JPH03289939A - Rf automatic adjusting method for mri - Google Patents

Abstract

PURPOSE:To correctly obtain RF intensity in a short repeat time by changing the RF intensities of the first and second reverse pulses and operating data of a SE signal corresponding to a reverse pulse to obtain a RF intensity value. CONSTITUTION:An excitation pulse 21 with a flip angle of alpha deg./2, the first reverse pulse with a flip angle of alpha deg. and the second reverse pulse are made uniform in pulse width T and given as a pulse sequence for changing the RF intensity, and the first SE signal S324 and the second SE signal S425 from a subject are received. The input data is Fourier-transformed by FFT processing, a pass band where the deterioration of profile is comparatively small is set in a slice profile, and a coefficient of correction is obtained to obtain corrected SE signals S3, S4. The ratio of the obtained SE signals S3, S4 is obtained to obtain a sin<2> (alpha/2) curve. The peak detection is performed from the curve using the method of least squares to obtain the RF intensity whose flip angle is 180 deg., and the RF intensity value of an arbitrary flip angle alpha deg. is obtained by the proportional calculation.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is applicable to R
MRI that accurately determines F pulse intensity in a short repetition time
This invention relates to an automatic RF adjustment method.

(Prior art) When an atomic nucleus is placed in a static magnetic field, it precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of nucleus. Magnetic resonance occurs when a high-frequency rotating magnetic field of the above-mentioned frequency is applied to an axis perpendicular to this static magnetic field, and a group of specific atomic nuclei having the above-mentioned constant undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, resulting in energy energy. - Transition to a higher level. After resonance, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

MRI uses nuclear magnetic resonance (NM) based on this specific atomic nucleus.
This is a device that observes a phenomenon (referred to as R) and captures a tomographic image of a subject.

FIG. 7 shows a pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in the Fourier transform method in MRI. In the figure, (a) shows a state in which gradient magnetic fields of Gx, Gy, and Gz are applied to the x, y, and z axes, which are the single axis, warp axis, and slice axis, respectively, and a high frequency magnetic field is applied to the x axis. In the figure, (b) is a diagram showing the timing of applying each magnetic field. In period 1, spins in a slice plane perpendicular to two directions centered on z=0 are selectively excited by excitation pulse 1 and slice gradient 2. The rephase gradient 3 in the period 2 is for restoring the phase of the spins disturbed by the slice gradient 2.

The day phase gradient 4 in the same period 2 is for giving a phase difference to the spins depending on the location so that the center of the SE signal 7 coincides with the temporal center of the data read period 4.

In period 2, a warp gradient 6 is applied to further shift the phase of the spin in proportion to the position in the y direction, and the warp gradient 6 is applied with varying intensity every cycle. Thereafter, an inversion pulse 5 is applied to align the magnetic moments, and the SE signal 7 that appears thereafter is observed. In period 4, a lead gradient 8 is applied to the X axis. As a result, the phase difference given by the day phase gradient 4 is canceled out at the temporal center of the lid gradient 8 in the period 4, and an SE signal 7 appears. This sequence is called a view, and after the pulse repetition period TR, excitation pulse 1 is applied again to start the next view.

In the above pulse sequence, the SE signal 7 requires repetitions of about three times the longitudinal relaxation time T1 (ms) or more until it reaches a steady state, and the excitation pulse described above can originally obtain the correct signal strength earlier. In order to determine the flip angle of a pulse or an inverted pulse, it is necessary to repeat the same excitation sequence many times and wait until a steady state is reached.

The flip angle can be changed by keeping the pulse width of the RF pulse constant and changing the intensity of the RF pulse (RF intensity). When adjusting the RF intensity, the amplitude of this RF pulse is changed little by little, the SE multiplied signal intensity is measured, and the RF intensity value at which the SE multiplied signal intensity becomes the maximum or minimum is determined.

(Problem to be Solved by the Invention) By the way, when the repetition time is short, when trying to adjust the RF intensity as described above, the RF pulse with the same RF intensity is repeated many times to reach a steady state at each point. In addition, is it necessary to wait for a steady state to occur? If the steady state is not waited for in this way, the obtained RF intensity value will be inaccurate. Furthermore, if the process is repeated for a long time after waiting for sufficient relaxation, a long adjustment time is required.

In addition, in general, the selective excitation shape of RF varies depending on the waveform of the excitation pulse, and there are problems such as a linear relationship or non-existence between the RF intensity and the shape of the excitation pulse.
It was difficult to accurately determine the strength.

On the other hand, using the pulse sequence shown in Fig. 8,
A method has been reported in which the influence of relaxation is removed by taking the ratio of the SE signal to the stimulated echo. In the figure, the same parts as in FIG. 7 are given the same reference numerals. 10 is an excitation pulse with a flip angle or ao, 11 is a first inversion pulse with a flip angle of α°, and excitation pulse 10
to the time τ1. 12 is the subsequent time τ2
A second inversion pulse with the same flip angle of α° is applied later. 13 is a stimulated echo S2 that appears at time τ1 after application of the second inversion pulse 12.

In this pulse sequence method, (1) SE signal 517 (hereinafter simply referred to as signal S1) and stimulated echo 8213 (hereinafter simply referred to as signal S2)
), the weight given to the signal by T1 relaxation is different,
It is not possible to completely remove the influence by taking the ratio.

(2) The signal S1 is expressed by the following equation.

S, cx: sina sin2-- (1) Signal S
2 is as follows.

52QC-sin3a-(2)(1)
From equation (2), the ratio is as follows.

A curve as shown in the above equation is obtained, but what is calculated from this curve is α as the pole determined between 0≦α≦360°.
= 180°, and the signal S1 and signal S2
Both correspond to the point that gives the minimum value. Therefore, the signal SI
and signal S2 are both susceptible to noise, and
Since the signal S2 generally has an intensity about half that of the signal S1, the signal-to-noise ratio is poor and it is unstable.

(3) The slice shape obtained with signal S1 and the slice shape obtained with signal S2 will be different, and if the ratio is taken as is, the obtained ratio curve will be shifted and the accurate RF intensity will be cannot be asked for.

For the above reasons, it has been difficult to obtain accurate RF intensity.

The present invention has been made in view of the above points, and its purpose is to accurately determine the RF pulse intensity in a short repetition time.
The purpose of this invention is to realize an RF automatic adjustment method for RI.

(Means for Solving the Problems) The present invention to solve the above-mentioned problems includes a first inversion pulse that changes the flip angle α° by changing the RF intensity with a constant pulse width after applying an excitation pulse to the RF axis; A second inversion pulse is applied after a time TE from the first inversion pulse and has a flip angle α° equal to that of the first inversion pulse.
F condition, a first spoiler gradient applied to the warp axis and the lead axis before and after the application of the first inversion pulse in order to eliminate unnecessary signals generated immediately after the first inversion pulse, and the second inversion pulse. A second spoiler gradient having an opposite phase to the first spoiler gradient is applied to the warp axis and the lead axis before and after the second inversion pulse in order to eliminate unnecessary signals and stimulated echo signals that occur immediately after. applying a pulse sequence having a spoiler gradient and two SE signals S3.
A step of obtaining a sin2 (α/2) curve by calculating the ratio of S4, a step of detecting the peak of the curve and calculating an RF intensity value at which the flip angle is 180°, and a step when the flip angle is 180°. This method is characterized by comprising the step of calculating and obtaining an RF intensity value at an arbitrary flip angle α° from the RF intensity value.

In the second invention, the flip angle of the excitation pulse is set to α/2, and the RF intensity is changed together with the inversion pulse.

(Effect) 1st. By changing the RF intensity of the second inversion pulse and calculating the ratio of the SE multiplied signal data corresponding to both inversion pulses,
2(α/2) curve is obtained, the RF intensity value at a flip angle of 180° is obtained from the peak of the curve, and the RF intensity value at an arbitrary flip angle α° is obtained based on it. The flip angle of the excitation pulse may also be changed as α/2.

(Example) Hereinafter, an example of the method of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a pulse sequence used in the method of one embodiment of the present invention. In the figure, parts equivalent to those in FIG. 8 are given the same reference numerals. In the figure, 21 is an excitation pulse with a pulse width T/2 and a free angle α°/2 applied to the RF axis;
2 is the first inverted pulse with pulse width T and flip angle α°, 2
3 is a second inverted pulse having a pulse width T and a flip angle α°.

24 is excited by the excitation pulse 21 with a flip angle of α°/2, and the first SE signal S3.25 generated by the first inversion pulse 22 with a flip angle of α° has a full-flusop angle as in the multi-echo method. is α° second inversion pulse 2
This is the second SE signal S4 generated by S.3. 26 is a spoiler A for erasing unnecessary signals generated immediately after the first inversion pulse 22 applied to the warp axis and the lead axis; the spoiler A is applied before and after the first inversion pulse 22. Spoiler A with the influence of spoiler A26 applied later
This is because it is erased by 26. 27 is a spoiler B for erasing unnecessary signals generated immediately after the second inversion pulse 23 applied to the warp axis and the read axis and the stimulated echo 8213 shown in FIG.

The reason why the spoiler B27 is applied before and after the second inversion pulse 2B is the same as that for the spoiler A26. In this pulse sequence, the day phase gradient 4, lead gradient 8, warp gradient 6, etc. applied to the lead axis and warp axis are omitted.

Next, a method of automatic adjustment performed by an embodiment in which a gradient magnetic field and an RF magnetic field are applied to each axis as described above will be described. The automatic adjustment method to be performed here is that in the multi-echo method of the spin echo method, the influence of the T1 relaxation of the SE multiplied signal is the same, so the first inversion pulse 22
By taking the ratio of the 1st SE signal 5324 caused by the 28th E signal 5425 caused by the second inversion pulse 23, the influence of T and relaxation is reduced, and the accurate RF intensity is obtained by correcting the slice shape. It is something.

Here, data is collected by sequentially changing the RF intensity value, that is, the flip angle, according to the pulse sequence shown in FIG. The repetition period TR at this time is relatively short, for example, 20
It may be about 0+ns. The signal intensities of the first SE signal 5324 and second SE signal 5425 given by this pulse sequence can be given by the following equations, assuming that the initial magnetization magnitude is M'.
Physical Review Vol, 80 No.
, 4).

The first sine term in equations (4) and (5) is the excitation pulse 2
], and the second sine term is a term due to the first inversion pulse 22. Further, the third sine term in equation (5) is a term due to the second inversion pulse 23.

Here, the ratio S of S4 and S3 is determined by equation (4).

Equation (6) can be obtained from equation (5).

As is clear from equation (6), by taking the ratio, the magnitude of the initial magnetization that is affected by T1 relaxation can be removed.

What is obtained by determining S in equation (6) as described above is a sin2 (α/2) curve, and by determining this peak value, the RF intensity value at a flip angle of 180° is determined. Incidentally, FIG. 2 shows the relationship between the flip angle and the passband, that is, the slice width.

In the figure, the vertical axis represents the flip angle, and the horizontal axis represents the position on the two axes. This curve shows the slice shape of an 80° pulse with a sinc waveform. The dashed line between 35 and 35 indicates the pass area. In general, in the case of a 180° pulse, as shown in the figure, the relationship between the RF intensity and the flip angle is 1 Isofune linearity, which appears in a relatively large slice shape, so the average flip angle of this slice shape is always less than 180°, and ( 6) The maximum value of the curve of sin2 (α/2) in the equation is the original 180
The value is calculated deviating from the value of °.

To solve this problem, shape correction of the slice profile is performed as follows. First, the obtained signal is Fourier-transformed using FFT to draw a slice profile curve as shown in FIG. In the figure, the center flip angle is α
. , the slice flip angle is α(2), the error term is α,
(2)... (7) The following equation is obtained from equations (4), (5), and (7).

becomes. Here, α, (2) is considered to be relatively small, and the slice shape correction coefficient A(z) is obtained from equation (8).

Approximately, δ3 (z) Using this correction coefficient A (z), S3 (Z) , 5
Correct 4(z).

S3(Z)-33(Z)/A(Z).

S4 (Z)-34(Z)/A2 (Z)
−= In formula (10), 83 (Z) and S4 (Z) on the left side are the updated first
SE signal 24. This is the 23rd E signal 25.

The above-described correction is applied to a passband (a-b) in which the profile is relatively less degraded, as shown in FIG. 3, after determining the passband (a-b). 2nd SE signal 5 in the passband part
425 and the first SE signal 5324 is determined.

A sin2(α/2) curve is obtained from equation (11) or equation (11').

The peak value was detected by the least squares method from the sin2 (α/2) curve obtained as above, and the flip angle was 18
Obtain the RF intensity that becomes 0°.

The RF intensity value at an arbitrary flip angle α° is determined from the RF intensity at 180° by proportional calculation. If there is nonlinear distortion in the amplitude value of the RF pulse, this is corrected to obtain the RF intensity at an arbitrary flip angle α°. FIG. 4 is a graph of the slice shape obtained in this manner.

Next, MRI for carrying out the method of the embodiment as described above.
The main part configuration diagram is shown in Fig. 5.

In the figure, 31 has a space (hole) into which the subject is inserted, and the space is surrounded by a
A static magnetic field coil that applies a constant static magnetic field to the subject and a gradient magnetic field coil that generates a gradient magnetic field (the gradient magnetic field coil has x, y
, z-axis coils. ), an RF transmitting coil that provides an RF pulse to excite the spins of atomic nuclei within the subject, and a receiving coil that detects NMR signals from the subject, etc., are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power supply 32, a gradient magnetic field drive circuit 33, an RF power amplifier 34, and a preamplifier 35, respectively. The sequence storage circuit 36 operates the gate modulation circuit 38 (modulates the RF output signal of the RF oscillation circuit 39 at a predetermined timing) according to a command from the computer 37 in an arbitrary view, and converts the RF pulse signal based on the Fourier transform method. is applied from the RF power amplifier 34 to the RF transmitting coil. Also, the sequence storage circuit 36 stores the gradient magnetic field drive circuit 3 using a sequence signal based on the Fourier transform method.
3, set x, y, z as shown in Figure 5.
A gradient magnetic field is supplied to each of the three axes. A phase detector 40 detects the phase of the received signal output from the preamplifier 35 using the output of the RF oscillation circuit 39 as a reference signal. This output signal is converted into a digital signal by the AD converter 41 and input to the computer 37. 42 is an operation console for inputting instructions and various setting values for realizing various pulse sequences to the computer 37; 43 is a display device for displaying images reconstructed by the computer 37;

Next, the operation of the apparatus configured as described above will be explained.

The operation console 42 is operated to set the pulse sequence timing, RF pulse width, etc., and a signal based on the set values is input to the computer 37. The computer 37 generates a control signal based on the set value and sends it to the sequence storage circuit 36.

The sequence storage circuit 36 controls the gradient magnetic field drive circuit 33 to generate a gradient magnetic field of a predetermined pulse sequence based on the above signal, and also controls the gate modulation circuit 38. The gate modulation circuit 38 modulates the RF multiplied signal oscillated and outputted by the RF oscillation circuit 39 into a signal having a set pulse width and amplitude, and supplies the modulated RF pulse to the RF power amplifier 34. This modulated RF pulse is amplified in the RF power amplifier 34, and in the static magnetic field generated by the static magnetic field power supply 32 of the magnet assembly 31, together with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 33, an excitation pulse is generated. The spins excited by 21 are caused to resonate.

The signal generated by the resonance is received, amplified by a preamplifier 35, and input to a phase detector 40. Phase detector 40
inputs the outputs of three RF oscillator circuits as reference signals NM
The R signal is phase detected and the output signal is sent to the AD converter 41. The NMR signal converted into a digital signal by the AD converter 41 is processed by the computer 37. The computer 37 is programmed to apply RF pulses with varying intensities sequentially, and sequentially obtains data using RF pulses with different RF intensities.

Next, the procedure of the automatic RF adjustment method using this device will be explained using the flowchart shown in FIG.

Step 1 The calculator 37 generates the excitation pulse 21 with a flip angle α″/2, the first inversion pulse 22 with a flip angle α°, and the second inversion pulse 23 with a flip angle α°, while keeping the pulse width T constant. , to the sequence storage circuit 36 as a pulse sequence that changes the RF intensity.The gradient magnetic field drive circuit 33.The gate modulation circuit 38.The RF power amplifier 34 gives the given sequence of signals to the magnet assembly 31, and The 1st SE signal 5324 and the 2nd SE signal 5425 are received.This signal is detected by the phase detector 40, digitized by the AD converter 41, and input to the computer 37, which changes the signal according to the change in RF intensity. The SE multiplied signal reception due to the flip angle α° is included.

Step 2 The computer 37 subjects the input data to Fourier transform using FFT processing to obtain a slice profile.

Step 3 As shown in Figure 3, set a passband in the slice profile where the profile degradation is relatively small, and (7)
A correction coefficient is obtained by calculating equations (9) to (9), and a corrected SE signal S3+S4 is obtained using equation (10).

Step 4 SE signal S3 obtained in step 3. The ratio of S4 is determined by equation (11) or (11'), and is 51n2 (α/2).
get a curve.

Step 5 Perform peak detection from the sin2 (α/2) curve obtained in Step 4 using the least squares method, etc., and find that the full Sob angle is 180
' Find the RF intensity.

Step 6: Obtain the RF intensity value at an arbitrary flip angle α° from the RF intensity at a flip angle of 180° by proportional calculation.

As explained above, according to this embodiment, the following effects can be expected.

(1) Since the ratio of the SE signals is taken as the 1st SE signal 5324 and the 23rd E signal 5425 and no stimulated echo is used, recovery of magnetization or steady state is not dependent on the initial magnetization. Without waiting, a tuning scan can be performed immediately to determine the RF intensity at which the flip angle becomes 1800 (independent of T1 relaxation).

(2) Since the slice shape is corrected, the RF intensity value can be accurately determined without depending on the slice shape.

(3) Near where the signal reaches its maximum value (sin2 (α/2))
Since the peak of the echo ratio curve can be obtained at , stable RF intensity can be determined from the viewpoint of the S/N ratio.

Note that the present invention is not limited to the above embodiments, and may be implemented as follows.

(1) Even if the flip angle α° of the first inversion pulse and the second inversion pulse is changed by using an RF pulse with a flip angle α°/2 as a flip angle β0 that produces a signal of a certain value or more,
A method may also be used in which the flip angle β0 of the excitation pulse is not changed.

(2) The peak detection method for sin2 (α/2) is not limited to the least squares method, and any method may be used.

(3) Normalize the obtained sin2(α/2) curve and
By multiplying the curve, the curve may be made sharper to facilitate peak detection.

(Effects of the Invention) As described above in detail, according to the present invention, the RF pulse intensity can be accurately determined in a short repetition time, and the practical effects are great.

[Brief explanation of the drawing]

Fig. 1 is a diagram of a pulse sequence according to an embodiment of the present invention, Fig. 2 is a diagram showing the relationship between flip angle and slice shape, Fig. 3 is a diagram of a profile curve drawn by the obtained signal, and Fig. 4 is a diagram showing the relationship between the flip angle and slice shape. A diagram of the slice shape obtained by the method of the present example, FIG. 5 is a diagram of an apparatus for implementing the method of the present invention, FIG. 6 is a flowchart of the procedure for implementing the method of the present invention, and FIG. 7 is a conventional method. FIG. 8 is a diagram of a pulse sequence for conventional RF intensity adjustment. 21... Excitation pulse 23... Second inversion pulse 25... 23rd E signal 54 26... Spoiler A 27... Spoiler B3 magnet assembly 33... Gradient magnetic field drive circuit 34... RF power amplifier 36 ... Sequence storage circuit 37 ... Computer 22 ... First inversion pulse 24 ... First SE signal S3

Claims (1)

[Claims] (1) A first inversion pulse (22) that changes the flip angle α° by changing the RF intensity with a constant pulse width after applying an excitation pulse (21) to the RF axis; and the first inversion pulse. RF conditions for applying a second inversion pulse (23) that is applied after TE time from the pulse (22) and has a flip angle α° equal to the first inversion pulse (22), and immediately after the first inversion pulse (22); A first spoiler gradient (26) is applied to the warp axis and the read axis before and after the application of the first inversion pulse (22) in order to eliminate unnecessary signals generated during the application, and immediately after the second inversion pulse (23). The first spoiler gradient (26) is opposite to the first spoiler gradient (26) applied to the warp axis and the lead axis before and after the second inversion pulse (23) in order to eliminate unnecessary signals and stimulated echo signals generated in the applying a pulse sequence having a second spoiler gradient (27) in phase;
, the two SE signals S_3, S_4 (24, 25) obtained by the second inversion pulse (22, 23)
) to obtain a sin^2 (α/2) curve; a step of detecting the peak of the curve and calculating an RF intensity value at which the flip angle is 180°; and a step when the flip angle is An MRI RF automatic adjustment method comprising the step of calculating and obtaining an RF intensity value at an arbitrary flip angle α° from an RF intensity value of 180°. (2) The flip angle of the excitation pulse (21) is set to the flip angle α of the first inversion pulse (22) and the second inversion pulse (23).
2. The MRI RF automatic adjustment method according to claim 1, wherein the RF is changed so as to maintain 1/2 of .degree.