JP2762277B2 - Magnetic field correction method for nuclear magnetic resonance imaging and nuclear magnetic resonance imaging apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and an apparatus for measuring a non-uniform distribution of a static magnetic field and generating a correction magnetic field based on the non-uniform information. The present invention relates to an MRI magnetic field correction method and apparatus with an improved range of possible inhomogeneity.

(Prior Art) In an MRI (Magnetic Resonance Imaging) device,
The uniformity of the magnetic field is required to be several tens of ppm or less by the static magnetic field magnet. However, the uniformity of the magnetic field generated by the magnet alone is several hundred ppm or more due to the accuracy in manufacturing the magnet and the installation environment. Therefore, normally, a correction magnetic field is generated using a current shim coil for magnetic field correction or a magnetic shim made of iron or the like to improve the magnetic field to a required uniformity. On the other hand, to generate the correction magnetic field, it is necessary to use the NMR phenomenon in order to accurately measure the magnetic field distribution on the order of ppm, and the following two methods are usually used. FIGS. 5 (a) and 5 (b) are diagrams showing a magnetic field intensity distribution measuring method using a conventional NMR probe. FIG. 5 (a) is a schematic diagram showing a method of arranging NMR probes, and FIG.
It is a block diagram of an NMR probe. In FIG. 5 (a),
1 is an NMR probe installed at a magnetic field strength measurement location,
In FIG. 2B, reference numeral 2 denotes an approximately 1 cc water sample installed in the NMR probe 1, reference numeral 3 denotes an RF coil for transmitting and receiving NMR signals from the water sample 2, and reference numeral 4 denotes a resonance coil connected in parallel to the RF coil 3. A variable condenser 5 is a transmitting / receiving T / R (Transmitter / Receiver) circuit connected in series to the RF coil 3. As shown in FIG. 5 (a), the NMR probe 1 is placed at a magnetic field measurement location, RF pulses are transmitted and received from the RF coil 3, and the magnetic field strength at that location is measured from the resonance frequency. next,
This position is moved at a predetermined distance within the magnetic field measurement range, for example, in a cylindrical shape, and the magnetic field at each point is plotted to measure the magnetic field non-uniform distribution. According to this method, the measurable magnetic field inhomogeneity is not limited, so that there is an advantage that the measurement can be performed even if the inhomogeneity is large. On the other hand, to measure the magnetic field strength one by one,
There is a problem that it takes a long time to measure hundreds to thousands of measurement points while moving the NMR probe 1 one by one. Also, the process of organizing the measured data to calculate the value of the current flowing through each shim coil by the least squares method is troublesome. Furthermore, a jig for installing and moving the NMR probe 1 is necessary for normal measurement, and if there is another object such as a table device in the magnet, the jig cannot be installed. There is also a problem that the measurement must be performed in a hollow state, and the measurement becomes large.

FIG. 6 is a timing chart showing a pulse sequence of a magnetic field intensity distribution measuring method using the DIXSON method, where t is a time axis, and RF is a static magnetic field direction (excitation of a magnetization vector of protons oriented in the Z direction). RF (Radio-Frequ
ency) wave, which is called a 90 ° pulse or a 180 ° pulse depending on the rotation angle. SE is observed when, after application of a 90 ° pulse, the phase of the magnetization vector dispersed by the inhomogeneity of the static magnetic field and the gradient magnetic field is reversed by 180 ° pulse and converges again in the plane (XY plane) perpendicular to the static magnetic field direction. (Hereinafter referred to as SE signal). Gphase and Gread are respectively a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field. For example, three-dimensional position information is given to the SE signal in such a manner that the X and Y directions correspond to the phase and the Z direction corresponds to the frequency information. In the Dickson method, two types of pulse sequences having different 180 ° pulse application timings are used. That is, in the S1 scan, the application timing tp of the 180 ° pulse is set to tp = middle of the time TE from the 90 ° pulse until the spin echo is obtained.
Tp = ε earlier in S2 scan than in S1 scan
TE / 2−ε. S2 scan also obtain spin echo timing, the same T _E as S1 scan. In the data obtained from these two scans, in the S1 scan, the phase is inverted by the 180 ° pulse at the time TE / 2, which is the middle of TE, so that the phases of the magnetization vectors separated at the time TE coincide. In the S2 scan, the phase is inverted by a 180 ° pulse earlier by ε, so that each magnetization vector is additionally rotated by 2ε time, and a phase shift occurs according to the magnetic field intensity at each location. Therefore, the image data S1 and S2 obtained by performing three-dimensional Fourier transform on the data obtained by each scan and reconstructing the image are represented by the following equations.

S1 = ρ (x, y, z) · exp (i αo) (1) S2 = ρ (x, y, z) · exp (i αo + E (x, y, z))
(2) Here, i ² = −1, and exp (i αo) is a zero-order phase offset component unique to the device generated in a receiving system or the like, E (x,
(y, z) is the non-uniform distribution of the static magnetic field.

Next, a non-uniform static magnetic field distribution E (x, y, z) is obtained for each pixel of the measurement FOV according to the following equation.

According to the measurement method by this imaging, it can be measured in a few minutes, without mechanical operation such as the above-mentioned NMR probe, only by installing a water phantom, with a table etc. incorporated in a magnet. Easy to measure.

(Problems to be Solved by the Invention) However, the above-described measurement method using imaging has the following problems. Since the detection range of the phase value of the non-uniform distribution is from -π to π, the measurable non-uniformity is limited. For example, in S2 scan
Assuming that the resolution of the time setting of the 180 ° shift time ε is 0.1 msec and the resonance frequency of the proton is 21.29 MHz, the range E (ppm) of the obtained nonuniformity is 2ε · 21.29 × 10 ⁶ · E · 2π = ± π ... (4), E ≒ ± 110 ppm. Therefore, if the degree of non-uniformity is larger than E, it cannot be measured. Furthermore, it is limited by the band of the RF pulse. FIGS. 7A and 7B are diagrams showing the relationship between the pulse width and the band of the RF pulse. In FIG.
(A) shows the RF pulse waveform, and (b) shows its Fourier spectrum. The band of a rectangular waveform having a pulse width τ as shown in FIG. 7A can be estimated as its Fourier transform. As shown in FIG. 7B, the zero point is 1 / τHz. For example, if the pulse width is 0.5 msec, the band of the RF pulse is 2 KHz. In a 0.5 Tesla system, the measurement range L in this band is L = ± 2 × 10 ³ /(21.29×10 ⁶ ) ≒ ± 90 ppm (5) The measurement range L can be expanded by reducing the pulse width τ of the RF pulse, but the power of the RF pulse is 1 / τ
^Because it is proportional to ^2, it is limited by the RF power limit.
Further, as a result of the flip angle of the magnetic moment to be excited being reduced, the signal S / N deteriorates. Generally, in a system of 0.5 Tesla, the RF power is appropriately about 2 KW in terms of cost, so that the measurement lens L has a measurement range of about ± 110 ppm.

As described above, the measurement method using imaging has an advantage that the measurement can be performed more easily than the method using the NMR probe, but has a problem that the nonuniformity that can be measured is limited. Therefore, it was not practical because a non-uniform distribution of the magnetic field from the order of several hundred ppm could not be measured.

An object of the present invention is to provide an MRI magnetic field correction method and apparatus that solves the above-mentioned problems, performs magnetic field measurement in a short time, and performs accurate magnetic field correction.

(Means for Solving the Problems) In an MRI magnetic field correction method of performing imaging for magnetic field intensity distribution measurement, obtaining a static magnetic field non-uniform distribution from the scan data, calculating a correction magnetic field intensity and generating a correction magnetic field, The region of the non-uniform distribution used for the calculation is changed according to the non-uniformity of the non-uniform distribution of the static magnetic field. Further, the degree of non-uniformity to be measured is sequentially increased from a low order to a high order, and accordingly, a different weighting function depending on the magnitude of the FOV, or according to the magnitude of the signal strength of the static magnetic field non-uniform distribution. By sequentially increasing the FOV used for correction using a weighting function, and further, according to the non-uniformity, the value of the time ε, or by changing the band of the RF pulse, the measurement accuracy of the magnetic field measurement. It is characterized by raising.

(Operation) The magnetic field inhomogeneity is such that when the inhomogeneity is large, a component having a low-order location dependence is dominant. Therefore, even if the measurement area is reduced, a large error occurs in the corrected magnetic field strength determined from the measured value. This does not occur. On the other hand, as the degree of uniformity increases, higher-order location-dependent components become effective. Therefore, by expanding the area used for correction, higher-order components can be corrected with high accuracy. Therefore, the measurable nonuniformity can be increased by the above means.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing a magnetic field correction device according to one embodiment of the present invention. In FIG. 1, reference numeral 6 denotes a main magnetic field magnet for generating a static magnetic field in the Z direction. Reference numeral 7 denotes a magnet disposed inside the main magnetic field magnet, and distributes the static magnetic field distribution in a plurality of directions, that is, X, Y, Z (1).
Next), XY, ZX, ZY, Y 2, Z 2, X 2 -Z 2, Z 2 -Y 2 (2 -order), X ^3, Y ^3,
Z ³ of (tertiary), Z ⁴ (4-order), Z ⁵ (5-order), Z ⁶ (6-order) 16
A shim coil that corrects by a magnetic field having channel directionality and increases its uniformity; 8, a gradient magnetic field coil that applies a gradient magnetic field in each of X, Y, and Z directions for the purpose of giving three-dimensional position information to an RF signal; 9 is further arranged inside,
An RF coil that transmits and receives RF pulses. Reference numeral 10 denotes a phantom filled with homogeneous water arranged in the magnetic field measurement range of the static magnetic field column, 11 denotes a shim coil power supply for supplying a correction current to each channel of the shim coil 7, 12 denotes a gradient magnetic field coil power supply,
13 is an RF circuit for transmitting an RF pulse waveform and processing a signal received by an RF coil, 14 is a scan controller for controlling a scan sequence such as a pulse waveform and an application timing of an RF pulse and a gradient magnetic field pulse, and 15 is a scan obtained. The computer determines a non-uniform distribution of static magnetic field from data, changes a region of the non-uniform distribution used for the correction according to the non-uniformity to be measured, and determines a correction magnetic field intensity.

In the above configuration, the magnetic field measurement and correction method of this embodiment will be described below with reference to FIG. FIG. 2 is a flowchart showing a magnetic field correction method according to one embodiment of the present invention. First, a 180 ° shift time ε in the S2 scan and a pulse width τ of the RF pulse are set. These values are
It is determined by the range of the nonuniformity E (ppm) to be measured. For example, when E is ± 100 ppm, the shift time ε is 0.1 msec,
The pulse width τ is set to 0.5 msec, and when E is ± 50 ppm, the shift time ε is set to 0.2 msec, and the pulse width τ is set to 1.0 msec. Next, imaging for magnetic field measurement using the same Dickson method as in the past is performed. The scan sequence includes the S1 and S2 scans in FIG. 5C, that is, the S1 scan in which the application timing of the 180 ° inversion pulse is between the application of the RF pulse of 90 ° and the reception timing of the spin echo signal, and the time ε from the middle. Perform S2 scan with shifted timing. At this time, the shooting area FOV is set to the ultimately required maximum FOV. Next, each scan data thus obtained is image-reconstructed by 3D Fourier transform, and the static magnetic field non-uniform distribution E is obtained for each pixel of the measurement FOV.
(X, y, z) is obtained from tan ^-1 (S2) / (S1). here,
When the uniformity E is equal to or less than the finally required value, the correction is terminated. When the uniformity E is equal to or greater than the value, the static magnetic field inhomogeneous distribution E (x, y,
In z), specify the range of calculation data used for calculating the correction current value. Here, the designation of the FOV is based on the aforementioned ε, τ
Is determined so as to adapt to the nonuniformity E set by the above.
FIG. 3 is a schematic diagram showing the setting of the calculation FOV of this embodiment. As shown in Fig. 3, for example, the specification of the magnet is
FOV (solid line) = φ500mm Uniformity in DSV (Diameter Spher Volume) is 300ppm, uniformity in the above setting is 100ppm
Then, the FOV (dotted line) for calculation is determined as φ200 mmDSV. Field inhomogeneity E is, when uneven large X, Y, Z (1-order), XY, ZX, ZY, Y 2, Z 2, X 2 -Z 2, Z 2 -Y 2 (2
Since there is a low-order place dependency such as the following, even if the measurement area is made small and used for calculation, a large error does not occur in the correction magnetic field strength determined from the measurement value. Therefore, when the degree of inhomogeneity is large, the FOV for calculation is made small, low-order magnetic field correction is performed, and X ³ , Y ³ , Z ³ (3
Next), Z ⁴ (fourth order), Z ⁵ (fifth order), Z ⁶ (sixth order) and other high-order place-dependent components become effective, so the area used for correction is expanded, and finally Is the maximum when scanning
FOV. This is because, in order to perform approximation using a high-order function system, the error increases unless the number of data points far from the coordinate origin is large. Next, the inhomogeneous magnetic field ΔB
(X, y, z) is approximated by the correction magnetic field generated by the channel Si of the shim coil to be used, and the current value Ai of each channel is calculated by the least square method. Therefore, Ai is determined so that In the equation (6), the channel i to be used shifts from a low order to a high order as the range FOV of the calculation data is increased, and the uniformity is increased. Finally, according to the determined current value Ai, a current is supplied to each shim coil channel to generate a correction magnetic field,
The imaging for the magnetic field measurement is performed again to measure the magnetic field uniformity. In this way, the correction is repeated until the uniformity E becomes equal to or less than the finally required value.

As described above, according to the magnetic field correction method and apparatus of the present embodiment, when the degree of inhomogeneity is large, the FOV for calculation is reduced, and low-order magnetic field correction is performed. Since the magnetic field is increased and high-order magnetic field correction is performed, the measurable nonuniformity can be increased.
It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the claims. FIGS. 4 (a) and 4 (b) are diagrams showing a method of specifying a range of calculation data according to another embodiment of the present invention. In the above-described embodiment, the range of the calculation data used for determining the correction current is set by the operator. However, as shown in FIG. 4A, the inside of a certain FOV is set to 1 and the outside is set to 0. A weighting function having a weighting factor is used to weight the measured data of the non-uniform distribution, and as the number of times of magnetic field measurement increases, a weighting function having a wider FOV with a weighting factor of 1 is used to automatically perform the weighting. May be specified. Further, as shown in FIG. 4 (b), the signal strength of the obtained data of the non-uniform distribution becomes weaker as the degree of non-uniformity becomes larger. The range may be automatically specified by weighting the data of the non-uniform distribution measured in the same manner as described above by using a weighting function W having a weighting factor with 0 as the outside and 0 outside. .

(Effects of the Invention) As described above, according to the MRI magnetic field correction method and apparatus of the present invention, a magnetic field can be measured in a short time and accurate magnetic field correction can be performed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a magnetic field correction apparatus according to one embodiment of the present invention, FIG. 2 is a flowchart showing a magnetic field correction method according to one embodiment of the present invention, and FIG. FIGS. 4 (a) and 4 (b) are views showing a method of designating a range FOV of calculation data according to another embodiment of the present invention, and FIGS. 5 (a) and 5 (b) are conventional examples. FIG. 6 is a timing chart showing a pulse sequence of a conventional method for measuring a magnetic field intensity distribution using the Dickson method, and FIGS.
(B) is a diagram showing the relationship between the pulse width of the RF pulse and the band. 1 ... NMR probe, 2 ... water sample, 3 ... RF coil, 4 ... variable condenser, 5 ... T / R circuit, 6 ... main magnetic field magnet, 7 ... shim coil, 8 ... gradient magnetic field coil, 9… RF coil, 10… Phantom, 11… Sim coil power supply, 12… Gradient magnetic field coil power supply, 13… RF circuit, 14… Scan controller, 15… Computer

Claims (2)

(57) [Claims] A step of obtaining a static magnetic field non-uniform distribution by applying a pulse sequence for measuring a magnetic field distribution; and changing a region of the non-uniform distribution according to the non-uniformity of the static magnetic field non-uniform distribution. Generating a correction magnetic field for making the static magnetic field uniform, the method for correcting a magnetic field for nuclear magnetic resonance imaging. 2. A means for obtaining a static magnetic field non-uniform distribution by applying a pulse sequence for measuring a magnetic field distribution, and changing a region of the non-uniform distribution according to the degree of non-uniformity of the static magnetic field non-uniform distribution. A nuclear magnetic resonance imaging apparatus including a magnetic field correction unit that generates a correction magnetic field that makes a static magnetic field uniform.