JP2000287949A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a real time property of an image display as well as reducing a load for weight calculation when photographing continuously on an MRI apparatus to be used in combination with plural small-sized receiving coils such as multiple coils. SOLUTION: A signal processing system of an MRI apparatus is provided with a means for detecting changes of absolute positions of plural small-sized receiving coils. A navigation echo for correction of body movement is used for positional changes. When reconstituting an image by synthesizing NMR signals obtained from the plural small-sized receiving coils with weight adding, a weight map used for weight adding is calculated and renewed only at detection of moving between images by the detecting means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures a nuclear magnetic resonance (hereinafter, referred to as "NMR") signal from hydrogen, phosphorus, or the like in a subject, and visualizes a nuclear density distribution, a relaxation time distribution, and the like. The present invention relates to a magnetic resonance imaging (MRI) device.

[0002]

2. Description of the Related Art In an MRI apparatus, a reception probe (receiving coil) receives an NMR signal generated in an object when the object is irradiated with electromagnetic waves from an RF coil, and performs an arithmetic process such as Fourier transform on the signal. To reconstruct the image. As a receiving RF coil, a high-sensitivity coil called a “multiple RF coil” has been widely used in recent years (for example,
Tokiohei 2-500175). In multiple coils, a plurality of small receiving coils (hereinafter simply referred to as small coils) with relatively high sensitivity are arranged, and the signals received by each small coil are combined, so that the field of view can be maintained while maintaining the high sensitivity of the small coils. It is possible to increase the sensitivity and increase the sensitivity. In order to reconstruct an image from signals received by such multiple coils, first, a weight map for signal synthesis is created based on the reception sensitivity of each small coil constituting the multiple coil, and each small coil is formed based on the weight map. Are weighted and combined.

[0003]

However, in the case of continuous photographing using multiple coils, it is necessary to create a weighting map for each image in consideration of the change in sensitivity distribution due to body motion or the like between consecutive images. . This will be described with reference to FIG. 9A and 9B show four small coils 5021 to 5021.
This shows a case where the abdomen is continuously imaged using a multiple coil consisting of 24 coils. When synthesizing the signals acquired by the small coils 5021 to 5024, the sensitivity distribution of each small coil
From 6011 to 6014, a weight map Wi (x, y) (x,
y each represent the coordinates of the real space), weight the signal after the Fourier transform based on this weight map, and then combine to obtain an image. When the subject moves due to respiratory motion or the like during continuous imaging as shown in FIG. 3B, the sensitivity distributions 6011 and 6012 of the small coils 5021 and 5022 shift as indicated by arrows, and their overlaps also change. Therefore, the weight map obtained from the sensitivity distribution shown in FIG.

As described above, continuous photographing using multiple coils requires a process of creating a weighting map for each image, requiring a high-performance arithmetic unit and parallel processing. In addition, since the calculation time is long, there is a problem that the shortening of the image acquisition time interval is limited. Therefore, the present invention
An object of the present invention is to provide an MRI apparatus capable of reducing an image acquisition time in continuous imaging in an MRI apparatus using a multiple coil and capable of displaying a real-time image.

[0005]

According to the present invention, there is provided an MRI apparatus for achieving the above object, comprising: a receiving probe including a plurality of small receiving coils; and a weighting unit for each of the nuclear magnetic resonance signals received by the plurality of small receiving coils. In a magnetic resonance imaging apparatus comprising a signal processing unit that obtains image data by combining, the signal processing unit calculates a weight map used for signal weighting based on the nuclear magnetic resonance signal,
Means for detecting a change in absolute position of the plurality of small receiving coils based on the nuclear magnetic resonance signal, wherein the calculating means calculates a weight map when the detecting means detects the change in the absolute position. And updating.

In the present invention, the small receiving coil constituting the receiving probe is not only a structurally combined coil such as a phased array coil, but also fixed to a catheter or the like, and subjected to signal processing in combination with another small receiving coil. Also includes a small receiving coil. The weight map is updated only when it is detected that the absolute positions of a plurality of coils have changed due to body motion or the movement of the small receiving coil itself, so that the amount of image processing calculation can be greatly reduced, and the continuous image Real-time performance can be improved.

Means for detecting the absolute position of the coil is as follows:
The motion is detected from a change in the measured signal, and a navigation echo used for body motion correction can be used. In general, the navigation echo is used as a means for compensating for any body movement during signal acquisition for one image, and the MRI apparatus of the present invention uses this navigation echo as an absolute value of the coil. The weight map can be updated only when necessary while performing body motion correction of the image using the navigation echo by using the position change detection means.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments shown in the drawings. FIG. 1 shows an MRI to which the present invention is applied.
FIG. 1 is a diagram showing the overall configuration of the apparatus.
Magnet that generates a static magnetic field in a spacious space around
And a gradient coil 3 for generating a gradient magnetic field in this space
And an RF coil 4 for generating a high-frequency magnetic field in this space, and an RF probe (reception probe) 5 for detecting an NMR signal generated by the subject 1. The control / signal processing system includes a gradient magnetic field power supply 9, an RF transmission unit 10, a signal detection unit 6, a signal processing unit 7, a control unit 11, and a display unit 8. The subject 1 is transported to the measurement space inside the static magnetic field magnet while lying on the bed.

The gradient magnetic field coil 3 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 a gradient magnetic field power supply 9. RF coil 4 is RF
A high-frequency magnetic field is generated according to a signal from the transmission unit 10. The RF probe 5 is a multiple coil 501 including four small coils 502 and a preamplifier 503 connected to each of them, as shown in FIG. Detected by the converter and quadrature detector,
The signal is processed by the signal processor 7 and converted into an image signal by calculation.

The image is displayed on the display unit 8. The gradient magnetic field power supply 9, the RF transmission unit 10, and the signal detection unit 6 are controlled by the control unit 11 according to a time chart generally called a pulse sequence. The signal processing in the signal processing unit 7 includes calculation of sensitivity distribution based on signals from each small coil, creation of a weight map based on the sensitivity distribution, weighting and synthesis of signals from each small coil, presence / absence of body motion for signals from each small coil. And updating the weight map based on the detection result of the presence or absence of the body motion.

An embodiment of a sequence of signal processing in the continuous imaging using the MRI apparatus having the above configuration, particularly in a signal processing system, will be described below with reference to FIGS. FIG. 3 shows the signal processing system 7.
First, a pulse sequence including acquisition of a navigation echo is executed to photograph a predetermined region of the subject, and a set 106 of echo signals necessary for reconstructing one image is acquired. (Step 107). The pulse sequence used in continuous shooting is not particularly limited,
A sequence that includes the measurement of at least one navigation echo to detect body movement for each echo measurement is preferred. FIG. 4 shows an example of such a pulse sequence. The pulse sequence shown in FIG. 4 is a pulse sequence based on a general spin echo method.
Navigation echo 303 without phase encoding other than 3
And step 304 of generating and obtaining After applying the slice selection RF pulse 401 and the inversion RF pulse 402, the navigation echo 303 is generated using the inversion readout gradient magnetic field 301 without applying phase encoding, and measured at the sampling time 302. Thereafter, a phase echo gradient magnetic field 407 and a read gradient magnetic field 408 are applied to obtain a spin echo (main measurement echo) 403. Step 410 from the RF pulse 401 to the main measurement echo 403 is repeated, and as shown in FIG. 5, a set of echo signals necessary for reconstructing one image.
Get 1051. The number of repetitions is usually 128, 256, etc.
The set of echo signals 1051 has the same number of navigation echoes 102 (1021, 1022, 1023 ...) as the number of repetitions, and the main measurement echo
106 (1061, 1062, 1063 ...).

The main measurement echo 106 is corrected for body movement using the navigation echo 102 measured in the same repetition (FIG. 1, processes 103 and 104). That is, first, one of the navigation echoes included in the set of echo signals 1051 (for example, the first acquired navigation echo 1021)
Is the reference navigation echo 1011 for body motion correction. In response to this reference navigation echo 1011, the navigation echo 102 (1021, 1022, 1023
) Are detected (process 103), and echoes 106 (1061, 106) measured within the same repetition based on the change amount 105 are detected.
2,1063...) (Process 104), and the corrected signal S′1 (n,
t). Note that t represents a point in the reading direction, for example, 0 ≦
It is an integer satisfying t ≦ 256. n is a repetition number,
For example, it is an integer satisfying 0 ≦ n ≦ 256. If n is the same,
Indicates that the signal is obtained within the same repetition.

The body motion detection using the navigation echo (step 103) can be performed using, for example, a phase change of a signal of the navigation echo or a position change of data obtained by one-dimensional Fourier transform of the navigation echo. The body motion correction (process 104) can be performed by a method of removing a phase change of the main measurement echo signal by a complex difference, and the like.
The measurement may be performed in a measured space (measurement space) or a space in which signals are one-dimensionally Fourier transformed (hybrid space). Furthermore, motion detection and body motion correction may be performed based on the navigation echo acquired during the pre-scan.

In the continuous photographing, a set of echo signals 1052, 1053... As shown in FIG. 5 are sequentially obtained by repeating the pulse sequence shown in FIG. Signal (S'i (n, t)) (i
Represents a coil number. In this embodiment, 1 ≦ i ≦ 4.
Hereinafter the same). A set of such echo signals 108
Is obtained for each of the four small coils, weighted and combined to reconstruct one image. For this purpose, a weight map 115 used for weighting is calculated using a set of signals obtained from four small coils. The processing of the weight map calculation unit 111 mainly includes a processing 112 for applying a low-pass filter to the echo signal 108, a processing 113 for performing a Fourier transform on the signal after the low-pass filter is applied, and a processing 114 for performing a weight calculation based on the data after the Fourier transform. That is, a weight map Wi (x, y) in which the weight applied to the image data is obtained for each coordinate of the small coil is created. The weight map 115 used at the start of measurement may be a map calculated based on the coil sensitivity distribution measured in advance.

If there is no movement between the acquisition of the echo signal for one image and the acquisition of the echo signal for the next image (between images), the weight map 115 is also used for synthesizing the next image. The weight map used for the previous image synthesis is used as it is, but when motion is detected between the images, the weight map is updated using the newly acquired signal 108. For detecting the motion between the images, a navigation echo acquired to correct the motion between the echoes is used. That is, one of the navigation echoes included in each set of echo signals 1051, 1052, 1053,... Shown in FIG.
101 (1011, 1012, 1013...) Are used to detect the movement of the subject or the like generated between the images.

In order to detect the movement between the images, one of the navigation echoes 1011, 1012, 1013... Of each signal set, for example, the navigation echo 1011 included in the first echo signal set 1051 is displayed on the image. The reference navigation echo 109 is used to detect the movement between the two. This reference navigation echo 109 and navigation echo 101
Detect signal changes such as 1, 1012, 1013, ... (Process 11
0). The process 110 for detecting the signal change is basically the same as the above-described body motion detection process 103, but since no subsequent correction process is involved in this case, it can be determined only from the phase change of the signal. Then, for example, if there is a predetermined change in the signal of the reference navigation echo 109 and the navigation echo 1012, the position of the subject has changed between the acquisition of the set of echo signals 1051 and the acquisition of the set of echo signals 1052. Judgment is made, weight map calculation (process 111) is performed, and the weight map 115 is updated to a newly calculated one.

As described above, the movement between the images is monitored while the continuous photographing is being performed.
Is updated, and if no motion is detected, the previous weight map is used as it is. FIG. 6 shows processing of signal synthesis and image reconstruction using the weight map 115. In the figure, reference numeral 11
61 to 1164 represent the processing of FIG. 3 for the four small coils, and by these processings 1161 to 1164, the signals 1171 to 1174 obtained by the small coils and the body motion corrected, respectively, and the weight map 1181 to 1184 is output. signal
1171-1174 are subjected to Fourier transform (processing 8011-801
4) The signals are combined (process 802). Thus, an image 803 is obtained. In addition, the signal synthesis 104 is based on “MEDICAL IMAGING
TECHNOLOGY, Vol. 15, Mp.6, November 199
7)) may be applied.

As described above, in continuous photographing, the movement between images is detected by using the navigation echo used for body movement correction, and the weight map is updated only when the movement occurs. Can be greatly reduced, and the interval between continuous image formation can be shortened.

In the above embodiment, the case where a multiple coil composed of four small coils is used has been described. However, the number of small coils is not limited to the above embodiment. Also, the present invention reconstructs an image by synthesizing signals from a plurality of small coils.
The present invention can be applied to any MRI apparatus. For example, a part of a small coil may be physically independent of other small coils. Although the case where the absolute position of the coil changes due to the body movement of the subject has been described, the present invention can be applied to a case where the absolute position changes due to the movement of the coil itself.

FIG. 7 shows a second embodiment of the present invention in which four small coils 5021 to 5024 and a small coil 5025 mounted at the tip of a catheter 701 are used. In this case, even if the subject 602 does not move, the position of the coil in the image changes as the catheter 701 moves inside the subject 602. That is, the catheter moves from the position shown in FIG. 7 (a) to FIG. 7 (b), and with the movement of this catheter, the coil 5025 attached to the tip moves, and the sensitivity distribution 702 also changes.

Also in this embodiment, the point that the weight map is calculated from the sensitivity distributions 6011 to 6014 and 702 of each of the small coils 5021 to 5025 and the signals are synthesized to obtain an image is the same as the above-described embodiment. It is detected that there is a movement from the change of the measured navigation echo (FIG. 3, processing 11).
0), weight map calculation is performed (process 111). However, in this case, since the small coils 5021 to 5024 do not move, the weight map based on the sensitivity distributions 6011 to 6014 is used as it is, and only the small coil 5025 is updated to a new weight map. In the embodiment shown in FIG. 7, when the movement of the catheter is continuous, the weight map is always updated for the small coils attached to the catheter, and only the small coils 5021 to 5024 shown in FIG. Steps 110 and 111 may be performed. That is,
The process of FIG. 3 can be performed on a part of the plurality of small coils.

Although the present invention has been described based on the embodiments shown in the drawings, the present invention is not limited to these embodiments, and various modifications can be made within the scope described in the claims of the present invention. . For example, in addition to the spin echo sequence shown in FIG. 4, any sequence such as a sequence based on a fast spin echo method or an echo planar (EPI) method can be adopted as the imaging sequence. As for the navigation echo, not only the navigation echo in the readout direction but also a slice echo direction in 3D imaging, a navigation echo in phase encoding direction in 2D imaging, or an orbital navigation echo may be used. The navigation echo may be generated for two or more axes.

Further, in the embodiment shown in FIG. 3, a navigation echo is generated at each repetition, and the body movement of the main measurement echo is corrected at each repetition. Although the case of using for detection has been described, the present invention can be applied even when such a body motion correction process is not performed. That is, as shown in FIG. 8, a pulse sequence for generating at least one navigation 101 is executed for each set of echo signals constituting one image, and the movement between the images is determined from the signal change of the navigation echo with respect to the reference navigation echo. Weight map when motion is detected
Do 115 calculations. The embodiment of FIG. 8 employs a sequence that can measure a set of echo signals required for one image in one repetition, such as an EPI sequence, and is particularly suitable when there is no need to correct body motion between echoes. is there.

[0024]

According to the present invention, a weight map for signal synthesis is calculated only when the absolute position of a small receiving coil changes in continuous imaging of an MRI apparatus using a plurality of small receiving coils. Therefore, it is possible to reduce the load on the apparatus in signal processing and to reduce the amount of calculation, thereby improving the real-time performance of image display.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an outline of an MRI apparatus to which the present invention is applied.

FIG. 2 is a diagram showing a reception probe and a signal processing system of the MRI apparatus of FIG. 1;

FIG. 3 is a diagram showing one embodiment of signal processing in the MRI apparatus of the present invention.

FIG. 4 is a diagram showing an example of a pulse sequence for imaging.

FIG. 5 is a view for explaining signal processing in the MRI apparatus of the present invention.

FIG. 6 is a diagram illustrating signal processing in the MRI apparatus of the present invention.

FIG. 7 is a diagram showing another embodiment of the multiple coil to which the present invention is applied.

FIG. 8 is a diagram showing another embodiment of signal processing in the MRI apparatus of the present invention.

FIG. 9 is a diagram illustrating continuous imaging using multiple coils.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Subject 2 Static magnetic field magnet 3 Gradient magnetic field coil 4 Coil 5 Probe 6 Signal detection part 7 Signal processing part 8 Display 11 Control part

Claims (1)

[Claims] 1. A magnetic resonance imaging system comprising: a receiving probe including a plurality of small receiving coils; and a signal processing unit for weighting and combining each of the nuclear magnetic resonance signals received by the plurality of small receiving coils to obtain image data. In the apparatus, the signal processing unit calculates a weight map used for weighting a signal based on the nuclear magnetic resonance signal, and a change in an absolute position of the plurality of small receiving coils based on the nuclear magnetic resonance signal. A magnetic resonance imaging apparatus, wherein the calculating means calculates and updates a weight map when the detecting means detects a change in the absolute position.