JP3427059B2 - Magnetic resonance diagnostic equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance diagnostic apparatus which obtains anatomical information and biochemical information of a subject by utilizing a magnetic resonance phenomenon.

[0002]

2. Description of the Related Art In recent years, with the development of magnetic resonance diagnostic equipment, non-invasive observation and observation of minute metabolites in the living body are possible.
MRSI (Magnetic Resonance) that can be imaged
Spectroscopic Imaging) is in practical use.

In recent years, the imaging of proton compounds using MRSI has been actively carried out. For example, in some types of tumor tissue, the concentration of NAA (N-acetylalbaric acid) is decreased as compared with that in normal tissue,
Since the concentration of Choline tends to be higher than that in normal tissues, imaging these distributions makes it possible to understand the site of metabolic abnormality in the body before it exhibits morphological changes.

In order to calculate image information reflecting the concentrations of various metabolites contained in the living body, it is necessary to calculate the area of each chemical shift spectrum (real part of spectrum). However, in the collected spectrum, the phase rotation based on the system, the cause that depends on the sequence parameter, or various causes other than those is observed,
This phase must be corrected when calculating the area.

Generally, the phase is approximately 1 with respect to frequency.
Since it can be approximated by the following functions, conventionally, a method of calculating the phase characteristic at the peak of the observed spectrum using the least square method and correcting the phase has been widely adopted.

However, since the concentrations of these compounds in the living body are generally low, the signal-to-noise ratio (S / N ratio) of the spectrum is not always good. Therefore, since a large error is included in the peak detection and the phase calculation in the phase correction processing, the spectrum after the phase correction does not draw an accurate absorption curve (real part spectrum) or dispersion curve (imaginary part spectrum), In some cases, only mixed spectra of parts and imaginary parts can be obtained. In such a case, it becomes difficult to obtain an accurate metabolic distribution image, and finally it is necessary to artificially perform phase correction on each of the many voxels acquired in MRSI. For example,
In the case of 3D-proton MRSI, often 32 ×
Since 32 matrix images are used, there is a drawback that it takes a lot of time to process the spectrum data when the phase correction is artificially performed.

In the case of MRI images, a method has been reported in which the magnitude of phase rotation is measured in advance and the phase is corrected using this data. That is, this is a method in which the phase distribution is set such that the imaginary part of each pixel of the image obtained for the phantom becomes 0, and the phase of the image to be measured thereafter is corrected. However, when such a method is applied to MRSI, the phase characteristics largely change depending on the set position of the surface coil when the surface coil is used, for example, and thus it is difficult to apply it in practice.

In addition, Journal of Magnetic Resonance 69, 151-155 (1986), or Magne
tic Resonance in Medeicine 14, 26-30 (19
As described in 90), a method of correcting the phase of each voxel by utilizing the fact that the phase of the spectrum can be corrected from the phase characteristic calculated in the time domain is conceivable. In this method, in principle, the primary phase characteristic Has a drawback that it cannot be corrected in each voxel and that a phase calculation error is large due to the influence of the signal-to-noise ratio for obtaining the phase characteristic in the time domain.

Further, in order to observe the proton metabolites, it is necessary to suppress a water signal having a magnitude of about 10 ^{4 with} respect to the concentration of the metabolites. At this time, it is difficult to completely remove the water signal. In this case, the water signal remains and the phenomenon in which the water spectrum and the metabolite spectrum overlap with each other is observed as shown in FIG. In such a case, there is a problem that the phase at the peak 31 of the metabolite spectrum is distorted by the residual water signal, so that the phase cannot be accurately corrected.

[0010]

As described above, in the conventional magnetic resonance diagnostic apparatus, when collecting distribution images of metabolites using MRSA, the distribution images reflecting the concentration of each metabolite are accurately measured. In order to obtain the above, it is necessary to correct the phase of the spectrum, and conventionally, since the phase is artificially corrected for each voxel, there is a problem that much labor is required.

The present invention has been made to solve such conventional problems, and an object of the present invention is to provide a magnetic resonance diagnostic apparatus capable of easily performing phase correction.

In order to achieve the above object , the present invention applies a high frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field in accordance with a predetermined pulse sequence to collect information on spectra of metabolites in the subject. Then, in the magnetic resonance diagnostic apparatus for obtaining the distribution image of the metabolite after correcting the phase shift of the spectrum, a plurality of water spectra are generated at different resonance frequencies from each voxel divided when constructing the distribution image of the metabolite. Means for observing times, means for detecting the maximum value of the spectrum at each resonance frequency for each voxel, means for obtaining a phase characteristic for each resonance frequency from the phase value of the spectrum at the maximum value, and based on the phase characteristic And a means for correcting the phase of the spectrum forming the distribution image of the metabolite.

[0013]

In the present invention configured as described above, when correcting the phase shift of the spectrum, first the water spectrum is observed, and the phase of the chemical shift spectrum of the metabolite is corrected based on this observation result. There is. Therefore, it is possible to perform accurate phase correction without requiring a lot of manual labor.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a magnetic resonance diagnostic apparatus according to the present invention.

In the figure, a static magnetic field magnet 1 and a gradient coil 2 and a shim coil 4 provided inside the static magnetic field magnet 1 make a uniform static magnetic field on an object (not shown) and x, y, z which are orthogonal to each other in the same direction. A gradient magnetic field having a linear gradient magnetic field distribution in the direction is applied. The gradient coil 2 is driven by a gradient coil power supply 5, and the shim coil 4 is driven by a shim coil power supply 6. The probe 3 provided inside the gradient coil 2 applies a high-frequency magnetic field to the subject by being supplied with the high-frequency signal from the transmitter 7, and receives a magnetic resonance signal from the subject. The probe 3 may be used for both transmission and reception, or may be separately provided for transmission and reception. The magnetic resonance signal received by the probe 3 is detected by the receiving unit 8 and then transferred to the data collecting unit 9, where it is A / D converted and then the computer system 10
The data is processed.

The above gradient coil power supply 5, shim coil power supply 6, receiving unit 8 and data collecting unit 9 are all controlled by the sequence control unit 12, and the sequence control unit 1 is also provided.
2 is controlled by the computer system 10. The computer system 10 is controlled by a command from the console 11. Then, the magnetic resonance signal input from the data collection unit 9 to the computer system 10 is subjected to Fourier transform or the like, and image data of the density distribution of desired nuclei in the subject is reconstructed based on the Fourier transform. Then, this image data is sent to the image display 13 and displayed as an image.

In this embodiment, a method for automatically performing phase processing correction in MRSI will be shown.

FIG. 2 is a flow chart showing the operation procedure of this embodiment. First, the reference data for performing the phase correction of the target MRSI data is imaged (step ST1). The measurement of this phase data is performed using the pulse sequence shown in FIG. 3 excluding the water signal suppression process, that is, the pulse sequence shown in FIG. 4, from the example of the pulse sequence for observing proton metabolites. Be seen. Here, a localized MRSI pulse sequence for imaging a spatial two-dimensional metabolite distribution is shown. At this time, the repetition time of the pulse sequence is set shorter than when observing the proton metabolites, and the measurement time can be shortened. If possible, the observation time can be shortened by roughening the frequency resolution.

Next, the maximum value of the spectrum (absolute value) due to the water signal of each voxel is detected (step ST
2). At this time, it is preferable not to perform subsequent processing on voxels whose spectrum size is smaller than a preset threshold value.

Thereafter, the phase φ 1 (x, y, z) at the peak position of the spectrum of each voxel is calculated and stored (step ST3). Here, the phase in the vicinity of the peak of the spectrum usually changes abruptly as shown in FIGS. 5A and 5B, and therefore a large error may be included when the phase is calculated. Therefore, when the line width of the spectrum is narrow, such error can be reduced by performing the line loading process of the spectrum by multiplying the exponential function in the time domain as is well known.

Then, the frequency of the transmitter / receiver is changed by Δf, the processes from step ST1 to step ST3 are executed, and the phase φ 2 (x, y, z) at the peak position of the spectrum of each voxel at this time is obtained. , Remember this.

Then, the obtained phase φ 1 (x, y,
Based on z) and φ 2 (x, y, z), the phase characteristic θ (x, y, z, f) in each voxel is calculated and stored (step ST5). Here, the phase characteristic is usually 0
The phase characteristics up to the first and the first order are calculated, but if necessary, higher order characteristics can be obtained by changing the frequency Δf at several points.

Assuming that the frequency of the transceiver at the time of measuring the phase φ1 is f0, the phase characteristic θ (x, y, z,
f) and the phases φ1 (x, y, z), φ2 (x, y,
z) is the phase correction coefficient a (x, y,
z) and b (x, y, z), the following (1) to (3)
It is shown by the formula.

Θ (x, y, z, f) = a (x, y, z) f + b (x, y, z) (1) φ1 = a (x, y, z) · f0 + b (x, y, z) (2) φ2 = a (x, y, z) · (f0 + Δf) + b (x, y, z) (3) Therefore, the first-order phase correction coefficient a (x, y, z) ), And 0
The next phase correction coefficient b (x, y, z) is the following (4),
It is shown by the equation (5).

A (x, y, z) = (φ2-φ1) / Δf (4) b (x, y, z) = φ1- (φ2-φ1) · f0 / Δf (5) MRSI data is collected by the pulse sequence for metabolite observation shown in 3 (step ST6).
The spectrum of each voxel observed at this time Sm (x,
y, z) has the above-mentioned phase characteristic θ (x, y, z, f), so that the spectral shape S in which the real part and the imaginary part are separated
Spectral distortion is exhibited for (x, y, z).

Therefore, the spectrum Sm (x, y, z) is expressed by the following equation (6).

Sm (x, y, z) = S (x, y, z) .exp (-j.theta. (X, y, z, f)) (6) Then, the equations (4) and (5) above are obtained. If the phase term of the equation (6) is removed by using the phase characteristic shown in, the spectrum S (x, y, z) in which the real part and the imaginary part are separated from the observed spectrum Sm (x, y, z) is obtained. ) Can be obtained (step ST7).

Thereafter, the area of the obtained real part spectrum is calculated, and in some cases, the peak height of the spectrum is calculated (step ST8), and the metabolite distribution image is displayed based on this (step ST9). .

In this way, a phase-corrected metabolite distribution image can be obtained. In this way, in this embodiment, the phase characteristics are calculated in advance by using the water spectrum, and the phase rotation of the spectrum of the proton metabolite is corrected using this, so that no manual labor is required. The phase can be easily corrected. As a result, a physically meaningful image reflecting the concentration distribution of metabolites can be obtained in a short time.

In the present embodiment, when there is a second-order or higher-order phase characteristic depending on the system or the like, it is possible to measure the higher-order phase characteristic in advance and correct it as described above. However, if the filter characteristic that is the cause of the high-order phase characteristic is existing as shown in FIG. 6, such characteristic is stored and the phase characteristic obtained by the above method is superposed to perform the correction. You can also

If the procedure shown in this embodiment is used,
Due to the encoding time for adding the position information as shown in FIG. 7, it is possible to correct the first-order distortion of the phase generated in the spectrum acquired by the pulse sequence having a so-called dead time.

[0032]

As described above, according to the present invention, the MR
When obtaining spectral information using SI, the water spectrum is used to obtain the phase characteristics in advance, and the phase characteristics of the metabolites are corrected using the phase characteristics. Therefore, it is possible to easily perform the phase correction without requiring manual labor, and as a result, it is possible to obtain an accurate image that reflects the concentration distribution of the metabolite in a short time.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a magnetic resonance diagnostic apparatus according to the present invention.

FIG. 2 is a flowchart showing the operation of one embodiment of the present invention.

FIG. 3 is a pulse sequence diagram for observing proton metabolites.

FIG. 4 is a pulse sequence diagram for observing a proton metabolite without water suppression processing.

FIG. 5 is a phase characteristic diagram of a spectrum.

FIG. 6 is an explanatory diagram showing an example of filter characteristics.

FIG. 7 is a pulse sequence diagram of MRSI having a dead time.

FIG. 8 is an explanatory diagram showing overlapping of spectra.

[Explanation of symbols]

1 Static magnetic field magnet 2 gradient coil 3 probes 4 shim coils 7 Transmitter 8 Receiver 9 Data processing unit 10 Computer system 11 consoles

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-3-264049 (JP, A) JP-A-64-34345 (JP, A) JP-A-3-60640 (JP, A) JP-A-5- 115453 (JP, A) Patent 3197590 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) A61B 5/055 JISST file (JOIS)

Claims (2)

(57) [Claims] 1. A high-frequency magnetic field and a gradient magnetic field are applied to a subject placed in a static magnetic field according to a predetermined pulse sequence,
In a magnetic resonance diagnostic apparatus that collects information about the spectrum of a metabolite in a subject, corrects the phase shift of the spectrum, and obtains a distribution image of the metabolite, it is divided when the distribution image of the metabolite is constructed. Means for observing the water spectrum from each voxel at different resonance frequencies multiple times , and detecting the maximum value of the spectrum at each resonance frequency for each voxel, and from the phase value of the spectrum at the maximum value. A magnetic resonance diagnostic apparatus comprising: a means for obtaining a phase characteristic for each resonance frequency; and a means for correcting the phase of a spectrum forming the metabolite distribution image based on the phase characteristic . 2. A system of the magnetic resonance diagnostic apparatus or
Describes the phase characteristics of the spectrum depending on the pulse sequence.
A memory means for storing the metabolites based on the phase information.
It is possible to correct the phase of the spectra that make up the quality distribution image.
The magnetic resonance diagnostic apparatus according to claim 1, wherein: