JPH06285033A - Echo signal collecting method in magnetic resonance imaging system - Google Patents

Abstract

PURPOSE:To arbitrarily set an image pickup time, and also, to obtain an image of a high picture quality by collecting an echo signal from an examinee once or plural times, and integrating them by the number of times of the collection for every one projection, and making the number of times of the collection different for every projection. CONSTITUTION:This method is provided with a static magnetic field generating magnet 2, a gradient magnetic field generating system 3, a sequencer 4, a transmission system 5, a reception system 6, a signal processing system 7, a central processor 8 and an operation part 21. In the method, a gradient magnetic field is applied to an examinee 1 for every projection (i) collecting an echo signal for one image I, and also, the echo signal is collected once or plural times, and it is integrated by the number of times of the collection for every one projection. The number of times of the collection is made different for every projection or at every projection of a certain area, thereby the collected and integrated echo signal is made as the echo signal of the projection concerned (i).

Description

Detailed Description of the Invention

[0001]

The present invention relates to nuclear magnetic resonance (hereinafter referred to as "N
(Abbreviated as "MR") phenomenon for collecting echo signals detected by a receiving system in a magnetic resonance imaging apparatus for obtaining a tomographic image of a desired portion of a subject (human body), and particularly, an imaging time can be arbitrarily set. Method for collecting echo signals.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus uses an NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in an object, and uses the measured data to measure an arbitrary cross section of the object. Is displayed as an image. The conventional magnetic resonance imaging apparatus is shown in FIG.
As shown in FIG. 1, the static magnetic field generating means (2) for applying a static magnetic field to the subject 1, the gradient magnetic field generating means (3) for applying a gradient magnetic field to the subject 1, and the biological tissue of the subject 1 are configured. A sequencer 4 that repeatedly applies a high-frequency pulse that causes nuclear magnetic resonance to atomic nuclei in a predetermined pulse sequence, and to cause nuclear magnetic resonance to atomic nuclei of a biological tissue of a subject by the high-frequency pulse from the sequencer 4. A transmission system 5 for irradiating a high frequency signal, a reception system 6 for detecting an echo signal emitted by the above-mentioned nuclear magnetic resonance, and a signal processing system for performing an image reconstruction calculation using the echo signal detected by the reception system 6. 7 and the operation unit 21 for inputting the control information of the processing, the echo signal emitted by the nuclear magnetic resonance is repeatedly measured to obtain a tomographic image.

In such a magnetic resonance imaging apparatus, a typical spin echo method pulse sequence for collecting echo signals emitted from a subject is as shown in FIG. That is, in FIG.
As shown in (a) to (d), a high frequency magnetic field and a gradient magnetic field G
s, Gp, and Gf are applied to the subject, and echo signals emitted from the subject are collected as shown in (e). At this time,
The phase encoding direction gradient magnetic field Gp shown in FIG.
The amplitude was changed for each projection for collecting echo signals for one image. Also,
In order to improve the SN ratio (signal-to-noise ratio) of the echo signals to be collected, the same echo signal is collected a plurality of times for one projection, the echo signals are integrated for the number of times of collection, and this collection integration is performed. The echo signal was used as the echo signal of the projection. Then, by subjecting this echo signal to Fourier transform, the subject 1
A tomographic image of

Now, as in the case of FIG. 2, for picking up one image I, the number of projections is P, the projection number is i (i = 1 to P), and the echo signal for each projection number i is taken. Letting A be the cumulative number of times and TR be the repetition time of signal measurement as shown in FIG. 5A, the imaging is completed by repeating this repetition time TR A / P times. Therefore, the imaging time t in this case is t = TR · A · P (1) Here, the cumulative number A is a positive integer (A = 1, 2,
3, ...).

[0005]

However, in such a conventional echo signal collecting method, since the number A of times of echo signal integration is set to a common fixed number for the projection i of the image I shown in FIG. The imaging time t shown in the above formula (1) can be taken as an integral multiple imaging time for each image I, even if the constant integration number A is changed for the appropriate image I. For example, the repetition time TR is 1000 milliseconds and the projection number P is 256.
If the integration number A is 2, then the imaging time t _{2 in} this case is t ₂ = 1000 × 2 × 256 = 512 (seconds) (2) from the equation (1). In addition, if the number of integrations A is set to 1 under the same conditions,
Similarly, the imaging time t _{1 in} this case is t ₁ = 1000 × 1 × 256 = 256 (seconds) (3) from the equation (1). Therefore, t ₂ = 2 · t ₁ .

In such a state, the imaging time t is t
_In order to set between ₂ and t ₁ , the repetition time TR and the projection number P must be changed to appropriate values as long as the integration number A is constant for each image I.
When R is changed, the contrast of the image is changed, and when P is changed, the resolution of the image is deteriorated, and the image quality may not be maintained. Therefore, the imaging time t that can be set while maintaining the image quality without changing TR and P can take only an integral multiple of TR · P, as is clear from the equation (1).
From this, the imaging time t could not be set arbitrarily.

Further, in the conventional method, the number of times A of echo signal integration is set to a common fixed number for each projection i of the image I shown in FIG. 2, so that the high frequency region and the low frequency region are all the same number of times. The SN ratio cannot be improved so much in a high-frequency region where the amount of noise is large without integration, and high-frequency noise remains and the SN ratio as a whole cannot be improved sufficiently. Therefore, a high quality image cannot be obtained.

Therefore, the present invention addresses the above problems and provides an echo signal acquisition method in a magnetic resonance imaging apparatus capable of arbitrarily setting an imaging time and obtaining a high quality image. And

[0009]

In order to achieve the above object, an echo signal acquisition method in a magnetic resonance imaging apparatus according to the present invention comprises a static magnetic field generating means for applying a static magnetic field to a subject and a gradient magnetic field to the subject. A gradient magnetic field generating means, a sequencer for repeatedly applying a high-frequency pulse that causes nuclear magnetic resonance to the atomic nuclei of the above-described biological tissue of the subject in a predetermined pulse sequence, and a high-frequency pulse from this sequencer A transmission system that irradiates a high-frequency signal to cause nuclear magnetic resonance in the atomic nuclei of the biological tissue of the sample, a reception system that detects an echo signal emitted by the above-mentioned nuclear magnetic resonance, and an echo signal detected by this reception system A signal processing system for performing image reconstruction calculation using
In a magnetic resonance imaging apparatus including an operation unit for inputting control information of processing performed by this signal processing system, a gradient magnetic field is applied to a subject for each projection for collecting the echo signal for one image, and The echo signal from the subject is collected once or multiple times, and this echo signal is integrated by the number of times of collection for each projection, and the number of times of collecting the echo signal is different for each projection or for the projection of a certain region. The echo signal thus collected and integrated is used as the echo signal of the projection.

[0010]

The echo signal acquisition method configured as described above is
The gradient magnetic field is applied to the subject for each projection that collects the echo signal for one image, and the echo signal from the subject is collected once or a plurality of times, and the echo signal is collected once for each projection. And the number of times the echo signal is collected is made different for each projection or for a projection in a certain region, and the echo signal thus collected and integrated is used as the echo signal of the projection. As a result, the imaging time can be set arbitrarily and a high quality image can be obtained.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus to which an echo signal acquisition method according to the present invention is applied. This magnetic resonance imaging apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon, and as shown in FIG.
, Gradient magnetic field generation system 3, sequencer 4, and transmission system 5
, Reception system 6, signal processing system 7, and central processing unit (CP
U) 8 and an operating portion 21.

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the body axis direction or in a direction orthogonal to the body axis, and has a certain spread around the subject 1. A magnetic field generating means of a permanent magnet type, a normal conducting type or a superconducting type is arranged in the space. The gradient magnetic field generation system 3 is composed of a gradient magnetic field coil 9 wound in three axial directions of X, Y and Z, and a gradient magnetic field power source 10 for driving each coil, and each of them is instructed by a sequencer 4 described later. By driving the gradient magnetic field power supply 10 of the coils, the gradient magnetic fields Gs, Gp, G in the three axial directions of X, Y, Z
f is applied to the subject 1. The slice plane for the subject 1 can be set by the method of applying the gradient magnetic field.

The sequencer 4 serves as control means for repeatedly applying a high-frequency pulse that causes magnetic resonance to the atomic nuclei of the atoms constituting the biological tissue of the subject 1 in a predetermined pulse sequence, and operates under the control of the CPU 8. Then, various commands necessary for collecting the data of the tomographic image of the subject 1 are sent to the transmission system 5, the gradient magnetic field generation system 3 and the reception system 6. Furthermore, it is possible to execute a pulse sequence for depicting the blood flow in the subject 1.

The transmission system 5 irradiates a high-frequency signal in order to cause magnetic resonance to atomic nuclei of the atoms constituting the living tissue of the subject 1 by the high-frequency pulse sent from the sequencer 4, and a high-frequency oscillator 11 and It is composed of a modulator 12, a high frequency amplifier 13, and a high frequency coil 14a on the transmitting side. The high frequency pulse output from the high frequency oscillator 11 is amplitude-modulated by the modulator 12 according to the instruction of the sequencer 4, and the amplitude-modulated high frequency pulse is generated. High frequency amplifier 1
After being amplified in 3, the electromagnetic waves are radiated to the subject 1 by supplying them to the high-frequency coil 14a arranged close to the subject 1.

The receiving system 6 is an echo signal (NMR signal) emitted by magnetic resonance of atomic nuclei of the living tissue of the subject 1.
Which includes a high-frequency coil 14b on the reception side, an amplifier 15, a quadrature detector 16 and an A / D converter 17, and the response of the subject 1 to the electromagnetic wave emitted from the high-frequency coil 14a on the transmission side. Of the electromagnetic wave (NMR signal) is detected by the high-frequency coil 14b arranged close to the subject 1, and is detected via the amplifier 15 and the quadrature phase detector 16
It is input to the D / D converter 17, converted into a digital amount, and further converted into two series of collected data sampled by the quadrature phase detector 16 at the timing according to the instruction from the sequencer 4, and the signal is sent to the signal processing system 7. It is designed to be used.

The signal processing system 7 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a CRT.
And the like, and the CPU 8 performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction,
The signal intensity distribution of an arbitrary cross section or the distribution obtained by performing an appropriate calculation on a plurality of signals is imaged to display the
It is designed to be displayed as a tomographic image. The operation unit 21 is for inputting control information of processing performed by the signal processing system 7 and includes a keyboard 22.

The echo signal acquisition method of the present invention is as follows.
In the magnetic resonance imaging apparatus having the above configuration, FIG.
, A gradient magnetic field is applied to the subject for each projection i that collects echo signals for one image I, echo signals from the subject are collected once or a plurality of times, and the echo signals are One projection i
For each projection i or for a projection in a certain area, the echo signals collected are integrated for each projection i.
Of the echo signal.

The image capturing time t'by such an echo signal collecting method is shown in FIG. 2, where P is the number of projections when capturing one image I, and the total number of echo signals for each projection number i. Letting each be Ai (A ₁ , A ₂ , A ₃ , ...) And the repetition time of signal measurement be TR, Becomes Here, the cumulative number Ai is a positive integer (Ai = 1,
2, 3, ...). Thereby, the imaging time t'can be set arbitrarily.

A specific example of this will be described with reference to FIG. In this example, the number of projections P for one image I is set to 256, the total number of projections is divided into three regions, and the number of times the projections are integrated in the high frequency region is larger than that in the low frequency region. For the area E ₁ of 1 to 64, the cumulative number of echo signals Ai is set to 2 times, for the area E ₂ of the projection number i = 65 to 192, the cumulative number of times Ai is set to 1, and the projection number i = 193 to 256. Area E
_{For 3} , the integration number Ai is set to 2 and the signal measurement repetition time TR is set to 1000 milliseconds. The imaging time t ′ in this case is calculated from the above equation (4) as Becomes

That is, in the above description of the conventional example,
If the number of integrations A = 2 over the entire projection number P = 256 for one image I, then equation (2)
When the image pickup time t ₂ = 512 (seconds) and the number of integrations A = 1 once over the entire projection number P = 256, the image pickup time t ₂ = 512 (seconds) Since t ₁ = 256 (seconds) and the imaging time between 256 seconds and 512 seconds could not be taken, according to the present invention,
As determined by the equation (5), it is possible to set an arbitrary imaging time t ′ between the two.

Next, a specific procedure of the echo signal collecting method according to the present invention will be described with reference to the flow chart shown in FIG. Here, as shown in FIG. 3, the projection number P is 256 for one image I.
And then, the entire projection is divided into three regions, and 2 times the integration number Ai of the echo signals to the region E ₁ of the projection number i = 1 to 64, integrated for area E ₂ of the projection number i = sixty-five to one hundred ninety-two A case will be described in which the number of times Ai is set to 1, the region E ₃ of the projection number i = 193 to 256 is set to 2 times the number of times of integration A, and the signal measurement repetition time TR is set to 1000 milliseconds. In addition, when the number of times of accumulation Ai is a plurality of times,
The content of the current number of times is represented by a.

First, the operator operates the keyboard 22 of the operation unit 21 shown in FIG. 1 to set the number of projections P = 256, each area E ₁ , as the parameters necessary for this imaging.
The cumulative number Ai of each projection of E ₂ and E ₃ and the signal measurement repetition time TR = 1000 (milliseconds) are set. Then, imaging is started from this state. First, FIG.
In FIG. 4, since the imaging starts from the point where the first echo signal is collected for the first projection i, i = 1 in step A and a = 1 in step B in FIG. Then, in this state, the echo signal of the a-th (a = 1) th time is collected for the i-th (i = 1) projection (step C). Next, in step D, a =
Determine if it is 1. Since the first collection has just started, step D advances to the "YES" side. Then, the echo signal of the first projection of the first projection measured in the above step C is used as the cumulative number of times Ai (=
The value divided by 2) is used as the echo signal of the first projection (step E).

Next, in step F, it is determined whether the preset cumulative number Ai is greater than 1 and the cumulative number of contents a currently being executed is a number other than 1. Since the state is a = 1 now, step F proceeds to the "NO" side and jumps to step H. Then, in this step H, it is judged whether or not the content a of the integrated number currently being executed is smaller than a preset integrated number Ai. I just collected the first echo signal of the first projection,
Since a = 1 and Ai = 2, step H is “YES”.
Go to the side. Then, in step I, the number of a's is increased by 1 to make a = 2, and the process returns to step C.

Next, in this state, the a-th (a = 2) th echo signal for the i-th (i = 1) projection is collected (step C). Next, in step D, it is determined whether or not a = 1. Since the second collection has been entered here, step D proceeds to the “NO” side. Then, in step F, it is determined whether or not the preset integration number Ai is greater than 1 and the currently executed integration number content a is a number other than 1. Since the state is a = 2 at present, step F proceeds to the “YES” side. Then, a value obtained by dividing the echo signal of the first projection captured in step E by the second echo signal of the first projection measured in step C by the number of integration times Ai (= 2) is integrated, The result is used as an echo signal of a new first projection (step G).

Next, at step H, it is judged whether or not the content a of the integrated number currently being executed is smaller than a preset integrated number Ai. Here, the second echo signal of the first projection is just collected, and a = 2
Since Ai = 2, step H advances to the "NO" side and jumps to step J. Then, in step J, it is determined whether or not the current projection number i is smaller than the preset projection number P. At this time, since the echo signals have only been collected twice for the first projection and i = 1 and P = 256, step J is "YE".
Then, the number of i is increased by 1 in step K to set i = 2, and the process returns to step B.

Next, in step B, a
= 1 and in this state, the a-th (a = 1) -th echo signal for the i-th (i = 2) projection is collected (step C). Hereinafter, the above operation is performed in the order of C → D → E → F →
Repeating G->H->I->J->K-> B, the echo signal is measured while advancing the projection number i in FIG. And the projection number i
Reaches 256, in step J, i = 256 and P =
Since the number becomes 256, the step J advances to the “NO” side, and the imaging is ended. The imaging time t ′ at this time is calculated by the above-mentioned equation (4).
The time is calculated by the equation (5) in the specific example shown in FIG.

The number of projections P for one image I, the method of dividing the area for all projections, and the setting of the cumulative number of times Ai for each area are as follows.
Not limited to that shown in FIG. 3, by setting an arbitrary numerical value, it is possible to set an arbitrary imaging time t ′ by the above-mentioned formula (4).

[0028]

Since the present invention is constructed as described above,
The gradient magnetic field is applied to the subject for each projection that collects the echo signal for one image, and the echo signal from the subject is collected once or a plurality of times, and the echo signal is collected once for each projection. However, the number of times the echo signal is collected is made different for each projection or for a projection in a certain area, and the echo signal thus collected and integrated can be used as the echo signal of the projection. The imaging time in this case is given by the above equation (4), and the imaging time can be arbitrarily set by making the number of echo signal collections different for each projection or for a certain area of one image. You can

Further, as described above, the number of echo signal collections is made different for each projection or projection of a certain region for one image, so that the noise amount becomes a different number of collections in the high frequency region and the low frequency region. It is possible to improve the SN ratio particularly in a high-frequency region where there are a lot of noises, reduce the high-frequency noise, and sufficiently improve the SN ratio as a whole. Therefore, a high quality image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus to which an echo signal acquisition method according to the present invention and a conventional example is applied;

FIG. 2 is a diagram for explaining the operation of the echo signal acquisition method according to the present invention;

FIG. 3 is a diagram for explaining a specific operation example of the echo signal collecting method,

FIG. 4 is a flowchart for explaining a specific operation procedure of the echo signal collection method,

FIG. 5 is a timing diagram showing a typical spin echo method pulse sequence for collecting echo signals emitted from a subject in a magnetic resonance imaging apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation magnet, 3 ... Gradient magnetic field generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal processing system, 8 ... CPU, 21 ... Operation part, I ... Image, i ... Projection number, Ai
… Total number of times, E _{1 to} E ₃ … area.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 9219-2J G01N 24/02 Y

Claims (1)

[Claims] 1. A static magnetic field generating means for applying a static magnetic field to a subject, and a gradient magnetic field generating means for applying a gradient magnetic field to the subject,
A sequencer that repeatedly applies a high-frequency pulse that causes nuclear magnetic resonance to atomic nuclei of the biological tissue of the subject in a predetermined pulse sequence, and a nucleus in the atomic nucleus of the biological tissue of the subject by the high-frequency pulse from the sequencer. A transmission system that radiates a high-frequency signal to cause magnetic resonance, a reception system that detects the echo signal emitted by the above-mentioned nuclear magnetic resonance, and an image reconstruction calculation using the echo signal detected by this reception system. In a magnetic resonance imaging apparatus comprising a signal processing system to perform and an operation unit for inputting control information of processing to be performed by the signal processing system, tilting the subject for each projection collecting the echo signal for one image. A magnetic field is applied and echo signals from the subject are collected once or a plurality of times, and the echo signals are collected into a single project. It is characterized in that the number of times of collection of each echo is integrated for each projection, and the number of times of collection of the echo signal is made different for each projection or projection of a certain region, and the echo signal thus collected and integrated is used as the echo signal of the projection. Echo signal acquisition method in magnetic resonance imaging apparatus.