1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus using a two- or three-dimensional Fourier transform.
2. Description of the Prior Art
When an atomic nucleus (a hydrogen atom or the like) is subjected to a uniform magnetic field, the spin of the nucleus shows precession in the magnetostatic field. If radio-frequency pulses (RF pulses) having the same frequency as that of the precession are applied to a subject in this state, magnetic resonance phenomenon occurs. The magnetic resonance imaging apparatus utilizes this phenomenon to acquire the images of cross-sections or the like of a subject. If a gradient magnetic field whose intensity varies along the specified direction is superimposed on the above mentioned magnetostatic field, each atom in a subject resonates or relaxes at different frequencies according to the intensity of the magnetic field. The frequency analysis of the signals thus acquired is performed to obtain the required images by using a Fourier transform.
The following examples are known techniques about the above-mentioned Fourier transform method.
(1) Fourier Transform Zeugmatography (original method)
A. Kumar, I. Welti, and R. R. Ernst: "NMR Fourier Zeugmatography" J. Magn, Reson, 18, p. 69 (1975).
(2) Spin Warp Method
W. A. Edelstein, J. M. S. Hutchison, G. Johnson, and T. W. Redpath: "NMR Imaging and Applications to Human Whole-Body Imaging" Phys. Med. Biol. 25, p. 751 (1980).
This method uses a field echo method to generate an echo, but this method is a mathematical equivalent of the method (1).
(3) Improved Spin Warp Method (an example thereof is shown in FIG. 5)
This method uses 180.degree. RF pulses to generate an echo, but the other principles are identical to the methods (1) and (2).
FIG. 1 is a block diagram showing a conventional magnetic resonance imaging apparatus.
A subject 1 is placed in a magnetosatic field coil 2 which generates a uniform magnetostatic field. A computer system 11 comprises a pulse sequencer 13, a CPU 14, a hard disk 15, a console 16, a digital input interface 17, an array processor 18, etc. These devices are combined with one another by a system bus 12.
When an imaging command is fed to the system from the console 16, the CPU 14 starts the pulse sequencer 13. The pulse sequencer 13 controls gradient magnetic field power sources Gx 5, Gy 6, and Gz 7, and an RF transmitting system 8. A gradient magnetic field coil 3 and an RF coil 4 generate a prescribed gradient magnetic field and a radio-frequency magnetic field, respectively. Thus, NMR (nuclear magnetic resonance) signals are generated from the subject 1. These NMR signals are received by the RF coil 4, are detected and amplified by an RF receiving system 9, and are converted into digital signals by an A/D converter 10. The digital signals are read into the computer system 11 through the digital input interface 17. The read data (raw data) are temporarily accumulated in the hard disk 15. When all the data have been acquired, these data are sent to the array processor 18, where an image reconstruction operation is performed. The operation result is displayed on the console 16 as an image.
FIG. 2 shows a typical example of pulse sequences used by the two-dimensional Fourier transform method.
A pulse sequence (a) is that of an RF magnetic field, and pulse sequences (b), (c) and (d) are those of the outputs of the gradient magnetic field Gz, Gx, and Gy, respectively. The reference numeral 201 designates 90.degree. RF pulses, 202 denotes a gradient magnetic field for selecting a slice, 203 designates a gradient magnetic field for frequency encoding, and 204 denotes a gradient magnetic field for phase encoding. To acquire an image of an L.times.M matrix, NMR signal 205 is sampled at L sampling points by the A/D converter 10 (see reference numeral 206). Pulse sequences (e) and (f) in FIG. 2 show NMR signals, and the operation of the A/D converter 10, respectively. This combination of sequences 201 to 206 is repeated a total of M times at the repeated intervals of T.sub.R, with varying phase encoding as indicated by the reference numeral 204. Finally, two-dimensional image data are acquired by conducting an image reconstruction operation including a two-dimensional Fourier transform to the L.times.M point data thus obtained.
FIG. 3 shows common pulse sequences in a three-dimensional Fourier transform method.
As in FIG. 2, a pulse sequence (a) in FIG. 3 shows that of an RF magnetic field. Pulse sequences (b), (c), and (d) in FIG. 3 show the outputs of the gradient magnetic fields Gz, Gx, and Gy, respectively. Pulse sequences (e) and (f) in FIG. 3 show NMR signal and the operation of the A/D converter 10, respectively. A reference numeral 302 designates a gradient magnetic field for frequency encoding, and reference numerals 303 and 304 denote gradient magnetic fields along the X and Y axis for phase encoding. To acquire an image associated with an L.times.M.times.N matrix, the NMR signal 305 is sampled at L sampling points by the A/D converter 10 as indicated by reference numeral 306. Then, this combination of sequences 301 to 306 is repeated a total of M times at the repeating interval of time T.sub.R with varying phase encoding in the direction of the Y axis as indicated by the reference numeral 304. This M time repetition is repeated a total of N times with varying phase encoding in the direction of the Z axis as shown by the reference numeral 302. By achieving image reconstruction operation including three-dimensional Fourier transform to the L.times.M.times.N sampling points thus obtained, three-dimensional image data can be acquired.
The magnetic resonance imaging apparatus described above carries out the image data reconstruction operation after all the data about the subject has been collected. This poses a problem that too much time is taken to generate an image: the total time required for testing a subject is the sum of the data acquisition time and the image reconstruction time, which is very time-consuming.