JPH0373131A - Scanogram generating method by mri device - Google Patents

Abstract

PURPOSE:To enhance the space resolution in a short photographing time without necessitating the relative movement of a body to be examined and an MRI device by using a non-selection excitating pulse by integrating a wide area, and operating an oblique magnetic field for, what is called, phase encoding in the direction intersecting orthogonally with a slice surface. CONSTITUTION:First of all, by applying an impulsive 90 deg. pulse to the body, the magnetization of a specific atomic nucleus pin in an excitating area in the body determined by the frequency of a non-selection excitating 90 deg. pulse is laid down by 90 deg.. An oblique magnetic field for determining a slice part is not applied. Secondly, an oblique magnetic field Gz for adding phase information whose strength is variable in the direction running along a first axis is generated, and also, an oblique magnetic field obtained by inverting Gxy is generated as a magnetic field for bringing the magnetization to 180 deg. inversion. Thirdly, an oblique magnetic field Gxy for collecting a signal along a second axis or a third axis is generated, and also, a magnetic resonance signal is collected. While varying the strength of the oblique magnetic field Gz for adding the phase information, a two-dimensional Fourier-transformation processing is performed to the magnetic resonance signal obtained by repeating it by the prescribed number of times.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field of the invention] The present invention relates to magnetic resonance (MR)
MRI (Ma
This invention relates to a method of imaging the distribution of a specific 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 GNETLC resonance imaging) device. .

[Technical Background of the Invention] Conventionally, in order to obtain a scanogram in an MHI apparatus for medical diagnosis, a -dimensional projection image (rPD) has been used. In order to obtain PD, as shown in Fig. 1, a very uniform static magnetic field 1 (. A linear magnetic field gradient in two axial directions is added.A specific atomic nucleus resonates with the static magnetic field Ho at an angular frequency ω expressed by the following equation.

ω. −γ■. ...... (1) (
In equation 1), T is the gyromagnetic ratio, which is specific to the type of atomic nucleus. Angular frequency ω that makes only a specific atomic nucleus resonate. The rotating magnetic field Hl is transmitted through a pair of transmitting coils 2A and 2B to form the linear magnetic field gradient (slice determination gradient magnetic field G).
A specific slice portion S in which a tomographic image is obtained by acting on the subject P within the illustrated x-y plane set using S)
(Although it is a planar part, in reality it has a certain thickness) that causes the MR phenomenon. The MR phenomenon is caused by free induction attenuation (FIDi f
ree 1inductior+decay) signal (hereinafter referred to as "rFID signal"), and by Fourier transforming this signal, a single spectrum for a specific nuclear spin rotation frequency can be obtained. After exciting the slice portion S to produce an MR phenomenon in order to obtain a projection image of the slice portion S in a predetermined direction in the x-y plane, the magnetic field Ho is applied in the X'-axis direction (
When a linear magnetic field gradient Gxy (which gives phase information and is a gradient magnetic field GE for phase encoding) having a linear gradient is applied to a coordinate system rotated by θ° from the X axis,
The isomagnetic field lines E within the sliced portion S of the subject are straight lines,
The rotational frequency of a particular nuclear spin of the confinement is expressed by equation (1). Here, for convenience of explanation, signals are obtained from each of the isomagnetic field lines E (referred to as El to E1), and ~D.

(a kind of FID signal). Signal.

The amplitude of ~D□7 is proportional to the nuclear spin density on the isomagnetic field lines E, -E, which penetrate the slice portion S, respectively. However, the FID signal actually observed is ~D,
(i.e. composite FID signal)
Therefore, 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 is obtained as shown in FIG. 3.

In order to satisfy the spatial resolution of the scanogram SG in the axial direction of a single subject, the slice thickness must be sufficiently thin (
Therefore, it is necessary to eliminate gaps between slices and perform multiple projections. 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 Fl, I signal is proportional to the excited magnetization plate, the thinner the slice thickness is, the very small the FiD signal becomes, and the signal/noise ratio (hereinafter referred to as rs/N ratio) is ) becomes very bad.

In order to improve this, it is necessary to increase the S/N ratio by observing the same signal many times and integrating it, resulting in the problem of longer imaging time to obtain the PD of one slice. occurs.

Especially, Ml? ! The data collection time of the apparatus is longer than that of other diagnostic equipment such as a CT scanner, and reducing the time for restraining a subject is a major clinical challenge.

[Purpose of the invention]

The present invention provides a method that does not require relative movement between the subject and the MHI device when obtaining a scanogram of the subject, and can realize an image with high spatial resolution in as short an imaging time as possible compared to 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. 6, and this z-axis is , which 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 the vertical region as the image range of the scanogram, there is no need to apply a gradient magnetic field to determine the slice region. The feature is that the timing of applying the gradient magnetic field for collection is advanced.

[Embodiments of the invention]

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

If f (x, y, 2) is the distribution of a certain nuclear spin density existing in the object P, then the transmitting coil 2A,
A high frequency magnetic field (90”
region V to obtain a scanogram excited by pulse)
The motion of the macroscopic magnetization M of is expressed by the following equation if relaxation is ignored.

M (x, y, z, t) − f (x, y, z)
exp(jωt) ・(2) Here, M is a complex number representing macroscopic magnetization, and although it is usually a vector quantity, a complex number is used to simplify the expression of o f (x,)
'+z) is a real number representing the nuclear spin density, j is an imaginary unit (= F-'), ω is the rotational angular frequency of magnetization determined by equation (11), and t is time, each of which is a real number.

Next, the magnetic field gradient G in the two-axis direction of the static magnetic field that satisfies equation (3)
z(t) is applied to the subject P for 11 hours.

Here, T is the gyromagnetic ratio of a specific atomic nucleus, l is the representative length of the subject P in the body axis direction, and here, the transmitting coil 2
The lengths A and 2B are taken (see FIG. 4), ξ is an arbitrary constant representing an angle (in radians) other than 0, and n is a variable.

Gz(t) is, for example, a sine curve gz si in ngl
If n - t, then equation (3) can be satisfied by changing the size of gz.

As a result, the magnetization M shown in FIG.
Initially, it rotates in the same direction (however, 2) at the angular frequency of the 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 magnetic field at point Zt shown in Figure 4 is larger than at point 28, and the angular frequency of the rotation of magnetization increases. It also becomes more expensive. However, the gradient magnetic field is τ.

Since only time is applied, when the gradient magnetic field is cut off,
2. The magnetization of 29 points rotates at the same angular frequency. However, the phases of their respective magnetizations differ by an angle φ expressed by the following equation.

φ−ξ−・・・・・・(4) Here, ξ and l are the same as in equation (3), and 2 is Z, and Zt
The interval is .

Therefore, by applying a gradient magnetic field in the Z direction for 11 hours, the magnetization is twisted as shown in FIG. 5 (C1).

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, and t are the same as in equation (2), and h is the same as in equation (3). C is a proportionality constant.

By utilizing this phase difference φ, signals in two directions, 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 GK is applied to the static magnetic field in the direction of the X' axis, which forms an angle of θ with the can get.

F, (t, n) +y' cosθ, Z)dy' ・exp (j(ω
(x')t+φ(Z)n))dx'dZ ・Old...(6) Here, K is a proportionality constant.

From equation (1), ω(X)-γCXV' is obtained, and from equation (4), ξ φ(Z)--Z, and from these relationships, equation (6) (%) (6 scanogram S, (x ', Z) is expressed by the following equation.

S, (x', Z) +y' cos θ, Z) dy' ...... (7
) (7), equation (6') can be rewritten as the following equation.

Ed (t, n) ・dωdφ (8) Therefore, by performing a two-dimensional Fourier transform on F (t, n) in equation (8), a scanogram S9 is obtained as shown in the following equation.
(x', z) is obtained.

S, (x',,z) −exp (Hωt+φn)) dnclt
-(9) FiD signals are usually generated by setting the C value in equation (3) to a value large enough to represent the change in Fa (t, ri) in the n direction, and setting n to an integer value (-N...・-2',-
1,0,1°2...N).

Here, the method of applying the magnetic field will be explained in detail with reference to FIG. In the first step, the linear magnetic field gradient G, (
A gradient magnetic field for slicing) is provided, and a specific atomic nucleus in a certain part of the object is excited by a 90″ pulse (selective excitation pulse) that uses only certain frequency components.In the second step, the above equation (3) is satisfied. G1 is applied.The negative Gxy applied at this time is to generate a signal after the third step τS time, and the reversal magnetic field in step 2 is to invert the magnetization by 1806, and the magnetic field in step 3 is 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 FIG. 6, as a first step, by applying an impulse-like 90'' pulse (non-selective excitation pulse) to the subject, the frequency of the 90° non-selective excitation pulse is determined by the frequency of the 90° pulse for non-selective excitation. The magnetization of a specific nuclear bin within the excitation region (substantially, a sufficiently wide region) is lowered by 90''. At this time, non-selective excitation pulse 9
Since 06 has many frequency components, the region excited in the subject is wide. In this first step, no ω gradient magnetic field (usually G) is applied to determine the slice site.

Next, in a second step following this first step, a gradient magnetic field (in this case, G,
) and to invert the magnetization by 180'', a gradient magnetic field in which G is added to increase the magnetization to 18
0” can be reversed.

Next, in a third step following this second step, a gradient magnetic field G□ is generated for collecting signals along at least one of the second axis and the third axis L, and magnetic resonance is generated. What you do is collect signals. Gxy in this third step is G, G, ,G depending on the projection direction.
X can be selected as appropriate.

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

In this way, when collecting each signal, the magnetization in every region from which a 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,
is not applied, so by the amount that this gradient magnetic field is not applied,
The steps after the second step can be shifted to the left in the diagram.

In other words, even if the time reduction is short in one encoding process, the degree of time reduction is large when it comes to all encodings.

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 certain nuclear spin density present in a subject can be determined with little relative movement of the subject to the MRI apparatus, while effectively shortening the imaging time compared to conventional methods, and in addition, in the body 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 drawings]

Figure 1 is a diagram of the basic principle of an example of an MRI device, Figure 2 is a diagram of the principle of an example of obtaining projection information using magnetic resonance phenomena, and Figure 3 is a schematic diagram of the principle of obtaining a scanogram using a one-dimensional photographed image. , FIG. 4 is a principle configuration diagram for explaining one embodiment of the present invention, and FIGS. 5(a) to (0) are schematic diagrams showing the movement of magnetization in the same embodiment ((al; static magnetic field is applied When (bl; 90' pulse is applied, (
'); After a gradient magnetic field in the z-axis direction is applied for a certain period of time), FIG. 6 is a diagram showing an embodiment of the method of applying a magnetic field of the present invention. P...Object, Ho...Static magnetic field, IA, IB...
Gradient magnetic field coil, H+...High frequency magnetic field, 2A, 2B
...Transmission coil, S...Slice part for obtaining PD,
3A. 3B...Receiving coil.

Claims (1)

[Claims] The subject is placed in the static magnetic field of the MRI device, and the first and second axes are orthogonal to each other with respect to the magnetic field generation direction of the static magnetic field.
In the method, a 90° pulse for non-selective excitation is applied to the subject without generating a gradient magnetic field in order to determine a slice site in a direction along the first axis. By applying it to the sample,
a first step of tilting the magnetization of a specific nuclear spin in an excitation region of the subject by 90 degrees, which is determined by the frequency of the non-selective excitation 90 degrees pulse; and a step performed subsequent to this first step, A gradient magnetic field for adding the phase information by generating a gradient magnetic field for adding phase information with variable intensity in the direction along the first axis and generating a magnetic field for reversing the magnetization by 180 degrees. a second step of reversing the magnetization by 180° by adding phase information according to the intensity of 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 Fourier transform process is performed on the group of magnetic resonance signals.