JPH01192344A - Blood vessel imaging method by nuclear magnetic resonance picture diagnosing device - Google Patents

Abstract

PURPOSE:To obtain a picture without distortion while phase distortion is corrected once by obtaining the difference of two pictures obtained by a pulse sequence to change a flow encoding grade, obtaining a phase image to show the phase difference, operating and obtaining the residual phase distortion from the phase image and adding and subtracting so as to remove it. CONSTITUTION:Setting is executed so as to scan to an operation console with an ordinary pulse sequence and so as to scan with the pulse sequence to add a flow encoding grade 16. The picture obtained by an echo signal includes a phase distortion 21 due to a blood vessel 20 and a flow encoding grade. The average value of the phase on a segment a-a' in a Y axis direction may be obtained and the phase on the segment may be corrected in order to remove the phase distortion 21. In sections b-b', the blood vessel is finer that the position of the periphery generally, when the average value of the sections b-b' is obtained, the phase change part of the blood vessel 20 can be neglected by the average value, and by reducing the average value, the whole phase distortion is excluded. A computer receives an echo signal, obtains the residual phase distortion in an (x) axis direction, subtracts from the data of two pictures, adds and subtracts two original pictures again, removes the phase distortion due to the flow encoding grade and re-constitutes the picture.

Description

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

(Prior Art) A nuclear magnetic resonance imaging diagnostic apparatus (hereinafter referred to as NMR-CT) that uses nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon to obtain a tomographic image of a subject focusing on a specific atomic nucleus is conventionally known. An outline of the principle of this NMR-CT will be briefly explained.

An atomic nucleus can be seen as a spinning top that is magnetic, but if it is placed in a static magnetic field Ha in two axial directions, for example, the atomic nucleus will precess at an angular velocity ω0 as shown by the following equation. . This is called Larmor precession.

ω0 - γHo However, γ: nuclear gyromagnetic ratio Now, a high-frequency coil is placed on an axis perpendicular to the z-axis where the static magnetic field is located, for example, the X ring, and the high-frequency rotating magnetic field with the angular frequency ω0 rotating in the X1 plane is When this is applied, magnetic resonance occurs, and the population of atomic nuclei undergoing Zeeman splitting under the static magnetic field Ho undergoes a transition between units due to the high-frequency magnetic field that satisfies the resonance conditions, and becomes a unit with a higher energy unit. Transition. Here, since the nuclear gyromagnetic ratio γ depends on the type of the atomic nucleus, the atomic nucleus can be specified by the co-optic frequency. Furthermore, by measuring the strength of that resonance, we can find out the existence mother of that nucleus. During the time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to the higher unit returns to the lower unit and radiates energy.

Part 4 about the medium pulse method for observing this NMR phenomenon.
This will be explained with reference to the figures.

A high frequency pulse (Hl) that satisfies the resonance condition as described above
When applied in the X-axis direction, not perpendicular to the static magnetic field (2 axes),
As shown in FIG. 4(a), the magnetization vector M starts rotating in the zy plane at an angular frequency of ω'-γHI in the rotating coordinate system.

Now set the pulse width to 1. Then, the rotation angle θ from Ha is expressed by the following equation.

θegγH1jo...(1)
The first pulse shown in FIG. 4 (c) is called an excitation pulse,
In particular, a pulse having to such that θ=90° in equation (1) is called a 90° pulse. In this 90'' pulse straight, the magnetization vector M moves along the xy plane at ω as shown in Figure 4 (b).
This means that it is rotating at zero, and an induced electromotive force is generated in the coil placed on the z-axis, for example. 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, Figure 4 (
As shown in c), a pulse applied after a time τ after the 90° pulse is called an inversion pulse, and in particular, a second pulse having a pulse width of θ-180″ is called a 180″ pulse. iso'' pulse, the scattered magnetic moments refocus in the -y direction after a time τ and a signal is observed. This signal is called a spin echo (hereinafter referred to as SE).
It is called a double signal). A desired image can be obtained by measuring this SE multiplied signal intensity. The resonance condition for NMR is given by γHo/2π. Here, ν is the resonance frequency IHO is the strength of the static magnetic field. Therefore, it can be seen that the resonance 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 to give a magnetic field of different strength depending on the position, and the resonance frequency is changed 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. (a) In the figure, X.

A state is shown 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.

(b) The figure shows the timing of applying each magnetic field. In the figure, RF is a high frequency rotating magnetic field of 90"
Apply a pulse and a 180° pulse to the z-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 depending on time applied to the z axis called the warb axis, and GZ is a fixed gradient applied to the z axis called the slice axis. It is a magnetic field. The signal shows the SE multiplied signal after 1800 pulses. 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 on Z-Q are selectively excited by the 90''' pulse and the gradient magnetic IGZ÷.Therefore, the 90' pulse is called an excitation pulse. .G×+ in period 2 is for disturbing the spin phase and inverting it with a 180 @ pulse.
This is called the dayphase gradient. Also, the 1800 pulse is called an inversion pulse. GZ- is for restoring the phase of the spins disturbed by Gz÷. During period 2, a phase encode 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, 1800 pulses are applied to align the magnetic moments again, and the SE and signals that appear thereafter are observed. Gx+ in period 4 is a gradient magnetic field for aligning the disturbed phases and generating an SE multiplied signal, which is called a rephase gradient, and an SE multiplied signal appears where the areas of the rephase gradient and the dayphase gradient become equal.

This sequence is called a view, and the pulse repetition period T
After R, apply a 90° pulse again and start the next view. The above-mentioned warb gradient is changed corresponding to each view.

In NMR-CT as described above, blood flow imaging applies a gradient magnetic field in the flow direction 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. Gradient magnetic field 1 at time τ1
12 is applied, and the reversal gradient magnet 13 is applied at time τ2 after Δτ. A reversed gradient magnetic field is a magnetic field that has the same magnitude but reverses sign. Since stationary object 4 does not move, time τ
1 and 2 have the same magnitude and opposite sign <G1
and -Gt), and their effects cancel each other out, resulting in the same state as when no gradient magnetic field is applied. On the other hand, since there is movement in the blood flow area, time τ! and τ2 feel different magnetic fields G15 and G26, and their effects are not canceled, giving a change in phase to the spins. A gradient magnetic field that provides phase information to the movement of blood flow by providing gradient magnetic fields with the same magnitude and opposite sign is called a 70-encoded gradient.

FIG. 1 shows a pulse sequence when blood flow is measured using a 70-encode gradient. In the figure, the spins excited by the excitation pulse 11 are at the day-face gradient position 2.
However, the phase is returned by the readout gradient device 3 and an echo signal 14 is generated. At this time, information on the position in the V-axis direction is given to the spins as phase information by the warp gradient position 5. In this way, one image (referred to as image A) is obtained. Next, the flow encode gradient position 6 is applied to the X axis as shown in the figure, and one more image
) is obtained. Since the 70-encode gradient is a gradient with opposite polarity and equal area as shown in Figure 3, 70-
Among the images obtained by applying the encode gradient 6, the change in the spin phase of the image of a stationary object is expressed by the following equation.

Here, γ...Gyromagnetic ratio G(t)...Output of the gradient magnetic field Therefore, the spin of the stationary object is determined by the flow encode gradient position 6
For stationary objects, the image m
A and image B are the same.

For a spin moving in the flow encoding direction, in this case, the x-axis direction, X (t)...The position of the spin and the movement of the spin are expressed by a linear function, for example, X (t) - x + vt. When, Equation (2) becomes as follows.

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 indicated 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) Error in the rise of the gradient magnetic field and deviation in the sample position origin of the NMR signal due to the residual gradient magnetic field due to eddy currents (2) Phase offset of the device (3) Ununiformity of the static magnetic field Above image 111A and the image Consider the phase of the complex numbers of the two images B. If the phase of the pixel is exEl(ie(X,V)), the phase of the image when the flow encoding gradient is applied is eXp(iel(x,y)) -exp(ieo(X,V) +iθt(X, y)+i θ!+ (×, y)+iθp
(X, V)) ・-(4) However, G
0... Phase offset θl of the device... 70 - Phase change due to gradient other than encode gradient θB... Phase change due to magnetic field inhomogeneity θF... Phase change due to flow encode gradient When flow encode gradient is not applied e) U
(ie2 (X, V)1 -exp(i θ0 (x, y) +1θ1(X * V) +i θa(X, V))・
...(5) The phase of the phase image representing the phase difference obtained by calculating the difference between the two images is (eXl from equation 4 - equation (5)) (iet (X, V)) exp(-iθz( X, y〉) −exp(iθF (X, V)) −(6
) That is, as is clear from equation (6), 2 of image B is
All differences between the images cancel out except for phase changes due to the 70-encoding gradient. The phase change included in θF (x, y) due to this flow encode gradient is (1) phase change due to flow, (2) phase distortion due to rise of flow encode gradient and area error caused by eddy current, and (3) eddy current. There are phase distortions due to the location dependence of the current. The image due to the phase change that is being sought is the phase change due to the flow (1), and the phase distortions (2) and (3) are unnecessary and their presence has an adverse effect. An example of an image containing this phase distortion is shown in FIG. (a) In the figure, blood vessel 20
is different from the original image of the blood vessel 20 shown in FIG.

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-mentioned problems, and its purpose is to obtain two
The object of the present invention is to realize a blood vessel imaging method using NMR-CT that corrects phase distortion caused by various causes at once to obtain a distortion-free image in blood vessel imaging that is created using a number of images.

(Means for Solving the Problems) The present invention solves the above problems in a blood vessel imaging method using a nuclear magnetic resonance imaging apparatus that creates an image based on a phase change that occurs over time in a certain portion of flow velocity.
A phase image representing a phase difference is obtained by determining the difference between two images obtained by a pulse sequence in which the flow encoding gradient is changed, and a residual phase distortion is calculated and determined from the phase image.
The method is characterized in that subtraction is performed from any one of the images, and addition and subtraction are performed between the obtained image and another image so as to remove the residual distortion.

(Operation) Obtain two images using a pulse sequence with a different flow encoding gradient, find the difference between them, calculate the residual distortion from the difference image, subtract it from one of the two images, and then calculate the difference between the two images. Perform addition and subtraction with.

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

FIG.
It is a block diagram of Aw1.

In the figure, 31 has a space (hole) into which the subject is inserted, and the space is surrounded by a
A static magnetic field coil that applies a constant static magnetic field to the subject and a gradient magnetic field coil that generates a gradient magnetic field (the gradient magnetic field coils are
, Z-axis coils. ), an RF transmitting coil that provides an RF pulse to excite the spins of atomic nuclei within the subject, and a receiving coil that detects NMR signals from the subject, etc., are arranged.

The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power supply 32, a gradient magnetic field driving circuit 33, an RF power amplifier 34, and a preamplifier 35, respectively. 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 the command from the computer 37 in the transmission view, and calculates <RF pulses based on the Fourier transform method. A signal is applied from the RF power amplifier 34 to the RF transmit coil. Also, the sequence storage circuit 36 stores the gradient magnetic field drive circuit 3 using a sequence signal based on the Fourier transform method.
3 to supply gradient magnetic fields to the x, y, and z axes, respectively, as shown in FIG. 5, and also operate the gate modulation circuit 3 and the AD converter 41. 40 is the RF oscillation circuit 3
This is a phase detector that detects the phase of the received signal output of the preamplifier 35 using the output of the preamplifier 35 as a reference signal. This output signal is converted into a digital signal by the AD converter 41 and input to the computer 37. -42 is an operation console for inputting instructions and various setting values for realizing a series of pulse sequences to the calculator 11137;
7 switches the operation of the sequence storage circuit 36, rewrites the memory, and performs image reconstruction calculations using each data from the AD converter 41 based on the data. 43 is a display device that displays the image reconstructed by the computer 37.

Next, the method of the embodiment will be explained while explaining the operation of the apparatus configured as described above.

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 sends it to the sequence storage circuit 36. Sequence storage circuit 36
illuminates the gradient magnetic field drive circuit 33 based on the above signal.
III to generate a gradient magnetic field of a predetermined pulse sequence, and also to control the gate modulation circuit 38. The gate modulation circuit 38 modulates the RF multiplied 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 RF power amplified! In the static magnetic field generated by the static magnetic field power supply 32 in the magnet assembly 31, the excited spins are caused to resonate in conjunction with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 33.

The SE multiplied signal produced by the resonance is received and sent to the preamplifier 3.
5 and input to the phase detector 4o. The phase detector 40 phase-detects the input NMR signal using the output of the RF oscillation circuit 39 as a reference signal, and converts the output signal into an AD signal.
It is sent to converter 41. AD converter 41. The NMR signal that has been converted into a digital signal is reconstructed into an image by the computer 37 through predetermined processing according to the scan sequence, and is displayed on the display device 43. calculator 37
can rewrite the contents of the sequence storage circuit 36, thereby realizing an accurate scan sequence.

In the above NMR-CT, the operation console 42 scans with the normal pulse sequence shown in FIG. Set it as follows. If two images are obtained, they are subtracted by the computer 37.

The obtained echo signal includes a phase change θF (x, v) due to the 70-encoding gradient shown in equation (6),
The image obtained by this echo signal is an image including a blood vessel 20 and a phase distortion 21 due to the encoding gradient 70, as shown in FIG. 6(A). Since this phase distortion 21 is due to the 70-encoding gradient, the directional dependence in the axial direction to which the flow encoding 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. 7. The figure shows the case where a flow encode gradient is applied to the X axis, and V
In the section a-a' represented by the line segment a-a' parallel to the axis, the amount of phase distortion is equal to the point on the y-axis.

Therefore, in order to remove the phase distortion 21 due to the 70-encoding gradient, it is sufficient to take the average value of the phase on the line segment aa' in the Y-axis direction and correct the phase on that line segment. interval b-b'
In general, blood vessels are thinner than the surrounding areas;
If you take the average value of the interval b-b', the average value is blood vessel 20
The phase change can be ignored, and by reducing the average value, the entire phase distortion can be removed without damaging the information on the phase change due to blood flow, as shown in FIG. 6(b).

Calculation 137 receives the echo signal, calculates the residual phase distortion shown in the above equation (6) in the X-axis direction, subtracts it from the data of image A or image B, and adds and subtracts the two original images again. - Reconstruct the image by removing the phase distortion caused by the encoding gradient, and display it on the image display device 43.

It should be noted that the present invention is not limited to the method of the above embodiment, but may be carried out as follows.

■In the example, for example, the average value of phase distortion was determined by using the interval bb' as a straight line, but in reality it may not be a straight line, so the curve of the interval bb' can be expressed as a function. The function may be used to express the interval bb' for each point on the x-axis.

■It is also possible to calculate the function Δφ−(x) of the curve formed by the phase distortion surface by cutting along a plane parallel to the x-axis and perpendicular to the xy plane. This is equivalent to finding a function in the X-axis direction using the average value of the phase distortion in the direction.

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

(2) The flow encode gradient is not limited to the x-axis, but may be applied to the (2) axis or the Z-axis.

(2) Although the flow encode 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.

(2) Although the pulse sequence was performed using the gradient inversion method in which the day phase gradient and the readout gradient are reversed as shown in FIG. 1, the pulse sequence may be performed by using 1804 pulses to reverse the gradient without inverting the gradient.

(Effects of the Invention) As described above in detail, according to the present invention! In addition to eliminating field inhomogeneity and phase distortion caused by the device, phase distortion due to flow encoding gradients can also be removed by calculation, making it possible to completely eliminate t phase distortion only when scanning 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, even if the condition of the subject changes, blood vessel imaging using NMR can be easily performed without being affected by the change, which has a great practical effect.

[Brief explanation of the drawing]

Figure 1 is a diagram of the pulse sequence used to implement the method of this R. Fig. 2 is a block diagram of an apparatus for carrying out the method of the present invention, Fig. 3 is an explanatory diagram of flow encoding gradient, Fig. 4 is an explanatory diagram of the principle of the NMR-CT pulse method, and Fig. 51 is a diagram of the NMR-CT 17. )li! 1JIa
) A diagram showing a pulse sequence, and Figure 6 is a diagram of a blood vessel image obtained using the flow encoding method. FIG. 7 is a diagram of an image having −-like phase distortion in the y-axis direction, and the eighth factor is a diagram of the principle of blood vessel imaging using the flow encoding method. 1.20... Blood vessel 11... Excitation pulse 1
2... Day phase gradient 13... Readout gradient device 4... 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 ... RFF force amplifier 35 ... Preamplifier 3
6...Sequence storage circuit 37...Computer 38...Gate modulation circuit 39...RF generation note circuit 40...Phase detector 41
...AD converter 42...operation console 43
... Display device patent applicant Yokogawa Medical Systems Co., Ltd. No. 1 - (A) Figure (B) Figure 6 (A) (B) Cocoon Figure 7 Capture rope 8 Figure

Claims (1)

[Claims] In a vascular imaging method using a nuclear magnetic resonance imaging system that creates images based on phase changes that occur over time in a certain portion of flow velocity, the difference between two images obtained by a pulse sequence that changes the flow encoding gradient is calculated. Obtain a phase image representing the phase difference, calculate residual phase distortion from the phase image, subtract it from any one of the two images, and combine the obtained image with another image. A blood vessel imaging method using a nuclear magnetic resonance imaging apparatus, characterized in that addition and subtraction are performed on and to remove the residual strain.