JPH0444740A - Mri device - Google Patents

Abstract

PURPOSE:To obtain an excellent image by shifting the frequency of input NMR after excitation with an exciting pulse of a frequency shifted in the generating state of offset magnetic field, and adding it to the input NMR signal after excitation with the exciting pulse of the frequency not shifted. CONSTITUTION:When an offset magnetic field is given to shift a resonance frequency by its magnetic field strength, and the frequency of an exciting pulse is shifted by its shift amount followed by exciting, the same slice surface as in case of general photographing can be selectively excited without giving no offset magnetic field, and the frequency of NMR signal is shifted by the above frequency amount. An external fixed frequency noise is admixed in the input signal as its frequency noise. When the frequency of the input signal at the time of giving the offset magnetic field is shifted by the shift amount, the frequency of the contained NMR signal is conformed with the frequency of NMR signal contained in an input signal which is obtained by general photo-graphing, but the noise of the above frequency is shifted in frequency by the shift amount. Thus, when the input signal of general photographing and the input signal at the time of applying the offset magnetic field in which the frequency is shifted are averaged and added, the NMR signal is added, but the noise is not added, resulting in an enhancement in S/N ratio.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an MRI apparatus that obtains images using nuclear magnetic resonance (NMR).

[Conventional technology]

Conventionally, in MHI equipment, the signal-to-noise ratio (S/N
The average addition method is adopted to increase the ratio). This is based on the fact that if you add the NMR signals that have been excited and received twice for the same object, the signal will double, but the white noise will only increase by 5 times, so the S/N ratio will increase by 57 times. This is what I did.

[Problem to be solved by the invention]

However, it is said that simply adding averages as in the past cannot reduce the constant frequency noise from amateur radio and external equipment that enters from outside the RF shield or gets on the coaxial cable. There's a problem. In view of the above, an object of the present invention is to provide an improved MRI apparatus that can effectively reduce noise at a constant frequency that is not white noise.

[Means to solve the problem]

In order to achieve the above object, the MRI apparatus according to the present invention includes means for generating a uniform offset magnetic field;
means for shifting the frequency of the excitation pulse in response to the offset magnetic field strength; and a means for shifting the frequency of the excitation pulse in response to the offset magnetic field strength; It is characterized by having means for shifting the frequency of the signal by the above-mentioned deviation, exciting it with an excitation pulse of the unshifted frequency without generating the offset magnetic field, and then adding it to the received NMR signal. There is.

[For production]

By applying an offset magnetic field, the resonance frequency can be shifted by the strength of the offset magnetic field. Therefore, if the frequency of the excitation pulse is shifted by the difference of the resonance frequency for excitation, the same slice plane can be selectively excited as in the case of normal imaging without applying an offset magnetic field and without shifting the frequency of the excitation pulse. At this time, the frequency of the NMR signal is shifted by the above frequency. In both imaging with an offset magnetic field applied and normal imaging, external noise at a certain frequency is mixed into the received signal as noise at that frequency. Therefore, if the frequency of the received signal when the above offset magnetic field is applied is shifted by the above deviation, the frequency of the NMR signal contained therein will be included in the received signal obtained by normal imaging. The frequency will be the same as that of the NMR signal that was previously used, but the frequency of the noise at the above frequency will be shifted by the amount of the shift. As a result, if we take the average addition of the received signal from normal imaging and the frequency-shifted received signal when a magnetic field is applied, we find that although the NMR signals are added, the noise at the constant frequency is Since the signals are scattered and not added, the S/N ratio increases.

【Example】

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an MRI apparatus according to an embodiment of the present invention, in which a subject is placed in a static magnetic field space generated by a static magnetic field magnet 21. In FIG. A gradient magnetic field coil 22 and a transmitting/receiving coil 23 are arranged in this space, and an offset magnetic field coil 24 is also arranged. A current is supplied to the gradient magnetic field coil 22 from a gradient magnetic field power supply 32, and a gradient magnetic field for slice selection, a gradient magnetic field for reading (frequency coding), and a gradient magnetic field for phase coding are generated. Signals from the waveform generator circuit 3 are sent to a magnetic field gradient source 32 to define the waveforms of these magnetic field gradients. On the other hand, according to the waveform sent from the waveform generation circuit 33, the carrier wave (high frequency) from the carrier wave generation circuit 35 is amplitude-modulated in the amplitude modulation circuit 34, and sent from the R, F transmission circuit 36 to the RF transmission/reception coil 23, and is R for sample 1
F excitation pulse irradiation is performed, and the spin of magnetization of the subject 1 is excited. The NMR signal generated in the subject 1 is sent and received by f,
After being received by one coil 23 and detected by a detection circuit i37, it is sampled by an A/D converter 38, converted into a digital signal, and taken into a computer 39. This digital data is stored in the computer 39 at 2
The image is reconstructed by dimensional Fourier transform processing. On the other hand, the offset magnetic field coil 24 is offset &! Connected to field power supply 10, static magnetic field magnet! = An offset magnetic field of uniform strength is superimposed on the static magnetic field created by 21. This computer 39 not only reconstructs an image by processing the captured data as described above, but also controls gradient magnetic field waveforms and RF excitation pulse waveforms, controls gradient magnetic field and R, F excitation pulse generation timing, and performs data sampling. Performs timing control, etc. The operation will be explained with reference to the flowchart shown in FIG. First, normal imaging is performed. Here, as shown in FIG. 3, it is assumed that one plane perpendicular to the seven directions of the subject 1 is used as the slice plane 11 and an image of the slice plane 11 is obtained by selective excitation [,5]. That is, while applying a gradient magnetic field for slice selection in which the magnetic field strength is gradient to the Z force, an RF excitation pulse having a carrier wave of a frequency corresponding to the magnetic field strength on the slice surface 11 is applied to select only the slice surface 11. Selective excitation. Then, the frequency encoding direction is set to the X direction, and a frequency encoding gradient magnetic field whose magnetic field strength is tilted in the X direction is applied. At the same time, a gradient magnetic field whose magnetic field strength is inclined in the Y direction is applied to phase encode the position information in the Y direction. In this way, data is collected by repeating the excitation/reception sequence many times while changing the phase encoding gradient magnetic field in the Y direction. Next, while an offset magnetic field is superimposed on the static magnetic field, the carrier wave frequency is shifted by a frequency corresponding to the intensity of the offset magnetic field, and the excitation/reception sequence is repeated in the same manner as described above for the same slice plane 11 to perform imaging. Here, Go is the strength of the static magnetic field, Gz is the gradient magnetic field strength in the Z direction at the position of the slice plane 11, and G(
x), the combined magnetic field strength G at the position
It becomes 2π. However, γ is the gyromagnetic ratio of the hydrogen atom. Therefore, one received signal obtained in normal imaging includes NMR signals of frequencies fa to fb, as shown in FIG. 4A. The frequencies fa and fb correspond to the positions of the ends of the slice plane 11 in the X direction. Furthermore, when the offset magnetic field ΔG is superimposed, the combined magnetic field strength G at the position Wx in the slice plane 11 is G=Go+ΔG+Gz10 (x>), so the frequency of the NMR signal at that position is Δf−
The frequency deviates from the above frequency by γΔG/2π. Therefore,
At this time, by shifting the frequency of the carrier wave by Δf, the same slice plane 11 is selectively excited, and the fourth
As shown in Figure B, the received signal has a frequency fa+Δf~f
An NMR signal of b+Δf is included. On the other hand, assuming that the frequency of external noise is fc, the frequency of this noise included in the received signal does not change in any imaging, so it is the same in the frequency spectrum of the received signal shown in Fig. 4A and B. This noise component appears at frequency fc. Therefore, by shifting the received signal when an offset magnetic field is superimposed by Δf along the frequency axis, the frequency of the NMR signal is made the same as the frequency of the NMR signal in the received signal for normal imaging, as shown in Figure 4C. do. Then, the frequency of the noise will shift to fc-Δf. When the frequency of the received signal is shifted when an offset magnetic field is applied in this way, and the re-received signals are averaged, the NMR signals of frequencies fa to fb are added up to the length of 2 as shown in Figure 4. Although it is twice as large, the noise has frequencies fc and f
Since it becomes c-Δf, it is not added and remains at the original size. As a result, the S/N ratio will double. Note that the above shift of the received signal along the frequency axis can be performed after performing the first Fourier transform in the frequency direction.
The image may be reconstructed by performing Fourier transform in the phase direction, or the image may be reconstructed by performing two-dimensional Fourier transform on the data obtained by normal imaging, and the image may be reconstructed by performing imaging when an offset magnetic field is applied. The obtained data may be subjected to 10 Fourier transforms, then frequency shifted and a second Fourier transform may be performed to reconstruct the image, and the S/N ratio may be increased by adding the average of both images. If a uniform offset magnetic field can be provided using a gradient magnetic field coil or the like, an offset magnetic field coil is not necessary. If the same operation as above is repeated while changing ΔG1Δf and average addition is performed, the S/N ratio will improve in proportion to the number of repetitions. [Effects of the Invention] According to the MRI apparatus of the present invention, it is possible to improve the S/N ratio with respect to constant frequency noise and reconstruct an image with excellent image quality.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a flowchart for explaining the operation, Fig. 3 is a schematic perspective view showing the positional relationship, and Fig. 4 shows the frequency spectrum of the signal. FIG. DESCRIPTION OF SYMBOLS 1... Subject, 11... Slice surface, 21... Static magnetic field magnet 1-122... Gradient magnetic field coil, 23...
... Transmission/reception coil, 24... Offset magnetic field coil,
31... Gradient magnetic field waveform generation circuit, 32... Gradient magnetic field power supply, 33... RF excitation signal waveform generation circuit, 34
... Amplitude modulation circuit, 35 ... Carrier wave generation circuit, 36
...RF transmission circuit, 37...detection circuit 538...
A/D converter, 39... Computer, 40...
Offset magnetic field power supply.

Claims (1)

[Claims] (1) means for generating a uniform offset magnetic field; means for shifting the frequency of an excitation pulse in accordance with the strength of the offset magnetic field; and excitation with the excitation pulse of the shifted frequency while generating the offset magnetic field. and means for shifting the frequency of the received NMR signal by the above-mentioned deviation and adding it to the received NMR signal after exciting it with an excitation pulse of the unshifted frequency without generating the offset magnetic field. Features of MRI equipment.