JPS6029646A - Device for obtaining signal containing information on spin-spin relaxation - Google Patents

Abstract

PURPOSE:To make it possible to obtain a signal contg. plural sets of information on spin-spin relaxation time only by change-over of a 90 deg. pulse and an inclined magnetic field by exciting the nuclear spin on the selecting surface of an object to be measured, defacing the nuclear spin, inverting the direction of the inclined magnetic field and reading out the generated spin echo signal. CONSTITUTION:A control part 11 is connected through a part 12 for generating the signal for an inclined magnetic field in a direction (x), a part 13 for generating the signal for an inclined magnetic field in a direction (y), a part 14 for generating the signal for an inclined magmetic field in a direction (z), a part 15 for generating the signal for a 90 deg. pulse, an upper bus 16 and a lower bus 17. An address is assigned by the data on the busses 16, 17 in the part 15 and an ROM28 is read out. The read-out data is converted to an analog signal by a DA converter 29. A reference high frequency signal from an oscillator 32 is modulated by an AM modulator 31 from said analog signal. The modulated output is passed through a gate 33, is further amplified by an amplifier 34 and is supplied to a coil 7. Said output is amplified by the gate 33 and is supplied to the coil 7. The gate 33 prevents leakage of the high frequency signal from the oscillator 32 to the amplifier 34 when the output from the converter 29 is zero. The opening and closing of the gate 33 are accomplished by the part 11.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for obtaining time-dependent information of spin-spin relaxation through energy exchange between spins in nuclear magnetic resonance. Relaxation time (hereinafter T2
) is used for image formation.

Background> Conventional NMR image forming apparatus When forming two images or an image dependent on T2, a pulse train of so-called spin echo signal (time') -180° pulses is applied to the object to be measured, and the spin echo signal is This was then read in the presence of a gradient magnetic field.

However, it is difficult to generate a high-frequency magnetic field that provides a 90° pulse or a 180° pulse completely uniformly over the entire object to be measured, especially a large object such as a human body, and the field inevitably becomes non-uniform. The effect of this heterogeneity is 9
Although it is relatively slight in the case of a 0° pulse, it becomes noticeable when a 180° pulse is applied.

In this case, even if a 180° pulse is applied, nuclear spins are generated that do not completely invert by 180°. This nuclear spin that does not completely flip appears as noise in the spin echo signal. Therefore, in the spin echo method, since the aforementioned K 180° pulse is used, the noise in the spin echo signal increases, making it impossible to measure T2 correctly.

<Summary of the Invention> The purpose of the present invention is to generate 9
The object of the present invention is to provide an apparatus that obtains a signal containing T2 information by obtaining a plurality of spin echo signals only by switching 0° pulses and gradient magnetic fields.

According to this invention, when a static magnetic field is applied to the object to be measured,
A selective excitation pulse along with a P+ gradient magnetic field to the object to be measured;
That is, a 90° pulse is applied to excite nuclear spins on a selected surface of the object to be measured. A second gradient magnetic field is applied to the object to be measured to deface the excited nuclear spins, and the second gradient magnetic field is
Reverse the direction of the gradient magnetic field. A spin echo signal generated by the inversion is read out. If necessary, T2 is measured from the attenuation rate of this spin echo signal.

<Example> An example of the present invention will be explained below with reference to the drawings. As shown in FIG. 1, the direction of the magnetic field generated by a magnet 1 for generating a static magnetic field is the y-axis, and an object 2, for example, a human being, is placed within the static magnetic field of the magnet 1. The longitudinal direction of the object to be measured 2, that is, the body axis direction is the y-axis, and the y-axis and the direction perpendicular to the y-axes are the y-axis. A cylindrical coil bobbin 3 whose axis coincides with the y-axis is disposed within the static magnetic field of the magnet 1, and the object to be measured 2 is disposed within the coil bobbin 3. As shown in Fig. 2, a coil 4 wound on a bobbin 3 generates a gradient magnetic field Gx whose magnetic field strength is inclined in the X-axis direction, and a coil 5 generates a gradient magnetic field
The axial gradient magnetic field Gz is converted into a y-axis gradient magnetic field G by the coil 6.
y can be generated respectively. Furthermore, a coil 7 for forming a high frequency magnetic field and detecting a signal in the y-axis direction.
is wound on bobbin 3.

As shown in Figure 3, period 1. A 90' pulse is applied using the coil 7 while applying the y-axis gradient magnetic field Gy. As a result, at a specific point on the y-axis, nuclear spins within a selected plane of the object 2 to be measured perpendicular to the y-axis are excited. Due to this excitation, the nuclear spin "+" is generated in the y-axis direction as shown in Figure 4A.
＋ ``2...'' are gathered together.

Next, in period t2, the X-direction gradient magnetic field Gx is applied for a certain period of time to generate a fourth
As shown in Figure B, the nuclear spins m1, m2... are defaced. This period t2 is set to be an appropriately short time such that T2 relaxation does not substantially occur.

In period t3, the direction of the X-axis gradient magnetic field Gx is switched.

Due to this magnetic field Gx, the nuclear spins ml, m2, . . . U are rephased toward their original positions as shown in FIG. 4C. A spin echo signal P1 is obtained during this period t3.

Since this spin echo signal can be said to be an echo signal obtained from spins whose period t2 is short or immediately after <900 pulses, the spins do not undergo T2 relaxation.

In period t4 after this spin echo signal is generated, the fourth
As shown in the figure, nuclear speed once rephased/mJ, m
2... is again tee-phased by the gradient magnetic field Gx. When the X-axis gradient magnetic field Gx is switched again at the end of period t4, the nuclear spins ml, m2...
is rephased again as shown in FIG. 4E, and a spin echo signal P2 is obtained as shown in FIG.

This signal P2 undergoes T2 relaxation during τ time (period t4) compared to the initially obtained signal Pl, and becomes a signal containing T2 information.

These two signals P1. T2 can be increased from P2 without using a 180° pulse. That is, as mentioned above, since the period t2 is set sufficiently short, the spin echo signal P1. If the peak values of P2 are S+ and Sz, T2 is divided by the following formula.

In place of the peak values S+ and S2, the signals P+ and P2 may be subjected to a two-dimensional Fourier transform as is usually done.0 The spin echo signals P+ and P2 are the outputs of the coils 7,
For example, by comparing the pulse width with the signal P1. Find the peak value of P2, and then calculate T2
It can be configured to calculate It is preferable that the X-axis direction gradient magnetic field CX be kept constant during signal sampling during the period L3. When the object to be measured 2 is a human being, the sampling period is preferably 100 kHz and τ is preferably within 10 m5ec. Furthermore, the period from period t1 to the end of period t5 is within T2 hours, for example, 50 m5e when a human being is the object to be measured 2.
It is appropriate to finish within c. Also, period L in Figure 3
In step 2, when the nuclear spins on the selected surface are excited, in order to correct the phase disturbance of the excited nuclear spins, the gradient magnetic field in the y-axis direction is reversed as in the conventional method.

When performing image formation using the information P2 and T2 including T2 obtained in this way, in order to obtain information in the Z direction, the tilt in the Z-axis direction is adjusted in the period t2 as shown in Japanese Unexamined Patent Publication No. 54-158988. A magnetic field Gz is also applied at the same time. The present invention can be applied not only to this image forming method, but also to an image forming method using the so-called reconstruction glossejection method, in which, for example, a gradient magnetic field is rotated and the excited in-plane spin density is projected onto the gradient magnetic field and reconstructed. . That is, the fifth
As shown in the figure, this can be achieved by obtaining a spin echo signal by switching the direction of a composite projection magnetic field of an X-axis gradient magnetic field aGX and a Z-axis gradient magnetic field bGz instead of the X-axis gradient magnetic field Gx.

In this invention, the signal P2 containing T2 information may be obtained without necessarily obtaining T2, and this may be displayed as an image, for example. As understood from the explanation of FIG. 4B-F, T
By repeating rephasing and dephasing during 2, a spin echo signal containing T2 information may be obtained repeatedly. The point is to dephase the excited nuclear spins, reverse the direction of the gradient magnetic field applied for the dephasing, and perform rephasing to obtain a spin echo signal containing T2 information.

The aforementioned 90° pulse generation, magnetic field control, etc. can be performed with a configuration as shown in FIG. 6, for example.

The control section 11 includes an X-direction gradient magnetic field signal generation section 12. Y-direction gradient magnetic field signal generator 13. Z-direction gradient magnetic field signal generator 14
．． and 90° pulse signal generator 15. They are connected through an upper bus 16° and a lower bus 17. Furthermore, the data line 21゜2
2 and 23, the controller 11 connects the magnetic field signal generator 12 to the magnetic field signal generator 12.
, 13°14. magnetic field signal generator 12,
Since L3 and 14 have similar configurations, the X-direction gradient magnetic field signal generating section j2 will be explained. Data indicating the waveform of the generated magnetic field (for example, Gx in Fig. 3) is stored in ROM2.
4 and read out by addresses from buses 16 and 17. The read data is multiplied by amplitude information specified by a multiplier 25. The multiplication output of the multiplier 25 is converted into an analog signal by a DA converter 26, and the analog signal is amplified by an amplifier 27 and supplied to the X-direction gradient magnetic field generating coil 4 (FIG. 2).

Y-direction gradient magnetic field signal generator 13. Each output of the Z-direction gradient magnetic field signal generator 14 is supplied to the coils 6 and 4, respectively.

In the 90° pulse signal generating section 15, a node 16,
17, the ROM 28 is read out, the read data is converted into an analog signal by the DA converter 29, and the analog signal modulates the reference high frequency signal from the oscillator 32 by the AM modulator 31. . This modulated output passes through a gate 33, is further amplified by an amplifier 34, and is supplied to the coil 7. The gate 33 prevents the high frequency signal of the oscillation 2 and 32 from leaking to the amplifier 34 when the output of the DA converter 29 is zero. The gate 33 is opened and closed by the control section 11. The signal P1 or P2 detected by the coil 7 is amplified by the amplifier 35, and further amplified by the detector 3.
The wave is detected at 6. The detection puff is converted into a digital signal by an AD converter 37 and written into a memory 38.

The control unit 11 is composed of, for example, a microcomputer,
The central processing unit (CPU) 39 reads and decodes a program memory (not shown), and outputs the signals to the respective powers q of the magnetic field signal generators 12, 13, and 14 through the input/output port 41 and through the data lines 21, 22, and 23. - to vessel 25;
Set amplitude information for. Further, the CPU 39 controls the opening and closing of the gate 33 through the input/output port 41, issues a start-up command to the AD converter 37, and transmits the ROM 24 of each of the magnetic field generators 12, 13, and 14 through the upper node 16.
, and the storage areas of the ROM 28 of the 90° pulse signal generator 15, that is, the areas where the waveforms to be generated are stored, and further issues a start command to the address counter 42. The wave number contents of the address counter 42 are supplied to each ROM 24 and ROM 28 as a lower address through the lower bus 17. When the counter 42 finishes reading one area of each ROM (each area has the same address), it notifies the CPU 39 of a count end signal.Next, the operation will be explained with reference to the flowchart of FIG. 7. When activated, the CPU 39 sets an amplitude value for each multiplier 25 of the magnetic field signal generators 12, 13, and 14 in step S1. In step S2, the CPU 39 sends information to each ROM 24 and ROM 2 of the magnetic field signal generation units 12, 13, and 14.
Eight read areas are designated via the upper bus 16. In step S8, the CPU 39 opens the gate 33, and in step S4, sets an initial value to the address counter 42 and starts a counting operation. Therefore, each ROM24, RO
M28 are read out all at once, for example during period 1 in FIG.
．． , 12 and t3 are generated by the magnetic field signal generators 12, 13, 14 and the 90° pulse signal generator 15, respectively, and are supplied to each corresponding coil.

In step S5, the completion of the counter 42 is monitored, and when the X-direction stretching magnetic field Gx reaches a constant value for a period t3r, the counter 42 is
2 is completed, and this is transmitted to the CPU 39 as an interrupt. Note that when the magnetic field is zero, zero is stored in the corresponding ROM address.

When the CPU 39 finishes reading the counter 42, the CPU 39 closes the gate 33 in step S6, and closes the gate 33 in step S7.
3, and a conversion start command is given to the AD converter 37 in step S8. From this, the output obtained by detecting the signal from the coil 7 is sequentially sampled, and each sampling value is written into the memory 38 as a digital signal. In this way, signal P1 is captured. In step S9, it is checked whether or not the AD converter 37 has performed a certain number of samplings, and when the sampling is completed, 't' is notified to the CPU 39. From this, step S+o measures the passage of time τ in period t4, and then, in step So, the CPU
39, the multiplier 25 of the X-direction gradient magnetic field signal generation section 12
, the sign (polarity) of the set amplitude is inverted. After this, time is taken until sampling of the signal P2 is started in step SI2, and a start command is issued to the AD converter 37 again in step SI3. From this, signal P2 is sampled. In step SI4, the end of the AD conversion operation is monitored, and when the end is notified to the CPU 39, the signal acquisition operation is ended.

Note that after the signal P2, the magnetic field Gx is further reversed and T
To obtain a signal including 2 relaxation, step 5ho-8+< may be repeated after step S14.

As explained above, according to the present invention, a spin echo signal is obtained only by switching between a 90° pulse and a gradient magnetic field without using a 180° pulse. Although the echo signal is greatly affected by noise, the device of the present invention is not affected by such noise and can accurately obtain a signal containing T2 information and T2.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an overview of an example of a nuclear magnetic resonance apparatus, Fig. 2 is a perspective view showing an example of a gradient magnetic field generating coil, Fig. 3 is a time chart showing an example of the operation of the apparatus of the present invention, and Fig. 4 is a diagram showing the movement of nuclear spins during the operation of this inventive device, FIG. 5 is a time chart showing another example of the operation of this inventive device, and FIG. 6 is an example of the configuration of each magnetic field generation and signal acquisition reservoir. FIG. 7 is a flowchart showing an example of the operation of FIG. 11: Control unit, 12: X-direction gradient magnetic field generation unit 13: Y-direction gradient magnetic field generation unit, 14: Direction gradient I magnetic field generation unit, 15
:90° pulse signal generator. Patent Applicant: Asahi Kasei Industries Co., Ltd. Agent Takashi Kusano

Claims (1)

[Claims] In an apparatus for obtaining a signal including spin-spin relaxation time information, the apparatus includes a static magnetic field generating means for applying a static magnetic field to an object to be measured, three mutually orthogonal gradient magnetic field generating means, and a selective excitation pulse generating means. means for exciting nuclear spins on a selected surface of the object to be measured by applying a selective excitation pulse together with a first gradient magnetic field generated by the gradient magnetic field generating means under a magnetic field; means for applying a second gradient magnetic field to the object to be measured to cause the excited nuclear spins to tee-phase, and reversing the direction of the second gradient magnetic field in a state in which spin spin relaxation occurs due to the tee-phase; , and a means for reading out a spin echo signal generated by the reversal of the spin echo signal.