JP2811891B2 - Adjustment method of static magnetic field uniformity - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a method for adjusting static magnetic field uniformity in an MR device such as an NMR imaging device or an NMR spectrometer.

[Prior art]

MR such as NMR imaging equipment and NMR spectrometer
The device requires a uniform static magnetic field. Conventionally, as a technique for improving the uniformity of the static magnetic field, there is known a method of measuring a magnetic field distribution at each point in space in the static magnetic field and calculating a current value flowing through the shim coil by calculation. In the NMR spectrometer, as shown in FIG. 3, an NMR signal (FID signal) 32 generated by irradiating an RF pulse (90 ° pulse) 31 is observed, and the signal amount is maximized. There is known a method of controlling a current value flowing through a shim coil. In this case, the repetition time TR of the RF pulse 31 is set longer than the relaxation time, and is usually selected to be about 1 second.

[Problems to be solved by the invention]

However, measuring the magnetic field distribution at each point in space requires a very long measurement time, and has a problem that the calculation is also difficult. Further, although the method of observing the FID signal is simple, it usually has a problem that the pulse repetition time TR cannot be shortened because the waiting time after the 90 ° pulse is longer than the relaxation time. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for adjusting the uniformity of a static magnetic field, which is simple and can reduce the time for adjustment by shortening the pulse repetition time.

[Means for Solving the Problems]

In order to achieve the above object, in the method for adjusting the uniformity of the static magnetic field according to the present invention, the magnetization is brought into a steady precession state by repeatedly irradiating an RF pulse with a repetition time shorter than the relaxation time of the sample. By controlling the value of the current flowing through the shim coil for correcting static magnetic field uniformity while maintaining it, the observed NM
It is characterized by selecting a current value such that the R signal has a maximum value.

[Action]

By repeatedly irradiating the RF pulse with a repetition time shorter than the relaxation time of the sample, a steady precession state of magnetization can appear. In this steady precession state, the magnetization at each point of the sample rotates with a phase corresponding to the static magnetic field strength at that point. Therefore, the NMR signal from each point includes the static magnetic field strength at that point as phase information. Then, the observed NMR signal is considered to be the sum of the NMR signals from each point of the entire sample. Therefore, while maintaining this steady precession state, the current value flowing through the uniformity correction shim coil was changed to
If the current value at which the signal has the maximum value is selected, the uniformity of the static magnetic field in the sample becomes optimal when the current value is passed through the shim coil.

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In an MR apparatus such as an NMR imaging apparatus or an NMR spectrometer, as shown in FIG. 1, a correction shim coil 2 for correcting the uniformity of a static magnetic field generated by the main magnet 1 is arranged in the main magnet 1. Then, a correction current is supplied from the shim coil power supply 3 to the shim coil 2. A probe 4 is arranged in a static magnetic field formed by the main magnet 1, and a sample (not shown) is placed in the probe 4. An NMR transceiver 5 is connected to the probe 4, and an RF signal is transmitted and received through the probe 4.
The pulse programmer 6 controls the NMR transmitter / receiver 5 based on a predetermined pulse sequence under the control of the CPU 7, and causes the NMR transmitter / receiver 5 to supply an RF signal to the probe 4. Thus, the sample is irradiated with the RF signal. An NMR signal is emitted from the excited nuclear spin of the sample, and the NMR signal is received by the probe 4 and sent to the transceiver 5.
The NMR data obtained via the transceiver 5 is sent to the CPU 7. The CPU 7 monitors the NMR data and controls the shim coil power supply 3 online via the pulse programmer 6. As a result, the current flowing through the shim coil 2 is increased or decreased. In the method of adjusting the uniformity of the static magnetic field according to one embodiment of the present invention, as shown in FIG.
Irradiated repeatedly. The repetition time TR is sufficiently shorter than the relaxation times T1 and T2 of the sample in the probe 4. Thus, the repetition time TR is equal to the relaxation time T1, T
If it is sufficiently shorter than 2, the magnetization will be in a state of performing a steady precession called SSFP, and as shown in FIG.
The state is such that the NMR signal 22 is generated on both sides before and after.
Assuming that T1 = T2 = 500 ms as a representative value, this SSFP state is obtained when TR = about 100 ms. The magnitude of the NMR signal at this time is Where, Mx, Mr; transverse magnetization in a rotating system ω ₀ ; NMR resonance frequency Φ; rotation amount of transverse magnetization during TR (∝H ₀ ) Ak; constant (for example, see MLGyngell, J. Mag. Reson, IA US
81,474 (1989)). Considering each part of the sample at this time, each part has Φ determined by the magnitude of the static magnetic field at that location, and as a result, each part of the sample has a signal having a phase corresponding to the value of the static magnetic field at that location And the sum of the signals from the entire sample is observed as an NMR signal received by the probe 4. Therefore, while controlling the pulse repetition time TR to an extent that the SSFP state can be maintained (about 100 ms in the above example), the current value flowing through the shim coil 2 is controlled online, and the reception NMR is controlled.
The current value is determined so that the signal has the maximum value. Then, in such a state where the NMR signal has the maximum value, the phase angle Φ of each part of the sample becomes uniform over the entire sample, and an NMR signal having the same phase is generated from the entire sample. . As a result, the magnitude of the static magnetic field to which each part of the sample is sensitive becomes uniform, and a highly uniform static magnetic field can be formed.

【The invention's effect】

According to the static magnetic field uniformity adjusting method of the present invention, a highly uniform static magnetic field can be obtained simply and at high speed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a static magnetic field uniformity correction system used in one embodiment of the present invention, FIG. 2 is a waveform diagram in the embodiment, and FIG. 3 is a waveform diagram in a conventional example. 1 ... Main magnet, 2 ... Shim coil, 3 ... Shim coil power supply, 4 ... Probe, 5 ... NMR transceiver, 6 ...
... pulse programmer, 7 ... CPU.

Claims (1)

(57) [Claims] 1. A repetition time shorter than a relaxation time of a sample.
The RF pulse is repeatedly irradiated to set the magnetization to a steady precession state, and while maintaining this state, the current flowing through the shim coil for static magnetic field uniformity correction is manipulated so that the observed NMR signal becomes the maximum value. A method for adjusting the uniformity of the static magnetic field, characterized by selecting a value.