JPH0871060A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To ensure a larger number of multi-slices by controlling the timing in such a way that within the reversely recovering time of a preceeding basic pulse sequence, a reverse 180 deg. pulse of the successive basic pulse sequence is inserted when a multi-slicing method is performed. CONSTITUTION: In a nuclear magnetic resonance imaging apparatus, NMR signals generated in a subject to be inspected are received by an RF coil 12 and are successively sampled by a specified sample rate to send them to a host computor 51. In this case, on a basic pulse sequence wherein a time consisting of addition of the reversely recovering time TI from the top 180 deg. pulse to the 90 deg. pulse and a time α until sampling for collecting data thereafter is finished is necessary, the timing is controlled in such a way that within the reversely recovering time TI, a reverse 1 180 deg. pulse of the successive basic pulse sequence is inserted and each pulse is not overlapped in time.

Description

Detailed Description of the Invention

[0001]

This invention relates to the nuclear magnetic resonance phenomenon (M
The present invention relates to a nuclear magnetic resonance imaging apparatus that performs imaging by utilizing the (R phenomenon).

[0002]

2. Description of the Related Art Various pulse sequences have been considered in a nuclear magnetic resonance imaging apparatus, but a 180 pulse sequence at the beginning of a normal spin echo (SE) method pulse sequence is used.
The iterative recovery (IR) method, in which a pulse is applied to enhance the contrast between tissues as compared with the SE method, is one of the well known methods. Also, by devising the parameters of this sequence, short-term repetitive recovery (S
TIR method and FLAIR (Flui) that suppresses water components
d Attended Inversion Rec
The OVERY method is also known. Furthermore, this 18
0 ° pulse is echo planer (EPI) method or Fast
An example applied to the SE method is also known.

On the other hand, in the multi-slice method, the free time in the repetition time (TR) that arises from the fact that the next excitation must be performed after the signal is generated until it is recovered is utilized, and another slice is used in that free time. By embedding the pulse sequence of, the imaging of a large number of slices is performed in a short time.

Therefore, it is also known to perform the multi-slice method by using the above-mentioned IR method or another pulse sequence in which a 180 ° pulse is added to the head as a basic pulse sequence.

[0005]

However, in these IR methods and other pulse sequences in which a 180 ° pulse is added to the beginning, the time from the leading 180 ° pulse to the first excitation 90 ° pulse, that is, the inversion recovery time (T
Since the time of the pulse sequence is extended by I), there is a problem that the number (multi-slice number) of this pulse sequence inserted as a basic pulse sequence in TR is reduced. Particularly, in the FLAIR method having a long TI, the decrease in the number of multi-slices is extremely large, which is a serious problem.

In view of the above, the present invention has been improved so that a larger number of multi-slices can be obtained when the multi-slice method is performed using a pulse sequence having a long TI as a basic pulse sequence. An object of the present invention is to provide a resonance imaging device.

[0007]

To achieve the above object, in a nuclear magnetic resonance imaging apparatus according to the present invention, a main magnetic field generating means for generating a static magnetic field and a means for generating a gradient magnetic field pulse for slice selection are provided. , Means for generating a gradient magnetic field pulse for reading, means for generating a gradient magnetic field pulse for phase encoding, means for irradiating a subject placed in a magnetic field with an RF pulse, and receiving an NMR signal from the subject. When the multi-slice method is performed with a receiving means for detecting and sampling and then collecting data, a means for processing the collected data to reconstruct an image, and a pulse sequence with an inversion 180 ° pulse added as a basic pulse sequence, The timing of the inversion 180 ° pulse of the following basic pulse sequence is inserted into the TI of the preceding basic pulse sequence. Is the distinctive feature that a control means for controlling.

[0008]

In the prior art, even when the multi-slice method is performed by using a pulse sequence to which an inverted 180 ° pulse is added as a basic pulse sequence, the next basic pulse sequence is not started unless all the basic pulse sequences are completed. Is added, one T
The number of basic pulse sequences inserted in R is reduced. In contrast, the T of the preceding basic pulse sequence
If the timing of each basic pulse sequence is determined by inserting an inversion 180 ° pulse of the subsequent basic pulse sequence into I, it is possible to put a large number of basic pulse sequences in one TR without interfering with each other. Even in the multi-slice method in which the basic pulse sequence is a pulse sequence to which an inversion 180 ° pulse is added, the time of which is extended by TI, a large number of slices can be imaged.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described in detail with reference to the drawings. A nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention is configured as shown in FIG. In FIG. 1, the main magnetic field magnet 10 is for generating a static magnetic field. Normal,
It consists of a superconducting magnet. A gradient magnetic field coil 11 is provided to generate a gradient magnetic field that is superimposed on this static magnetic field. The gradient magnetic field coil 11 is composed of three sets of coils,
By each of them, three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths are respectively inclined in the three axis directions of X, Y, Z are generated. These three-axis gradient magnetic fields Gx, Gy, G
By selecting one of z or combining them, a gradient magnetic field Gs for slice selection in any direction, a gradient magnetic field Gr for reading (and frequency encoding), and a gradient magnetic field Gp for phase encoding can be created.

An object (not shown) is placed in the space to which the static magnetic field and the gradient magnetic field are applied. For this subject,
An RF coil 12 is attached for irradiating the subject with RF pulses and for receiving the NMR signal generated in the subject.

Each set of gradient magnetic field coil 11 includes:
Currents of predetermined waveforms are respectively supplied from the gradient magnetic field power amplifier 23. Under the control of the sequence controller 52, the digital gradient magnetic field waveform generator 21 outputs Gs, Gr, Gp.
Each digital pulse waveform of is generated, and this is D / A
It is converted into an analog signal by the converter 22 and input to the gradient magnetic field power amplifier 23. An amplified version of this analog signal is supplied to each set of coils of the gradient magnetic field coil 11, so that Gs, Gr, Gp pulses corresponding to the desired waveform output from the digital gradient magnetic field waveform generator 21 are generated. It will be. As a result, it becomes possible to generate Gs, Gr, and Gp pulses having a pulse waveform required in the pulse sequence described later.

The RF pulse is applied to the subject from the RF coil 12. Therefore, the RF from the RF oscillation circuit 34
The carrier signal is amplitude-modulated by the amplitude modulation circuit 33, the modulated output is amplified by the RF power amplifier 35, and then supplied to the RF coil 12. The RF oscillation circuit 34 is controlled by the sequence controller 52 and generates an RF carrier signal having a frequency corresponding to the resonance frequency of the subject. The amplitude modulation signal is obtained by converting the digital RF pulse waveform generated from the digital RF waveform generator 31 under the control of the sequence controller 52 into an analog signal by the D / A converter 32. 90 degree pulse and 1
The 80 ° pulse is obtained by defining the waveform and magnitude of this amplitude modulated signal.

When excited by such an RF pulse, an NMR signal is generated in the subject and the N
The MR signal is received by the RF coil 12, passed through the preamplifier 41 and the anti-aliasing filter 42, and is sent to the phase detection circuit 43 for phase detection. The RF signal from the RF oscillation circuit 34 is sent as a reference signal for this phase detection. The signal obtained by the phase detection is sent to the A / D converter 44, sequentially sampled at a predetermined sampling rate, and converted into a series of digital data. The data obtained from the A / D converter 44 is taken into the host computer 51,
An image is reconstructed by receiving a three-dimensional Fourier transform process or the like. The host computer 51 also controls the sequence controller 52 in accordance with pulse sequences that make up various imaging scans.

In such a nuclear magnetic resonance imaging apparatus, an imaging sequence by the multi-slice method is performed under the control of the host computer 51 and the sequence controller 52. FIG. 2 shows a time chart when a pulse sequence of the IR method is used as a basic pulse sequence for inserting a plurality of pulses in one TR.
2A is related to the embodiment of the present invention, while FIG. 2B is related to the conventional example given for reference. Although the gradient magnetic field pulse is omitted in this figure, the Gs pulse is added simultaneously with each of the 180 ° pulse and the 90 ° pulse, the Gr pulse is added so that the phases are aligned after TE from the 90 ° pulse, and the Gp pulse is added. T
This is changed for each R, and in this respect, normal IR
It is no different from the modal pulse sequence.

In this embodiment, as shown in FIG. 2A, the basic pulse sequence of the IR method for the first slice is performed, and within the TI in that pulse sequence,
The first 180 ° pulse for inversion of the basic pulse sequence of the IR method for the second slice is input so that the second pulse
A pulse sequence for a slice is performed.
In this case, each basic pulse sequence has an inversion recovery time T from the first 180 ° pulse to the 90 ° pulse.
I and the time TE from the 90 ° pulse to the echo center,
After that, it is necessary to add time α until the sampling for data collection is completed. And
In order to prevent these basic pulse sequences from affecting each other, the conditions must be such that the pulses do not overlap in time and the times (TE + α) do not overlap in time with each other. is necessary.

Therefore, in the example shown in this figure, another leading 180 ° pulse is put in the TI of the first slice, and the basic pulse sequence for the third slice is started from this 180 ° pulse. There is. In this example, the first 180 ° pulse of the next basic pulse sequence cannot be put in the TI of the first slice if the above condition is satisfied, so that the third pulse
The basic pulse sequence for a slice is (TI + T
(E + α), the basic pulse sequence for the next fourth slice is started when the processing is completed, and the first 180 ° pulse in the basic pulse sequence for the fifth slice is added to the TI of the basic pulse sequence for the fourth slice. I am trying to insert it.

Therefore, in this embodiment, five basic pulse sequences can be inserted in one TR,
Imaging can be performed on five slices.

FIG. 2B shows a conventional example for reference, and in the conventional case, the time (T
Since the timing of each basic pulse sequence is determined so that (I + TE + α) does not overlap, the subsequent basic pulse sequence cannot be started unless the previous basic pulse sequence ends, so the time required for the basic pulse sequence and TR When the conditions such as the above are the same as those in the above-mentioned embodiment, only three Ts are provided as shown in FIG.
The basic pulse sequence cannot be put in R and 3
Only one slice can be imaged.

FIG. 3 shows a time chart in the case where a pulse sequence of the IR-fast spin echo method is adopted as a basic pulse sequence to be put in one TR. Also in this case, FIG. 3A shows the second embodiment of the present invention, and FIG. 3B shows the conventional example given for reference. In the pulse sequence of the IR-fast spin echo method, a 180 ° pulse is added to the beginning to invert it, a 90 ° pulse is added in the recovery process, and then multiple 180 ° pulses for refocusing are sequentially applied to each spin echo. Generate. In this figure, the gradient magnetic field pulse is omitted, but the Gs pulse is 18
The 0 ° pulse and the 90 ° pulse are applied simultaneously, the Gr pulse is added so that the phases are aligned when refocused by the 180 ° pulse, and the Gp pulse is changed for each spin echo and for each TR. Change. These are similar to the pulse sequence of the normal IR-fast spin echo method.

As shown in FIG. 3A, first, a basic pulse sequence of the IR-fast spin echo method is performed on the first slice, and TI in the pulse sequence is performed.
The first 180 ° pulse of the basic pulse sequence of the IR-fast spin echo method for the second slice is inserted therein. In this basic pulse sequence of the IR-fast spin echo method, if the time TM after 90 ° pulse is added to the time TI, the first 180 ° pulse in the later sequence enters the TI of the earlier sequence, In addition, it is necessary that the times TM do not overlap.

Therefore, if the basic pulse sequence of the IR-fast spin echo method having the time relationship as shown in this figure is used, only the first 180 ° pulse of the next second slice is included in the TI of the first slice. First, the 180 ° pulse at the beginning of the third pulse needs to be applied after the basic pulse sequence of the second slice is completed. Fourth
The 180 ° pulse at the beginning of the slice can be placed within the TI of the basic pulse sequence for the third slice. Therefore, in (a) of this FIG.
By inserting one basic pulse sequence, it is possible to perform imaging for four slices.

On the other hand, in the conventional example, under the same conditions, the next basic pulse sequence is started after all the basic pulse sequences are completed, as shown in FIG. Since only three basic pulse sequences are included in one TR, only three slices can be imaged.

For example, the maximum value N of the number of multi-slices in the above-mentioned example of FIG. 2 can be obtained as follows according to the present invention. N = NTI × NBTR + NRES NTI = int {TI / (TE + α)} NBTR = int [TR / {NTI × (TE + α) + T
I}] NRES = int [{TR− (NTI × (TE + α) +
TI) × NBTR-TI} / (TE + α) where NTI; the number of leading 180 ° pulses that enter TI, NBTR; the number of pulse sequences that have other leading 180 ° pulses in TI, and NRES ;
It is the number of pulse sequences that enter an extra time when NTI × NBTR pulse sequences are inserted in TR.

On the other hand, in the conventional example, N = int {TR / (TI + TE + α)}.

TR = 2500 ms, TI = 600 ms,
Considering a specific example of TE = 30 ms and α = 10 ms, according to the present invention, NTI = 15, NBTR =
2. Since NRES = 0, N = 30, whereas according to the conventional example, only N = 3, and the number of multi-slices can be significantly increased.

In the example of FIG. 3, (TE + α) in the above equation may be replaced with TM.

When the imaging scan of the multi-slice method is set and the pulse sequence to be adopted as the basic pulse sequence is selected, the host computer 51 is made to satisfy the above conditions, and the start timing of each basic pulse sequence (leading The timing for inserting the 180 ° pulse in the TI of the preceding pulse sequence) is determined, and the imaging scan by the multi-slice method is executed in cooperation with the sequence controller 52.

Therefore, since the imaging scan is performed with the number of multi-slices increased, it is possible to image a large number of slices in the same imaging time. This effect is more remarkable as the TI becomes longer, and the clinical efficacy has recently been revealed.
In the IR method, TI is about 2000 m as its parameter.
Since it is s (TI is about 600 m in the normal IR method)
s), if it is applied to the FLAIR method, it is possible to image 10 or more slices, whereas the conventional technique was able to image only 3 slices.

The multi-slice method in which several types of pulse sequences are adopted as the basic pulse sequence has been described above.
The present invention can be applied to the multi-slice method in which all other pulse sequences with 180 ° inversion pulse added, such as the SE or EPI method with inversion 180 ° pulse added, are adopted as the basic pulse sequence.

[0030]

As described in the above embodiments, according to the nuclear magnetic resonance imaging apparatus of the present invention, a larger number of slices can be imaged as compared with the conventional one. The number of multi-slices does not decrease even when an imaging scan of the multi-slice method is performed using a pulse sequence of the IR method capable of enhancing contrast as a basic pulse sequence, so that it is practically used clinically.

[Brief description of drawings]

FIG. 1 is a block diagram of a nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a time chart showing an example of a pulse sequence.

FIG. 3 is a time chart showing another example of a pulse sequence.

[Explanation of symbols]

 10 Main Magnetic Field Magnet 11 Gradient Magnetic Field Coil 12 RF Coil 21 Digital Gradient Magnetic Field Waveform Generator 22, 32 D / A Converter 23 Gradient Magnetic Field Power Amplifier 31 Digital RF Waveform Generator 33 Amplitude Modulation Circuit 34 RF Oscillation Circuit 35 RF Power Amplifier 41 Preamplifier 42 Filter 43 Phase detection circuit 44 A / D converter 51 Host computer 52 Sequence controller

Claims (1)

[Claims] 1. A main magnetic field generating means for generating a static magnetic field, a means for generating a gradient magnetic field pulse for slice selection, a means for generating a gradient magnetic field pulse for reading, and a means for generating a gradient magnetic field pulse for phase encoding. , Means for irradiating a subject placed in a magnetic field with an RF pulse and N from the subject
Receiving means for receiving MR signals, detecting and then sampling to collect data, means for processing the collected data to reconstruct an image, and a multi-slice method using a pulse sequence with an inversion 180 ° pulse as a basic pulse sequence. The basic pulse sequence T
And a control means for controlling the timing so that an inversion 180 ° pulse of the subsequent basic pulse sequence is inserted into I.