B, in which a part of the object 52 of interest outside of a field of view appears on the opposite side of the image area. This phenomenon is called a signal aliasing phenomenon which is caused in accordance with the Nyquist's sampling theorem.
For example, JP-A-60-207045 filed on Mar. 30, 1984 by Shimadzu Seisakusho, Ltd. has proposed a method in which a field of view is shifted in a frequency encoding direction, as shown in FIG. 2, by selecting a reference signal frequency used for detecting a resonant signal by homodyne-detection as a center frequency. The frequency encoding direction is represented by a line which goes through the center of the gradient magnetic field and the center of the object of interest.
The center frequency is a resonant frequency calculated from a distance between the center of the object of interest and the center of the gradient magnetic field and the intensity of the gradient magnetic field for frequency encoding. Since data involving an aliasing phenomenon is obtained if the detection is made with the conventional reference signal f.sub.0 and a Fourier transformation is made, the proposed method obtains data free of the aliasing phenomenon by making the detection with (f.sub.0 +.alpha.) being taken as a reference signal (see FIG. 3). This reference signal is produced by means of hardware. Accordingly, a frequency changeover mechanism for selecting the resonance frequency is newly required.
Also, JP-A-63-160641 filed on Dec. 24, 1986 by Hitachi, Ltd. has proposed a method in which a field of view is shifted in a frequency encoding direction or a phase encoding direction through data scroll which is one of image processing techniques. In this method, shifting with a pitch finer than the resolution in the frequency encoding direction or the phase encoding direction is impossible.
Further, JP-A-63-111848 filed on Oct. 31, 1986 by Toshiba, Ltd. (U.S. Pat. No. 4,772,850) has disclosed a technique in which after image data involving the aliasing of signal has been Fourier-transformed in each of a phase encoding direction and a frequency encoding direction, readout start addresses of the image data in the respective directions are controlled so that an object of interest imaged is displayed at the center of an image area. In this case, too, since the shifting is made after the Fourier transformation, finer shifting cannot be attained.