JPH02109542A - Method for decreasing influence of eddy current by magnetic field gradient of mri - Google Patents

Abstract

PURPOSE:To decrease the influence of eddy current by a magnetic field gradient by means of scan sequence with which the eddy current component of a long time constant hardly remains by adopting the pulse sequence in which the gradient magnetic fields to be impressed to a slice axis and a lead axis are so inverted as to be the same in positive and negative numbers while the same codes are obtd. at every view with respect to the same slice to operate the gradient magnetic fields. CONSTITUTION:The pulse sequence by an even number of sheets of multislices indicates only the magnetic field gradient pulses to be applied to the RF axis, the x axis and the z axis. The slide gradient impressed by inverting the code from the code of the slice gradient 2 impressed by inverting the code to the slide gradient 2 impressed to the 1st slice in the 2nd slice is 22 and the rephase gradient impressed by inverting the code to the rephase gradient 2' in the 2nd slice is 22'. Similarly, the dephase gradient impressed by inverting the code to the dephase gradient 3 is 23 and the lead gradient impressed by inverting the code to the lead gradient 7 is 27. The positive gradient magnetic field is impressed at the time of the odd slide and the negative gradient magnetic field at the time of the even slice in such a manner.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for reducing the influence of eddy currents due to magnetic field gradients generated when switching gradient magnetic fields in MRI.

(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 Transition to the higher level. After resonance, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

A nuclear magnetic resonance imaging apparatus (hereinafter referred to as MRI) is an apparatus that observes the nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon caused by this specific atomic nucleus and images a tomographic image of a subject.

FIG. 3 shows a pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in the Fourier transform method in MRI. (a) In the figure, Gx and Gy are on the X+y+Z axes, respectively.
, Gz, and a high frequency magnetic field is applied to the X axis. (b) The figure shows the timing of applying each magnetic field. In period 1, spins in a slice plane centered at z-0 and perpendicular to the Z direction are selectively excited by excitation pulse 1 and slice gradient 2.

The rephase gradient 2' in the period 2 is for restoring the phase of the spins disturbed by the slice gradient 2. The day phase gradient 3 in the same period 2 is for disturbing the phase of the spins and inverting them with the inversion pulse 4. In period 2, a warp gradient 5 is applied to further shift the phase of the spin in proportion to the position in the y direction, and the warp gradient 5
is applied with varying intensity every cycle. Thereafter, an inversion pulse 4 is applied to align the magnetic moments, and the SE signal 6 that appears thereafter is observed. 7 aligns the disturbed phase and SE
The SE signal 6 appears at the lead slope for generating the signal 6, which is equal to the area of the day phase slope 3. FID signal 8 appears after excitation pulse 1 and can be used as a signal, but it attenuates quickly, so S
This is not a good idea in terms of N ratio. This sequence is called a view, and after the pulse repetition period TR, excitation pulse 1 is applied again to start the next view.

The waveform of the magnetic field gradient applied in this way has a step waveform, and when the gradient magnetic field is switched, an eddy current is generated in the highly conductive material of the magnet, and the gradient magnetic field is canceled by this eddy current and weakened, resulting in an NMR image. A problem has arisen in which image quality deterioration is unavoidable.

Conventionally, as a method to reduce the influence of the above eddy current, the phase of the FED signal 8 after applying a step gradient is measured, the residual magnetic field due to the eddy current is calculated, and this is decomposed into multiple exponential functions. Using this value, a gradient was generated in advance that took the eddy current component into account, and the eddy current was corrected. This conventional method is shown in FIG. In the figures, (a) is a diagram without correction, and (b) is a diagram with correction. (a) In the figure, eddy current 1 is applied to gradient output 10.
It shows the waveform of the gradient magnetic field 12 when 1 is superimposed,
The gradient magnetic field 12 is distorted by the eddy currents 11 . (
b) The figure shows a gradient magnetic field 1 which is obtained by adding an eddy current 11 to a gradient output 10' whose waveform has been corrected in advance.
2' is output.

(Problems to be Solved by the Invention) However, the above correction method has the following problems.

(+) Location-dependent eddy currents occur differently depending on location, so they cannot be completely corrected within the magnet.

(if) Difficulty in measuring long time constants Measuring long time constants requires a very long repetition time.

As a countermeasure to this problem, a shield gradient magnetic field method can be considered in which the gradient coils are doubled and the magnetic field of the outer coil is applied in the opposite direction to the magnetic field of the inner coil, but (1) it is expensive.

(2) Power limitations 2 Difficult to apply to small diameter magnets due to heat generation and noise.

There is a problem. In the case of multi-slice or short TR, many gradients occur within the repetition period TR, so in the case of a long time constant component, the residual magnetic field lingers and accumulates until the next gradient magnetic field is applied, as shown in Figure 5. If this happens, the image quality will deteriorate. In FIG. 5, parts equivalent to those in FIG. 3 are given the same reference numerals. In the figure, reference numeral 13 indicates residual eddy current that remains after the eddy current has not been completely eliminated by the lead gradient 7. The remaining eddy currents 13 that cannot be completely erased are accumulated, and the residual gradient of the slice axis causes image positional deviation, and the residual gradient of the lead axis causes deterioration in image quality, typified by expansion and contraction of the image.

The present invention has been made in view of the above points, and its purpose is to realize a method of reducing the influence of eddy currents due to magnetic field gradients by using a scan sequence in which eddy current components with long time constants are less likely to remain.

(Means for Solving the Problems) The present invention solves the above-mentioned problems in a method for reducing the influence of eddy currents due to magnetic field gradients generated when switching gradient magnetic fields in MRI. This method is characterized in that the magnetic field is operated by a pulse sequence that is alternately reversed so that the magnetic field has the same sign for each view and has the same number of positive and negative values for the same slice.

(Function) The signs of the magnetic field gradients applied to the slice axis and the read axis are reversed so that the same number of positive and negative magnetic field gradients are applied, and the remaining eddy currents are canceled out and reduced.

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

FIG. 1 is a diagram of a pulse sequence of one embodiment of the method of the invention. The figure shows a pulse sequence with an even number of multi-slices, RF axis, X axis.

Only the magnetic field gradient pulse applied to the Z-axis is shown.

In the figure, parts equivalent to those in FIG. 3 are given the same reference numerals. In the figure, 22 is a slice gradient applied in the second slice with the sign reversed from the slice gradient 2 applied to the first slice.

22' is a rephasing gradient applied in the second slice with the sign reversed with respect to the rephasing gradient 2'.

Similarly, 23 is a day phase gradient applied with the sign reversed to the day phase gradient 3, and 27 is a read gradient applied with the sign reversed to the lead gradient 7.

In this way, when a positive gradient magnetic field is applied for odd-numbered slices and a negative gradient magnetic field is applied for even-numbered slices, the influence on the remaining eddy current is as shown in FIG. 2. In the figure,
Components equivalent to those in FIGS. 1 and 4 are given the same reference numerals. (A) The figure shows the state of the remaining eddy current 13 in the slice axis in the pulse sequence of the embodiment of FIG. . (B) is a diagram showing the state of the remaining eddy current 13 in the pulse sequence of the conventional method.

(b) In the conventional case shown in the figure, the slice gradient 2 is positive in both the first slice and the second slice, so the remaining eddy current 13 is also positive in both cases, so the remaining eddy current 13 accumulates as the sequence progresses. However, in the case of the method of the embodiment shown in FIG. They cancel each other out and decrease. Therefore, it is not superimposed on the next gradient.

In Fig. 2, only the slice gradient was explained, but other gradients such as rephase gradient 2', dayphase gradient 3°, lead gradient 7
The same behavior applies to . At this time, on the same slice plane, the sign of the applied gradient is made constant regardless of the view, so that artifacts do not occur even when there is a shift in the center frequency.

As described above, according to this embodiment, it is possible to reduce the influence of eddy currents that could not be completely corrected.

Note that the present invention is not limited to the above embodiments. In this embodiment, the case where the number of slices is an even number has been described, but there is no problem if the number of slices is an odd number or a single slice. In that case, after the last slice of each view, a dummy gradient is generated with a size that cancels out the long time constant component during the view, so that the amount of positive and negative gradients is equalized.
Achieve the same effect as an even number of multi-slices.

(Effects of the Invention) As explained in detail above, eddy currents with long time constants can also be generated by applying gradients to the lead axis and slice axis alternately so that the number of positive gradients and negative gradients are equal. This makes it possible to reduce the effects of this, which has a great practical effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram of a pulse sequence of an embodiment of the method of the present invention, FIG. 2 is an explanatory diagram of the influence on residual eddy current obtained by the embodiment, FIG. 3 is an explanatory diagram of an MRI pulse sequence, and FIG. The figure is an explanatory diagram of a conventional method of correcting the influence of eddy currents, and FIG. 5 is an explanatory diagram of the accumulation of eddy currents that cannot be eliminated. 1...Excitation pulse 2.22...Slice gradient 2
．． 22'...Rephase gradient 3.23...Day phase gradient 4...Reverse pulse 5...Warb gradient 6...S
E times signal 7.27...Lead slope 8...FID
Signal 10.10'...Gradient output 11...Eddy current 12.12'...Gradient magnetic field 13...Remaining eddy current Patent applicant Yokogawa Medical Systems Co., Ltd. Liu 2 Figure (A) 22' slice gradient

Claims (1)

[Claims] In a method for reducing the influence of eddy currents caused by magnetic field gradients generated when switching gradient magnetic fields in MRI, the gradient magnetic fields applied to the slice axis and read axis are made to have the same sign and negative numbers for each view for the same slice. A method for reducing the effects of eddy currents due to magnetic field gradients in MRI, characterized in that the method operates with an alternately reversed pulse sequence.