JPH0422094B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a nuclear magnetic resonance imaging diagnostic apparatus that obtains a blood vessel image based on a phase change that occurs over time in a certain portion of a flow velocity.

(Prior art) Nuclear magnetic resonance imaging diagnostic imaging equipment (hereinafter referred to as NMR-CT) that uses the phenomenon of nuclear magnetic resonance (hereinafter referred to as NMR) to obtain a tomographic image of a subject focusing on specific atomic nuclei.
has been known for a long time. The principle of this NMR-CT will be briefly explained.

An atomic nucleus can be seen as a spinning top that is magnetically charged, but it can be interpreted by a static magnetic field in the z-axis direction.
When placed in H ₀ , the above-mentioned atomic nucleus precesses at an angular velocity ω ₀ given by the following equation. This is called Larmor precession.

ω ₀ = γH ₀ However, γ: nuclear gyromagnetic ratio Now, a high-frequency coil is placed on an axis perpendicular to the z-axis with a static magnetic field, for example, the x-axis, and the high-frequency coil with the above-mentioned angular frequency ω ₀ rotates in the xy plane. Magnetic resonance occurs when a rotating magnetic field is applied, and a population of atomic nuclei undergoing Zeeman splitting under the static magnetic field H ₀ undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, and the energy level changes. Transition to a higher level. Here, since the nuclear gyromagnetic ratio γ differs depending on the type of atomic nucleus, the atomic nucleus can be specified by the resonance frequency. Furthermore, by measuring the strength of that resonance, we can determine the amount of that nucleus present. During a time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

The medium pulse method for observing this NMR phenomenon will be explained with reference to FIG.

As mentioned above, when a high-frequency pulse (H ₁ ) that satisfies the resonance condition is applied in the (x-axis) direction perpendicular to the static magnetic field (z-axis), the magnetization vector M changes in the rotating coordinate system as shown in Figure 4A. It starts rotating in the zy plane with an angular frequency of ω' = γH ₁ . Letting the pulse width be _tD , the rotation angle θ from _H0 is expressed by the following equation.

θ=γH ₁ t _D …(1) The first pulse shown in Figure 4 C is called an excitation pulse, and in particular, the pulse with t _D such that θ=90° in equation (1) is called a 90° pulse. call. Immediately after this 90° pulse, the magnetization vector M rotates at ω ₀ in the xy plane as shown in FIG. However, since this signal attenuates over time, this signal is called a free induction attenuation signal (hereinafter referred to as an FID signal). By Fourier transforming the FID signal, a signal in the frequency domain can be obtained. Next, as shown in Figure 4 C.
A pulse applied τ time after the 90° pulse is called an inversion pulse, and in particular, a second pulse with a pulse width such that θ=180° is called a 180° pulse. When a 180° pulse is applied, the scattered magnetic moments refocus in the -y direction after τ time, and a signal is observed. This signal is called a spin echo (hereinafter referred to as SE)
It is called a signal). A desired image can be obtained by measuring the intensity of this SE signal. NMR
The resonance condition is given by ν=γH ₀ /2π. Here, ν is the resonant frequency and H ₀ is the strength of the static magnetic field. Therefore, it can be seen that the resonant frequency is proportional to the strength of the magnetic field. For this reason, there is an NMR imaging method in which a linear magnetic field gradient is superimposed on a static magnetic field, giving a magnetic field of different strength depending on the position, and changing the resonance frequency to obtain positional information. Of these, the Fourier transform method will be explained. The pulse sequence for applying the high frequency magnetic field and gradient magnetic field used in this method is shown in FIG. In figure A, x,
It shows a state in which gradient magnetic fields of Gx, Gy, and Gz are applied to the y and z axes, respectively, and a high frequency magnetic field is applied to the x axis. The figure B is a diagram showing the timing of applying each magnetic field. In the figure, RF is a high-frequency rotating magnetic field that applies 90° pulses and 180° pulses to the x-axis.
Gx is a fixed gradient magnetic field applied to the x-axis called the lead axis, Gy is a gradient magnetic field whose amplitude changes over time and applied to the y-axis called the warp axis, and Gz
is a fixed gradient magnetic field applied to the z-axis called the slice axis. The signal shows the SE signal after the 180° pulse. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. In period 1
Spins in the slice plane in the tomography perpendicular to the z-axis centered at z=0 are selectively excited by the 90° pulse and the gradient magnetic field Gz ⁺ . For this reason, the 90° pulse is called an excitation pulse. Gx ⁺ in period 2 is used to disrupt the spin phase and invert it with a 180° pulse, and is called a day phase gradient. or,
The 180° pulse is called an inverted pulse. Gz ^- is Gz ⁺
This is to restore the phase of spins that have been disturbed by During period 2, a phase encoding gradient Gyn is also applied. This is to shift the phase of the spin in proportion to the position in the y direction, and its intensity is controlled to be different every cycle. In period 3
Apply a 180° pulse to align the magnetic moments again,
Observe the SE signal that appears after that. period 4
Gx ⁺ is a gradient magnetic field that aligns the disturbed phases and generates an SE signal, which is called a ref-phase gradient. An SE signal appears when the areas of the ref-phase gradient and the day-phase gradient become equal.

This sequence is called a view, and after the pulse repetition period _TR , a 90° pulse is applied again to start the next view. The warp gradient described above is changed corresponding to each view.

In NMR-CT as described above, blood flow imaging applies a gradient magnetic field in the direction of flow that does not affect stationary objects but only affects moving objects, and adds different phase information depending on the flow velocity. This is what I am trying to do. The principle will be explained with reference to FIG. Now, assume that blood flow is flowing in the blood vessel 1 in the x direction. A gradient magnetic field 2 is applied at time τ ₁ , and a reversal gradient magnetic field 3 is applied at time τ ₂ after Δτ.
A reversed gradient magnetic field is a magnetic field that has the same magnitude but reverses sign. Since the stationary object 4 does not move, it feels magnetic fields (G ₁ and -G ₁ ) with the same magnitude and opposite sign at times τ ₁ and τ ₂ , and their effects cancel each other out, and no gradient magnetic field is applied. The situation is the same as in the case. On the other hand, since the blood flow part moves, different magnetic fields G ₁ 5 and G ₂ 6 are felt at times τ ₁ and τ ₂ , and their influence is not canceled, causing a change in the phase of the spins. A gradient magnetic field that provides phase information to the movement of blood flow by providing a gradient magnetic field with the same magnitude and opposite sign is called a flow encode gradient.

FIG. 1 shows a pulse sequence when blood flow is measured using a flow encode gradient. In the figure, spins excited by an excitation pulse 11 are disturbed by a dephasing gradient 12, but are brought back in phase by a readout gradient 13, producing an echo signal 14. At this time, information on the position in the y-axis direction is given to the spins as phase information by the warp gradient 15. In this way, one image (referred to as image A) is obtained. Next, a flow encoding gradient 16 is applied to the x-axis as shown in the figure to obtain yet another image (referred to as image B). As shown in Figure 3, the flow encoding gradient is a gradient with opposite polarity and equal area, so the change in the spin phase of the image of a stationary object among the images obtained by applying the flow encoding gradient 16 is as follows: become.

Δθ=γ∫ ^T _-T G(t)dt=0 Here, γ...Gyromagnetic ratio G(t)...Output of gradient magnetic field Therefore, the spin of a stationary object is influenced by the flow encode gradient 16. First, with respect to a stationary object, images A and B are the same.

For spins moving in the flow encoding direction, in this case the x-axis direction, Δφ=γ∫ ^T _-T G(t)X(t)dt...(2) Here, X(t)...position of the spin When movement is expressed by a linear function such as X(t)=X+vt, equation (2) becomes as follows.

Δφ=γvT∫ ^T ₀ G(t)dt (3) As is clear from equation (3), the spin phase change Δφ is proportional to the flow velocity v. When the difference between image A and image B obtained in this manner is determined, the image of a stationary object disappears, and a blood vessel image represented by blood flow having a phase change proportional to the flow velocity is obtained.

(Problem to be Solved by the Invention) By the way, as described above, a blood vessel image is obtained by the difference between images obtained with different pulse sequences, but in reality, blood vessel imaging using a method that changes the phase depending on the blood flow velocity If there is phase distortion, the obtained image will be adversely affected, such as static parts other than blood vessels remaining unerased. The reasons for this are as follows.

(1) Shift in the sample position origin of the NMR signal due to errors in the rise of the gradient magnetic field and residual gradient magnetic fields due to eddy currents (2) Phase offset of the device (3) Inhomogeneity of the static magnetic field The difference between image A and image B above Consider the phase of complex numbers of two images. Expand the pixel phase {i(x,y)}
Then, the phase of the image when the flow encoding gradient is applied is exp{i ₁ (x, y)} = exp {iθ ₀ (x, y) + iθ ₁ (x, y) + iθ _B (x, y) + iθ _F (x, y)} ...(4) However, θ ₀ ... Phase offset of the device θ ₁ ... Phase change due to gradient other than flow encode gradient θ _B ... Phase change due to magnetic field inhomogeneity θ _F ... Phase due to flow encode gradient If no change flow encoding gradient is applied, exp{i ₂ (x, y)} = exp {iθ ₀ (x, y) + iθ ₁ (x, y) + iθ _B (x, y)} …(5) 2 The phase of the phase image representing the phase difference obtained by calculating the difference between the two images is obtained from equations (4) and (5) as follows: exp{i ₁ (x, y)} ・exp{−i ₂ (x, y)} = exp{iθ _F (x, y)} ...(6) That is, as is clear from equation (6), the two images of image A and image B
All differences between the two images are canceled out except for phase changes due to flow encoding gradients. The phase change included in θ _F (x, y) due to this flow encode gradient is (1) phase change due to flow (2) phase distortion due to area error caused by rising flow encode gradient and eddy current (3) There are phase distortions due to the location dependence of eddy currents. The image we are looking for due to the phase change is due to the phase change due to the flow (1), and the phase distortions (2) and (3) are unnecessary and their presence has a negative effect.
An example of an image containing this phase distortion is shown in FIG.
In Fig. A, the blood vessel 20 differs from the original image of the blood vessel 20 shown in Fig. B due to phase distortion 21 due to the flow encoding gradient. It is difficult to correct the overall phase distortion so that information on phase changes due to blood flow is not lost, and it is a laborious and time-consuming task.

The present invention has been made in view of the above problems, and
The purpose is to create a blood vessel image using two images obtained by changing the flow encoding gradient magnetic field.
The object of the present invention is to realize NMR-CT, which can correct phase distortion caused by various causes at once and easily obtain a blood vessel image free of phase distortion.

(Means for Solving the Problems) The present invention solves the above problems by using a flow encoding gradient in NMR-CT that creates a blood vessel image based on a phase change that occurs over time at a certain portion of the flow velocity. a pulse sequence means for executing a changed pulse sequence to obtain two images; a means for obtaining phase image data representing a phase difference by determining a difference between the two images obtained by the pulse sequence means; Residual phase distortion of data at each coordinate of the phase image obtained by the means is determined using data at coordinates located on a straight line passing through the coordinate and perpendicular to the direction in which the flow encoding gradient is applied. a distortion calculation means, and a residual phase distortion obtained by the residual phase distortion calculation means is subtracted from one of the two images, and the image obtained by the subtraction and the other of the two images are (Operation) The pulse sequence means obtains two images using a pulse sequence in which the flow encoding gradient is changed.
A means for calculating the phase difference obtains phase image data representing the phase difference from the difference between the two images. The residual phase distortion calculation means calculates the residual phase distortion of data at each coordinate of the phase image using data at coordinates located on a straight line passing through the coordinate and perpendicular to the direction in which the flow encoding gradient is applied, and The correction means subtracts this residual phase distortion from one of the two images, and the image obtained by the subtraction and the second
Addition and subtraction are performed between the two images and the other image so as to remove the residual phase distortion.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram of the apparatus of the present invention.

In the figure, reference numeral 31 has a space (hole) into which the subject is inserted, and surrounding this space, a static magnetic field coil applies a constant static magnetic field to the subject and generates a gradient magnetic field. A gradient magnetic field coil (the gradient magnetic field coil is equipped with coils for the three axes of X, Y, and Z), an RF transmitter coil that provides an RF pulse to excite the spin of the atomic nucleus within the subject, and a This is a magnet assembly in which receiving coils and other components for detecting NMR signals are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are each connected to a static magnetic field power supply 3.
2. Gradient magnetic field drive circuit 33, RF power amplifier 34
and a preamplifier 35. The sequence storage circuit 36 operates the gate modulation circuit 38 (modulates the RF output signal of the RF oscillation circuit 39 at a predetermined timing) in accordance with a command from the computer 37 in an arbitrary view, and generates an RF pulse signal based on the Fourier transform method. is applied from the RF power amplifier 34 to the RF transmitting coil. Further, the sequence storage circuit 36 operates the gradient magnetic field drive circuit 33 using a sequence signal based on the Fourier transform method, and supplies gradient magnetic fields to the three axes of x, y, and z, respectively, as shown in FIG. At the same time, the gate modulation circuit 38 and AD converter 41 are operated. A phase detector 40 detects the phase of the received signal output from the preamplifier 35 using the output of the RF oscillation circuit 39 as a reference signal. This output signal is converted into a digital signal by the AD converter 41 and input to the computer 37. Reference numeral 42 denotes an operation console for inputting instructions and various setting values to the computer 37 for realizing various pulse sequences, and the computer 37 switches the operation of the sequence storage circuit 36 and rewrites the memory based on the instructions. and image reconstruction calculations using each data from the AD converter 41. 43 is calculator 3
This is a display device that displays the image reconstructed in step 7.

Next, the operation of the apparatus configured as described above will be explained.

The operation console 42 is operated to set the timing of the pulse sequence, the amplitude of the RF pulse, the pulse width, etc., and a signal based on the set values is input to the computer 37. The computer 37 generates a control signal based on the set value and outputs the control signal to the sequence storage circuit 3.
Send to 6. The sequence storage circuit 36 controls the gradient magnetic field drive circuit 33 to generate a gradient magnetic field of a predetermined pulse sequence based on the above signal, and also controls the gate modulation circuit 38. Gate modulation circuit 38
modulates the RF signal oscillated and output by the RF oscillation circuit 39 into a signal having a set pulse width and amplitude, and supplies the modulated RF pulse to the RF power amplifier 34. This modulated RF pulse is amplified in the RF power amplifier 34, and in the static magnetic field generated by the static magnetic field power source 32 in the magnet assembly 31, it is combined with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 33. This causes the excited spins to resonate.
The SE signal generated by the resonance is received, amplified by a preamplifier 35, and input to a phase detector 40. The phase detector 40 is connected to the RF oscillation circuit 39
The input NMR signal is phase-detected using the output as a reference signal, and the output signal is sent to the AD converter 41.
The NMR signal converted into a digital signal by the AD converter 41 is subjected to predetermined processing according to the scan sequence in the computer 37 to reconstruct an image, and is displayed on the display device 43. The computer 37 can rewrite the contents of the sequence storage circuit 36, thereby realizing various scan sequences.

In the above NMR-CT, the operation console 42 is set to scan with the normal pulse sequence shown in FIG. If two images are obtained, they are subtracted by the computer 37. The obtained echo signal includes a phase change θ _F (x, y) due to the flow encode gradient shown in equation (6), and the image obtained from this echo signal shows the blood vessel as shown in Figure 6A. 20 and phase distortion 21 due to the flow encoding gradient. Since this phase distortion 21 is due to the flow encode gradient, the direction dependence in the axial direction to which the flow encode gradient is applied is dominant, and the phase distortion in other directions is slight. Therefore, the distribution of phase distortion is substantially as shown in FIG. The figure shows the case where a flow encoding gradient is applied to the x-axis. In the section a-a' expressed by the line segment a-a' parallel to the y-axis, the amount of phase distortion is equal. Therefore, in order to remove the phase distortion 21 due to the flow encoding gradient, Y
It is sufficient to take the average value of the phase on the line segment a-a' in the axial direction and correct the phase on that line segment. In the interval b-b', the blood vessel is generally thinner than its surrounding parts, and if the average value of the interval b-b' is taken, the phase change of the blood vessel 20 can be ignored, and the average value is By subtracting the value, the entire phase distortion can be removed without impairing information on phase changes due to blood flow, as shown in FIG. 6B.

The calculator 37 receives the echo signal and calculates the residual phase distortion in the x-axis direction as shown in equation (6) above, subtracts image A from the data of image B, adds and subtracts the two original images again, and calculates the residual phase distortion in the x-axis direction. The image is reconstructed by removing the phase distortion caused by the encoding gradient, and is displayed on the image display device 43.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and may be modified as follows.

In the embodiment, for example, the average value of the phase distortion was determined using the interval b-b' as a straight line, but in reality it may not be a straight line, so the interval b-b' is used as a straight line.
The curve b' may be obtained in the form of a function, and the interval b-b' may be expressed by the function, and the calculation may be performed for each point on the x-axis.

Cut along a plane parallel to the x-axis and perpendicular to the xy plane,
The function Δφ=f(x) of the curve formed by the phase distortion surface may be calculated. This is y in Figure 7.
This is equivalent to finding a function in the x-axis direction using the average value of phase distortion in the axial direction.

In the embodiment, images were obtained using a normal pulse sequence and a pulse sequence using a flow encoding gradient, but the pulse amplitude of the flow encoding gradient may be changed. The embodiment is a special case in which one side has a pulse amplitude of 0.

The flow encoding gradient is not limited to the x-axis, but may be applied to the y-axis or the z-axis.

Although the flow encoding gradient was corrected by being applied only in one direction of the x-axis, it may be applied in two of the three axes, the x, y, and z axes.

The pulse sequence was performed using a gradient inversion method in which the dephase gradient and readout gradient are reversed as shown in Figure 1, but it may also be performed by a pulse sequence in which the slope is reversed using a 180° pulse without inverting the slope. .

(Effects of the Invention) As explained in detail above, according to the present invention,
In addition to eliminating magnetic field inhomogeneity and phase distortion caused by the device, it is also possible to easily remove phase distortion caused by flow encoding gradients, making it possible to completely eliminate phase distortion by scanning only two images. Furthermore, since phase distortion is removed using the obtained image data itself, there is no need to have scan data in advance using a phantom or the like. Furthermore, it is not affected by changes in the condition of the subject,
It has become easier to perform NMR blood vessel imaging, which has a great practical effect.

[Brief explanation of drawings]

Fig. 1 is a diagram of a pulse sequence, A is a diagram of a pulse sequence of the normal spin warp method, Fig. 2 is a diagram of a pulse sequence using a flow encode gradient, Fig. 2 is a block diagram of the device of the present invention, and Fig. 3 is a diagram of a pulse sequence using a flow encode gradient. The figure is an explanatory diagram of flow encoding gradient, and Figure 4 is
Figure 5 is a diagram explaining the principle of the NMR-CT pulse method.
Diagram showing the pulse sequence of the magnetic field of NMR-CT,
Figure 6 is a diagram of a blood vessel image obtained by the flow encoding method, A is a diagram of a blood vessel image that includes phase distortion, B is a diagram of an image of only blood vessels that does not include phase distortion, and Figure 7 is a diagram of a blood vessel image that includes phase distortion.
FIG. 8, which is a diagram of an image having uniform phase distortion in the axial direction, is a diagram of the principle of blood vessel imaging using the flow encoding method. 1, 20... Blood vessel, 11... Excitation pulse, 12... Day phase gradient, 13... Readout gradient, 14... Echo signal, 15... Warp gradient, 21... Phase distortion due to flow encode gradient, 31... Magnet assembly, 32... Static magnetic field. Power supply, 33... Gradient magnetic field drive circuit, 34... RF power amplifier, 35... Preamplifier,
36...Sequence storage circuit, 37...Computer, 38
...Gate modulation circuit, 39...RF oscillation circuit, 40...
Phase detector, 41...AD converter, 42...operation console, 43...display device.

Claims (1)

[Scope of Claims] 1. In a nuclear magnetic resonance imaging apparatus that creates a blood vessel image based on a phase change that occurs over time in a certain portion of flow velocity, a pulse sequence with a varying flow encoding gradient is executed. 2 pulse sequence means for obtaining two images; means for obtaining phase image data representing a phase difference by determining the difference between the two images obtained by the pulse sequence means; and a phase image obtained by the means. residual phase distortion calculation means for calculating residual phase distortion of data at each coordinate using data at coordinates located on a straight line passing through the coordinate and perpendicular to the direction in which the flow encoding gradient is applied; The residual phase distortion obtained by the calculation means is subtracted from one of the two images, and the image obtained by the subtraction and the other of the two images are added or subtracted so as to remove the residual phase distortion. What is claimed is: 1. A nuclear magnetic resonance imaging diagnostic apparatus comprising a phase correction means. 2. As a residual phase distortion calculation means, calculate the average value of the data of the coordinates located on the same straight line in the direction perpendicular to the direction in which the flow encoding gradient is applied, and calculate the average value of the data of all the coordinates on the same straight line. The nuclear magnetic resonance imaging apparatus according to claim 1, characterized in that a residual phase distortion calculation means is used to calculate residual phase distortion.