JPH08191817A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE: To provide an MRI imaging method capable of obtaining an image different in magnetization relaxed state with respect to a plurality of slices within a short time. CONSTITUTION: A 180 deg. RF pulse 200 is applied to reverse all of nuclear magnetizations in a subject to repeatedly apply θ deg. RF pulses 201-1, 202-1, 203-1... to a plurality of slices successively at a short time interval. One, image is obtained with respect to one application of a θ deg. (θ<90) FR pulse by an echo planar method. By this constitution, an image different in magnetization relaxed state can be obtained with respect to a plurality of slices during one relaxation process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures the magnetic resonance signals (MR signals) from the atomic nuclei of hydrogen, phosphorus, carbon and the like contained in a subject and visualizes the information on the tissues in the living body. The present invention relates to an imaging method (MRI imaging method), and more particularly, to an MRI imaging method capable of acquiring a plurality of images having different longitudinal relaxation states of magnetization for a plurality of slices in a short time.

[0002]

2. Description of the Related Art In an MRI apparatus, magnetic resonance signals from atomic nuclei such as hydrogen, phosphorus, and carbon contained in a region of interest of a subject are acquired to noninvasively image the inside of the subject. A lot of physicochemical information is mixed in this magnetic resonance signal, and each has a clinically important meaning. One of such information is spin-lattice relaxation (hereinafter referred to as longitudinal relaxation). The spatial distribution of the time constant T ₁ of vertical relaxation can be obtained by acquiring a plurality of images having different states of vertical relaxation. If this time constant T ₁ for longitudinal relaxation is used, there is a possibility that the clinical condition of the subject can be quantitatively evaluated numerically, and there is also a possibility that it can be used for temperature measurement deep inside the body. However, in the method currently used in clinical practice, it takes a lot of time to obtain the spatial distribution of T ₁ , which is a problem. The following method has been reported as a method for solving this problem and acquiring a plurality of images having different vertical relaxation states in a short time.

A direction of magnetization is applied by applying a 180-degree inversion RF pulse to the magnetization of atomic nuclei in a subject placed in a static magnetic field and aligned in one direction (this direction is referred to as a vertical direction in this specification). Invert. After the time Ta has elapsed, the first θ degree (θ <
90) An RF pulse is applied to invert the magnetization by θ degrees with respect to the longitudinal direction. The collapsed magnetization is decomposed into a component in the vertical direction and a component in the direction orthogonal to it (hereinafter referred to as the horizontal direction), and from the component in the horizontal direction, an image in the first longitudinal relaxation state is created. Acquire all necessary echo signals by the echo planar method. Following the first θ-degree (θ <90) RF pulse, application of the θ-degree RF pulse is repeated n times at time Tb intervals. Every echo signal applied for each θ-degree RF pulse is acquired to create an image of each relaxation state. As a result, n images in different longitudinal relaxation states can be acquired during one relaxation process. From the acquired images of different relaxation states, the distribution image of T ₁ can be obtained with the time constant T _{1 of} longitudinal relaxation and the nuclear density M ₀ as unknowns. For the detailed principle of this method, see Magnetic Resonance in Medicine Vol. 30, 351-.
Pp. 354 (1993).

[0004]

Considering the clinical application of various images obtained by MRI, it is necessary to determine the exact position of the region of interest of the subject. To achieve this, it is generally required to measure on multiple slices. In the above-mentioned conventional technique, the echo planar method is used at short time intervals to perform the measurement in one relaxation process, and therefore, there is no repetitive waiting time unlike the ordinary measurement.
Therefore, it is difficult to perform the measurement using the waiting time for relaxation, which is performed in the normal multi-slice measurement. That is, in the above-mentioned conventional method, since information for only one slice can be obtained for each relaxation process of nuclear magnetization, there arises a problem that the time resolution deteriorates depending on the number of slices. An object of the present invention is to provide an MRI imaging method capable of acquiring a plurality of images in different relaxation states for a plurality of slices without deteriorating the temporal resolution.

[0005]

In order to achieve the above object and acquire n images having different relaxation states for N slices, the present invention employs the following means.
First, apply a 180 degree RF pulse in a wide frequency band,
The magnetization of nuclei contained in all the slices of interest in the subject is reversed. Each slice is selected by a slice selection gradient magnetic field. After the lapse of time Ta, the first θ degree (θ <9
0) An RF pulse is applied and the image of the first longitudinal relaxation state for the first slice is acquired by the ultrafast imaging method. After a lapse of time Tb, a second θ-degree RF pulse whose frequency is adjusted to the second slice is applied, and an image of the first longitudinal relaxation state for the second slice is acquired by the echo planar method. Similarly, the θ degree RF pulse is applied up to the Nth time at intervals of the time Tb to acquire the first relaxed image for the Nth slice. As the ultra-high speed imaging method, for example, an echo planar method can be used.

After that, while changing the frequency in accordance with each slice, the θ degree RF pulse is applied at the time interval Tb,
From the second relaxed image for the first slice to the Nth
Acquire images up to the second relaxed state for the slice. By repeating this operation until the image acquisition of the nth relaxation state of the Nth slice is completed, n different relaxation states can be obtained for N slices during one relaxation process.
Acquire each of the images.

Then, from the n images having different relaxation states, n pieces of information are obtained for each pixel in the image, and the spin density M ₀ and the longitudinal relaxation time constant T ₁ are known as a known relational expression. Then, the distribution image of the spin density M ₀ and the longitudinal relaxation time constant T ₁ can be obtained for each slice. The angle θ at which the magnetization falls is also an unknown
By solving a relational expression in which M ₀ , T ₁ and θ are unknowns by using, for example, a nonlinear least squares approximation method, a distribution image of the spin density M ₀ and the time constant T ₁ of longitudinal relaxation is obtained with higher accuracy for each slice. be able to.

[0008]

According to the present invention, it is possible to acquire a plurality of images having different relaxation states for a plurality of slices without degrading the time resolution as compared with the conventional method. Furthermore, a distribution image of M ₀ and T ₁ can be obtained for a plurality of slices from the obtained image.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of the overall configuration of a magnetic resonance apparatus according to the present invention. This magnetic resonance apparatus uses MR
To obtain a tomographic image of the subject 101 using the phenomenon,
The static magnetic field generating magnet 102, the signal processing unit 103, the RF transmitting unit 104, the RF receiving unit 105, the gradient magnetic field generating unit 106, the display unit 107, and the control unit 108 for controlling them.

The static magnetic field generating magnet 102 is for generating a strong and uniform static magnetic field in the space in which the subject 101 is placed.
It is arranged in the space around the subject 101. The output of the RF transmitter 104 is sent to the transmitter coil 109 to generate an RF magnetic field. The output of the gradient magnetic field generation unit 106 is sent to the gradient magnetic field coil 110 and generates gradient magnetic fields Gx, Gy, Gz in three directions of X, Y, and Z. A tomographic plane with respect to the subject 101 can be set depending on how to apply the RF magnetic field and the gradient magnetic field. The RF receiving unit 105 includes the receiving coil 1
11 signal is received. The output of the RF receiving unit 105 is subjected to processing such as image reconstruction in the signal processing unit 103, and then displayed on the display unit 107. Here, the image reconstruction means a process of creating one image by performing a two-dimensional complex Fourier transform in the measurement space. Note that, in FIG. 1, the transmission coil 109, the reception coil 111, and the gradient magnetic field coil 110 are arranged in a space around the subject 101.

FIG. 2 shows a sequence (R
A part of an example of (F pulse and gradient magnetic field application procedure) is shown. How to use this sequence to obtain a plurality of images having different relaxation states in three slices will be described. Step 1. A 180 degree RF pulse 200 having a wide frequency band is applied to invert the nuclear magnetization of all slices in the subject. Step 2. A dephasing gradient magnetic field 214 is applied to cancel the remaining magnetization in the lateral direction.

Step 3. θ degree (θ <90) RF pulse 20
1-1 and the slice selection gradient magnetic field 215 are applied at the same time, and the magnetization of the nucleus in the first slice of the subject is inclined by θ degrees with respect to the longitudinal direction. Step 4. A rephasing gradient magnetic field 216 for the slice selection gradient magnetic field 215, a dephasing gradient magnetic field 217 for an encode gradient magnetic field for adding position information, and a dephasing gradient magnetic field 219 for a readout gradient magnetic field for signal reading are applied. Step 5. After applying the encode gradient magnetic field 218 and the read-out gradient magnetic field 220 as shown in the figure and acquiring an image of the first relaxation state of the first slice, a dephasing gradient magnetic field 214 is applied in the same manner as in step 2 to remain in the lateral direction. Cancels the magnetization.

Step 6. The θ-degree RF pulse 202-1 and the slice selection gradient magnetic field 215 are simultaneously applied to invert the magnetization of the nucleus in the second slice of the subject by θ degrees with respect to the longitudinal direction. At this time, the frequency of the θ-degree RF pulse 202-1 is
Only the frequency corresponding to the difference in the gradient magnetic field strength for one slice differs from the frequency of the θ-degree RF pulse 201-1. Step 7. Similar to steps 4 and 5, after acquiring the image of the first relaxation state of the second slice, the magnetization remaining in the lateral direction is canceled. Step 8. The θ-degree RF pulse 203-1 and the slice selection gradient magnetic field 215 are simultaneously applied to invert the magnetization of the nucleus in the third slice of the subject by θ degrees with respect to the longitudinal direction. At this time, the frequency of the θ-degree RF pulse 203-1 differs from the frequency of the θ-degree RF pulse 202-1 by a frequency corresponding to the difference between the gradient magnetic field strength of one slice and the θ-degree RF pulse 201.
The frequency of -1 differs from the frequency of -1 by the frequency corresponding to the difference in gradient magnetic field strength for two slices.

Step 9. Similar to steps 4 and 5, after acquiring the first relaxed state image of the third slice, the magnetization remaining in the lateral direction is canceled. Step 10. The θ-degree RF pulse 201-2 and the slice selection gradient magnetic field 215 are simultaneously applied to invert the magnetization of the nuclei in the first slice of the subject by θ degrees with respect to the longitudinal direction. The frequency of the θ-degree RF pulse 201-2 is the θ-degree RF pulse 20.
It is equal to the frequency of 1-1. Step 11. Same as steps 4 and 5, 2 of the first slice
After acquiring the th relaxation state image, the magnetization remaining in the lateral direction is canceled.

Similarly, images of the second relaxed state of the second slice and the third slice are acquired in the same manner. Furthermore, by repeating these procedures, it becomes possible to acquire a plurality of images in which the relaxation states of the three slices are different. Here, the 180 degree RF pulse 200 and the θ degree RF pulse 20
The time between 1-1 is represented by Ta, and the time between the θ-degree RF pulses is represented by Tb. The method of image acquisition after each θ-degree RF pulse used in this method is generally called an echo planar method. However, the image acquisition method is not limited to the echo planar method shown in FIG. 2, and any method capable of acquiring an image at high speed can be used.

FIG. 3 shows the application timing of the RF magnetic field when five images with different relaxation states are acquired from three slices using the same sequence as in FIG. The procedure until the application of the θ-degree RF pulse 201-2 is the same as that shown in FIG. 2, and thereafter, the θ-degree RF pulse 202-2 to the θ-degree RF pulse 205-3 are repeated at the time interval Tb. One image is obtained by the above-mentioned echo planar method or the like for each application of the θ-degree RF pulse, and in this measurement,
Five images with different relaxation states of three slices, that is, a total of 15 images are acquired.

In the method described so far, adjacent slices are imaged in order. However, it is not necessary to image adjacent slices in order, and it is preferable to image every other slice in consideration of mutual interference between slices. . For example, when there are four slices, the first slice, the third slice, the second slice,
The images are taken in the order of the fourth slice. FIG. 4 shows the behavior of longitudinal magnetization in the first slice when the sequence of FIG. 3 is executed, in accordance with the RF pulse application timing. A 180-degree RF pulse 200 is applied to reverse the initial magnetization 300 by 180 degrees. The reversed magnetization is longitudinally relaxed for time Ta to become magnetization 301. A θ degree RF pulse 201-1 is applied to the magnetization 301 to reverse the magnetization by θ degrees. The lateral component of the magnetization tilted by θ degrees is detected by the echo planar method as a signal that contributes to the image formation in the first relaxation state. The remaining vertical component is further longitudinally relaxed to become the magnetization 302 for 3 Tb. In the same manner, the signal acquisition for the third magnetization 303, the fourth magnetization 304, and the fifth magnetization 305 is repeated, and five images in different relaxation states in the first slice can be acquired.

FIG. 5 shows the behavior of longitudinal magnetization in the third slice when the sequence of FIG. 3 is executed in accordance with the RF pulse application timing. A 180-degree RF pulse 200 is applied to set the initial magnetization 300 to 180
Flip it once. The reversed magnetization takes time (Ta + 2Tb)
During that period, longitudinal relaxation occurs and the magnetization becomes 401. A θ degree RF pulse 203-1 is applied to the magnetization 401 to reverse the magnetization by θ degrees.
The lateral component of the magnetization tilted by θ degrees is detected by the echo planar method as a signal that contributes to the image formation in the first relaxation state. The remaining vertical component is further longitudinally relaxed to become the magnetization 402 for 3 Tb. Similarly, the third magnetization 403, the fourth magnetization 404, and the fifth magnetization 405.
It is possible to acquire images with five different relaxation states in the third slice by repeating the steps up to the signal acquisition for.

The time constant T ₁ of the longitudinal relaxation time, the spin density M ₀ , and the tilt angle θ of the magnetization can be obtained by using a plurality of images obtained by the above method and having different relaxation states. Next, the method will be described. The following expression (1) is an expression representing the behavior of the longitudinal magnetization shown in FIGS. 4 and 5, and M _Nn is the magnitude of the magnetization before applying the nth θ-degree RF pulse in the Nth slice, M _{Nn 0} represents the magnitude of initial magnetization (spin density), and n represents the relaxation state for each slice.

[0020] _{M Nn = M 0 [(1} -β) [1- (αβ) n-1] / (1-αβ) -γ N (αβ) n-1 + (cosψ) (1-γ N) ( αβ) ^n-1 ] (1) In the formula (1), α is the following formula (2), β is the following formula (3), and γ
_N is represented by the following formula (4). In the formula, Ta and Tb are waiting times for the above-mentioned relaxation, N ₀ is the total number of slices, and N is the target slice number. Also, ψ represented by the following equation (5)
Is the angle at which the nuclear magnetization falls with the first 180 degree RF pulse. θ ₀ is the angle set by the device to defeat nuclear magnetization,
θ represents the angle at which the nuclear magnetization actually falls.

Α = cos θ (2) β = exp (−N₀Tb / T₁) (3) γ_N= Exp [-[Ta + (N-1) Tb] / T₁] (4) ψ = (θ / θ₀) π (5) In equation (1), the relaxation state of the Nth slice is different.
Pay attention to each pixel of n images (n ≧ 3 in this case)
From that signal strength change, T₁, M₀, Θ with unknown
Each value can be determined by performing a linear least squares approximation.
You can Thus T for each slice₁, M₀And θ
It is possible to form a distribution image of M ₀From the image of
The molecular number distribution can be evaluated.

The angle θ at which the magnetization falls by the RF pulse
Is proportional to the strength of the applied RF pulse and varies depending on the location in the subject, and it is generally difficult to determine the absolute value of θ. However, the distribution of θ can be calculated by solving equation (1) using θ as an unknown number as in the present embodiment. By treating θ as a parameter in this way, the values of M ₀ and T ₁ can be evaluated with high accuracy.

Further, the angle θ at which the magnetization falls is a known constant,
That is, it can be treated as being equal to the angle θ ₀ set by the device. In this case, α becomes a constant and ψ =
π, that is, cos ψ = −1 in Expression (1), and T ₁ and M ₀ are similarly obtained from the signal intensity change of each pixel of n images (n ≧ 2 in this case) having different relaxation states imaged for each slice. Can be determined to form a respective distribution image. When treating θ as a constant,
Although the accuracy of the values of T ₁ and M ₀ is reduced, there is an advantage that image formation can be performed at high speed.

[0024]

According to the present invention, it is possible to provide an MRI imaging method capable of acquiring a plurality of images having different states of vertical relaxation in a plurality of slices in a short time.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an apparatus for carrying out the present invention.

FIG. 2 is an explanatory diagram of a sequence according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating application timing of an RF magnetic field.

FIG. 4 is a diagram for explaining the behavior of longitudinal magnetization in the first slice.

FIG. 5 is a diagram for explaining the behavior of longitudinal magnetization in the third slice.

[Explanation of symbols]

101 ... Subject, 102 ... Static magnetic field generating magnet, 103 ... Signal processing unit, 104 ... RF transmitting unit, 105 ... RF receiving unit,
106 ... Gradient magnetic field generation unit, 107 ... Display unit, 108 ... Control unit, 109 ... Transmission coil, 110 ... Gradient magnetic field coil,
111 ... Receiving coil, 200, 201-1, 202-
1, 203-1, 201-2, 202-2, 203-
2, 201-3, 202-3, 203-3, 201-
4,202-4,203-4,205-1,205-
2, 205-3 ... RF pulse, 214 ... Spoiler gradient magnetic field, 215 ... Slice selection gradient magnetic field, 216 ... Rephase gradient magnetic field, 217, 219 ... Dephase gradient magnetic field, 218 ... Encoding gradient magnetic field, 220 ... Readout gradient magnetic field, 300 ... Initial magnetization, 301 ... First longitudinal magnetization of first slice, 302 ... Second longitudinal magnetization of first slice, 303 ... Third longitudinal magnetization of first slice, 304
... the fourth longitudinal magnetization of the first slice, 305 ... the fifth longitudinal magnetization of the first slice, 401 ... the first longitudinal magnetization of the third slice, 402 ... the second longitudinal magnetization of the third slice, 40
3 ... Third longitudinal magnetization of third slice, 404 ... Fourth longitudinal magnetization of third slice, 405 ... Fifth longitudinal magnetization of third slice

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshitaka Bito 1-280 Higashi Koigakubo, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. Central Research Center

Claims (4)

[Claims] 1. A magnetic resonance imaging method for acquiring an image in which longitudinal relaxation states of nuclear magnetization are different for a plurality of slices in a subject, and a 180 degree RF pulse having a predetermined frequency band is applied to the magnetic resonance of all slices. After the step 1 of reversing the conversion, a θ degree (θ <90) RF pulse is sequentially applied to each slice at an arbitrary time interval, and a step 2 of acquiring a magnetic resonance image by the ultrafast imaging method is performed. A magnetic resonance imaging method comprising repeating step 2. 2. A plurality of slices obtained by the magnetic resonance imaging method according to claim 1 having different relaxation states (n ≧ n).
Using the image of 2), determine the values of M ₀ or T ₁ from relationship between constant T ₁ time of spin density M ₀ and longitudinal relaxation for each pixel for each slice, the distribution image of M ₀ or T ₁ A magnetic resonance imaging method characterized by obtaining the same. 3. A plurality of slices obtained by the magnetic resonance imaging method according to claim 1, which have different relaxation states (n ≧ n).
Using the image of 3), the spin density M ₀ , the time constant T ₁ of the longitudinal relaxation, and the tilt angle θ of the magnetization for each slice are used as parameters for the three parameters, and M ₀ is calculated by the nonlinear least square approximation method. obtaining the values of T ₁ or theta, a magnetic resonance imaging method characterized by obtaining a distribution image of M _0, T ₁ or theta. 4. The magnetic resonance imaging method according to claim 1, wherein the ultrafast imaging method is an echo planar method.