JPH0368344A - Magnetic resonance photographing - Google Patents

Abstract

PURPOSE:To obtain the magnetic resonance photographing method free from the influence of deflection when an image is reconstructed, by correcting an echo signal by obtaining the difference from the observed frequency according to the resonance frequency obtained in the region in the vicinity in each image sequence and putting the corrected echo signal into the two-dimensional Fourier transformation. CONSTITUTION:In the front placed sequence T01, an FID signal S3 is taken in after the lapse of a prescribed time after the excitation of a slice surface outside a diagnosis region, and put into the unidimensional Fourier transformation, and the resonance frequency (f0) is determined according to the peak position of the spectrum. In the main sequence T02, the spin echo signal S11 is received, applying the Gx gradient magnetic field S10, and stored into the memory of a computer. Then, the same procedures in the main sequence T02 is repeated, varying the magnetic field gradient Gyi, and the resonance frequency (fi) and the echo signal S11 are obtained each time. When an image is reconstructed, the echo signal is corrected according to the deflection of the resonance frequency, and put into the two dimensional Fourier transformation, and a magnetic resonance imaging image in which the deflection of the resonance frequency is corrected is reconstructed.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a magnetic resonance imaging system (hereinafter referred to as rMRI system) that uses nuclear magnetic resonance phenomena to create a tomographic image of a subject. The present invention relates to a resonance imaging method.

<Conventional technology> An MRI apparatus 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).
selectively excites the nuclear spins of the N MR
This is a device that receives a double signal and converts the signal strength difference into an image as a-light information.

To explain an example of the imaging sequence by the above-mentioned MRI apparatus, first, a −-like static magnetic field and a gradient magnetic field Gz having a gradient in the direction Z of the static magnetic field are applied to reduce the nuclear spins of the slice plane by 9.
Selectively excite with 0"' RF pulse. Next, a gradient magnetic field Gy in the phase encoding direction y is applied to cause a rotational phase shift of the spins (phase encoding). Subsequently, 18
Apply a 0' RF pulse to invert the phase of the spins,
After a predetermined time, a magnetic field Gx in the reading direction is applied to observe the echo signal.

The echo signal of the above procedure is measured many times by changing the magnitude of the gradient magnetic field Gy, that is, by changing the amount of phase encoding.

The reception of the echo signal is performed using R
This is done through the F receiving coil. A reception signal received by the RF reception coil is detected by a receiver and converted into a digital signal.

A two-dimensional tomographic image can be reconstructed by Fourier transforming this digital signal, collecting it as spatial frequency domain data (echo data), and performing a two-dimensional inverse Fourier transform.

By the way, when performing imaging with the above-mentioned MRI apparatus, conventionally, before entering the imaging sequence, the resonant frequency fo of the slice plane is determined using an adjustment sequence, and this value is set in the RF transmitter/receiver as the observation frequency. After this, imaging begins, and this value is not changed during imaging. Therefore, this method is based on the premise that the resonant frequency set in the adjustment sequence before imaging does not change during imaging, or even if it changes, it is so small that it can be ignored.

However, for example, in the case of an MHI device that has a magnetic field generator using a normal conducting magnet or a permanent magnet, if the temperature control of the cooling water or the temperature control of the air conditioner at the location where the magnet body is installed does not work well, the static magnetic field Intensity may fluctuate gradually. Furthermore, even if the amount of variation is small, when imaging is carried out for a long time, the variation may not be able to be ignored as it may affect the image.

Figure 4 (a) = (b) is a graph showing the spatial frequency spectrum, and when the initially set resonance frequency does not change (see Figure 4 (a)), the signal in all projection data is The observed frequency fo and the center frequency of the spectrum after the Fourier transform match. However, if the resonant frequency changes due to changes in the static magnetic field (see Figure 4(b)), the observed frequency fo and the center frequency of the spectrum become
Amount of change in resonance frequency Δfi+ Δf2+ ..., Δf
21. When reconstructing an image using these data, this effect may appear as an artifact.

An object of the present invention is to provide a magnetic resonance imaging method in which the shift in resonance frequency due to fluctuations in static magnetic field strength and the like during imaging is corrected, so that the shift does not affect image reconstruction.

<Means for Solving the Problems> In order to achieve the above object, the magnetic resonance imaging method of the present invention selectively modulates nuclear spins in the imaging region in each imaging sequence Ti, which is performed while changing the amount of phase encoding each time. Before excitation, nuclear spins in the vicinity of the imaging region are excited, the FID signal from the excited nuclear spins is received, the resonance frequency fi is determined, and the resonance frequency fi and the resonance frequency fi obtained in the first sequence are calculated. The echo signal Sl is corrected based on the difference Δfi from the resonant frequency (observation frequency) fo, and the corrected echo signal Sl
This is a method of reconstructing an image from ′.

<Operation> According to the magnetic resonance imaging method having the above configuration, in each imaging sequence Ti, before selectively exciting the nuclear spins in the imaging region, the nuclear spins in the vicinity of the imaging region are excited, and the excited Since the resonant frequency fi is determined by receiving the FID signal from the nuclear spin, even if the resonant frequency changes over time due to a change in the magnetic field, the amount of change can be determined.

Then, the resonance frequency fi and the observation frequency f.

Since the echo signal S1 is corrected based on the difference Δfi between the two and the image is reconstructed using the corrected echo signal Si', the image is not affected by the shift in the resonant frequency due to fluctuations in the static magnetic field, etc. during imaging. Can you get it?

Note that when measuring the resonance frequency fi, nuclear spins in a region near the imaging region are excited because if the same region is excited, there is a risk that the imaging sequence will begin before the spins have sufficiently relaxed. It is from.

<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 MR,I apparatus. The MHI device includes a Gz gradient magnetic field coil 16a1 for slice plane selection, a GX gradient magnetic field coil 16b1 for continuous output, a Gy gradient magnetic field coil 16c1 for phase encoding, a magnetic field generator 16, and an R for oscillating RF pulses and detecting signals.
F coil 11, preamplifier 13, RF transmitter/receiver 14, gradient magnetic field power supply circuits 17a to 17c that supply excitation power to each gradient magnetic field coil 16a to 1.6c, gradient magnetic field power supply circuit 17a to], 7c to a predetermined value A controller 18 that is driven based on the timing, and a computer 19 that controls the controller 18 and processes the NM'R signal output from the RF transceiver 14 to create an MHI tomographic image.
It is composed of a memory 20 for storing MR signal information and a display 21 for temporarily storing and displaying image information.

The magnetic field generator 16 uses a normal conducting magnet or a permanent magnet, and has a characteristic that the strength of the static magnetic field changes depending on the temperature of the cooling water and the temperature of the installation location. Therefore, temperature management of the system is necessary, but according to the magnetic resonance imaging method described below, this temperature management can be made unnecessary or the accuracy of this temperature management can be reduced.

FIG. 1 shows one unit of the imaging sequence adopted in the program loaded into the computer 19.
The imaging sequence TiJ is hereinafter referred to as "imaging sequence TiJ".The imaging sequence Ti is repeated 256 times, for example from i-0 to 1-255, while changing the value of the phase encode cy gradient magnetic field.The imaging sequence T+ is used to observe the resonance frequency. A pre-sequence Tit for obtaining an echo signal, and a Hon-ken sequence Ti2 for obtaining an echo signal.

First, in the first pre-sequence TOI, a selective excitation pulse S2 is applied while the slice direction Gz gradient magnetic field S1 is being applied to excite the slice plane other than the diagnostic region. After a predetermined period of time, the FID signal S3 is taken in.

The FID signal S3 obtained by the above prefix TOI is subjected to one-dimensional Fourier transform, and the resonance frequency fo is determined based on the peak position of its spectrum. Note that when determining the peak position of the spectrum, the frequency resolution (FID
(expressed as the reciprocal of the sampling interval when sampling the signal) is finite, so there is a possibility that the data will be given as a point P2 different from the original peak value P shown by the broken line in Figure 3 (a). There is. To prevent this, it is assumed that the data near the maximum value is given by a quadratic function, and three pieces of data P2 . P3
．． P4 is used to find the maximum value P4 of the data (Figure 3 (
b)).

Thereafter, the resonant frequency fO given in this first presequence TO+ is fixed as the observation frequency during the subsequent imaging sequence.

Subsequently, the main sequence TO2 is entered. In this main sequence TO2, the frequency set in the RF transceiver is the value fo observed in the prefix sequence TO+. First, with the Gz gradient magnetic field S4 applied, the RF coil 11 applies a 90" RF pulse S7 to the subject to tilt the nuclear spins of the desired slice plane by 90 degrees. After this, the Gx gradient magnetic field S6 and the Gy gradient magnetic field S5 are applied. The gradient magnetic field G is applied while continuously changing the phase of the nuclear spins in the slice plane.
By inverting the sign of 2, the phases of spins that are disordered during excitation are aligned. Subsequently, with the Gz gradient magnetic field S8 applied, a 180° RF pulse S9 is applied from the RF coil 1 to invert the nuclear spin.

After a predetermined time, the spin echo signal S1.1 is received while applying the Gx gradient magnetic field SIO. This signal S11
is stored in one computer's memory.

Next, the second imaging sequence T1 is entered. The imaging sequence T1, like the imaging sequence To described above, is composed of a prefix sequence Tit and a main sequence T12. The FID signal obtained in the pre-sequence T is subjected to one-dimensional Fourier transform, and the resonance frequency fi is detected. The value of the resonant frequency 11 is the observed frequency f determined by the prefix sequence TO1.

stored on one computer's memory in the form of a difference between The imaging procedure in this sequence T12 is the same as that in the above-mentioned main sequence T12, and the observation frequency also uses the value fo determined in the first pre-sequence TQL.

Thereafter, the same procedure is repeated while changing the gradient magnetic field G yl (i=2 to 255), and each time the resonance frequency fi and the echo signal Sll are determined.

Table 1 is a table in memory that stores the difference Δf 1 (1-1 to 255) between the resonance frequency f 1 (1 = 0 to 255) found in each imaging sequence Ti and the observed frequency fO. .

Table 1 The phase of the observed echo signal is shifted by the difference between these resonance frequencies. FIG. 4(b) is a graph showing a state in which the phase of the echo signal 5j (i-0 to 255) is shifted on the spatial frequency axis, and compared to the graph in FIG. 4(a) with no phase shift, the signal is You can see that the center is off.

Therefore, during image reconstruction, the echo signal is corrected based on this phase shift. When the resonance frequency shift is Δfi, the phase shift φl (Δ1.n) is given by the following equation (1).

φ1(Sl, n) −2n ・Δf+−Δt−n(1)
Δt: Signal sampling interval n: Number of points (-number of sampling points 72≦
(specified in the range of 05 sampling number/2) To correct the echo signal S1(ty, tz) on the real time axis using the phase shift given by equation (1),
Formula (2) is used.

S i' (ty, tz) = S 1 (ty, tz)
e-lyd(61,°ゝ= f f f
(y, z) e −It(Gyly+G+++1
e-1γφ(Δ'-n>dt, dt, (2) f
(y, z): Nuclear magnetic distribution Gy: Phase encoding amount ty: Application time of Gy Gz: Frequency encoding amount tz: Application time of Qz γ: Nuclear gyromagnetic ratio S' l (ty , tz) can be subjected to two-dimensional Fourier transformation to reconstruct an MRI image in which the resonance frequency shift is corrected.

Note that the present invention is not limited to the above-described embodiments, and is also applicable to imaging methods other than the spin echo method, for example, the gradient echo method. Various design changes can be made without changing the gist of the invention.

<Effects of the Invention> As described above, according to the magnetic resonance imaging method of the present invention, in each imaging sequence, the difference Δfi from the observed frequency fo is determined using the resonance frequency fi determined in the nearby region, and the echo signal S 1 (ty, tz) and the obtained S'
By performing a two-dimensional Fourier transform on i (ty, tz), it is possible to reconstruct an MRI image in which the resonance frequency shift has been corrected. Therefore, an image with excellent S/N without artifacts can be obtained.

[Brief explanation of drawings]

Fig. 1 is a waveform diagram showing the imaging sequence of the magnetic resonance imaging method, Fig. 2 is a schematic configuration diagram of the MRI apparatus, Fig. 3 is a graph explaining the method of calculating the pic value from the spectrum waveform of the FID signal, Fig. 4 is a graph showing the phase shift of echo signals on the spatial frequency axis. Special; 1-Applicant Sanyo Electric Co., Ltd. (Q) Figure (b)

Claims (1)

[Claims] 1. A gradient magnetic field is applied to the object placed in a static magnetic field. and applying RF pulses to selectively excite nuclear spins, and the excited For the imaging region, the phase encoding gradient Phase encoding is performed by applying a magnetic field, After that, apply a gradient magnetic field in the reading direction. The imaging process receives echo signals from The phase encoding gradient magnetic field Repeat each time, changing the size of The echo signal Si obtained twice is Fourier-transformed. data in the spatial frequency domain by data, and use this data as an inverse Fourier. The two-dimensional tomographic image is regenerated by In the magnetic resonance imaging method comprising: In each of the above imaging sequences Ti, Selectively excite nuclear spins in the imaging region Before, the nuclear spin in the vicinity of the imaging region is from the excited nuclear spin. Receive FID signal and find resonance frequency fi seek, The above resonance frequency fi and the first sea Difference from resonance frequency fo observed with Kens Correct the echo signal Si using Δfi Based on this corrected echo signal Si′ A magnetic device that reconstructs an image using Air resonance imaging method.