JPH0647014A - Imaging method by mr - Google Patents

Abstract

PURPOSE:To obtain a diffusion emphasizing image having quantitativeness by executing an arithmetic processing for eliminating the influence of T1 relaxation (time constant of longitudinal relaxation) in a spoiling period, and the influence of an eddy current generated by a diffusion emphasizing gradient magnetic field. CONSTITUTION:By a sequence controller 3, a gradient magnetic field driving circuit 4 is operated and a static magnetic field and a gradient magnetic field are generated in a magnet assembly 5. Also, a gate modulating circuit 7 is controlled, an RF pulse is modulated to a prescribed waveform and applied to a transmission coil of the magnet assembly 5, an NMR signal obtained by a resin coil is inputted to a computer 2 through a phase detector 10 and thus an image is reconstituted. Subsequently, at the time of reconstituting the image by this computer 2, data containing only the influence of T1 relaxation in a spoiling period, and data containing the influence of diffusion information and an eddy current superposed thereto and the influence of longitudinal relaxation in the spoiling period, etc., are collected and by executing an arithmetic processing of each data thereof, a diffusion emphasizing image is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging method, and more particularly to an imaging method capable of obtaining a quantitative diffusion-weighted image and T2-weighted image.

[0002]

2. Description of the Related Art FIG. 10 is an explanatory diagram of a conventional pulse sequence for obtaining a diffusion weighted image. This pulse sequence is the preparation sequence PA of the previous stage.
And a pulse sequence PF of the FRASH method in the latter stage. The preparation sequence PA includes a preparation period TEp and a spoiling period Ts.

In the preparation period TEp, an RF pulse sequence of {π / 2 [x]} → {π} → {π} by the SE method is applied, and before and after the third RF pulse {π} pulse. A diffusion enhancing gradient magnetic field DG having the same polarity is applied. Note that {π / 2 [x]} represents a {π / 2} pulse having a transmission phase of [x]. In this preparation period TEp, the transverse magnetic charge excited by the {π / 2 [x]} pulse responds to the diffusion by the diffusion enhancing gradient magnetic field DG before and after the {π} pulse which is the third RF pulse. Diffusion information is emphasized due to signal attenuation. The transverse magnetic charge also attenuates T2.

During the spoiling period Ts, the {π / 2 [-x]} pulse is applied and the spoiler SP is applied. The transverse magnetic charge is returned to the longitudinal magnetic charge by the {π / 2 [-x]} pulse. Further, the longitudinal magnetic charge generated in the preparation period TEp becomes a lateral magnetic charge due to the {π / 2 [-x]} pulse and is erased by the spoiler SP. Also, the above (π / 2 [-
x]} The unnecessary components caused by the imperfections of the pulse are erased by the spoiler SP.

In the pulse sequence PF of the FRASH method, in order to effectively reflect the diffusion information emphasized in the preparation period TEp in the image, data is collected from the phase encoding gradient of the lowest frequency.

The signal strength I of the data collected with the phase encoding gradient of the lowest frequency is approximated by the following equation. I = M0 * exp {-b * D} * exp {-TEp / T2} * exp {-Ts / T1} ... (1) Here, M0 is the magnitude of the magnetic charge in an equilibrium state. b
Is a constant determined by the intensity of the diffusion enhancing gradient magnetic field DG, the application time, and the like. D is the diffusion coefficient of the subject.
T2 is the time constant of lateral relaxation. T1 is the time constant of longitudinal relaxation. Therefore, changing the constant b in the above equation (1) (that is,
Diffusion coefficient D is obtained by collecting data I1 and I2 by adding a gradient magnetic field DG for diffusion enhancement having different strength and application time) and D = Log {I1 / I2} / (b2-b1) (2) I got the image of diffusion emphasis by.

On the other hand, although the T2-weighted image is used for diagnosis, the conventional pulse sequence for obtaining the T2-weighted image requires several minutes for scanning.

[0008]

The above equation (1) ignores the influence of longitudinal relaxation during the spoiling period Ts. That is, the above equation (1) neglects the second term on the right side in the following equation. I = M0 ・ exp {-b ・ D} ・ exp {-TEp / T2} ・ exp {-Ts / T1} + M0 ・ {1-exp {-Ts / T1}]… (3) However, the right side of the above equation The second term is so large that it cannot be ignored in practice, and the conventional technique that ignores this has a problem that the quantitativeness of the diffusion coefficient D is impaired.

Further, for example, depending on the coil structure, there is an influence of an eddy current generated by the diffusion enhancing gradient magnetic field DG, and at this time, the signal intensity I of the data is I = M0.exp {-b.D} .exp { -TEp / T2} ・ exp {-Ts / T1} cosθ ＋ M0 ・ {1-exp {-Ts / T1}]… (4). That is, the phase of the transverse magnetic charge is shifted by θ, and the shading (shad
ing) occurred.

On the other hand, in the conventional pulse sequence for obtaining a T2-weighted image, it took several minutes to scan, so
There were problems such as being an obstacle in improving throughput and being affected by body movement.

Therefore, the first object of the present invention is to achieve the above.
Effect of the second term on the right side of equation (3) (T1 during spoiling period)
An object of the present invention is to provide an imaging method by MR capable of removing the effect of relaxation. A second object of the present invention is to provide an MR imaging method capable of removing the influence of θ in the above formula (4) (the influence of the eddy current generated by the diffusion enhancing gradient magnetic field DG). A third object of the present invention is to provide an MR imaging method capable of obtaining a T2-weighted image in a few seconds.

[0012]

SUMMARY OF THE INVENTION In a first aspect, the present invention is a preparation for applying a diffusion enhancing gradient magnetic field having the same polarity before and after a second and subsequent predetermined RF pulses in an RF pulse sequence by the SE method. A first preparation sequence consisting of a period and a spoiling period in which an RF pulse having a transmission phase for canceling the transmission phase of the first RF pulse in the RF pulse sequence is applied and a spoiling gradient magnetic field is applied; FR that can collect data at high speed
The first added to the previous stage of the pulse sequence such as ASH method
The first data collection step of collecting the first data by the pulse sequence of No. 3, the preparation period of applying the magnetic field gradient magnetic field for opposite polarity before and after the predetermined RF pulse, and the spoiling period. Become second
A second data collecting step of collecting second data by a second pulse sequence added to the preceding stage of the pulse sequence capable of collecting data at high speed, and the second data collecting step from the first data. And a subtraction step of subtracting the data of 1.

According to a second aspect, the present invention provides a preparation period in which a gradient magnetic field for diffusion enhancement having the same polarity is applied before and after a second and subsequent predetermined RF pulses in an RF pulse sequence by the SE method, and the RF pulse. 1st R in the series
RF with a transmission phase that cancels the transmission phase of the F pulse
A first preparation sequence consisting of a spoiling period in which a pulse is applied and a spoiling gradient magnetic field is applied is added to a first pulse sequence that is added before the pulse sequence such as the FRASH method capable of collecting data at high speed. A first data collecting step of collecting data of No. 1, and a preparation period of applying a gradient magnetic field for magnetic charge elimination having opposite polarities before and after the predetermined RF pulse,
A second data collection step of collecting second data by a second pulse sequence in which a second preparation sequence consisting of the spoiling period is added to the preceding stage of the pulse sequence capable of high-speed data collection; A third preparation sequence in which the transmission phase of the RF pulse in the spoiling period in the first preparation sequence is different by 90 ° is added to the preceding stage of the pulse sequence capable of collecting data at high speed.
A third data collection step of collecting third data by the pulse sequence of, first subtraction data obtained by subtracting the second data from the first data, and the third data collection step.
And a second addition data obtained by subtracting the second data from the second data, and then performing a square addition step of adding and then adding.

According to a third aspect, the present invention relates to a preparation period for applying an RF pulse sequence by the SE method,
A preparation sequence consisting of a spoiling period in which an RF pulse having a transmission phase that cancels the transmission phase of the first RF pulse in the RF pulse sequence is applied and a spoiling period in which a gradient magnetic field for spoiling is applied can be collected at high speed. An image data collecting step of collecting image data by the image pulse sequence added to the front stage of the pulse sequence such as the FRASH method;
The preparation period in which the gradient magnetic field for eliminating the magnetic charge having the opposite polarity is applied before and after the second and subsequent RF pulses in the RF pulse sequence by the E method and the transmission phase of the first RF pulse in the RF pulse sequence are offset. R with a transmission phase
The correction data is collected by the correction pulse sequence in which the second preparation sequence consisting of the spoiling period in which the F pulse is applied and the gradient magnetic field for the spoil is applied is added to the preceding stage of the pulse sequence capable of high-speed data acquisition. And a subtraction step for subtracting the correction data from the image data.

[0015]

The MR-based imaging method according to the first aspect of the present invention is applied when the influence of the eddy current generated by the diffusion enhancing gradient magnetic field DG can be ignored. First,
In the first pulse sequence, the first
Data IA of. This first data IA is represented by the above equation (3). Next, in the second pulse sequence, a magnetic field erasing gradient magnetic field of opposite polarity is applied before and after the {π} pulse to collect the second data IB. Since the polarity of the diffusion enhancing gradient magnetic field is reversed before and after the {π} pulse, the transverse magnetic charge is not focused in the xy plane and is completely scattered. Therefore, the first term on the right side of the above equation (3) becomes “0”, and only the second term on the right side remains. That is, IB = M0 · {1-exp {-Ts / T1}] (5) Next, the first data IA to the second data IB
By subtracting, the second term on the right side of equation (3) disappears, and
It becomes (1). Therefore, the effect of T1 relaxation during the spoiling period can be eliminated.

The MR-based imaging method according to the second aspect of the present invention is applied when the influence of the eddy current generated by the diffusion enhancing gradient magnetic field DG cannot be ignored.
First, in the first pulse sequence, the first data IA is collected in the same manner as in the conventional case. This first data I
A is represented by the above formula (3). Next, in the second pulse sequence, a magnetic field erasing gradient magnetic field of opposite polarity is applied before and after the {π} pulse to collect the second data IB. The second data IB is represented by the above equation (5). next,
RF during the spoiling period with the third pulse sequence
A third data IC is acquired in the same manner as the first pulse sequence except that the transmission phase of the pulse is changed by 90 °. This third data IC is IC = M0 ・ exp {-b ・ D} ・ exp {-TEp / T2} ・ exp {-Ts / T1} sinθ ＋ M0 ・ {1-exp {-Ts / T1}]… ( 6). Next, the first data IA to the second data IB
If is subtracted, the second term on the right side of the above equation (3) disappears. Further, when the second data IB is subtracted from the third data IC, the second term on the right side of the equation (6) disappears. Furthermore, if the subtraction results are squared, then added, and further square rooted, θ
The term of disappears and the above (1) is obtained. Therefore, the influence of the eddy current generated by the diffusion enhancing gradient magnetic field DG and the influence of T1 relaxation during the spoiling period can be removed.

The MR-based imaging method according to the third aspect of the present invention is applied when it is desired to obtain a T2-weighted image at high speed. First, the image data IE is collected by the image pulse sequence. Since the image data IE is b = 0 in the equation (3), IE = M0.exp {-TEp / T2} .exp {-Ts / T1} + M0. {1-exp {- Ts / T1}] (7). Next, the correction data IB is collected by the correction pulse sequence. Since the correction pulse sequence is the same as the second pulse sequence, the correction data IB is expressed by the equation (5). Next, by subtracting the correction data IB from the image data IE, the above (7)
The second term on the right side of the equation disappears, and only the first term on the right side remains. here,
Since T1 and T2 of the in-vivo tissue have a positive correlation, the term T1 of the first term on the right side contributes to the direction in which the contrast due to T2 is not masked. Therefore, a T2-weighted image can be obtained, but the time required for scanning is several seconds or less.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail based on the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an MRI apparatus 1 for carrying out the imaging method of the present invention. The computer 2 controls the overall operation based on the instruction from the console 13. The sequence controller 3 operates the gradient magnetic field driving circuit 4 based on the stored sequence, and causes the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5 to generate a static magnetic field and a gradient magnetic field. Also,
The gate modulation circuit 7 is controlled to modulate the RF pulse generated by the RF oscillation circuit 6 into a predetermined waveform, and the RF power amplifier 8
To the transmitter coil of the magnet assembly 5.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to the phase detector 10 via the preamplifier 9 and further to the computer 2 via the AD converter 11. The computer 2 reconstructs an image based on the data of the NMR signal obtained from the AD converter 11, and displays it on the display device 12. The imaging method of the present invention is carried out by the procedure stored in the computer 2 and the sequence controller 3.

When the user gives an instruction using the console 13 after setting the subject on the magnet assembly 5, the computer 2 executes the processing of the flow chart shown in FIG. In step V1, pulse sequences A and B are created. 3 and 4 illustrate pulse sequences A and B. The pulse sequence A in FIG. 3 is the same as the conventional pulse sequence A (FIG. 10). That is, the data IA collected by this pulse sequence A is expressed by the above equation (3). The pulse sequence B of FIG. 4 is the same as the pulse sequence A except that the polarities of the diffusion enhancing gradient magnetic fields DG and DG are the diffusion enhancing gradient magnetic fields DG and dg that are inverted before and after the {π} pulse. is there. However, the data I collected by this pulse sequence B
B is represented by the above formula (5).

At step V2, the processing parameter k =
Set to 1. In step V3, the diffusion enhancing gradient magnetic field DG1 is set such that b = b1.

In step V4, the data IB is collected by the pulse sequence B. The data IB collected is
It is represented by the above formula (5). In step V5, the pulse sequence A is used to collect the data IA. The collected data IA is represented by the above equation (3). In step V6,
Data IB is subtracted from data IA, and the result is data Ik.
And The first time is Ik = I1. At step V7, it is determined whether k = 2. If k ≠ 2, the process proceeds to step V8. If k = 2, the process proceeds to step V10.

At step V8, the processing parameter k =
Set to 2. In step V9, the diffusion enhancing gradient magnetic field DG2 is set so that b = b2. Then, the above step V
Return to 5. Data I by the second step V5, V6
2 is obtained. In step V10, a diffusion weighted image is obtained by the above equation (2).

On the other hand, the user can use the magnet assembly 5
After setting the subject on the
If another instruction is given, the computer 2 executes the processing of the flow chart shown in FIG. In step S1, pulse sequences A, B, C are created. The pulse sequences A and B are shown in FIGS. 3 and 4. FIG. 6 illustrates the pulse sequence C. The pulse sequence C of FIG. 6 uses the {π / 2 [-x]} pulse of the pulse sequence A instead of the {π / 2 [-x]} pulse, which uses a {π / 2 [y]} pulse whose transmission phase differs by 90 °. Except for this, it is similar to the pulse sequence A.
Data IC collected by this pulse sequence C
Is expressed by the above equation (6). In (a) and (b) of FIG.
The difference between the pulse sequence A and the pulse sequence C is conceptually shown.

In step S2, the processing parameter k =
Set to 1. In step S3, a diffusion enhancing gradient magnetic field DG1 that sets b = b1 is set.

In step S4, the data IB is collected by the pulse sequence B. The data IB collected is
It is represented by the above formula (5). In step S5, the pulse sequence A is used to collect the data IA. The collected data IA is represented by the above equation (3). In step S6,
The data IC is collected by the pulse sequence C. The collected data IC is represented by the above equation (6). In step S7, the data IB is subtracted from the data IA, the data IB is subtracted from the data IC, the respective results are squared and then added, and the square root is set as the data Ik. The first time is Ik = I1. In step S8, it is determined whether k = 2. If k ≠ 2, the process proceeds to step S9. If k = 2, the process proceeds to step S11.

In step S9, the processing parameter k =
Set to 2. In step S10, the diffusion enhancing gradient magnetic field DG2 is set such that b = b2. Then, the process returns to step S5. The data I2 is obtained by the second steps S5, S6 and S7. In step V11, the above (2)
A diffusion weighted image is obtained by the formula.

Now, the user selects the magnet assembly 5
After setting the subject on the
If another instruction is given, the computer 2 executes the processing of the flow chart shown in FIG. In step Q1, pulse sequences E and B are created. FIG. 9 shows a pulse sequence E
Is illustrated. The pulse sequence B is the one shown in FIG. In the pulse sequence E shown in FIG. 9, the preparation sequence PE is a diffusion enhancement gradient magnetic field D.
Same as the preparation sequence PA (FIG. 3) except that G is not applied. The pulse sequence PF subsequent to the preparation sequence PE is F
It is a pulse sequence of the RASH method. The data IE collected by this pulse sequence E is represented by the above equation (7).

In step Q2, the data IE is collected by the pulse sequence E. The collected data IE is
It is expressed by the above equation (7). In step Q3, the data IB is collected by the pulse sequence B. The collected data IB is represented by the above equation (5). In step Q4,
Data IB is subtracted from data IE, and the result is data Ir
And This data Ir is expressed by the following equation. Ir = M0 ・
exp {-TEp / T2} ・ exp {-Ts / T1}
(8) That is, a T2-weighted image can be obtained. The time required for scanning at this time is 1 second or less, which is a very short time.

[0030]

According to the MR-based imaging method of the present invention, the influence of T1 relaxation during the spoiling period is removed, and a diffusion-weighted image with good quantitativeness can be obtained. Also,
The influence of the eddy current generated by the diffusion-enhancing gradient magnetic field DG is removed, and a diffusion-enhanced image with good quantitativeness can be obtained. Furthermore, it becomes possible to obtain a T2-weighted image in a few seconds or less.

[Brief description of drawings]

FIG. 1 is a block diagram of an MRI apparatus for implementing an MR imaging method of the present invention.

FIG. 2 is a flowchart of an imaging method according to an embodiment of the present invention.

FIG. 3 is an exemplary diagram of a pulse sequence used in the imaging method of the present invention.

FIG. 4 is an exemplary diagram of a pulse sequence used in the imaging method of the present invention.

FIG. 5 is another flowchart of the imaging method according to the embodiment of the present invention.

FIG. 6 is an exemplary diagram of a pulse sequence used in the imaging method of the present invention.

FIG. 7 is a conceptual diagram showing a difference in effect between the pulse sequences of FIGS. 3 and 6.

FIG. 8 is another flowchart of the imaging method according to the embodiment of the present invention.

FIG. 9 is an exemplary diagram of a pulse sequence used in the imaging method of the present invention.

FIG. 10 is an exemplary diagram of a pulse sequence for obtaining a conventional diffusion weighted image.

[Explanation of symbols]

1 MRI device 2 Computer 3 Sequence controller A, B, C, E Pulse sequence DG, dg Diffusion enhancement gradient magnetic field PA, PB, PC, PE Preparation sequence PF FRASH pulse sequence SP Spoiler TEp Preparation period Ts Spoiling period

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 9118-2J G01N 24/08 Y

Claims (3)

[Claims] 1. A preparation period in which a gradient magnetic field for diffusion enhancement having the same polarity is applied before and after a second and subsequent predetermined RF pulses in an RF pulse sequence by the SE method, and the RF.
A spoiling period in which an RF pulse having a transmission phase that cancels the transmission phase of the first RF pulse in the pulse sequence is applied and a spoiling gradient magnetic field is applied.
The preparation sequence of 1) is added to the front stage of the pulse sequence such as the FRASH method capable of collecting data at a high speed by a first pulse sequence, and a first data collection step of collecting first data; A second pulse obtained by adding a second preparation sequence consisting of a preparation period for applying a gradient magnetic field for magnetic charge elimination having opposite polarities to the front and back and the spoiling period to the preceding stage of the pulse sequence capable of high-speed data acquisition. An imaging method by MR, comprising: a second data collection step of collecting second data by a sequence; and a subtraction step of subtracting the second data from the first data. 2. A preparation period in which a gradient magnetic field for diffusion enhancement having the same polarity is applied before and after the second and subsequent predetermined RF pulses in the RF pulse sequence by the SE method, and the RF.
A first spoiling period in which an RF pulse having a transmission phase that cancels the transmission phase of the first RF pulse in the pulse sequence is applied and a spoiling gradient magnetic field is applied.
The preparation sequence of 1) is added to the front stage of the pulse sequence such as the FRASH method capable of collecting data at a high speed by a first pulse sequence, and a first data collection step of collecting first data; A second pulse obtained by adding a second preparation sequence consisting of a preparation period for applying a gradient magnetic field for magnetic charge elimination having opposite polarities to the front and back and the spoiling period to the preceding stage of the pulse sequence capable of high-speed data acquisition. A second data collecting step of collecting second data by a sequence, and a third preparation sequence in which the transmission phase of the RF pulse in the spoiling period in the first preparation sequence is different by 90 °, To the front of the pulse sequence that can collect data The third pulse sequence, and a third data collection step of collecting the third data, the obtained by subtracting the second data from the first data 1
Subtraction data and second subtraction data obtained by subtracting the second data from the third data are squared, and a square addition step of adding the squared data is added.
Imaging method by R. 3. A gradient magnetic field for spoiling, in which a preparation period for applying an RF pulse sequence by the SE method and an RF pulse with a transmission phase for canceling the transmission phase of the first RF pulse in the RF pulse sequence are applied. A preparation sequence consisting of a spoiling period for applying the image is added to a pulse sequence for the image that is added before the pulse sequence such as the FRASH method, which enables high-speed data collection, and an image data collecting step for collecting the image data, and an SE method. And a preparation period in which a gradient magnetic field for eliminating magnetic charges having opposite polarities is applied before and after the second and subsequent RF pulses in the RF pulse sequence, and transmission for canceling the transmission phase of the first RF pulse in the RF pulse sequence. Applying a phased RF pulse and marking the gradient magnetic field for the spoil A second preparation sequence of the spoiling period, the correction pulse sequence added upstream of the data collection can be a pulse sequence to the high speed,
An imaging method by MR, comprising: a correction data collection step of collecting correction data; and a subtraction step of subtracting the correction data from the image data.