JPH03286736A - Adjusting method for uniformity of magnetostatic field - Google Patents

Abstract

PURPOSE:To simply obtain the magnetostatic field of high uniformity and at a high speed by setting magnetization to a stationary precession motion state by radiating repeatedly an RF pulse in a shorter repeat time than a relaxation time of a sample, operating a current value allowed to flow to a magnetostatic field uniformity correction shim coil, while holding this state, and selecting such a current value as an observed NMR signal becomes the maximum value. CONSTITUTION:To a probe 4, an NMR transmitter/receiver 5 is connected, and an RF signal is sent and received. A pulse programmer 6 controls the NMR transmitter/receiver 5, based on a prescribed pulse sequence under the control of a CPU 7, and supplies the RF signal to the probe 4. In such a way, a sample is irradiated with the RF signal. From a nuclear spin of the excited sample, an NMR signal is emitted, and this NMR signal is received by the probe 4 and sent to the transmitter/receiver 5. NMR data obtained through this transmitter/receiver 5 is sent to the CPU 7. The CPU 7 monitors this NMR data, and also, controls a shim coil power source 3 by on-line through the pulse programmer 6.

Description

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

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

[Conventional technology]

MR devices such as NMR imaging devices and NMR spectrometers require a uniform static magnetic field. Conventionally, as a technique to increase the uniformity of this static magnetic field,
A known method is to measure the magnetic field distribution at each point in space within a static magnetic field and calculate the value of the current to be passed through the shim coil. In addition, in the NMR spectrometer, as shown in Fig. 3, the NMR signal (FID signal) 32 generated by irradiating the RF pulse (90° pulse) 31 is observed, and the signal amount is maximized. A method of controlling the current value flowing through the shim coil is known.In this case, the RF
The repetition time TR of the pulse 31 is set longer than the relaxation time, and is usually selected to be about 1 degree.

[Problems to be Solved by the Invention] However, there are problems in that measuring the magnetic field distribution at each point in space requires a very long measurement time and is also difficult to calculate. Furthermore, although the method of observing the FID signal is simple, it usually has the 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 static magnetic field uniformity that is simple and can shorten 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 static magnetic field uniformity according to the present invention, RF pulses are repeatedly irradiated with a repetition time shorter than the relaxation time of the sample to maintain magnetization at a steady state. The observed N
The feature is that the current value is selected such that the MR signal has a maximum value.

[For production]

By repeatedly irradiating an RF pulse with a repetition time shorter than the relaxation time of the sample, a steady state of precession of magnetization can appear. In this steady precession state, the magnetization at each point on 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. The observed NMR signal is then considered to be the sum of NMR signals from each point on the entire sample. Therefore, while maintaining this steady precession state, the current value flowing through the uniformity correction shim coil was changed, and the NMR
If the t-current value at which the signal has a maximum value is selected, the uniformity of the static magnetic field in the sample will be optimal when that t-current value is applied to the shim coil.

【Example】

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. In an MR device such as an NMR imaging device or an NMR spectrometer, as shown in FIG. 1, a correction shim coil 2 is arranged within a main magnet 1 to correct the uniformity of the static magnetic field generated by the main magnet 1. A correction current is supplied to the shim coil 2 from the shim coil power supply 3. A probe 4 is placed within the static magnetic field formed by the main magnet 1, and a sample (not shown) is placed inside the probe 4. An NMR transceiver 5 is connected to this probe 4, and RF multiplied signals are transmitted and received through this probe 4. The pulse programmer 6 controls the NMR transceiver 5 based on a predetermined pulse sequence under the control of the CPU 7 and causes the NMR transceiver 5 to supply an RF multiplied signal to the probe 4 . In this way, the sample is irradiated with the RF signal. An NMR signal is emitted from the excited nuclear spins of the sample, and this NMR
The signal is received by probe 4 and sent to transceiver 5. The NMR data obtained through this transmission/reception 815 is sent to the CPU
Sent to 7. The CPU 7 monitors this NMR data and controls the shim coil source 3 online via the pulse programmer 6. As a result, shim coil 2
The current flowing through the circuit will be increased or decreased. In a method for adjusting static magnetic field uniformity according to an embodiment of the present invention, as shown in FIG. 2, an RF pulse 21 is repeatedly irradiated with a repetition time TR. This repetition time TR is
It is assumed that the relaxation time TI of the sample in the probe 4 is sufficiently shorter than T2. In this way, the repetition time TR is the relaxation time Tl of the sample. If it is sufficiently shorter than T2, the magnetization will undergo a steady precession called 5SFP, and as shown in Figure 2, the RF
9 Typical values that result in a state where the NMR signal 22 is generated on both sides of the pulse 21 are T 1 = T 2
, = 500 m s, and if TR = about 100 m5, this 5SFP state is obtained. The magnitude of the NMR signal at this time is: Mxy=My+ i Mx However, 1χ, My: Transverse magnetization in the rotating system ωo: NMR resonance frequency Φ; Rotation amount of transverse magnetization during TR (echo) Ak: expressed as a constant (for example, M, L, Gyngel
I; J, Mag. Re5on, 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 portion of the sample will generate a signal with a phase corresponding to the value of the static magnetic field at that location, and the sum of the signals from the entire sample will be observed as the NMR signal received by the probe 4. Therefore, the current value flowing through the shim coil 2 is controlled online while keeping the pulse repetition time TR at a level that allows the 5SFP state to be maintained (about 100m5 in the above example).
The current value is determined so that the received NMR signal reaches its maximum value. Then, when the NMR signal reaches its maximum value like this, the phase angle Φ of each part of the sample becomes uniform throughout the sample, and
This means that NMR signals with the same phase are 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 created.

【Effect of the invention】

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

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a static magnetic field uniformity correction system used in an embodiment of the present invention, FIG. 2 is a waveform diagram in the same 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)

[Claims] (1) Repeatedly irradiating RF pulses with a repetition time shorter than the relaxation time of the sample to bring the magnetization into a steady precession state;
A method for adjusting static magnetic field uniformity, which is characterized in that, while maintaining this state, a current value flowing through a shim coil for correcting static magnetic field uniformity is operated to select a current value such that the observed NMR signal has a maximum value.