Method and Apparatus for Generation of High Frequency Pulses

(1) Field of the Invention

The present invention relates to a method and apparatus for generation of a high frequency pulse and for excitation of nuclear magnetic resonance of a sample and, in particular, relates to the above method and apparatus suitable for nuclear magnetic resonance imaging and nuclear magnetic resonance analysis.

(2) Prior Art

In the nuclear magnetic resonance imaging and the nuclear magnetic resonance analysis, a high frequency pulse is typically applied to the sample for excitation of nuclear magnetic resonance of the sample.

According to the well known technique, a high frequency or radio frequency wave (carrier wave) having a predetermined frequency is modulated with a modulation wave defined by a sinc function both ends of which are restricted in order to have a time width T and thereby while centering the above frequency, a high frequency pulse is generated which provides a frequency spectrum having a frequency band inversely proportional to the time width T.

However, since the time width of the modulation wave and the frequency band of the frequency spectrum are inversely proportional to each other, it was difficult to simultaneously satisfy both shortening of the time width of the modulation wave and hence the high frequency pulse width (which indicates radiation time on the sample with the high frequency pulse) and narrowing the frequency band.

An object of the present invention is to provide a method and apparatus for generation of a high frequency pulse and for excitation of nuclear magnetic resonance of a sample which are suitable for making the shortening of the high frequency pulse compatible with the narrowing of the frequency band.

According to the present invention, a plurality of modulated high frequency waves are generated which have different frequencies from each other, and the generated high frequency waves are so combined as to generate a high frequency pulse which provides a frequency spectrum having a predetermined band.

By obtaining the high frequency pulse through the combination of the plurality of the modulated high frequency waves having different frequencies from each other, the frequency band of the frequency spectrum provided by the respective high frequency waves can be made broader than the frequency band of the frequency spectrum provided by the obtained high frequency pulse, whereby the compatibility of the shortening of the high frequency pulse width with the narrowing of the frequency band is achieved.

The foregoing objects and features and other objects and features of the present invention will become apparent from the following explanation which is made with reference to the attached drawings.

FIG. 1 is a block diagram of one embodiment of a high frequency pulse generation circuit according to the present invention;

FIG. 2 and FIG. 3 are views each illustrating one example of a modulation wave used in the embodiment of FIG. 1;

FIG. 4, FIG. 5, FIG. 6 and FIG. 7 are views each illustrating one example of frequency spectrum provided by the high frequency pulse used in the embodiment in FIG. 1;

FIG. 8 is a block diagram of one embodiment of a nuclear magnetic resonance imaging device incorporating the embodiment of FIG. 1; and

FIG. 9 is a view illustrating one example of pulse sequence used in the embodiment of FIG. 8.

Referring to FIG. 1, a first radio frequency wave or high frequency wave (carrier wave) generated by a high frequency oscillator 1 is amplitude-modulated in a modulator 3 with a first modulation wave generated by a modulation wave generator 2. Second and third radio frequency waves or high frequency waves (carrier waves) generated by high frequency oscillators 4 and 5 are respectively amplitude-modulated in modulators 7 and 8 with a second modulation wave generated by a modulation wave generator 6 and the modulated second and third high frequency waves are added to each other by an adder 9. The output of the adder 9 is added to the output from the modulator 3 by an adder 10.

Sinc function waves as shown in FIG. 2 and FIG. 3 are used as the first and second modulation waves. Although the polarities of the sinc function waves shown in FIG. 2 and FIG. 3 are opposite to each other, the both ends of the both sinc function waves are cut so as to each include only a main lobe and two pairs of side lobes at its both ends within respective time intervals T.sub.1 and T.sub.2.

When the first high frequency wave is amplitude-modulated with the modulation wave of the since function as shown in FIG. 2, the first high frequency pulse having the time width T.sub.1 is obtained, and when the second and third high frequency waves are ampltude-modulated with the modulation wave of the sinc function as shown in FIG. 3, the second and third high frequency pulses having the time width T.sub.2 are obtained. Therefore, the second and the third high frequency pulses are added by the adder 9, and the output of the adder 9 is added to the first high frequency pulse by the adder 10, so that a high frequency pulse having a time width T.sub.2 obtained by synthesizing the first, second, and third high frequency pulses is outputted from the adder 10. Accordingly, the first, second, and third high frequency pulses are called component high frequency pulses with reference to the high frequency pulse of the output signal from the adder 10.

Assuming that the frequency of the first high frequency wave is fo and this high frequency wave is modulated with the sinc function wave shown in FIG. 2, the first high frequency pulse thus obtained provides a frequency spectrum of substantially rectangular shape having a frequency band of f.sub.B.sbsb.1 while centering fo as shown in FIG. 4. Likely, assuming that the frequency of the second and third high frequency waves are f.sub.1 and f.sub.2 and these high frequency waves are modulated with the sinc function wave shown in FIG. 3, the second and third high frequency pulses thus obtained provide respectively frequency spectra of substantially rectangular shape having a frequency band f.sub.B.sbsb.2 while centering f.sub.1 and f.sub.2 as shown in FIG. 5 and FIG. 6. Accordingly, the high frequency pulse obtained by synthesizing the first, second and third high frequency pulses also provides a frequency spectrum of substantially rectangular shape.

For example, assuming that T.sub.2 =2.5T.sub.1 ; f.sub.1 =f.sub.0 -3f.sub.B.sbsb.0 /2, f.sub.2 =f.sub.0 +3f.sub.B.sbsb.0 /2, f.sub.B.sbsb.1 =5f.sub.B.sbsb.0, and f.sub.B.sbsb.2 =2f.sub.B.sbsb.0, the synthesized pulse provides a frequency spectrum as shown in FIG. 7, that is, a frequency spectrum having the central frequency of f.sub.0 and the frequency band of f.sub.B.sbsb.0.

In contrast, when a high frequency pulse is simply obtained by modulating the high frequency wave having the frequency of f.sub.0 with, for example, a sinc function wave of FIG. 3 as in conventional art, the frequency band f.sub.B.sbsb.3 of the frequency spectrum obtained thereby having the center frequency of f.sub.0 is proportional to 1/T.sub.2, and, since T.sub.2 =2.5T.sub.1, and f.sub.B.sbsb.1 (=5f.sub.B.sbsb.0) which is proportional to 1/T.sub.1, f.sub.B.sbsb.3 become 2f.sub.B.sbsb.0.

From the above facts, it will be understood according to the above example of the present invention, the frequency spectrum having the frequency band f.sub.B.sbsb.0 is obtained in time T.sub.2, but with the conventional method, the frequency band of the frequency spectrum obtained in time of T.sub.2 is twice broader than the example of the present invention. In other words, in the conventional method, time of 2T.sub.2 is needed for obtaining the frequency spectrum having the frequency band f.sub.B.sbsb.0, while in the example of the present invention half of the time is needed, and accordingly, when selecting T.sub.1, T.sub.2, f.sub.B.sbsb.1, and f.sub.B.sbsb.2 properly, the shortening of the time as well as the narrowing of the frequency band may be achieved.

The number of high frequency waves to be combined is three in the embodiment as first, second and third high frequency waves. However the number is not restricted to the three and may be two. Further, the second and the third high frequency waves in the above embodiment may be modulated by sinc function waves of different time width. Further, though in the embodiment the polarity of the second modulation wave is opposite to that of the first modulation wave, this polarity inverting may be performed after the modulation, not at the stage of the modulation wave generation. Furthermore while maintaining the polarities as the same, and using a subtracter in place of the adder 10, the difference of both these input signals may be taken. Still further, a Gaussian wave or a rectangular wave other than the sinc function wave may be used as the modulation wave.

Referring to FIG. 8, a static magnetic field generation device 11 into which samples are inserted, may be either of a superconductivity type, an electromagnetic type and a permanent magnetic type. In the static magnetic field generation device 11 a gradient magnetic field coil 12 is disposed so as to surround a sample, and is connected to a magnetic field gradient control device 13 which controls the application of the gradient magnetic field in the x, y and z directions to the static magnetic field generated by the static magnetic field generation device 11. A high frequency pulse generation device 14 is the same as that shown in FIG. 1 and is connected to a transmitter-receiver coil 16 through an amplifier 15 so as to apply the high frequency pulse generated thereby to the sample. To the transmitter-receiver coil 16 also a receiver 17 is connected for receiving a nuclear magnetic resonance signal obtained from the sample.

A sequence control device 18 is connected to the magnetic field gradient control device 13, the high frequency pulse generation device 14, and the receiver 17, and controls the respective timings of generation of the high frequency pulses, of generation of the gradient of magnetic field in the x, y, and z directions, and of reception of the nuclear magnetic resonance signal. The sequence control device 18 also functions to have a data processing device 19 carried out an image reconstruction based on the nuclear magnetic resonance signal received at the receiver 17 and a display device 21 displayed an image through a console 20 which performs transmission and reception of information.

Referring to FIG. 9, a selective high frequency 90.degree. pulse is applied under the existence of a magnetic field gradient Gs in the Z axis direction for slice selection use. Thereby, a slice perpendicular to the Z axis is selected. Namely, the nuclear spins in the slice are nutated by 90.degree.. Thus the nutated spins are gradually dispersed.

Following above, a selective high frequency 180.degree. pulse is applied under the existence of the magnetic field gradient Gs, thereby the nuclear spins in the slice are inverted. The dispersed nuclear spins are gradually converged, and the nuclear magnetic resonance signal called a spin echo are generated from the whole of the slice.

Between the selective high frequency 90.degree. pulse and the selective high frequency 180.degree. pulse a magnetic field gradient Gp in the Y axis direction for phase encoding use is applied, and further, after the application of the selective high frequency 180.degree. pulse, a magnetic field gradient G.sub.R in the X direction for read-out use is applied, and during that time the spin echo signal is read.

The above step is repeated N times so as to generate N spin echo signals, wherein Gp is changed at every time so that its time integration changes at a predetermined rate.

For the respective spin echo signals, N samplings are applied, and the N spin echo signals each consisting of the respective signals sampled are subject to two dimensional Fourier transformation processing. Thereby, a two dimensional nuclear magnetic resonance signal image of the slice of the X-Y plane is obtained.

The magnetic field gradient G.sub.R for readout use is also applied between the both selective high frequency pulses. This application is made for compensating for dephase of the nuclear spins caused by the earlier half of the magnetic field gradient G.sub.R which is applied after the application of the selective high frequency 180.degree. pulse.

From the above explanation, it will be apparent that the present invention provides a method and apparatus for generation of high frequency pulses and for excitation of nuclear magnetic resonance of a sample which are suitable for shortening of high frequency pulse width as well as narrowing of frequency band.

While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.