JP2905569B2 - MRI equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus that corrects a temporal variation in the intensity of a static magnetic field and eliminates a positional shift in measurement of an MR image.

[Conventional technology]

The resonance frequency W in the MRI apparatus can be determined based on the target nuclide and the magnetic field strength B. That is, if the gyromagnetic ratio of the nuclear spins of the target nuclide is γ, W = γB (1)

When a linear gradient magnetic field is applied to the static magnetic field, the resonance frequency is proportional to the magnetic field strength according to the equation (1), and is therefore proportional to the distance from the center of the magnetic field. Therefore, when the center of the magnetic field is set as the reference position and the resonance frequency at that position is set as the center frequency, the difference between the frequency of the measured MR signal and the center frequency corresponds to the relative distance between the MR signal generation position and the reference position.

Here, the gradient applied magnetic field is set to zero at the reference position so that the reference position becomes the value of the static magnetic field, and a positive linear gradient magnetic field is applied to one half of the reference position, and the other is applied to the other half. Applies a negative linear gradient magnetic field. Therefore, the center frequency at the reference position may be defined as a value determined by the static magnetic field. As the gradient magnetic field, a positive or negative linear gradient magnetic field may be applied from one end in the measurement space instead of the center position. At this time, this end is the reference position, but the center frequency is still a value determined by the static magnetic field.

However, the intensity of the static magnetic field may change due to temperature fluctuation. This tends to occur when a permanent magnet is used for generating a static magnetic field.

Various materials have been devised for the permanent magnet used for the static magnetic field generating magnet. The rare earth magnet (Nd-Fe-B) recently used has the highest maximum energy product, but has the highest energy product. On the other hand, the temperature coefficient is large. In general, when the ambient temperature increases, the generated static magnetic field decreases. It has a so-called negative temperature coefficient. As an example, there is one whose temperature coefficient reaches −1000 ppm / ° C. In this case, when the ambient temperature rises by 1 ° C., the static magnetic field intensity decreases by 1000 ppm. For example, a magnetostatic strength of 1000 Gauss corresponds to 1 Gauss.
As a result, the center frequency of the static magnetic field also changes. Since the true center frequency is shifted compared to the center frequency determined by such a static magnetic field, the result is equivalent to the movement of the reference position, and it is not necessary to measure the desired slice position but to measure another slice position. Become. This is an anatomical misalignment, leading to a misdiagnosis.

Similarly, if a difference between the assumed center frequency and the true center frequency occurs, a shift in the frequency encoding tilt direction occurs.

To solve this problem, a magnetic field locking method (a method in which when the static magnetic field strength fluctuates from the virtual central magnetic field strength, the static magnetic field strength is increased or decreased by the difference to match the virtual central magnetic field strength) and a special application As described in 1986-255666, measurement was performed without applying a gradient magnetic field to the subject prior to imaging.
There is a frequency locking method for preventing the displacement by performing Fourier transform on the NMR signal, performing complex absolute value processing, and determining the current center frequency from the peak of the obtained spectrum.

In the magnetic field locking method, when the magnet body has a negative temperature coefficient as in the case of the permanent magnet method, the intensity of the static magnetic field is kept constant by flowing a current through the static magnetic field correction coil. But,
The heat generated by flowing the current further reduces the static magnetic field strength, so that the current may need to be increased. In order to prevent this, the vicious circle can be broken if the static magnetic field strength generated by the permanent magnet is configured so that a current is always passed through the correction coil to slightly cancel the static magnetic field. In other words, when the temperature of the external environment rises and the static magnetic field strength generated by the permanent magnet decreases, the static magnetic field that has been canceled can be recovered and used by reducing the current flowing through the correction coil, and the static magnetic field is also virtual center. As the magnetic field strength returns, the heat is reduced because the current is also reduced.

However, with such a configuration, it is necessary to compensate for the magnetic field strength equivalent to canceling out by the static magnetic field correction coil by strengthening the permanent magnet. This increases the cost of a permanent magnet type static magnetic field generator as an NMR imaging device.

Therefore, a new proposal has been made in Japanese Patent Application No. 61-255666 filed by the present applicant. The frequency lock method according to this application performs measurement for directly obtaining the center frequency prior to the original measurement of the subject. Therefore, first, without applying the frequency encoding gradient magnetic field and the phase encoding gradient magnetic field, only the slice direction gradient magnetic field is applied, and 90 ° pulse and 180 ° pulse are applied in synchronization with the slice direction gradient magnetic field, NMR generated by this
The signal is subjected to Fourier transform, and among the frequencies obtained by the Fourier transform, a frequency having a peak spectrum is set as an actual center frequency.

[Problems to be solved by the invention]

In the frequency locking method, the center frequency is obtained from the peak value in the spectrum after the Fourier transform. However, when the subject is not uniform, such as a human body, the signal source is not uniform due to the inhomogeneity of the static magnetic field. As shown in (1), the peak may not converge at one point but may have a spectrum having a plurality of peaks. The peak position of a spectrum having a plurality of peaks becomes unstable due to noise, and the position of the peak may shift every measurement. The peak position shift is the position shift of the slice position. Even if it is assumed that there is no fluctuation in the static magnetic field, the position shift of the slice position is very slight but occurs.

As image quality improves with the advancement of imaging technology and there is an increasing demand to capture the fine structure of the subject, it is a problem that the displacement of the slice position occurs, and it is necessary to reduce this displacement as much as possible. .

The object of the present invention is to improve the setting accuracy of the center frequency by picking up a wide range of spectra in view of the fact that the accuracy is deteriorated when the peak spectrum is set to the center frequency.
An MRI apparatus is provided.

[Means for solving the problem]

The present invention further comprises means for setting an actual center frequency absorbing a change in the center frequency determined by the static magnetic field strength, and the setting means performs the slice direction without applying the frequency encoding gradient magnetic field and the phase encoding gradient magnetic field. Means for applying only a gradient magnetic field, means for irradiating 90 ° and 180 ° pulses in synchronization with the application of the slice-direction gradient magnetic field, and means for Fourier transforming a nuclear magnetic resonance signal obtained by the detection means for the irradiation Means for calculating an average of the frequencies of the obtained spectrum and setting a center frequency from the average frequency.

(Operation)

According to the present invention, the average of the frequency of the spectrum obtained from the Fourier transform of the MR signal is calculated, and since this is set as the center frequency, it is possible to obtain an accurate center frequency, and eliminate the displacement in imaging. Let

〔Example〕

FIG. 3 is a block diagram showing the overall configuration of the magnetic resonance imaging apparatus according to the present invention. This magnetic resonance imaging apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon. As shown in FIG. 3, a static magnetic field generating magnet 2, a magnetic field gradient generating system 3, It comprises a transmission system 4, a reception system 5, a signal processing system 6, and a sequencer 7.

The static magnetic field generating magnet 2 generates a strong and uniform static magnetic field around the subject 1 in the body axis direction or a direction perpendicular to the body axis, and has a certain space around the subject 1. A permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means is disposed in the apparatus. The magnetic field gradient generating system 3 includes a gradient magnetic field coil 9 wound in three directions of X, Y, and Z axes.
And a gradient magnetic field power supply 10 for driving each coil. By driving the gradient magnetic field power supply 10 for each coil in accordance with an instruction from the sequencer 7 described later, X, Y,
The gradient magnetic fields Gx, Gy, Gz in the three axial directions of Z are applied to the subject 1. Then, a slice plane for the subject 1 can be set depending on how to apply the gradient magnetic field. The transmission system 4 irradiates a high-frequency signal to cause nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue of the subject 1, and includes a high-frequency oscillator 11, a modulator 12, a high-frequency amplifier 13, and a high-frequency signal on the transmission side. And a high-frequency pulse output from the high-frequency oscillator 11.
The amplitude is modulated by the modulator 12 in accordance with the instruction, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then supplied to the high-frequency coil 14a arranged close to the subject 1, whereby the electromagnetic wave is 1 is illuminated. The receiving system 5 is a high-frequency signal (NMR signal) emitted by nuclear magnetic resonance of nuclei of living tissue of the subject 1
The high-frequency coil 14b on the receiving side and the amplifier 1
5, a quadrature detector 16 and an A / D converter 17. An electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave emitted from the high-frequency coil 14 a on the transmitting side is close to the subject 1. Detected by the arranged high-frequency coil 14b, input to the A / D converter 17 via the amplifier 15 and the quadrature phase detector 16, converted into a digital quantity, and further converted to a quadrature phase detector at a timing according to a command from the sequencer 7. The collected data is collected into two series of data sampled by 16, and the signal is sent to the signal system 6. This signal processing system 6
A central processing unit (CPU) 8, a recording device such as a magnetic disk 19 and a magnetic tape 18, and a display 20 such as a CRT. The CPU 8 performs processing such as Fourier transform and correction coefficient calculation image reconstruction. The vibration intensity distribution of the cross section or the distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 20 as a tomographic image. The sequencer 7 operates under the control of the CPU 8 and sends various commands necessary for data collection of tomographic images of the subject 1 to the transmission system 4, the magnetic field gradient generation system 3 and the reception system 5, and measures the high-frequency signal. This is a means for generating a sequence.

FIG. 1 shows an example of processing by the setting means of the present invention. This setting means can be realized by software processing by the CPU 8. The time chart of the measurement is shown in FIG.

Step 2-1 ...... First, setting the appropriate center frequency W ₀ in the high-frequency oscillator 11. This may be manual.

Step 2-2: Next, the sequencer 7 is started through the CPU 8 and measurement is started. At this time, the gradient magnetic fields Gz, Gx, Gy
Among them, only the slice-direction gradient magnetic field Gz is applied, and the phase encode and frequency encode gradient magnetic fields Gy and Gx are not applied. Synchronously with the application of the gradient magnetic field in the slice direction,
W ₀ 90 ° pulse and 180 ° pulse are high frequency pulse 14a
Is applied.

Step 2-3: The MR signal is detected by the high-frequency coil 14b, and the detected signal is transmitted to the A / D converter 17 and received by the CPU 8.

Step 2-4... The MR data captured via the A / D converter 17 is transferred and stored in the CPU 8 or a one-dimensional buffer provided separately.

Step 2-5: After the measurement is completed, the NMR data transferred to the one-dimensional buffer is subjected to Fourier transform by the CPU 8 or a dedicated FET calculator.

Step 2-6... Since the measured data is complex data consisting of a real part and an imaginary part, an absolute value is calculated by the CPU 8 or a dedicated complex absolute value calculator and transferred to another one-dimensional buffer.

Step 2-7: A peak value is detected from the absolute value (spectrum) in another one-dimensional buffer.

Step 2-8 ... Since the spectrum obtained in step 2-7 contains a noise component, the noise component is removed. Therefore, the noise determination threshold V _th
Is provided, and a spectrum larger than this threshold is extracted as a signal component. FIG. 4 shows an example of the peak value and the threshold value. An example (shaded area) in which a component having a peak value smaller than the threshold value is removed and a component having a peak value larger than the threshold value is extracted as a signal component is shown.

Step 2-9: An average of frequencies is obtained from the extracted signal components to obtain an average frequency. The formula for calculating the average frequency is
According to the method of calculating the center of gravity, the following is obtained.

Here, i = 1, 2,..., N, indicating the frequency number or the frequency value itself, and a _i indicates the signal level (spectrum size) at each frequency. However, the average calculation method may be another method such as the least square method. In the example of FIG. 4, P ₁ , P ₂ , P ₃ , and P ₄ are examples of peaks as signal components.
Was a W ₁ according to the conventional maximum peak method, a W ₂ according to the averaging method of the present embodiment (W _1> W _2).

Step 2-10 and 2-11 ...... the obtained average frequency determines the difference W ₂ -W ₁ and the initial set frequency W _0, obtains the new center frequency. This step may not be necessary. W ₂ is because the set as the center frequency. Only when the difference is used for calculation, steps 2-10 and 2-
11 exists.

Step 2-12 to determine the level value of the spectrum at ...... said center frequency W ₂ is greater than the second threshold value is a noise level which is preset in advance, is smaller than the second threshold value, correct and Nakatto peak detection is made, the process proceeds to step 2-13, is set to another frequency range W _01. Specifically, a certain increment is added to or subtracted from the current center frequency. In step 2-14, it is determined whether or not the result is outside the predetermined frame.
If it is outside the frame, the process ends abnormally. If it is within the frame, the process returns to step 2-1 and the same is repeated to continue detecting the center frequency.

The level value of the spectrum at the center frequency is the second
If it is not less than the threshold value, the process proceeds to step 2-15, where the center frequency at this time is set as a new center frequency in the high-frequency oscillator 17, and the process is terminated.

As described above, according to the present embodiment, when detecting the center frequency prior to imaging, by obtaining the center frequency from the average of the spectrum, more accurate measurement without displacement can be performed without using the magnetic field lock. However, it can be easily implemented.

 A modification of the present embodiment will be described below.

(1) 90 ° and 18 without applying a gradient magnetic field in the slice direction
At 0 °, a high frequency pulse is applied to measure a nuclear magnetic resonance signal to obtain a spectrum, from which a center frequency can be obtained.

(2) Instead of using a 180 ° high frequency pulse for forming an echo signal, a gradient magnetic field may be switched.

(3) Instead of using a 180 ° high-frequency pulse for forming an echo signal in (1), a gradient magnetic field may be switched.

〔The invention's effect〕

According to the present invention, even if the static magnetic field fluctuates, the center frequency after the fluctuation can be accurately obtained. In addition, there is no need to attach a shim coil for static magnetic field correction to the device, and the circuit required for magnetic field lock is not required.Moreover, even when the small subject reception signal is small and the S / N ratio is poor, the precise slice position can be specified. This makes it possible to provide an MRI apparatus that performs simple and accurate position correction.

[Brief description of the drawings]

FIG. 1 is a processing flow chart of the setting means of the present invention, FIG. 2 is a measurement time chart thereof, FIG. 3 is an overall configuration diagram of the MRI apparatus of the present invention, and FIG. 4 is an explanatory diagram of an averaging method of the present invention. FIG. 5 is an explanatory view of a conventional example. Reference Signs List 1 subject 2 static magnetic field generating magnet 3 magnetic field gradient generating system 4 transmitting system 5 receiving system 6 signal processing system 7 sequencer 8 CPU , 9 ... gradient coil, 10 ... gradient power supply, 11 ... high frequency oscillator, 12 ...
... Modulator, 13 ... High-frequency amplifier, 14a ... High-frequency coil on transmission side, 14b ... High-frequency coil on reception side, 15 ... Amplifier, 16 ... Quadrature phase detector, 17 ... A / D converter, 18 ……
Magnetic disk, 19 ... magnetic tape, 20 ... display.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) A61B 5/055

Claims (1)

(57) [Claims] A means for applying a static magnetic field to the subject; a means for applying a slice direction gradient magnetic field, a phase encoding gradient magnetic field, and a frequency encoding gradient magnetic field to the subject in a predetermined sequence; Application means for applying a high-frequency pulse to cause nuclear magnetic resonance in the nucleus; nuclear magnetic resonance signal detecting means for detecting a nuclear magnetic resonance signal; and an image obtained by performing a Fourier transform on the obtained nuclear magnetic resonance signal to obtain an image. Means for reconstructing, and means for setting an actual center frequency absorbing a change in the center frequency determined by the static magnetic field strength, wherein the setting means comprises a frequency encoding gradient magnetic field and a phase encoding gradient. Means for applying only the slice-direction gradient magnetic field without applying a magnetic field, and a 90 ° pulse and 1 pulse in synchronization with the slice-direction gradient magnetic field application. Means for irradiating an 80 ° pulse, means for Fourier transforming a nuclear magnetic resonance signal obtained by the detection means for the irradiation, calculating an average of frequencies of the obtained spectrum, and setting a center frequency from the average frequency And an MRI apparatus comprising: