JP2003325477A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the total measurement time for cardiac imaging, or to increase the number of images acquired in one cardiac cycle. <P>SOLUTION: In a process to reach a steady state free precession after repeating sequences in a short repetition time (TR) and going through a state of transient oscillation of magnetization, the amount of phase encoding to be applied is controlled to acquire echo signals in the state of transient oscillation and to have the acquired signals placed in a high region of a measurement space (k space). <P>COPYRIGHT: (C)2004,JPO

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 more particularly to a technique for imaging a moving organ such as a heart of a subject using an SSFP sequence.

[0002]

2. Description of the Related Art FIG. 3 shows the structure of a typical MRI apparatus. MR
The device I includes a magnet 302 that generates a static magnetic field around the subject 301.
And a gradient magnetic field coil 303 for generating a gradient magnetic field in the space
Further, an RF coil 304 that generates a high-frequency magnetic field and an RF probe 305 that detects an MR signal generated by the subject 301 are provided in this region. The gradient magnetic field coil 303 is composed of gradient magnetic field coils in three directions of X, Y, and Z, and generates a gradient magnetic field according to a signal from the gradient magnetic field power source 309. RF coil 304 is R
A high frequency magnetic field is generated according to the signal from the F transmitter 310. The signal of the RF probe 305 is detected by the signal detection unit 306, processed by the signal processing unit 307, and converted into an image signal by calculation. The image is displayed on the display unit 308. The gradient magnetic field power supply 309, the RF transmission unit 310, and the signal detection unit 306 are controlled by the control unit 311, and the control time chart is generally called a pulse sequence. The bed 312 is for the subject to lie on, and 313 is a console for the operator to input operation commands.

At present, the object to be imaged by MRI is proton, which is the main constituent substance of the subject, which is widely used clinically. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon of the excited state, the shape or function of the human head, abdomen, limbs, etc., can be photographed two-dimensionally or three-dimensionally.

Next, the photographing method will be described. Different phase encoding amounts are applied to the excited nuclear spins by the gradient magnetic field, and echo signals given the respective phase encoding amounts are detected. The number of phase encodes is usually 128, 256, 512, etc. per image. Each echo signal is usually obtained as a time-series signal composed of 128, 256, 512, 1024 sampling data. Two-dimensional Fourier transform is performed on these data to create one MR image.

FIG. 7 shows an example of a gradient echo sequence. The pulse sequence shown in FIG. 7 is a SSFP type gradient echo sequence, and is a known technique in which the gradient magnetic fields of all axes are rephased at the repetition time TR of the pulse sequence (true FISP, balanced SG, FIES.
Sometimes called TA). In FIG. 7, RF is a high frequency excitation pulse, Gs is a slice direction gradient magnetic field, Gp is a phase encoding direction gradient magnetic field, Gr is a readout direction gradient magnetic field, A / D is a timing and a sampling period for reading an echo signal,
Also, echo represents an echo signal.

Explaining the operation of this pulse sequence, first, a high frequency excitation pulse 102 is applied while applying a slice selection gradient magnetic field 101 to a subject placed in a static magnetic field. Next, after applying the phase encoding gradient magnetic field 103 and the dephasing pulse 104 in the reading direction, the echo signal 107 is read during the A / D interval 106 while applying the reading gradient magnetic field 105. Then, after measuring the echo signal 107, a phase encoding direction gradient magnetic field 108 and a reading direction gradient magnetic field 109 are applied to rephase the nuclear spins. Then, when the repetition time TR has passed, the nuclear spins are excited again, that is, from the application of the high-frequency excitation pulse 102 to 1
The application of 10 is repeated at a repetition time TR with a change in the amount of phase encoding given to the echo signal.

[0007] When imaging the heart using the SSFP sequence, the TR
It is said that ≦ 3 ms is preferable. The reason is TR ≤ 3
This is because in ms, good contrast between blood and myocardium can be obtained.

In the gradient echo method, TE / TR can be set shorter than in the spin echo method. On the other hand, if the TR is shortened, the transverse magnetization remains at the end of each TR without disappearing and is carried over to the next TR. Short TR
When RF excitation is continued at intervals, after the magnetization vibrates for a while,
A steady state is reached after a process about the size of T1 or T2. This state is called Steady State Free Precession.
Free Precession: SSFP) state. Figure 5 shows the change in the intensity of the echo signal from the start of excitation of the nuclear spins until the steady state is reached. In Fig. 5, the horizontal axis is excitation (repeated TR)
The number of times and the vertical axis represent the signal strength, that is, the absolute value of the magnetization. It takes several tens of excitations until the magnetization reaches a steady state, and in FIG. 5, the steady state is reached by repeating the excitations about 90 times. The process until the magnetization reaches a steady state is called the transient state.However, since the signal oscillation is strongly unstable in the transient state, it was considered unfavorable to use the signal in this transient state for image reconstruction ( JP 2
000-350714).

When performing cardiac imaging, an electrocardiographic synchronization method is used as shown in FIG. This is, the adjacent R wave of the electrocardiographic waveform, for example, the time interval between the R waves 201, 202 (cardiac cycle, RR interval) is usually about 1 s, this heart cycle is divided into a plurality of time phases, A measurement window is provided in each time phase, an image signal (echo signal) is acquired from the inside of the subject, and an image corresponding to each time phase is created. During this measurement window, the TR of the SSFP sequence shown in FIG. 7 is repeated a plurality of times to obtain a plurality of phase-encoded echo data.

In the lower part of FIG. 8, a high frequency excitation pulse (RF) and an echo signal (echo) of the SSFP sequence simplified from FIG.
Only the timing of occurrence of is shown. This is repeated for a plurality of cardiac cycles to obtain an echo signal required for image reconstruction.

Each measurement window is usually set to a time of about 100 ms to 200 ms, but the shorter the measurement window is, the more the number of heart images that can be acquired within one cardiac cycle increases.

[0012]

As described above, since the signal oscillation is strong and unstable in the transient state, it is considered not preferable to use the signal in this state for image reconstruction. Conventionally, in the transient state, the echo signal is not acquired and only the excitation of the magnetization by the high-frequency excitation pulse is performed (since the echo signal is not measured, it will be referred to as “blank firing” hereinafter). Blanking time) =
The time required for measurement is extended by TR × (number of blank shots).
On the other hand, since cardiac imaging is strongly affected by respiratory motion, breath-hold imaging may be performed to suppress the effect of respiratory motion. At this time, it is desirable that the measurement time is short because the breath-holding time that the patient can take is limited. When breath-holding is not performed, respiratory motion correction using navigation echo is generally performed, but in this case, the steady state of magnetization is destroyed each time a navigation echo is acquired, so blanking is performed each time. It is necessary to return the magnetization to the steady state before measuring. In this case, since the measurement window is extended, the number of time phases for which an image can be acquired within one cardiac cycle is reduced.

[0013]

In order to solve the above problems, the present invention devises the order of phase encoding of the echo signals to be acquired. That is, in the MRI apparatus of the present invention, the measurement of the echo signal shall be started if the magnetization is in the transient state after the excitation of the nuclear spins is started, and the echo signal measured immediately after the transition state is entered. Gives the amount of phase encoding so that they are arranged at the ends (high range) of k-space. Then, in the echo signal measured during the transient state during which the transient state is shifting to the steady state, phase encoding is performed so that the echo signal is sequentially arranged in a region closer to the center of the k space than the echo signal immediately after the start of the measurement. Give the amount. A phase encoding amount is added to the echo signals that finally reach the steady state so that they are arranged in the central region (low band) of the k-space.

By measuring the echo signal as described above,
In cardiac imaging, the total measurement time can be shortened. In addition, since the number of images that can be acquired within one cardiac cycle can be increased, the movement of the heart can be observed with high temporal resolution.

Further, since the echo signal in the transient state is arranged in the high band of the k-space and the stable echo signal reaching the steady state is arranged in the low band, artifacts can be suppressed.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. The MRI apparatus used in the present embodiment has a configuration in which an electrocardiograph 320 is combined with the one shown in FIG. 6, and the output of the electrocardiograph is input to the control unit 311. The control unit 311 detects the R wave as a characteristic time phase of the electrocardiographic waveform, and measures one cardiac cycle from the detected R wave. The technology for measuring this R wave and one cardiac cycle is
Since it is known as an electrocardiographic synchronization technique in the field of medical equipment, description thereof will be omitted.

Next, a first embodiment of the present invention will be described with reference to FIG. The operator considers how many images are to be acquired for one cardiac cycle measured by the electrocardiograph 320 from the console 313 equipped with an input key such as a keyboard, and the like. Perform the input operation to set the time phase. In the present embodiment, description will be given assuming that the number of time phases is eight.

When the setting of the number of time phases is completed, the operator moves the subject to the imaging position, that is, the imaging space of the MRI apparatus,
Set the imaging section of the heart. Then shooting is started. The imaging is performed in synchronization with the electrocardiographic detection of the subject by the electrocardiograph 320.

The MRI apparatus has an SS for nuclear spin shown in FIG.
Based on the behavior data reaching the FP state, the number of blank hits to the transient state, time phase division with the R wave of the electrocardiogram as a reference,
And the association of SSFP sequences to them is incorporated as software. Therefore, when an imaging start command is issued from the control unit 311 to each unit of the MRI apparatus,
When a predetermined time has elapsed from the detection of the R wave, which is not shown in FIG. Then, the R wave (R1) shown in FIG. 1 is detected at the time when the blanking is performed a certain number of times. This R1
Is a trigger signal for setting the time phase, and measurement of the NMR signal is started for acquiring the image of the time phase 1 after Δt time after R1 is detected. At the time when the measurement of the NMR signal is started, the magnetization is in a transient state due to the blanking. In the present embodiment, the time interval of one time phase is set to 100 to 120 ms in order to observe the movement of the heart in eight images in a cardiac cycle of about 1 second. As mentioned above, since the TR of SSFP sequence is preferably about 3 ms, it is possible to measure up to about 40 echo signals during one phase. The number of heartbeats to measure one image is set based on the number of phase encodes forming one image and the number of echo signals measured in one time phase. In the example of FIG. 1, this is set to 3 heartbeats.

When measuring the echo signals, in the present embodiment, if the echo signals to be measured are in a transient state of magnetization, they are sequentially arranged from the both ends (high range) of the k space toward the center in the order of measurement. As described above, the phase encode amount is applied.

According to FIG. 5, the transient state is about 90 times from the start of the excitation, but when the excitation is repeated about 30 to 40 times, the oscillation of the absolute value of the magnetization is reduced to some extent. Therefore,
The signal acquired from 40 times to about 90 times of the excitation indicated by 401 is suppressed to about 40 times and is arranged in the k-space high region as shown in FIG. 2 (a). In the example of FIG. 1, the measurement is performed in the time phase 1 from about 40 to 80 times of excitation and from the time phase 2 from about 80 to 90 times of excitation.

A specific k-space arrangement method for echo signals,
That is, the amount of phase encoding added to the echo signal is
Shown in 2 (b) and (c). 2 (b) and 2 (c) show the case where the k space is divided into eight regions. In the order in which the signals are acquired from the start of measurement, the k space regions are 1, 2, 3, 4 ,. Place them in numerical order. There are roughly two methods of arranging at this time.

(I) A method in which a signal acquired in one measurement window is directly associated with a divided region of k space, and (ii)
In this method, the echo signal acquired first in each measurement window is arranged in the area 1 and the echo signal acquired second is arranged in the area 2 ...

12 measurement matrices in the phase encode direction
8 is 16 in each of the divided areas in Fig. 2 (b).
The echo signal of is arranged. In the case of FIG. 2 (c), the area 1
There are two areas 7 to 7, but 8 echoes are arranged in each area, and a total of 16 echoes are arranged in the area of the same number. 16 echoes are arranged in the area 8.

In the case of the method (i), 16 echo signal acquisitions are repeated for 8 cardiac cycles in one measurement window in a predetermined time phase of the cardiac cycle. Here, the echo signal acquired in the first cardiac cycle is shown in Fig. 2.
In regions 1 of (b) and (c), the echo signal acquired in the second cardiac cycle is in region 2 ..., the echo signal acquired in the eighth cardiac cycle is in the region
Placed at 8. In the method (ii), one of the predetermined time phases of the cardiac cycle
The acquisition of 8 echo signals is repeated for 16 cardiac cycles in the measurement window. The echo signal acquired first in each measurement window of each cardiac cycle is in region 1 of Figures 2 (b) and (c), the echo signal acquired second is in region 2 ... The echo is placed in area 8. FIGS. 4 (a) and 4 (b) show examples of arrangement of echo signals when the k space is divided as shown in FIG. 2 (b). Figure 4
(a) corresponds to the method (i), and FIG. 4 (b) corresponds to the method (ii).

Although it is possible to further shorten the signal acquisition start time by further reducing the number of blank shots, if the number of blank shots is reduced, the vibration of the signal absolute value immediately after the start of signal acquisition becomes large, so the image quality of the obtained image is high. descend.

In the case of the first embodiment shown in FIG. 1, the transient state of the signal exists only once immediately after the start of measurement. Therefore, the method (i) described above is suitable as a method of arranging the echo acquired at this time in the high frequency band of the k space.

Returning to FIG. 1, the description will be continued. In phase 1 to period
After repeating the TR a predetermined number of times to measure the echo signals and performing the above-mentioned k-space arrangement of those echo signals, the time phase 2
Is measured. Since the echo signal measurement in the transient state is also included in the time phase 2, the measured echo signal is taken into the k space different from the k space in which the echo signal of the time phase 1 is arranged by the above k space arrangement method. Be done. Further subsequently, measurement is performed for each measurement window of time phase 3, time phase 4, ..., Time phase 8. If the echo signal after time phase 3 is measured in a steady state, then (i) or (ii) above
It is considered that there is no need to perform the k-space arrangement of the above, but if the phase encoding assignment procedure different from the time phase 1 and the time phase 2 is incorporated into the software, the software becomes complicated. Would be preferred.

When the measurement of the time phase 8 in the first measurement heartbeat is completed, the control unit 311 waits for R2 detection of the trigger signal, and when R2 is detected, the measurement of the second heart cycle is started. Also for the second heart cycle, the echo signals are measured from time phase 1 to time phase 8. The echo signal measured in this second heart cycle is (i) above.
Alternatively, it is taken into the k space by the k space arrangement method of (ii). Hereinafter, the echo signal measurement for the third cardiac cycle is similarly performed.

When the measurement of the last cardiac cycle is completed, eight k spaces are filled with echo signals. Two-dimensional Fourier transform of each of these eight k-spaces,
Eight images are obtained corresponding to each time phase of the cardiac cycle. These images are displayed on the display screen of the display unit 308 by an image display command input by the operator from the console 313. The display mode can be displayed by a method of displaying each image individually or a cine display method.

FIG. 3 shows a second embodiment of the present invention. In this embodiment, a navigation echo is acquired or a prepulse is sandwiched for each measurement window. for that reason,
The steady state of magnetization is broken for each measurement window.
Therefore, it is necessary to perform blanking for each measurement window so that the magnetization reaches a steady state. In order to reach a steady state, dozens of blank hits must be performed, so 1
The time required to measure the time phase becomes extremely long, and the number of time phases that can be acquired decreases. Therefore, the blanking is reduced and the acquisition of the data signal is started. That is, the echo signal acquired in the transient state is also used for reconstruction. The echo signal acquired in the transient state is arranged in the high frequency band of the k-space, but in the case of the second embodiment shown in FIG. 3, since the transient state of the signal exists for each measurement window, the arrangement of the k-space is arranged. As the method, it is preferable to use the above case (ii).

As a result, in the second embodiment, the time of each measurement window can be shortened and the number of time phases for acquiring an image can be increased. Therefore, in the transient state, when the number of time phases is the same as when the echo signal is not measured, the number of phase encodes that can be acquired in one cardiac cycle increases, so the number of repetitions of the cardiac cycle decreases and the imaging time is reduced. Can be shortened. In addition, since the stable echo signal that has reached the steady state is brought to the low range of the measurement space, the echo signal
The image quality is improved compared to the case where the arrangement in the k space is not considered. This embodiment is suitable for imaging a coronary artery using respiratory motion correction by a navigation signal.

Although the embodiment of the present invention discloses the SSFP sequence that acquires one echo with one excitation, the present invention is applicable to essentially all SSFP sequences. For example, TR
A similar effect to that of the above-mentioned embodiment can be expected even in a multi-echo sequence that operates in a steady state, such as a spiral sequence with a shortened TR and a multi-shot EPI with a shortened TR.

The present invention can also be applied to three-dimensional imaging. The present invention can be applied not only to the heart but also to an electrocardiographically synchronized imaging (for example, diffusion weighted imaging).

[0035]

Since the present invention is constructed as described above,
In imaging such as cardiac imaging, the total measurement time can be shortened or the number of images that can be acquired within one cardiac cycle can be increased.

Further, since the echo signal in the transient state is arranged in the k-space high region and the stable echo signal reaching the steady state is arranged in the low region, artifacts can be suppressed.
The image obtained can also be good.

[Brief description of drawings]

FIG. 1 is a diagram showing a signal measuring method and an image forming method according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the arrangement of echo signals in k space according to the present invention.

FIG. 3 is a diagram showing a signal measuring method according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a detailed example of an arrangement of echo signals in k space according to the present invention.

FIG. 5 is a diagram showing the relationship between the number of excitation repetitions and the absolute value of magnetization.

FIG. 6 is a block diagram of an MRI apparatus to which the present invention is applied.

FIG. 7 is a diagram showing an SSFP type gradient echo sequence.

FIG. 8 is a data acquisition time chart using conventional electrocardiographic synchronization.

[Explanation of symbols]

301 ... Subject 302… Static magnetic field magnet 303 ... Gradient field coil 304 ... RF coil 305 ... RF probe 306 ... Signal detector 307 ... Signal processing unit 308 ... Display 309… Gradient magnetic field power supply 310 ... RF transmitter 311 ... Control unit 312 ... Bed 313 ... Console 320 ... Electrocardiograph

Claims (3)

[Claims] 1. A sequence is repeated for a short repetition time (TR), and after a magnetization reaches a steady state free precession through a transient oscillation state, a phase encoding gradient magnetic field is applied to acquire an echo signal and form an image. In the magnetic resonance imaging apparatus having a means for providing, to the echo signal of the transient vibration state, so that the signal is arranged in the high region of k space, means for controlling and measuring the amount of phase encoding to be applied, A means for arranging the echo signal in the steady state and an echo signal in the transient state in the same k space, and means for two-dimensionally Fourier-transforming the echo signal group arranged in the same k space to form an image. A magnetic resonance imaging apparatus comprising: a means for displaying an image. 2. The echo signal is measured so that an image is formed for each time phase set by dividing the heart cycle into several, and the echo signals acquired in each time phase are divided into a plurality of heart cycles. The magnetic resonance imaging apparatus according to claim 1, wherein the images are collected and reconstructed. 3. The magnetic resonance imaging apparatus according to claim 2, wherein a preparation pulse is inserted for each time phase.