JPH0723932A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To eliminate small DC offset components under effective precision of measurement signal data by setting a computation range separately for each odd number/even number projection in a phase encoding direction in computing and eliminating DC offset values in measured number signal data. CONSTITUTION:In an MRI device, a static magnetic field and an inclined magnetic field are given to a sample 1, and high frequency waves are radiated to the sample 1, where measured magnetic resonance signal data are Fourier- transformed in a signal processing system 16 for recomposition of an image. In the signal processing system 16, the magnetic resonance signal data are Fourier-transformed, and DC offset values are eliminated from zero-frequency components after Fourier-transformation. In this case, in calculating and eliminating the DC offset values in the magnetic resonance data and frequency spectrum after the Fourier-transformation, a computation range is set for each odd number/even number projection in a phase encoding direction, thereby the DC offset values are computed and eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention aims to improve image quality by removing components from measured nuclear magnetic resonance (hereinafter referred to as "NMR") signals and the frequency spectrum after Fourier transform of the NMR signal data. And a magnetic resonance imaging apparatus.

[0002]

2. Description of the Related Art As a method for removing a component from a measured NMR signal in a magnetic resonance imaging apparatus and a frequency spectrum after Fourier transform of the NMR signal data, a method described in Japanese Patent Laid-Open No. 1-256944 has been disclosed in the past. There is something. In this conventional method, a component having an effective precision or less, which cannot be completely removed from the measured NMR signal and which is determined by the conversion precision of the A / D converter, has a zero frequency position in the frequency spectrum after the Fourier transform of the NMR signal. In this method, the spectrum that is the value corresponding to the DC offset is calculated and removed.

[0003]

However, the above-mentioned conventional method has the following problems. That is, in general, the magnetic resonance imaging apparatus is composed of various devices, and a modulator that changes the amplitude of the gradient magnetic field in the phase encode direction is supposed to have a characteristic of having a DC offset component in the NMR signal. In some cases, this DC offset amount is mixed in the NMR signal in the state where the sign is inverted in the phase encode direction depending on the measurement sequence of the magnetic resonance imaging apparatus. in this case,
In the conventional method, accurate DC offset value (especially, measurement NMR
A DC offset component below the effective accuracy of the signal data) cannot be calculated and removed, and when an image is constructed using this NMR signal, it appears as bright spot artifacts (bright spot artifacts) on the image. There was a problem.

An object of the present invention is to accurately determine the DC offset component in the NMR signal and in the frequency spectrum after the Fourier transform, without being restricted by the characteristics of the DC offset component which changes in the phase encoding direction depending on the measurement sequence. It is an object of the present invention to provide a magnetic resonance imaging apparatus that can be removed and that can remove bright spot artifacts of an image formed by using an NMR signal from which this DC offset component is removed.

[0005]

Means for Solving the Problems The above-mentioned object is to provide a magnetic field applying means for applying a static magnetic field and a gradient magnetic field to an object, a high frequency irradiating means for irradiating the object with a high frequency, and an object for applying the high frequency to the object. NMR to measure the NMR signal of
The image reconstructing means comprises: a signal measuring means; and an image reconstructing means for Fourier-transforming the NMR signal data measured by the NMR signal measuring means to reconstruct an image.
A magnetic resonance imaging apparatus comprising: a Fourier transform unit for performing a Fourier transform on the NMR signal data; and a DC offset value removing unit for removing a DC offset value from the zero frequency component after the Fourier transform by the Fourier transform unit. When calculating and removing the DC offset value in the NMR signal data and in the frequency spectrum after the Fourier transform, the calculation area set to calculate the DC offset value is an odd number in the phase encoding direction.
This is achieved by configuring the DC offset value removing means to calculate and remove the DC offset value separately set for each even projection.

[0006]

The DC offset value removing means, when calculating and removing the DC offset value in the NMR signal data and in the frequency spectrum after the Fourier transform of the NMR signal data, sets a calculation area set to calculate the DC offset value. It is set separately for odd and even projections in the phase encoding direction, and the DC offset value is calculated and removed. This allows
The DC offset component can be removed without being restricted by the characteristics of the DC offset component that changes in the phase encoding direction depending on the measurement sequence, as well as the component below the effective accuracy of the measurement data of the NMR signal. The bright spot artifact can be removed by constructing an image using the NMR signal.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of the overall configuration of a magnetic resonance imaging apparatus according to the present invention. This magnetic resonance imaging apparatus obtains a tomographic image of the subject 1 by using the NMR phenomenon, and includes a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as CPU) 11, and a sequencer 12.
, Transmission system 13, magnetic field gradient generation system 14, and reception system 15
And a signal processing system 16.

The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 1 in the body axis direction or in the direction orthogonal to the body axis. A magnetic field generating means of permanent magnet type, normal conducting type or superconducting type is arranged in the space provided. The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmission system 13, the magnetic field gradient generation system 14, and the reception system 15.

The transmission system 13 includes a high frequency oscillator 17, a modulator 18, a high frequency amplifier 19, and a high frequency coil 2 on the transmission side.
0a, and the high frequency pulse output from the high frequency oscillator 17 is modulated by the modulator 1 according to the instruction of the sequencer 12.
8, the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19 and then supplied to the high-frequency coil 20a arranged close to the subject 1, whereby the subject 1 is irradiated with the high-frequency pulse. It is supposed to be done.

The magnetic field gradient generation system 14 is composed of a gradient magnetic field coil 21 wound in three axial directions of X, Y and Z, and a gradient magnetic field power source 22 for driving each coil, and an instruction from the sequencer 12 is given. By driving the gradient magnetic field power sources 22 of the respective coils in accordance with the above, gradient magnetic fields GX, GY, GZ in the triaxial directions of X, Y, Z are 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 receiving system 15 comprises a high frequency coil 20b on the receiving side, an amplifier 23, a quadrature phase detector 24 and an A / D converter 25. The high-frequency coil 20b is arranged close to the subject 1, and the high-frequency coil 20 on the transmission side is provided.
An NMR signal of the response of the subject 1 due to the high frequency pulse emitted from a is detected. The detected NMR signal is sent to the A / D converter 2 via the amplifier 23 and the quadrature detector 24.
The quadrature phase detector 2 is input to the signal 5 and converted into a digital quantity, and further at a timing according to an instruction from the sequencer 12.
Two series of collected data are sampled by 4, and the signals thereof are sent to the signal processing system 16.

The signal processing system 16 includes a CPU 11 and a recording device such as a magnetic disk 26 and a magnetic tape 27.
The display 28 such as a CRT and the above CPU
The Fourier transform is performed at 11, and 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 (image reconstruction) and displayed on the display 28. Further, the signal processing system 16 according to the present invention is a DC offset value removing means for removing a DC offset value from the zero frequency component after the Fourier transform, in particular,
When calculating and removing the DC offset value in the NMR signal data and its frequency spectrum after Fourier transformation, the calculation area set to calculate the DC offset value is divided into odd and even projections in the phase encoding direction. It is also used as a DC offset value removing means for setting and setting, calculating and removing the DC offset value.

In FIG. 1, the high frequency coils 20a and 20b and the gradient magnetic field coil 21 on the transmitting side and the receiving side are
Static magnetic field generating magnet 10 arranged in the space around the subject 1.
It is located in the magnetic field space.

Here, X, sliced in the z direction,
Imaging in the Y plane will be described. FIG. 2 shows a pulse sequence in spin echo measurement using the two-dimensional Fourier transform method. First, a high frequency pulse R is applied while applying a gradient magnetic field GZ having a linear gradient in the Z direction.
The slice plane is selected by irradiating F and changing the spin direction only on the target slice plane. After that, a gradient magnetic field GY having a linear gradient in the Y direction is applied in order to have position information in the Y direction, and X in order to further have position information in the X direction.
NM while applying a gradient magnetic field GX with a linear gradient in the direction
The R signal is detected and the signal data is stored in the buffer memory.

That is, in the image reconstruction method based on the two-dimensional Fourier transform method, when the measurement is performed in which the Y direction gradient magnetic field is changed stepwise, for example, 256 times of acquisition of the NMR signal is carried out, 512 times are acquired at the time of one acquisition of the signal. When the data sampled at the points are obtained and the data read out 256 times are arranged in the vertical direction, the measured signal data becomes 512 × 256 two-dimensional data.

At this time, a gradient magnetic field having a linear gradient in the X direction is applied to have position information in the X direction at the time of reading a signal, which is called frequency encoding. Also, applying a gradient magnetic field by changing the amplitude stepwise in order to have position information in the Y direction is called phase encoding. That is, in image reconstruction,
NMR encoded in the frequency and phase directions
An image is obtained by performing two-dimensional Fourier transform on the signal data.

FIG. 3 shows a flow of image reconstruction in which the NMR signal is obtained by a two-dimensional Fourier transform method. It is mainly composed of a fast Fourier transform (FFT) processing 101 in the frequency encoding direction and an FFT 103 in the phase encoding direction.
Before and after the process 101, the DC offset removing processes 100 and 102 according to the present invention are included.

That is, in the image reconstruction, the FFT processing 101 in the frequency encoding direction is first performed. FIG. 4 shows the flow of FFT data in the frequency encoding direction. The NMR signal 110 is N-point measurement signal data of a complex number in which one row in the horizontal direction is read at once, and N pieces of measurement signal data are arranged in the vertical direction while changing the amount of phase encoding. Therefore, the FFT in the frequency encode direction is the FFT of the horizontal direction (column direction) data, and the FFT in the phase encode direction is the FFT of the vertical direction (row direction) data. The unit of measurement signal data in the phase encoding direction is called a projection.

First, F1 is applied to the columns a11 to a1N in the column direction.
Transfer to the FT memory 111. At this time, due to the nature of the Fourier transform, it is necessary to bring the data whose time t is zero to the head of the FFT memory 111. However, the NMR signal 110
The time origin at is the center of the echo signal, and if M = N / 2, it is the (M + 1) th point aiM + 1 (i = 1 to N) of the NMR signal data in the column direction. Further, the FFT calculation assumes a periodic function, and after all, the NMR signal data from a11 to a1N from the center may be crossed and transferred. That is, a11 to a1N of the NMR signal 110 may be transferred to the latter half of the FFT memory 111, and a1M + 1 to a1N may be transferred to the first half. Then, the FFT processing 112 is performed to perform the post-FFT complex data 11 from a'11 to a'1M.
3 is obtained. One-dimensional FF for complex data after the FFT
The data is transferred to the memory 114 after T. At this time, as in the previous case, the one-dimensional FFT is performed by crossing the NMR signal data from the center so that the zero frequency (component) becomes the (M + 1) th position in the center.
It is transferred to the later memory 114. That is, N after FFT
Memory 114 after one-dimensional FFT of the latter half of MR signal data
In the first half a'11 to a'1M, the first half of the NMR signal data after the FFT is converted into the second half a of the memory 114 after the one-dimensional FFT.
Transfer each from '1M + 1 to a'1N.

Next, with data from a21 to a2N, F similarly.
FT is performed to obtain NMR signal data from a'21 to a'2N, and this operation is repeated N times from aN1 to aNN's FFT, and a'ij (i = i = i = i = i) after (N × N) one-dimensional FFT. 1
.About.N, j = 1 to N).

Now, consider a case where the NMR signal contains a DC offset component. FIG. 5 shows a profile of the NMR signal 120 of the real part and the NMR signal data 130 after the FFT in the frequency encoding direction. In addition, the profiles 121, 122, and 123 show how the DC offset component exists in the NMR signal.

The DC offset component of the NMR signal as described above has a zero frequency, that is, (M
It is collected in the (+1) th data and added to the original zero frequency component to appear. The DC offset component of this NMR signal has a constant value or a variable value in the phase encoding direction depending on the measurement sequence, and as a result, the appearance of the DC offset component collected on the zero frequency of the data after the FFT in the frequency encoding direction. Different people will be different.
Also in the imaginary part data of the complex data, the DC offset component on the NMR signal appears after addition to the original zero frequency component after FFT as in the real part data. When the FFT-processed data including the DC offset component is subjected to FFT in the vertical direction (phase encoding direction) as it is, an image 141 having a bright spot artifact 143 at the image center is obtained. The brightness of the bright spot artifact 143 varies depending on the DC offset component.

Therefore, in the present invention, as shown in FIG. 3, before and after performing the FFT processing 101 in the frequency encoding direction,
The DC offset removal processing 100 and 102 for each odd / even projection is performed. This process 100,
The purpose of 102 is to remove the DC offset component for each odd / even projection on the NMR signal and the frequency space. N for both odd and even projections
For the profile on the MR signal, profile 1
21, 122 and 123 are profiles 124 and
The profiles 131, 132, and 133 may be profiles 134, 135, and 136 in the frequency space 125 and 126, respectively.

If the value of the DC offset component can be calculated, the value may be subtracted from the NMR signal data. Therefore, it is necessary to first obtain the DC offset amount. Here, as an example of obtaining the DC offset amount, a method of obtaining the average value of the data at the zero frequency in the data at the upper and lower ends where the phase encoding amount is large in the frequency space and using it as the DC offset value will be described.

FIG. 6 shows profile data after the FFT in the frequency encoding direction. As shown in this profile, it can be seen that the signal components are concentrated near the center where the phase encode amount is small, and there are almost no signal components near the upper and lower ends where the phase encode amount is large. Therefore, even at the zero frequency point, the data at the upper and lower ends are almost DC offset components. Therefore, A, B
By sampling several points for each odd / even projection in the vicinity of the points and averaging them, the DC offset value can be accurately obtained regardless of the measurement sequence.

FIG. 7 shows a flowchart of the DC offset removal processing 100, 102 in the frequency space. First, in the odd projection (160) of the real part data after the FFT in the frequency encoding direction, the sum of the data is obtained from several points at the upper and lower ends at the zero frequency (Process 1).
50). Next, the total sum is divided by the number of added data to obtain an average value, which is set as the value of the DC offset component (process 151). Furthermore, the DC offset value obtained from all the zero frequency data of the odd-numbered projections in the phase encoding direction is subtracted and stored in the original location (Process 1).
52). Next, steps 150 to 152 are similarly performed for the even projection (161) of the real part data after the FFT in the frequency encoding direction. Further, with respect to the imaginary part data after the FFT in the frequency encoding direction, the processes 150 to 152 are similarly performed for each odd-numbered and even-numbered projection. Thus, NMR signal data without a DC offset component is obtained.

Similarly, in the DC offset removal processing for the NMR signal data before FFT in the frequency encode direction, the sum of the data is similarly obtained from an arbitrary area in the phase encode direction for each odd / even projection.
63), the sum is divided by the number of the added data to obtain an average value, which is used as a DC offset value, which is subtracted from the NMR signal data.

In this way, the DC offset removal processing 160
The image 142 of FIG. 5 is obtained by performing FFT in the phase encoding direction on the data after performing .about.163 and taking the absolute value of the data of the real and imaginary parts. As shown in FIG. 3, the bright spot artifact 143 appearing in the image 141 by performing this process before and after the FFT process 101 in the frequency encoding direction does not appear in the image 142.

Further, in order to improve the resolution of the image, for example, when the estimation processing in the frequency encoding direction of the NMR signal is performed, the profiles 131 and 13 in FIG.
Frequency-encode direction FFT shown in the center of 2, 133
The DC offset component that appears by adding to the zero frequency component on the subsequent data may appear not only on the zero frequency data but also in the vicinity thereof. In such a case, the DC offset removal is performed after the FFT in the frequency encoding direction of the present process by an arbitrary number of points in the frequency encoding direction with the zero frequency as the center, thereby removing the profiles 131, 132, and 13.
3 can be profiles 134, 135 and 136, respectively.

As described above, according to the present invention, it is possible to remove a small DC offset component below the effective accuracy of the measured NMR signal data without being restricted by the measurement sequence, and to remove an artifact that becomes a bright spot on the image. effective.

[0031]

As described above, according to the present invention, when calculating and removing the DC offset value in the measured NMR signal data and in the frequency spectrum after its Fourier transform,
The calculation area set to calculate the DC offset value is set separately for odd and even projections in the phase encoding direction, and the DC offset value is calculated and removed. It is possible to remove a small DC offset component that is less than the effective accuracy of the signal data and remove bright spot artifacts in the image.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of the overall configuration of a device of the present invention.

FIG. 2 is a processing flow chart of DC offset removal of an NMR signal and data after FFT in the frequency encoding direction in the device of the present invention.

FIG. 3 is a pulse sequence diagram based on a spin echo method.

FIG. 4 is an explanatory diagram of FFT in a frequency encoding direction.

FIG. 5 is an explanatory diagram of an FFT when a DC offset component is mixed.

FIG. 6 is an explanatory diagram of calculating a DC offset value in data after FFT in the frequency encoding direction.

FIG. 7 is a flowchart for calculating a DC offset value on an NMR signal and data after FFT in the frequency encoding direction in the device of the present invention.

[Explanation of symbols]

1 subject 10 static magnetic field generating magnet 11 central processing unit (CPU) 12 sequencer 13 transmission system 14 magnetic field gradient generation system 15 reception system 16 signal processing system (image reconstruction means (Fourier transform means, DC offset value removal means)) 17 High-frequency oscillator 18 Modulator 19 High-frequency amplifier 20a Transmission-side high-frequency coil 20b Reception-side high-frequency coil 21 Gradient magnetic field coil 22 Gradient magnetic field power supply 23 Amplifier 24 Quadrature phase detector 25 A / D converter 26 Magnetic disk 27 Magnetic tape 28 Display

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location 9287-5L G06F 15/62 390 C

Claims (1)

[Claims] 1. A magnetic field applying means for applying a static magnetic field and a gradient magnetic field to a subject, a high-frequency irradiating means for irradiating the subject with a high frequency, and a nuclear magnetic resonance signal from the subject due to the irradiation of the high frequency. A nuclear magnetic resonance signal measuring means, and an image reconstructing means for performing an image reconstruction by Fourier transforming the nuclear magnetic resonance signal data measured by the nuclear magnetic resonance signal measuring means, and the image reconstructing means, Fourier transform means for Fourier transforming the nuclear magnetic resonance signal data,
In a magnetic resonance imaging apparatus comprising a direct current offset value removing means for removing a direct current offset value from a zero frequency component after the Fourier transform by the Fourier transform means, the offset value removing means is provided in the nuclear magnetic resonance signal data. When calculating and removing the DC offset value in the frequency spectrum after the Fourier transform and the Fourier transform, the calculation area to be set for calculating the DC offset value is set separately for odd and even projections in the phase encoding direction, A magnetic resonance imaging apparatus, which is a means for calculating and removing a DC offset value.