JPH08117206A - Magnetic resonance imaging equipment - Google Patents

Abstract

(57) [Summary] [Purpose] The acquisition time of a plurality of echo signals in the same sequence is made variable to achieve both the S / N ratio and the number of multi-slices. A means for sequentially applying a series of a plurality of signal acquisition pulses together with a slicing gradient magnetic field to a subject placed in a static magnetic field, and a plurality of means sequentially generated by being energized by the plurality of signal acquisition pulses. A means for applying a phase encoding gradient magnetic field and a readout gradient magnetic field to each of the echo signals to collect each of the plurality of echo signals. Acquisition time changing means (controller) for changing a part of acquisition time length in a plurality of acquisition time zones for collecting a plurality of echo signals
Was provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus, and in particular, uses a pulse sequence for applying a plurality of signal acquisition pulses for each sequence, such as a pulse sequence based on a fast SE (spin echo) method. The present invention relates to a magnetic resonance imaging apparatus.

[0002]

2. Description of the Related Art Various types of image data acquisition sequences are currently used in magnetic resonance imaging. One of them is a spin echo method (SE method) using 90 ° pulse (excitation pulse) and 180 ° pulse (refocus).

A single echo scan method is known as one of the spin echo methods. This single echo scan method applies T _E / 2 after applying a 90 ° pulse.
One (180) pulse is applied at the time when (T _E : a predetermined echo time) has elapsed, one spin echo signal accompanying this 180 ° pulse application is collected, and this collection is repeated for the required number of matrices. It is a thing.

The spin echo method further includes a multi-echo scan method (hereinafter referred to as "high-speed SE" for shortening the imaging time).
The law is also known). In this fast SE method, C
The PMG pulse sequence or the CP pulse sequence is used, and FIG.
As shown in, one 90 ° RF pulse and a plurality of 1
The 80 ° RF pulse is sequentially applied. As a result, a plurality of echo signals EC1 to EC5 (in the case of the five-echo method) are generated from high-frequency excitation based on one 90 ° RF pulse, so that each of the echo signals EC1 to EC5 has a different gradient magnetic field. Phase encoding by G _E is performed. This series of sequences is repeated the required number of times, and the echo data necessary for reconstructing one image is collected with a smaller number of excitations than the normal SE method (single echo scan method). Therefore, there is an advantage that the photographing time can be significantly shortened.

[0005]

However, the generation time of the third echo signal EC3 in FIG.
When E (TE _eff ) time is set, signal collection ends there in the normal SE method, but signal collection is further performed in the conventional high-speed SE method (fourth and fifth echoes EC4 and EC5 in FIG. 9).
Therefore, there is a problem that the number of multi-slices decreases under the same repetition time TR as compared with the normal SE method.

To avoid this problem, the repetition time TR
However, there is a problem in that it takes a lot of time because the whole photographing time is long and the photographing procedure is complicated.

The present invention has been made in view of the above problems of the prior art, and in magnetic resonance imaging using a pulse sequence in which a plurality of signal acquisition pulses are applied for each sequence, such as the fast SE method. In particular, the purpose is to shorten the signal acquisition time after the effective TE time TE _eff and reduce the number of multi-slices.

[0008]

In order to achieve the above object, the magnetic resonance imaging apparatus of the present invention is, in one aspect thereof, provided with a series of a plurality of signal acquisition pulses for a subject placed in a static magnetic field. Means for sequentially applying together with the slicing gradient magnetic field and each of a plurality of echo signals sequentially generated by being energized by the plurality of signal collecting pulses are applied and collected by applying a phase encoding gradient magnetic field and a reading gradient magnetic field. And a means for changing the collection time length of a part of a plurality of collection time zones for collecting the plurality of echo signals.

According to another aspect, there is provided means for applying one high-frequency excitation pulse together with the slice gradient magnetic field to the subject before applying the plurality of signal acquisition pulses. The collection time changing means is means for setting a part of the plurality of collection times shorter than the remaining collection times. Further, the acquisition time changing means is means for allocating the part of the acquisition time to the echo signal corresponding to the position on the Fourier space where the phase encoding amount by the phase encoding gradient magnetic field is low.

According to still another aspect, there is provided reconstruction means for reconstructing a plurality of images having different contrasts based on the plurality of collected echo signals, and the acquisition time changing means is provided for each of the plurality of images. It is a means for changing the acquisition time for each corresponding echo signal. The plurality of images are a proton density image and a T ₂ -weighted image, the acquisition time changing means sets a short acquisition time of the echo signal corresponding to the proton density image, and corresponds to the T ₂ -weighted image. This is a means for setting a long echo signal acquisition time. The means for collecting includes means for setting the number of applications of the phase encoding gradient magnetic field in the T ₂ -weighted image to be larger than in the proton density image.

[0011]

A series of a plurality of pulse for signal acquisition (for example, 180 ° RF pulse) is sequentially applied to the subject placed in the static magnetic field together with the gradient magnetic field for slicing. A plurality of echo signals (for example, spin echo signals) sequentially generated by being energized by the plurality of signal acquisition pulses are collected by applying a phase encoding gradient magnetic field and a reading gradient magnetic field. At this time, a part of the plurality of acquisition time zones corresponding to each phase encode amount, for example, another echo acquisition time length on the back side (for example, after the effective TE time TE _eff ) on the time axis (narrow band acquisition). ) Is set shorter (wideband collection), the collection time length of some of the plurality of collection time zones is changed. As a result, the number of multi-slices is
It can be as secure as the E method.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

A schematic structure of the magnetic resonance imaging apparatus of this embodiment is shown in FIG. This magnetic resonance imaging apparatus includes a magnet unit for generating a static magnetic field, a gradient magnetic field unit for adding position information to the static magnetic field, a transmitter / receiver unit for selective excitation and MR signal reception, system control and image reconstruction. It is equipped with a control / arithmetic unit.

The magnet section includes, for example, a superconducting magnet 1.
A static magnetic field power supply 2 for supplying a current to the magnet 1 is provided, and a static magnetic field H ₀ is generated in the Z-axis direction of the cylindrical diagnostic space into which the subject P is inserted.

The gradient magnetic field section is composed of the X,
Three sets of gradient magnetic field coils 3x to 3z in the Y and Z-axis directions, a gradient magnetic field power supply 4 that supplies a current to the gradient magnetic field coils 3x to 3z, and a gradient magnetic field sequencer 5 that controls the power supply 4.
With. The sequencer 5 receives a control signal in accordance with a sequence for controlling the strength of each gradient magnetic field in the X, Y, and Z axis directions from a controller 6 that controls the entire apparatus. As a result, a linear gradient magnetic field can be superimposed on the static magnetic field H _0, and a gradient magnetic field in each direction is formed. Where Z
The gradient magnetic field in the axial direction is referred to as a slice gradient magnetic field G _S , that in the X axis direction is referred to as a read gradient magnetic field G _R, and that in the Y axis direction is referred to as a phase encoding gradient magnetic field G _E.

The transmission / reception unit indicates the high-frequency coil 7 arranged in the vicinity of the subject P in the imaging space of the magnet 1, the transceiver 8 connected to this coil 7, and the operation timing of this transceiver 8. And an RF sequencer 9 for controlling. As will be described later, this transceiver 8 supplies an RF current pulse of Larmor frequency for exciting nuclear magnetic resonance (NMR) to the high frequency coil 7 under the control of the RF sequencer 9, while the high frequency coil 7 The received MR signal (high frequency signal) is subjected to various kinds of signal processing to generate a digital signal.

In addition to the controller 6 described above, the control / arithmetic unit receives the digital signal generated by the transceiver 8 and reconstructs the image data by using, for example, a two-dimensional Fourier transform method. A storage unit 11 for storing the image data, a display 12 for displaying the image data, and an input device 13. The controller 6 supplies a control signal indicating a predetermined sequence to the gradient magnetic field sequencer 5 and the RF sequencer 9, while switching the acquisition mode including the control of the sampling pitch, the filter band, and the readout gradient magnetic field strength shown in FIG. .

As shown in FIG. 1, the transceiver 8 comprises a transmitter 8T and a receiver 8R. The transmitter 8T includes two oscillators 20, 21, a phase selector 22, a frequency converter 23,
An amplitude modulator 24 and a high frequency power amplifier 25 are provided in this order. Of these, the two oscillators 20, 21 the frequency f ₁ that satisfies the Larmor frequency _{f 0 = f 1 + f 2} , f 2
Is output. The phase selector 22 controls the phase of high frequency. The frequency converter 23 synthesizes the _two frequencies f ₁ and f ₂ to generate the Larmor frequency f ₀ . Further, the amplitude modulator 24 is a part that determines the power condition of the RF pulse and the slice characteristic, and modulates the high frequency with a sink function or a square wave. The high frequency power amplifier 25 amplifies the amplitude-modulated high frequency and supplies the pulse current to the high frequency coil 7.

On the other hand, the receiver 8R includes a pre-stage amplifier 30, an intermediate frequency converter 31, a phase detector 32, a low frequency amplifier 33, a low pass filter 34, and an A / D converter 35, as shown in FIG. I have it.

Among these, the pre-stage amplifier 30 is the high frequency coil 7
The weak MR signal (frequency f ₀ ) received at is amplified and the amplified signal is output to the intermediate frequency converter 31. The intermediate frequency converter 31 uses the MR signal (frequency f ₀ ) and the reference frequency f.
₂ and are combined and converted into an intermediate frequency f ₁ = f ₀ −f ₂ . Receives the signal converted to the intermediate frequency f ₁ , 2
The channel type phase detector 32 further detects the received signal at the reference frequency f ₁ to form a signal in the audio frequency band. The signals of the two channels in the audio frequency band are amplified by the low frequency amplifier 33 and sent to the A / D converter 35 via the low pass filter 34.

The low-pass filter 34 (2 channels) limits the band of the passing signal to prevent aliasing of the MR signal and improve the S / N ratio. This low pass filter 3
In the present embodiment, 4 is capable of changing the band, that is, the cutoff frequency according to the band control signal from the controller 6. The cutoff frequency of the low-pass filter 34 is always set to a value of 1/2 of the Nyquist frequency that satisfies the sampling theorem in the A / D converter 35 in the next stage.

The A / D converter 35 (2 channels) has a commanded sampling pitch (sampling time period).
At, the analog signal whose audio band is converted is sampled and converted into a digital signal. This A / D converter 35
Also, the sampling pitch can be changed according to the sampling pitch variable signal from the controller 6. This digital signal is sent to the reconstruction unit 10 to reconstruct the image signal.

Next, the operation of this embodiment will be described.

In this embodiment, the controller 6 commands the RF sequencer 9 and the gradient magnetic field sequencer 5 to execute a sequence relating to the "high-speed spin echo method in which the number of echoes is" 5 "". As a result, selective excitation, phase encoding, and signal reading are repeated according to the sequence shown in FIG.

While repeating this signal collection, the controller 6 performs the process shown in FIG. This process starts when the acquisition of MR signals starts.

First, the controller 6 reads from the gradient magnetic field sequencer 5 the phase encode amount for the echo signal that is about to be collected (step S1). Next, it is determined whether the read phase encoding amount is high or low by comparing it with a threshold value. In this embodiment, as shown in FIG. 2, the second, third and fourth spin echo signals EC2, EC3,
EC4 is arranged in the zero encode on the k space and the regions R2 to R4 near the zero encode, and the first and fifth spin echo signals EC1 and EC5 are on the k space and on the higher encode side than the regions R2 to R4. They are arranged in R1 and R5 respectively. Therefore, when the read phase encode amount corresponds to the regions R2 to R4 on the k space, it is recognized as a "low phase encode amount" (step S2).

Next, the controller 6 determines whether or not the "narrow band acquisition" has already been instructed (step S
3) If it is still, command “narrow band acquisition”. That is, the A / D converter 35 has a sampling pitch = ΔT, and the low-pass filter 34 has a filter band =
1 / ΔT, and the gradient magnetic field sequencer 5 is instructed to read gradient magnetic field G _R = G _R1 determined from the sampling pitch = ΔT. Here, the sampling pitch = ΔT is selected so that a good S / N ratio can be obtained and a predetermined number of samples can be sampled. The filter band is also set to a value calculated from the sampling pitch = ΔT, and the read gradient magnetic field G _R = G _R1 is set according to the designated image size and sampling pitch.

If the result of the determination in step S3 is YES (that is, "already narrow band acquisition" has been instructed), the processes in steps S1 to S3 are repeated, and the narrow band acquisition is instructed to perform wide band acquisition to be described later. Continue until

On the other hand, if it is recognized in step S2 that the amount of phase encoding is high, it is then determined whether "wideband acquisition has already been commanded" (step S5). If the result of the determination is NO, that is, if the wideband acquisition has not been instructed yet, then a "wideband acquisition" is instructed instead of the "narrowband acquisition" up to then (step S6). That is, the sampling pitch = ΔT ′ (<ΔT) in the A / D converter 35, the filter band = 1 / ΔT ′ in the low-pass filter 34, and the read gradient magnetic field G _R = G determined in the gradient magnetic field sequencer 5 based on the sampling pitch ΔT ′. Command _R2 (> G _R1 ) respectively.

For this broadband acquisition, the sampling pitch of the A / D converter 35, which is a fast predetermined value, is determined. The filter band (= 1 / ΔT ') is set accordingly, and the read gradient magnetic field G _R = G _R2 is "ΔT × G".
_R1 = _ΔT '× G _R2 "is set to a high strength to satisfy the phase shift due to the gradient magnetic field G _R is kept constant for each sampling. That is, the roughness of reading the effective echo signal waveform is always kept constant.

This broadband acquisition command continues until it is recognized again in step S2 that the "low phase encoding amount" is recognized.

The narrow band acquisition and the wide band acquisition depending on the phase encode amount in one sequence are repeated for each sequence.

While receiving the signal acquisition mode information determined as described above from the controller 6, the gradient magnetic field sequencer 5 executes the slice gradient magnetic field G _S , the readout gradient magnetic field G _R , and the gradient magnetic field G _R based on the sequence shown in FIG. The phase encoding gradient magnetic field G _E is controlled. In parallel with this, the RF sequencer 9 which receives the command of the high-speed SE method from the controller 6 applies 90 ° RF pulse and 180 ° RF pulse to the subject P in the sequence shown in FIG.

That is, first, the slice gradient magnetic field G _S
Is applied from the gradient magnetic field power source 4 through the gradient magnetic field coils 3z and 3z, and when the gradient magnetic field G _S rises to a constant value, 90 ° R is applied through the transmitter 8T and the high frequency coil 7.
The F pulse is applied only once. As a result, a region having a predetermined slice width of the subject is selectively excited and the nuclear spins in the plane are flipped to the y ′ axis (rotational coordinate).

Then, along with the reversal of the slice gradient magnetic field G _S , the read gradient magnetic field G _R is changed to the gradient magnetic field coils 3x and 3x.
applied via x. This is applied for G _R direction nuclear spin phase aligned in the slice plane to align the center time of each echo.

Next, the first 180 ° RF pulse is applied together with the slicing gradient magnetic field G _S. This allows
The nuclear spin rotates 180 ° around the y'axis. Further, the first phase-encoding gradient magnetic field G _E = A is applied from the gradient magnetic field power source 4 to the subject P via the gradient magnetic field coils 3y and 3y, and then applied via the gradient magnetic field coils 3x and 3x. The first spin echo signal EC1 is collected via the radio frequency coil 7 together with the gradient magnetic field G _R = G _R2 for use.

After that, the inverted phase encoding gradient magnetic field G _E = -A is applied. This is a pseudo echo (stim
180 ° R to avoid image deterioration due to modulated echo)
This is for returning to the state of ke = 0 when the F pulse is applied.

Next, the second 180 ° RF pulse is applied together with the slice gradient magnetic field G _S , and then the second phase encoding gradient magnetic field G _E = B is applied. And
The second spin echo signal EC2 is collected via the high-frequency coil 7 together with the application of the read gradient magnetic field G _R = G _R1 .

Similarly, the third to fifth spin echo signals EC3 to EC5 are collected.

These five spin echo signals EC1 to EC
The collection of 5 is repeated for each high frequency excitation with a predetermined number of 90 ° RF pulses.

The echo signals thus collected are sequentially
The signal is sent to the receiver 8R, where it undergoes processing such as amplification, intermediate frequency conversion, phase detection, low frequency amplification, and filtering, and is then converted by the A / D converter 10 into digital echo data. At this time, as described above, the low-pass filter 34 and the A / D converter 35 are instructed to perform the "wideband acquisition" for the first and fifth spin echo signals EC1 and EC5, and the second to fourth spin echo signals. “Narrow band acquisition” is commanded to EC2 to EC4. Therefore, k
The first and fifth spin echo signals EC1 and EC5 arranged at a high encoding amount position in space are sampled in a short sampling time, and the signal acquisition time can be shortened without significantly lowering the S / N ratio. .

The echo data converted into a digital amount is sent to the reconstruction unit 10, and the data is arranged in a memory area corresponding to the k space, which can be Fourier transformed, and the image data in the real space is obtained by the two-dimensional Fourier transform. Reconfigured into. This image is stored in the storage unit 13 and displayed on the display unit 14.

As described above, by changing the signal acquisition band in the same sequence according to the amount of phase encoding, the entire signal acquisition time is shortened, especially by shortening the time after TE _eff , which is favorable. While maintaining the S / N ratio, it is possible to secure the number of multi-slices that is comparable to that of the normal SE method under the same TR.

The fast SE method described in the above embodiment was the "5" echo mode, but the effect of shortening the acquisition time becomes more remarkable as the number of echoes is increased beyond this (for example, 10 echoes or more).

The arrangement of the spin echo signal in the k space is not limited to the method shown in FIG.
When collecting a T ₁ -weighted image, three echoes may be arranged as shown in FIG.

Next, other modified examples of the present invention will be described with reference to FIGS.
It will be described based on. The hardware configuration of the magnetic resonance imaging apparatus according to these modified examples is the same as that in FIG.

The first modification relates to a magnetic resonance imaging apparatus which simultaneously collects a proton density image and a T ₂ -weighted image based on the fast SE method. As shown in FIG. 5, this device has three echoes in the first half: first to third spin echo signals EC1 to EC.
3 is collected in broadband acquisition mode and a proton density image is reconstructed. Then, the latter three echoes: the fourth to sixth spin echo signals are collected in the narrow band acquisition mode, and the T ₂ -weighted image is reconstructed. The spin echo signal is attenuated with a time constant of T ₂ ^* , but since the signal value is high in the first half of the sequence, even if it is acquired over a wide band (short sampling time), there is almost no effect on the S / N ratio reduction. Also by this, since the wideband acquisition mode is adopted in one sequence, good S / S
While maintaining the N ratio, the number of multi-slices can be increased and two types of images can be acquired simultaneously.

In the second modification, the proton density image and the T ₂ -weighted image are simultaneously acquired by the high-speed SE method similarly to the first modification, but the phase encode numbers used for both images are different from each other. It is a thing. As shown in FIG. 6, the proton density image is represented by three spin echo signals in the first half: EC1 to EC3 (broadband acquisition mode), and the T ₂ weighted image is represented by four spin echo signals in the latter half: EC4 to EC7 (narrow band acquisition mode). Mode). As a whole, the proton density image is 192 encoded (= 3 echoes × 64 times) and the T ₂ weighted image is 256 encoded (= 4 echoes × 64 times), and one image each is created. The number of echo signals used may be larger in the proton density image than in the T ₂ -weighted image.

Furthermore, the third modification is applied to the high-speed FE method, as shown in FIG. For example, by collecting / sampling the spin echo signals for the first half of 32 encodes in the narrow band acquisition mode and then collecting / sampling the remaining spin echo signals in the wide band acquisition mode, the advantages described above even in the high-speed FE method. Can enjoy.

Furthermore, the above-described embodiments and modifications are so-called Vari SCAN (: Variable TR SCAN), which is a signal acquisition method in which the repetition time TR is variable for each phase encoding.
Method (see FIG. 8).

[0051]

As described above, in the magnetic resonance imaging apparatus of the present invention, the collection time (sampling time) of a plurality of echo signals in the same sequence is made variable for each echo signal, for example, in the k space. Since the echo signals arranged near the zero encode of are collected in a narrow band (long sampling time) and the other echo signals in a wide band (short sampling time), while maintaining a good S / N ratio, It is possible to shorten the entire signal acquisition time and obtain the same number of multi-slices as the normal SE method.

[Brief description of drawings]

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

FIG. 2 is a pulse sequence based on the high speed SE method of the same embodiment.

FIG. 3 is a schematic flowchart showing a collection mode determination process in the controller of the embodiment.

FIG. 4 is a diagram illustrating another collection example.

FIG. 5 is a pulse sequence of a fast SE method according to a first modification.

FIG. 6 is a pulse sequence of a fast SE method according to a second modification.

FIG. 7 is a pulse sequence of a fast FE method according to a third modification.

FIG. 8 is an explanatory diagram of a Vari SCAN method according to another example.

FIG. 9 is a pulse sequence showing a fast SE method according to a conventional example.

[Explanation of symbols]

 1 magnet 2 static magnetic field power supply 3x-3z gradient magnetic field coil 4 gradient magnetic field power supply 5 gradient magnetic field sequencer 6 controller 7 high frequency coil 8 transceiver 8T transmitter 8R receiver 9 RF sequencer 34 low pass filter 35 A / D converter

Claims (7)

[Claims] 1. A means for sequentially applying a series of a plurality of signal acquisition pulses together with a gradient magnetic field for slicing to a subject placed in a static magnetic field, and a plurality of the signal acquisition pulses which are energized and sequentially applied. In a magnetic resonance imaging apparatus having means for applying a phase encoding gradient magnetic field and a readout gradient magnetic field to collect each of a plurality of echo signals generated, in a plurality of collection time zones for collecting the plurality of echo signals. A magnetic resonance imaging apparatus comprising a collection time changing means for changing a collection time length of a part of the magnetic resonance imaging apparatus. 2. The magnetic resonance imaging apparatus according to claim 1, further comprising means for applying one high-frequency excitation pulse together with the slice gradient magnetic field to the subject before applying the plurality of signal acquisition pulses. . 3. The magnetic resonance imaging apparatus according to claim 2, wherein the acquisition time changing means is means for setting a part of the plurality of acquisition times shorter than the remaining acquisition times. 4. The acquisition time changing means is means for allocating the part of the acquisition time to the echo signal corresponding to a position on a Fourier space where the phase encoding amount by the phase encoding gradient magnetic field is low. 3. The magnetic resonance imaging apparatus described in 3. 5. Reconstructing means for reconstructing a plurality of images having different contrasts based on the plurality of collected echo signals, wherein the collection time changing means is provided for each echo signal corresponding to each of the plurality of images. The magnetic resonance imaging apparatus according to claim 2, which is a unit that changes the acquisition time. 6. The plurality of images are a proton density image and a T ₂ image.
In the emphasized image, the acquisition time changing means is a means for setting a short acquisition time of the echo signal corresponding to the proton density image and a long acquisition time of the echo signal corresponding to the T ₂ emphasized image. The magnetic resonance imaging apparatus according to claim 5. 7. The magnetic resonance imaging apparatus according to claim 6, wherein the collecting means includes means for setting the number of applications of the phase encoding gradient magnetic field in the T ₂ -weighted image to be larger than in the proton density image. .