JPH0579329B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to magnetic resonance (MR) technology.
MRI (Magnetic Resonance), which diagnoses living bodies by using resonance (hereinafter referred to as "MR") phenomenon
The present invention relates to a method of imaging the distribution of a certain nuclear spin density present in a subject as a two-dimensional projected image (hereinafter referred to as a "scanogram") in a certain direction using a scanning imaging device.

[Technical background of the invention]

Conventionally, in order to obtain a scanogram in an MRI apparatus for medical diagnosis, a one-dimensional projection image (hereinafter referred to as PD) has been used. A very uniform static magnetic field H ₀ along the illustrated z-axis direction is applied to the subject P, and a pair of gradient magnetic field coils 1A,
1B adds a linear magnetic field gradient in the z-axis direction to the static magnetic field H ₀ . A specific atomic nucleus resonates with the static magnetic field H ₀ at an angular frequency ω ₀ given by the following equation.

ω ₀ =γH ₀ ...(1) In equation (1), γ is the gyromagnetic ratio, which is specific to the type of atomic nucleus. A rotating magnetic field _H1 with an angular frequency ω ₀ that makes only a specific atomic nucleus resonate is transmitted via a pair of transmitting coils 2A and 2B, and the illustrated x-y is set using the linear magnetic field gradient (gradient magnetic field GS for slice determination). A specific slice portion S that acts on the subject P within a plane to obtain a tomographic image (although it is a flat portion, it actually has a certain thickness)
It only causes MR phenomenon. The MR phenomenon is transmitted via a pair of receiving coils 3A and 3B to a free induction decay (FID) signal (hereinafter referred to as "FID").
By Fourier transforming this signal, a single spectrum for a specific nuclear spin rotation frequency can be obtained. In order to obtain a projection image of the slice portion S in a predetermined direction in the x-y plane, the slice portion S is excited to produce an MR phenomenon, and then the magnetic field H ₀ is applied in the x′-axis direction as shown in FIG. When a linear magnetic field gradient Gxy (which imparts phase information and is a gradient magnetic field GE for phase encoding) with a linear gradient is applied to the (coordinate system rotated by θ° from the x-axis), the sliced portion of the subject is The isomagnetic field line E in S is a straight line, and the rotational frequency of a particular nuclear spin on that line is expressed by equation (1). For convenience of explanation, it is assumed here that signals D ₁ to _{D n} (a kind of FID signal) are generated from each of the isomagnetic field lines E (denoted as E ₁ to _{E o} ). Signal D ₁ ~ _{D o}
The amplitudes of are the isomagnetic field lines passing through the slice portion S, respectively.
It is proportional to the nuclear spin density on E ₁ to E _o . However, the FID signal actually observed is D ₁ ~
The sum of D _o (i.e. the composite FID)
By Fourier transforming this FID signal, projection information (one-dimensional image) PD of the slice portion S onto the x' axis can be obtained.

By moving the slice position of the PD thus obtained and continuously projecting it, a scanogram SG as shown in FIG. 3 is obtained.

In order to achieve a satisfactory spatial resolution of the scanogram SG in the direction of the body axis of the subject P, the slice thickness must be made sufficiently thin, gaps between slices must be eliminated, and projections must be performed multiple times. However, since the slice position is usually moved by moving the subject, the mechanical part of the apparatus for moving the subject becomes complicated. In addition, since the magnitude of the FID signal is proportional to the amount of excited magnetization, the thinner the slice thickness, the smaller the FID signal becomes. ) becomes very bad. In order to improve this, it is necessary to increase the S/N ratio by observing and integrating the same signal many times, which results in a longer projection time to obtain the PD of one slice. occurs. In particular, the data acquisition time for MRI equipment is
Compared to other diagnostic equipment such as a CT scanner, for example, it takes a long time, and shortening the time in which a subject is restrained is a major clinical challenge.

[Purpose of the invention]

The present invention provides a method for obtaining a scanogram of a subject, which does not require relative movement between the subject and the MRI apparatus, and which can realize images with high spatial resolution in a much shorter imaging time than conventional methods. The purpose is to

[Summary of the invention]

A feature of the present invention is that a gradient magnetic field for so-called phase encoding is applied in a direction perpendicular to the slice plane, and this direction is the z-axis direction in FIG. is generally the direction of the static magnetic field and the direction of the subject's body axis. In addition, in the present invention, since a non-selective excitation pulse is used to collectively cover a wide area as the image range of the scanogram, there is no need to apply a gradient magnetic field to determine the slice site. It is characterized by the fact that the timing of applying the gradient magnetic field for this purpose is advanced.

[Embodiments of the invention]

FIG. 4 shows a basic configuration for explaining one embodiment of the present invention.

If f(x, y, z) is the distribution of a certain nuclear spin density existing in the subject P, then the high frequency magnetic field (90° pulse) applied to the subject P via the transmitting coils 2A and 2B The motion of the macroscopic magnetization M in the region V from which an excited scanogram is obtained is expressed by the following equation, if relaxation is ignored.

M(x,y,z,t)=f(x,y,z)exp(jωp)
...(2) Here, M is a complex number representing macroscopic magnetization, and although it is normally a vector quantity, a complex number is used to simplify the expression. f(x, y, z) is a real number representing the nuclear spin density, and j is an imaginary unit (=√−
1). ω is the rotational angular frequency determined by equation (1), t
are time and each is a real number.

Next, a magnetic field gradient Gz(t) in the z-axis direction of the static magnetic field that satisfies equation (3) is applied to the subject P for a time _τz .

^γl _∫ Take the length of coils 2A and 2B (4th
(see figure). ξ is an arbitrary constant representing an angle (radian) other than 0, and n is a variable. For example, if Gz(t) is a sine curve gz sinτ/τ _z t with n=1, equation (3) can be satisfied by changing the magnitude of gz.

As a result, the magnetization M shown in FIG.
Initially, it is tilted in the same direction (but rotating at the angular frequency of equation (2)) by a 90° pulse, but since a gradient magnetic field with, for example, a positive gradient in the z-axis direction is added to the static magnetic field, , the modulus of the Z _E point shown in Figure 4 is
The magnetic field becomes larger than the Z _S point, and the angular frequency of magnetization rotation also becomes higher. However, since the gradient magnetic field is only applied for τ _z time, when the gradient magnetic field is cut off, Z _S
The magnetizations of Z and _E rotate with the same angular frequency. However, the phases of their respective magnetizations differ by an angle φ expressed by the following equation.

φ=ξZ/l ...(4) Here, ξ, l are the same as equation (3), or Z is Z _S
The distance is Z _E.

Therefore, by applying a gradient magnetic field in the Z direction for a time τ _z , the magnetization is twisted as shown in FIG. 5c.
The motion of each magnetization at that time is expressed by the following equation.

M (x, y, z, t) = C・f (x, y
, z) exp[j(ωt+φn)]...(5) Here, M, f, j, ω, t are the same as equation (2), and n
is the same as equation (3). C is a proportionality constant.

By utilizing this phase difference φ, signals in the Z direction, that is, in the body axis direction of the subject P can be separated, and a scanogram (two-dimensional projected image) can be obtained.

Next, as in the conventional method described earlier, when a magnetic field gradient G _xy is applied to the static magnetic field in the x'-axis direction, which forms an angle of θ with the can get.

F _d (t, n)=K∫∫∫ ^∞ _-∞ f(x′cosθ−y′sinθ,
x′sinθ+y′cosθ,Z)・exp(j(ω(x′)t+φ
(5)(Z)n)]dx'dZ...(6) Here, K is a constant of proportionality.

From equation (1), ω(X)=γC _xy x′, and from equation (4), φ(Z)=
ξlZ, and rewriting equation (6) from these relationships, Fd (t, n) = Kl/ξγG _xy ∫∫∫ ^∞ _-∞ f(x′cosθ−
y′sinθ, x′sinθ+y′cosθ, Z)dy′・exp(j(ωtt+φn)
]
dωdφ ……(6)′ The scanogram S _g (X′, Z) is expressed by the following equation.

S _g (X′, Z)=∫ ^∞ _-∞ f(x′cosθ−y′sinθ,x′s
inθ＋
y′cosθ,Z)dy′...(7) Using equation (7), equation (6)′ can be rewritten as the following equation.

E _d (t, n) = Kl/ξγG _xy ∫∫ ^∞ _-∞ S _g (x′, Z) exp
[j (ωt+φn)]・dωdφ ...(8) Therefore, by performing two-dimensional Fourier transformation on F(t, n) in equation (8), the scanogram S _g (X', z) is obtained as shown in the following equation. can get.

S _g (x′, z)=1/(2π) ² ξγG _xy ／Kl∫∫ ^∞ _-∞ E _d
(t, n)・exp[−j(ωt+φn)]dndt……(9) The FID signal is usually calculated by converting the ξ value in equation (3) to F _d (t,
n) with a value large enough to represent the change in the n direction, and let n be an integer value (-N...-2, -1, 0,
1, 2...N).

Here, the method of magnetic field arrows will be summarized and explained with reference to FIG. In the first step, a linear magnetic field gradient G _z (gradient magnetic field for slicing) is provided in the z-axis direction, and a 90° pulse (selective excitation pulse) consisting only of a certain frequency component is used to target a specific atomic nucleus in a certain part of the specimen. excite. The second step satisfies equation (3) above.
Apply G _z . The applied _{negative G} This is for reading out signals (signal collection).

The example shown in FIG. 6 will be explained in more detail. That is,
FIG. 6 is a so-called pulse sequence showing the process of data collection.

First, in Figure 6, as the first step,
Impulse-like 90° pulse (pulse for non-selective excitation)
is applied to the specimen, magnetization of a specific nuclear pin within the excitation region (substantially a sufficiently wide region) in the specimen determined by the frequency of the 90° pulse for non-selective excitation is applied to the specimen. Tilt 90 degrees. At this time, since the non-selective excitation pulse 90° has many frequency components, the excited region within the subject becomes wide. In this first step, no gradient magnetic field (usually G _z ) for determining the slice site is applied.

Next, in a second step following this first step, a gradient magnetic field (G _z in this case) is generated to add phase information whose intensity is variable in the direction along the first axis (in this case, the z-axis). Further, as a magnetic field for inverting the magnetization by 180°, a gradient magnetic field in which G _xy is inverted is generated. Thereby, the magnetization can be reversed by 180° by adding phase information corresponding to the intensity of the gradient magnetic field G _z for adding the phase information.

Next, in a third step following this second step, a gradient magnetic field G _xy is generated for collecting signals along at least one of the second axis and the third axis, and a magnetic resonance signal is generated. Do what you collect. G _xy at this third step is
G _x , G _y , and G _xy can be selected as appropriate depending on the projection direction.

These first to third steps are repeated a predetermined number of times while varying the intensity of the gradient magnetic field _Gz for adding the phase information to obtain a group of magnetic resonance signals. Perform Fourier transform processing.

In this way, when collecting each signal, the magnetization of the region from which the scanogram is obtained is excited, so that the signal-to-noise ratio of the signals is good. Therefore, there is no need to improve the S/N ratio under the same acquisition conditions, so the imaging time is shortened compared to the conventional method.

Furthermore, by using magnetization phase information, images in the body axis direction can be separated, so a scanogram (two-dimensional projection image) over a wide range can be obtained without moving the subject.

Furthermore, in the first step, although a 90° pulse for excitation (impulse-like non-selective excitation pulse) is applied, a gradient magnetic field (G _z ) for slice determination is not applied, so this gradient magnetic field is applied. The procedure from the second step onwards can be shifted to the left in the figure by the amount not required. In other words, even if the time reduction is short in one encoding process, the degree of time reduction becomes large when the entire encoding process is performed.

This reduces data collection time and restrains the subject, which is clinically advantageous.

〔Effect of the invention〕

According to the present invention, the distribution of a specific nuclear pin density present in a subject can be determined with little relative movement of the subject to the MRI apparatus, and with the imaging time being effectively shortened compared to conventional methods, and moreover, in the opposite axis direction. It is possible to provide a method that allows imaging as a scanogram with a spatial resolution that is satisfactory to the user.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of an example of an MRI device, Figure 2
The figure is a principle diagram of an example of obtaining projection information by magnetic resonance phenomenon, Figure 3 is a schematic diagram of the principle of obtaining a scanogram using a one-dimensional photographed image, and Figure 4 is for explaining an embodiment of the present invention. Figures 5a to 5c are schematic diagrams showing the movement of magnetization in the same example (a: when a static magnetic field is applied, b: when a 90° pulse is applied, c: when a 90° pulse is applied, c: in the z-axis direction). FIG. 6 is a diagram showing an embodiment of the magnetic field application method of the present invention. P...Object, _H0 ...Static magnetic field, 1A, 1B...
...Gradient magnetic field coil, H ₁ ...High frequency magnetic field, 2A,
2B...Transmission coil, S...Slice part for obtaining PD, 3A, 3B...Reception coil.

Claims (1)

[Claims] 1. In a method of placing a subject in a static magnetic field of an MRI apparatus, determining a first axis, a second axis, and a third axis that are perpendicular to each other based on the magnetic field generation direction of the static magnetic field, and generating a scanogram of the subject. , by applying a non-selective excitation 90° pulse to the subject without generating a gradient magnetic field in order to determine a slice site in the direction along the first axis, the frequency of the non-selective excitation 90° pulse is A first step in which the magnetization of a specific nuclear spin in the excitation region of the object to be determined is tilted by 90 degrees, and a step that is performed subsequent to this first step and in which the intensity is varied in the direction along the first axis. Generate a gradient magnetic field for adding phase information and change the magnetization.
a second step of reversing the magnetization by 180° by generating a magnetic field for reversing the magnetization by 180°, adding phase information according to the strength of a gradient magnetic field for adding the phase information; This step is carried out following the second step, and is carried out following the second step.
a third step of generating a gradient magnetic field for collecting a signal along at least one of the axis and the third axis and collecting a magnetic resonance signal; 1. A scanogram generation method using an MRI apparatus, characterized in that the process is repeated a predetermined number of times while varying the strength of a magnetic field to obtain a group of magnetic resonance signals, and a two-dimensional Freeier transform process is performed on the group of magnetic resonance signals.