JPH06105825A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To provide a magnetic resonance imaging apparatus applying a multi- echo method which is free from the generation of ringing. CONSTITUTION:The maximum value of signal intensity of echoes is detected to determine a correction value of the signal intensity of the echoes according to a ratio between the maximum value of the signal intensity of the echo containing 0 encode and the maximum value of the signal intensity of the echoes. A reconstruction filter for FFT in the encode direction is multiplied by a correction filter having the correction value as coefficient to correct the reconstruction filter for FFT for the second time thereby correcting the signal intensity of the echoes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-echo method for magnetic resonance imaging in which a plurality of echoes are collected by one excitation and different echo amounts are given to the echoes to reconstruct one image from the plurality of echoes. Regarding the device.

[0002]

2. Description of the Related Art As a conventional example of such an apparatus, a RARE (Rapid Acquisition with a multi-echo method is applied.
There is a Relaxation Enhancement method. An example of the RARE method is MAGNETIC RESONANCE IN MEDICINE 3, 823-833 (1986)
It is described in.

In the RARE method, as shown in FIG. 9, after excitation by a 90 ° pulse once, a plurality of, here, five 18
A 0 ° pulse is sequentially applied to collect 5 echoes, and each echo is placed at a different encoding position on the K space.
This is to reconstruct a single image. The first echo is -12
7 encoding ~ is placed in the first area and the fifth echo is ~ 1
It is arranged in the fifth area of 28 encoding. Here, the signal strengths of the first echo to the fifth echo are not equal due to the influence of T2 attenuation, and as shown in FIG. 10, they are attenuated toward the first echo to the fifth echo. For this reason, a step difference occurs in the signal intensity between the echoes. If the signal strength is extremely different at the boundary between echoes, ringing easily occurs.

[0004]

As described above, the conventional RA
The RE method has a drawback that ringing occurs at the boundary between echoes. The present invention has been made to address the above-mentioned circumstances, and an object thereof is to provide a multi-echo magnetic resonance imaging apparatus in which ringing does not occur.

[0005]

A magnetic resonance imaging apparatus according to the present invention comprises means for detecting the signal strength of an echo, and the echo signal strength according to the ratio of the signal strength of the echo including 0 encoding and the signal strength of the echo. And means for correcting the signal strength.

[0006]

According to the magnetic resonance imaging apparatus of the present invention, the signal intensity of each echo can be made uniform by correcting the signal intensity of each echo on the basis of the signal intensity of the echo including 0 encoding, Ringing is prevented from occurring at the boundary between echoes.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of this embodiment. Gantry 20
A static magnetic field magnet 1, an X-axis / Y-axis / Z-axis gradient magnetic field coil 2 and a transmission / reception coil 3 are provided therein. Transmit / receive coil 3
Instead of being embedded in the gantry, it may be directly attached to the subject. The static magnetic field magnet 1 as the static magnetic field generator is configured by using, for example, a superconducting coil or a normal conducting coil. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is a coil for generating an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz. The transmission / reception coil 3 has a high frequency (R) as a selective excitation pulse for selecting a slice.
F) It is used to generate pulses (90 ° pulse, 180 ° pulse) and detect a magnetic resonance signal (MR signal) generated by magnetic resonance. The subject P on the bed 13 is inserted into an imageable region (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region) in the gantry 20.

The static magnetic field magnet 1 is driven by the static magnetic field controller 4. The transmission / reception coil 3 is driven by the transmitter 5 when magnetic resonance is excited, and is coupled to the receiver 6 when detecting a magnetic resonance signal. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is driven by an X-axis gradient magnetic field power source 7, a Y-axis gradient magnetic field power source 8 and a Z-axis gradient magnetic field power source 9.

The X-axis gradient magnetic field power source 7, the Y-axis gradient magnetic field power source 8, the Z-axis gradient magnetic field power source 9, and the transmitter 5 are driven by a sequencer 10 in a predetermined sequence, and an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, A Z-axis gradient magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence described later. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field Gz are mainly used as the phase encoding gradient magnetic field Ge, the reading gradient magnetic field Gr, and the slice gradient magnetic field Gs, respectively. The computer system 11 drives and controls the sequencer 10, captures the magnetic resonance signal received by the receiver 6 and performs predetermined signal processing to generate a tomographic image of the subject, and displays it on the display unit 12.

Next, the operation of the first embodiment will be described with reference to the flow chart shown in FIG. The operation of the first embodiment is roughly divided into three steps: prescan, main scan, and reconstruction. In the pre-scan step, the maximum signal value of each echo is detected and stored. R in the main scan step
Multiple echoes are collected by the ARE method. In the reconstructing step, a predetermined algorithm, for example, a two-dimensional Fourier transform method (2D
The FT method) is used to reconstruct the tomographic image. Here, assuming that the number of multi-echo is 5, the operation of the above three steps will be described in detail.

In the pre-scan, first in step # 1, FIG.
Encoding gradient magnetic field G using the pulse sequence shown in
Collect 5 echoes with e = 0. Since no encoding gradient magnetic field is applied, signals from the entire encoding direction can be collected without disturbing the phase. In step # 2, the maximum value of the signal for each echo is searched and stored as one piece of raw data information. The detected signal maximum values of the first to fifth echoes are EMAX (1), EMAX (2), and EMAX.
(3), EMAX (4), and EMAX (5).

In the main scan step, data acquisition by the RARE method is performed using the pulse sequence shown in FIG. 4 in step # 3. Each echo data collected in step # 5 is rearranged and stored in the raw data file. As for the rearrangement, the K space from -127 encoding to +128 encoding is equally divided into the first area to the fifth area as in the conventional example shown in FIG. Allocate to the fifth area. In the reconstruction step, the correction value for each echo is first obtained as follows.

In step # 5, the maximum value of the echo signal including 0 encoding is set to BASEMAX. Here, BA
SEMAX = EMAX (3). B in step # 6
ASEMAX is divided by the signal maximum value of each echo, and this is set as the correction value CORET (n) (1 ≦ n ≦ 5). CORET (n) = BASEMAX / EMAX (n) (1)

As a result, the correction value of the echo including the 0 encoding becomes 1.0. That is, the echo including 0 encoding is not corrected. This is because it is desirable not to correct the signal strength as this echo is most important in reconstructing the image.

By multiplying the collected data of each echo by this correction value, the signal intensity can be normalized by compensating for the difference in signal intensity of each echo. However, in this embodiment, only (1)
Not only the correction value is obtained from the equation, but the more optimal correction value can be obtained by further selectively adding the following functions (i) to (iii) in step # 7.

(I) The upper limit value (CORMAX) and the lower limit value (CORMIN) of the correction value are set, and when the correction value of the equation (1) exceeds the upper limit and the lower limit, they are matched with the upper limit and the lower limit. The correction value of the equation (1) may take a value of 1 or more, but the correction value of 1 or more amplifies the signal intensity of the collected data, and the S / N ratio deteriorates. .0
It is recommended to set to. That is, the correction for amplification can be omitted. Further, the lower limit value is set so that the correction value does not become zero. (ii) The presence or absence of correction can be specified for each echo. The parameter for this is COREX (n). COREXE (n) = 0 (no correction) = 1 (correction)

The first echo is an echo containing 0 encoding,
Since the signal strength is always greater than the third echo here,
If the first echo is corrected with the correction value of the equation (1), the signal strength will decrease. The reduction of the signal component of the first echo means that the high frequency component of the image is reduced and the image is blurred. Therefore, it is preferable not to correct the first echo (C
OREXE (1) = 0).

(Iii) Specify the echo range to be corrected,
The maximum and minimum correction values are used to suppress correction values outside the specified range. The parameters for this are RANGE (n1, n
2) (specify the range from the n1th echo to the n2th echo).

FIG. 5 and FIG. 6 show correction values for each echo when various settings (i) to (iii) are made. The horizontal axis corresponds to the encoding amount (echo). FIG. 5A shows (1)
The case where the correction value of the equation is unchanged is shown. Here, the correction value for the fourth and fifth echoes becomes 1 or more, the signal is amplified, and the S / N ratio deteriorates. Therefore, the upper limit value CORMAX is set to 1.
When set to 0, the correction value becomes as shown in FIG. That is, the fourth and fifth echoes are not corrected.
For this reason, a step difference in signal intensity remains between the fourth and fifth echoes, but since the signal intensity of these echoes is originally small,
Even if there is a step, no noticeable ringing occurs.

However, in FIG. 5B, since the signal intensity of the first echo is lowered and the image is blurred, if COREXE (1) = 0 is set so that the first echo is not corrected, the correction value is as shown in FIG. 5 (c). Here, only the second echo is corrected.

Further, in FIG. 5B, in order to limit the correction range from the second echo to the fourth echo, RANGE
When (2, 4) is set, the correction value shown in FIG. 6A is obtained. At this time, the correction value of the first echo is the same as the correction value of the second echo, and the first echo is also corrected. However, since the correction value is larger than that in the case of FIG. The degree of lowering the signal strength is smaller than that in the case of FIG. 5B, the correction of the high frequency component is weakened, and the blur in the encoding direction can be suppressed. Further, in FIG. 6A, if the upper limit value is eliminated (CORMAX = ∞), the correction value is
As shown in (b).

By thus setting the various parameters (i) to (iii), it is possible to determine the correction value suitable for the image to be reconstructed. 5 and 6 are merely examples, and the present invention is not limited to these.

Next, in step # 8, the following signal strength correction filter CORFIL (m) having the correction value determined as described above as a coefficient is created (m indicates an encoding step, -127≤m). ≦ 128). = CORECT (1) (-127 ≤ m ≤ -77) = CORET (2) (-76 ≤ m ≤ -26) COREIL (m) = CORET (3) (-
25 ≤ m ≤ 25) = CORET (4) (26 ≤ m ≤ 76) = CORET (5) (77 ≤ m ≤ 128)

The correction is performed by applying the correction filter to the collected data, but here, it is assumed that the data after the first FFT in the image reconstruction by the 2DFT is applied. That is, in step # 9, the reconstruction filter ENCFIL (m) for the second FFT (encoding direction FFT) is multiplied by the signal strength correction filter CORFIL (m), and the reconstruction filter ENCFIL (m) is corrected and encoded. The signal strength of the data is corrected by performing the FFT in the direction. Usually, the reconstruction filter is for correcting ringing due to truncation of echo signal collection. ENCFIL (m) = ENCFIL (m) × CORFIL (m) However, the correction may be performed before the first FFT.

Such a signal strength correction filter CORF
Reconstruction filter ENCFI corrected by IL (m)
The shape of L (m) is shown in FIGS. FIG. 7 shows FIG. 5 (b).
8 is a case where the correction values shown in FIG. 8 are used, and FIG. 8 is a case where the correction values shown in FIG.

As described above, according to the present invention, 0
The reconstruction filter for the encoding direction FFT is multiplied by the correction filter obtained from the correction value obtained by dividing the maximum value of the signal strength of the echo including the encoding by the maximum value of the signal strength of each echo, and the reconstruction filter is corrected. By doing
It is possible to prevent the occurrence of ringing due to the difference in signal strength between echoes. In addition, when the correction value is obtained, the upper limit of the correction value, the range of the echo to be corrected, or whether or not to perform the correction process for each echo is specified, so that the echo in the first half of the echo including the 0 encode is specified. The correction can be performed only for the ringing correction without causing the blur in the encoding direction and the deterioration of the S / N ratio.

The present invention is not limited to the above-described embodiments, but can be implemented with various modifications. For example, in the above description,
Although the RARE method has been described as the multi-echo method, it is not limited to this. Further, the number of echoes is not limited to 5, and may be any number. Furthermore, although the signal strength of the collected data is corrected by correcting the reconstruction filter, a correction filter may be independently applied to the collected data to correct the collected data separately from the reconstruction.

[0028]

As described above, according to the present invention, a multi-echo magnetic resonance imaging apparatus in which ringing does not occur is provided.

[Brief description of drawings]

FIG. 1 is a first magnetic resonance imaging apparatus according to the present invention.
The block diagram which shows the structure of an Example.

FIG. 2 is a flowchart showing the operation of the first embodiment.

FIG. 3 is a diagram showing a pulse sequence of a previous scan.

FIG. 4 is a pulse sequence of a main scan by the RARE method.

FIG. 5 is a diagram showing various modifications of correction values.

FIG. 6 is a diagram showing various modifications of correction values.

FIG. 7 is a diagram showing an example of a reconstruction filter.

FIG. 8 is a diagram showing another example of a reconstruction filter.

FIG. 9 is a diagram showing rearrangement of echoes in a general RARE method.

FIG. 10 is a diagram showing a step difference in signal intensity of each echo that causes ringing in the RARE method.

[Explanation of symbols]

1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 3 ... Transmitting / receiving coil, 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver, 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display part.

Claims (5)

[Claims] 1. A multi-echo magnetic resonance imaging apparatus for reconstructing one image from a plurality of echoes by collecting a plurality of echoes with one excitation and giving different encoding amounts to the echoes. A magnetic resonance imaging apparatus comprising: a unit for detecting a signal intensity; and a unit for correcting the signal intensity of the echo according to the ratio of the signal intensity of the echo including 0 encoding and the signal intensity of the echo. 2. The correction means corrects the signal strength by multiplying the signal strength of the echo by a correction value obtained by dividing the maximum value of the signal strength of the echo including 0 encoding by the maximum value of the signal strength of the echo. The magnetic resonance imaging apparatus according to claim 1, wherein: 3. The magnetic resonance imaging apparatus according to claim 2, further comprising means for setting an upper limit value and a lower limit value of the correction value. 4. The apparatus according to claim 2, further comprising means for designating whether or not the correction processing is executed for each echo.
The magnetic resonance imaging apparatus according to. 5. The apparatus further comprises means for designating an echo range for executing the correction processing, and means for setting an echo correction value within an adjacent designated range as a correction value for an echo outside the designated range. The magnetic resonance imaging apparatus according to claim 2, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus.