JPH02305549A - Receiving sensitivity adjusting device for nmr signal used for magnetic resonance photographing device - Google Patents

Abstract

PURPOSE:To eliminate deterioration of the picture quality and obtain an excellent dislocation image of it by varying the phase encode gradient magnetic field in an area around Gphi=0, measuring corresponding NMR signals, and by adjusting the signal receiving gain of NMR signal on the basis of the NMR signal with max. intensity. CONSTITUTION:The phase encode gradient magnetic field Gphi is varied with the size varied near Gphi=0, and the sequence is repeated. The NMR signal obtained is amplified by a preamplifier 13 and an RF transmitter/receiver 14, and the level is measured by a level measuring instrument 22. A controller 18 stores the max. level, and the reception level of the RF transmitter/receiver 14 is adjusted so that this stored max. level becomes the allowable input level for an A/D converter 15. Because the reception gain of the RF transmitter/ receiver 14 has been adjusted in this fashion at the time of photographing the NMR dislocation, the NMR signal can always be maintained so as not to exceed the allowable input value for the A/D converter. Accordingly two-dimensional Fourier transform is made on the basis of signal by the sequence, which enables producing an excellent dislocation image.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MR1 apparatus) that creates a tomographic image of a subject using nuclear magnetic resonance phenomena.
Regarding.

<Conventional technology> An MHI device places a subject in a static magnetic field and applies RF pulses while applying a gradient magnetic field to obtain a tomographic plane (slice plane).
This is a device that selectively excites the nuclear spins of , and receives the NMR signals generated therefrom, and images the signal intensity differences as gray scale information.

To explain in detail the principle of imaging by the above MRI device, first,
A uniform static magnetic field Ho and a gradient magnetic field Gr having an inclination in the direction 2 of the static magnetic field Ho are applied to reduce the nuclear spins of the sliced surface by 90
' Selectively excite with RF pulse. Next, the gradient magnetic field G
By reversing the sign of ``, the phases of the spins that are disordered during excitation are aligned, and then the gradient magnetic field Gφ in the phase encoding direction y and the gradient magnetic field Gf in the frequency encoding direction
Apply. This makes it possible to generate a shift in the rotational phase of the spins (phase encoding). Subsequently, TE/2 hours after application of the above 90° RF pulse, 180°
Applying an RF pulse to invert the phase of the spins, 180
TE/2 hours after applying the RF pulse, the spin echo signal is observed while applying the FIS gradient magnetic field G['.

The above procedure is performed many times by changing the magnitude of the gradient magnetic field Gφ, that is, by changing the amount of phase encoding, and each spin echo signal is obtained.

The spin echo signal is received through an RF receiving coil placed around the subject.

The received signal received by the RF receiving coil is input to a preamplifier, amplified by a predetermined factor, and then input to a receiver for detection. Then, it is input to an A/D converter and converted into a digital signal.

A two-dimensional tomographic image can be obtained by subjecting this digital signal to two-dimensional Fourier transform within a computer and performing image processing.

By the way, considering the level of the signal input to the A/D converter, if the level becomes too high, the A/D converter will become saturated and a correct digital signal will not be obtained. Therefore, it is desirable that the signal input to the A/D converter always be within a proper range. In other words, it is desirable that the gain of the receiver can be set according to the range of NMR signal strength.

However, the above-mentioned MRI apparatus performs measurement by changing the gradient magnetic field Gφ in the phase encoding direction many times, and the intensity of the detected signal changes depending on the amount of phase encoding.

Conventionally, in order to prevent the detection signal from exceeding the permissible level of the A/D converter, the reception gain is set to match the signal strength of Gφ = 0, where the maximum signal strength is obtained. there was.

<Problems to be Solved by the Invention> However, the electrical and magnetic characteristics of the coils that generate the slice direction gradient magnetic field G r , the frequency encode direction gradient magnetic field G', and the phase encode direction gradient magnetic field Gφ are different from those calculated, or are orthogonal. If the signals are not dense or are coupled to each other, the signal strength should be maximum Gφ=
There is a difference between the gradient magnetic field under the zero condition and the gradient magnetic field that actually occurs.

This tendency is particularly noticeable when three orthogonal gradient magnetic fields are combined to obliquely image a subject. Therefore, when the reception gain is adjusted in a state where Gφ=0, the signal increases after that and the reception system may become saturated, causing a problem of deterioration of image quality.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an NMR signal reception sensitivity adjustment device that can always obtain good image quality by improving reception gain adjustment. .

Means for Solving the Problems> To achieve the above object, the NMR signal reception sensitivity adjustment device of the present invention is capable of adjusting the reception sensitivity of NMR signals for magnetic resonance imaging on a slice plane in which nuclear spins are selectively excited. , a phase encoding means that applies a phase encoding gradient magnetic field while changing it each time in the vicinity of Gφ=0, and a phase encoding means that applies a phase encoding gradient magnetic field while changing it each time in the vicinity of Gφ=0, and a
[Intensity measuring means for determining the maximum value of the measured NMR signal intensity; discriminating means for determining the maximum value of the measured NMR signal intensity;
and gain adjustment means for adjusting the reception gain for NMR signal reception based on the R signal.

Effect〉 In actual tomographic imaging, the NMR signal with the maximum intensity is G
It is assumed that it is found in the vicinity of φ=0. Therefore, in the NMR signal reception sensitivity adjustment device described above, the phase encode gradient magnetic field is changed around Gφ=0 and the corresponding NMR signals are measured. By adjusting the gain of the NMR signal receiver so as not to exceed the human power tolerance, a signal at an appropriate level can always be obtained during actual tomographic imaging.

<Example> Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments.

FIG. 2 is a schematic diagram showing the overall configuration of the MRI apparatus.

The MRI apparatus includes: ・G gradient magnetic field coil 16a, Gr gradient magnetic field coil 16
b, a magnetic field generation system consisting of a Gφ gradient magnetic field coil 16C1 and a magnet for static magnetic field generation (not shown); - RF coil 11 for oscillating RF pulses and signal detection, preamplifier 13, RF transceiver 14, A/D A signal processing system consisting of the converter 15: Gradient magnetic field power supplies 17a, 17b, 17c that supply excitation power to each gradient magnetic field coil 16a.

- RF transceiver 14, level measuring device 22 that measures the output level of the RF transceiver 14, gradient magnetic field power supplies 17a to 17
A/D Processing the digital NMR signal output from the converter 15,
It is composed of a computer 19 for creating an NMR tomographic image, a memory 20 for storing digital NMR signal information, and a display 21 for temporarily storing and displaying image information.

The RF transmitter/receiver 14 supplies the RF coil 11 with an RF multiplied signal of the frequency corresponding to the slice plane to be imaged, and also transmits the NMR signal from the excited nuclear spins into R
Detected through F coil 11 and preamplifier 13, A/D
A signal of a level corresponding to the human power level allowed by the converter 15 is supplied to the A/D converter 15.

FIG. 1 shows a pulse sequence for adjusting the reception sensitivity of the NMR signal while changing the phase encode gradient magnetic field Gφ in the MHI apparatus having the above configuration.

First, while applying a gradient magnetic field G in two directions, the RF coil 11 applies a 90° RF pulse to the subject to tilt the nuclear spins of a desired slice plane by 90°.

After this, by reversing the sign of the gradient magnetic field G', the phases of the spins that were disturbed during excitation are aligned, and then the gradient magnetic field G' is set to 0, and the gradient magnetic fields Gφ and G' are applied to slice. Continuously change the phase of the nuclear spin in the plane.After this, while applying a gradient magnetic field G in two directions, R
A 180° RF pulse is applied from the F coil 11 to invert the nuclear spin. After a predetermined time, the spin echo 8 is received while applying the gradient magnetic field Gf'.

In the figure, only the pulse sequence in one repetition unit time TR2 is shown, or the sequence is repeated by changing the magnitude of the phase encode gradient magnetic field Gφ about 10 times in the vicinity of Gφ=0. The NMR signals obtained in these 10 sequences are then sent to a preamplifier 13,
The RF transceiver 14 amplifies the signal, and the level measuring device 22 determines the level n1.

The controller 18 stores the maximum level among the measured levels, and if this maximum level exceeds the allowable input level of the A/D converter 15, adjusts the reception level of the RF transceiver 14 to reach the allowable input level. adjust. Figure 3 shows
A method for adjusting the reception level of the RF transceiver 14 is illustrated. 3A shows a signal manually input to the RF transceiver 14, and FIG. 1B shows an output signal of the RF transceiver 14 detected by the RF transceiver 14 before receiving gain adjustment. If this output signal exceeds the allowable input level Va of the A/D converter 15, the reception gain of the RF transceiver 14 is adjusted so that the output level of the RF transceiver 14 is always maintained as shown in FIG. Do not exceed the allowable input level Va.

In the reception sensitivity adjustment sequence described above, the magnitude of the phase encode gradient magnetic field Gφ is changed about 10 times.

As the number of changes increases, the accuracy of adjusting the reception gain of the RF transceiver 14 improves; however, if the number of changes is too large, the adjustment takes time and the diagnosis time is significantly extended. On the other hand, it is considered that the received signal reaches the maximum level only when the amplitude of the pulse gradient magnetic field Gφ does not deviate significantly from the vicinity of Gφ=0. Therefore, by changing it about 10 times, it is possible to make adjustments without the risk of deteriorating the image.

It should be noted that the repetition time TR2 of the above reception sensitivity adjustment sequence is preferably set to be shorter than the repetition unit time TRI of the NMR imaging sequence, which will be described later, in order to shorten the diagnosis time. In the sequence, after receiving Scheen echo-8, the gradient magnetic field G'
You can image different slice planes by changing the value of 180”
In some cases, the attenuation of spin echoes is measured by applying RF pulses many times, but in the receiving sensitivity adjustment sequence according to the present invention, after acquiring the echo signal 8, the acquired signal is transferred to a computer and the data is calculated. When the minimum required time has elapsed, Gφ is changed and the next sequence is started immediately.

After adjusting the reception gain of the RF transmitter/receiver 14 in the reception sensitivity adjustment sequence described above, NMR tomography is performed, for example, in a sequence as shown in FIG. The sequence of FIG. 4 is the same as the sequence of FIG. 1, except that the number of repetitions of the phase encode gradient magnetic field Gφ is 256 and that one unit time TRI can be longer than TR2.

During NMR tomography, the reception gain of the RF transceiver 14 is adjusted as described above, so the NMR tomography is always
It can be ensured that the signal does not exceed the input tolerance of the A/D converter.

A tomographic image can be obtained by performing two-dimensional Fourier transformation on the signals obtained in the sequence shown in FIG. 4 above.

Note that the present invention is not limited to the above embodiments; for example, the number of repetitions of the phase encode gradient magnetic field Gφ is 10.
It is not limited to the number of times, but may be any number of times within the number of repetitions of the imaging sequence. In addition, in the imaging sequence, it is also possible to adopt a known modification such as continuously applying 180 @ pulses after detecting the spin echo and collecting spin echo signals. Various design changes can be made without changing the gist of the invention.

<Effects of the Invention> As described above, according to the NMR signal reception sensitivity adjustment device of the present invention, the phase encode gradient magnetic field is changed around Gφ=0 and the corresponding NMR
Since we decided to measure the signals and adjust the reception 1 gain of the NMR signal based on the NMR signal with the highest intensity, it is possible to always ensure that the NMR signal level does not exceed the input tolerance value during actual tomographic imaging. can. therefore,
It is possible to eliminate deterioration of image quality due to poor adjustment of reception gain and to always obtain tomographic images of excellent image quality.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the reception sensitivity adjustment pulse sequence of the present invention, FIG. 2 is a schematic block diagram showing the overall configuration of the reception sensitivity adjustment device for NMR signals, and FIG. 3 is a diagram showing the NMR signal waveform of each part. Figure 4 is N
FIG. 3 is a diagram illustrating an imaging sequence of the MR signal reception sensitivity adjustment device. Gr, Gr, Gφ... Gradient magnetic field, 12... Subject, 18... Controller, 22...
・Level measuring device, 14...RF transmitter/receiver Fig. 1 Fig. 2

Claims (1)

[Claims] 1. Applying a gradient magnetic field and an RF pulse to a subject placed in a static magnetic field to selectively excite nuclear spins on a sliced surface, and applying a phase encoding gradient magnetic field to the sliced surface. is applied while changing the magnitude of the phase encode gradient magnetic field each time, and each time the phase encode gradient magnetic field is applied, an NMR signal from the subject is received, and by processing this received signal, the tomographic image of the subject is detected. In a magnetic resonance imaging device that obtains images, a phase-encoding gradient magnetic field is applied to a slice plane where nuclear spins are selectively excited to perform magnetic resonance imaging.
a phase encoding means for applying a phase encoding gradient magnetic field while changing its magnitude near Gφ=0 each time; and an intensity measuring means for measuring the intensity of an NMR signal from a subject each time each phase encoding gradient magnetic field is applied. A magnetic resonance system characterized by comprising: a determining means for determining the maximum value of the measured NMR signal intensity; and a gain adjusting means for adjusting the NMR signal reception gain of the magnetic resonance imaging apparatus based on the maximum value of the NMR signal. NM used for imaging device
R signal reception sensitivity adjustment device. 2. Used in the magnetic resonance imaging apparatus according to claim 1, wherein a repetition time TR2 for applying the phase encoding gradient magnetic field by the phase encoding means is shorter than a repetition time TR1 of the phase encoding gradient magnetic field during tomographic imaging. NMR signal reception sensitivity adjustment device.