JPH0515510A - Method of adjusting RF power in MRI apparatus - Google Patents

Abstract

(57) [Abstract] [Purpose] An object of the present invention is to provide an RF power adjustment method capable of significantly improving the RF power adjustment accuracy as compared with conventional methods. When the RF power is adjusted, a pulse sequence by the field echo method is executed, and when the signal intensity of the MR signal is minimized, the flip angle is set to 180 °.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting the RF power in an MRI apparatus which adjusts the signal strength (RF power) of the RF pulse corresponding to the object in advance before the image is taken by the MRI apparatus.

[0002]

2. Description of the Related Art Conventionally, when adjusting the signal intensity of an RF pulse, a pulse sequence by the spin echo method is performed under the condition that the repetition time of the RF pulse is sufficiently longer than the longitudinal relaxation time: T _{1 of} the biological tissue. The magnetic resonance signal (M
When the signal strength of the R signal) is maximum, the flip angle is set to 9
It was set to 0 °.

[0003]

However, since the RF power changes in a form close to | sin θ | (θ: flip angle of RF pulse), when the flip angle is around 90 °, the RF power changes with respect to the flip angle (RF power). The level change of the MR signal is small. Therefore, in many cases, the power adjustment is set at a position deviating from the true 90 ° condition. Therefore, when the RF power is adjusted by the conventional RF power adjustment method, the 180 ° condition is inaccurate, so that an artifact may occur due to the flow of the free induction attenuation (FID) signal or the 90 ° -180 ° condition. Stimulated artifacts due to incorrect S / N
There was a problem such as deterioration.

The present invention has been made in view of the above problems, and an object thereof is to provide an RF power adjusting method capable of significantly improving the RF power adjusting accuracy as compared with the conventional method. It is in.

[0005]

In order to achieve the above-mentioned object, the present invention, when adjusting the RF power, implements a pulse sequence by the field echo method to minimize the signal strength of the MR signal, It is characterized in that the flip angle is set to 180 °.

[0006]

According to the RF power adjusting method of the present invention,
The pulse sequence by the field echo method which does not use the 180 ° pulse for the RF pulse is T than T ₁ of the biological tissue.
If the flip angle is set to 180 ° when the signal intensity of the MR signal becomes the minimum (0) when _R is set sufficiently long, the rate of change of the MR signal becomes maximum near the flip angle of 180 °. , RF power adjustment accuracy is significantly increased.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing the overall configuration of an MRI apparatus to which the RF power adjusting method of the present invention is applied.

In this MRI apparatus, an RF coil 2 is used to make a R
At the same time as the application of the F pulse, the gradient magnetic field coils 3 encode, read, and slice gradient magnetic fields G _E ,
By applying G _R and G _S , the MR signal generated in the subject P is received by the probe 4, and the received MR signal is detected by the receiver 5 and sent to the computer system 6. Then, the tomographic image is reconstructed based on the MR signals collected in the computer system 6, and the tomographic image is displayed on the monitor 7. At this time, under the control of the computer system 6, the sequencer 8 executes a pulse sequence to drive the transmitter 9 and the gradient magnetic field power supply 10, and the RF coil 2
To generate RF pulses, and the gradient magnetic field coils 3 generate respective gradient magnetic fields G _E , G _R , and G _S. MR like this
Before imaging the subject P with the I device,
It is necessary to adjust the RF power according to.

Therefore, in this embodiment, in order to adjust the RF power, the pulse sequence by the field echo method as shown in FIG. 2 is controlled by the sequencer 8 under the control of the computer system 6 to be more than T _{1 of} the biological tissue. _{Take R} long enough to collect MR signals. Since T ₁ of the human body is about 1 second, it is better to set T _R longer than that.

At this time, when the object P is scanned while slightly increasing the RF power, X, Y, Z in FIG.
When the flip angle is θ in three-dimensional polar coordinates of the axis, MR
The RF power dependence of the signal is approximately | sin θ |. Therefore, when T _R >> T ₁ , the signal strength of the MR signal is measured by further changing the RF power in the vicinity where the signal strength of the MR signal decreases and rises again as shown in FIG.
A flip angle of 180 ° is selected with RF power that minimizes the signal strength of the R signal. Therefore, for the 90 ° pulse, it may be set to 1/2 of the RF power that gives the minimum signal.

As described above, when the RF power is adjusted according to the present embodiment, the 180 ° condition becomes accurate, so that the artifact due to the inflow of the FID signal is suppressed.
Since the 0 ° -180 ° condition is accurate, stimulated artifacts are suppressed and the S / N is greatly improved.

[0012]

According to the above description, when the pulse sequence by the field echo method is executed and the signal intensity of the MR signal is minimized, the flip angle is set to 180 °, so that the RF power adjustment accuracy is improved. Significantly improved compared to the past.

[Brief description of drawings]

FIG. 1 is an M to which an RF power adjustment method of the present invention is applied.
It is a figure which shows the whole structure of RI apparatus.

FIG. 2 is a timing chart showing a pulse sequence of the field echo method.

FIG. 3 is a diagram showing a relationship between a flip angle and signal strength in three-dimensional polar coordinates.

FIG. 4 is a characteristic curve diagram showing a flip angle dependence of signal intensity and a selected position of a 180 ° condition in the field echo method.

[Explanation of symbols]

1 main magnet 2 RF coil 3 gradient coil 4 probes 5 receiver 6 computer system 7 monitors 8 sequencer 9 transmitter 10 gradient magnetic field power supply

Claims (2)

[Claims] 1. The flip angle is set to 180 ° when the signal intensity of the magnetic resonance signal is minimized by performing a pulse sequence by the field echo method when adjusting the signal intensity of the RF pulse. A method for adjusting RF power in an MRI apparatus. 2. The method of adjusting RF power in an MRI apparatus according to claim 1, wherein the repetition time of the RF pulse is set sufficiently longer than the longitudinal relaxation time of the biological tissue.