JP2000157507A - Nuclear magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable photographing with precision even concerning soft tissue by permitting an image re-constituting means to be provided with means for correcting the movement of a testee at every small RF reception coil through the use of a multiple coil as the reception coil, synthesizing signals which are corrected by means of the respective correcting means and obtaining the image where a body movement is corrected. SOLUTION: Each small RF coil 602 is connected to a signal detecting part 406 having an A/D converter and an orthogonal detection circuit 604 at every coil with each preamplifier 603. The signal detected by each small RF coil 602 is converted into two groups of digital signals by the signal detecting part 406 and transmitted to a signal processing part 407. The signal processing part 407 is provided with the means for executing a signal processing such as a phase correction or the like in the digital signal, an image re-constituting means(Fourier transform means) 605 and the means 606 for synthesizing image signals so that the signals are synthesized by re-constituting the images at every coil 602. The signal processing part 407 is provided with the body movement correcting means using a navigation echo as a signal processing means and the synthesized image is displayed in a display part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic field for measuring a nuclear magnetic resonance (hereinafter, referred to as NMR) signal from hydrogen, phosphorus, or the like in an object to visualize a nuclear density distribution, a relaxation time distribution, and the like. The present invention relates to a resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), and more particularly, to an MRI apparatus with addition of a navigation echo for correcting a body motion.

[0002]

2. Description of the Related Art In MRI, when measuring an NMR signal generated from nuclear spins constituting a tissue of a subject by an NMR phenomenon, a gradient magnetic field is applied to give positional information to the signal. As the gradient magnetic field, the slice encode direction,
Gradient magnetic fields in three axial directions of the phase encode direction and the frequency encode direction are used, and a signal encoded by these gradient magnetic fields can be reconstructed as a two-dimensional or three-dimensional image by performing a Fourier transform.

[0003] It is known that large artifacts occur in images when the subject moves during such MRI imaging. This is called a body movement artifact. The reason for the occurrence of this body motion artifact is that a predetermined phase encoding amount at a predetermined measurement point should be given, but the encoding amount is applied to another measurement point by motion, and the motion of the subject The phase of each acquired echo changes before and after, and this is caused by performing the Fourier transform in the phase encoding direction (or the slice encoding direction) including the changed echo.

As a technique for removing the body motion artifact, an imaging method using a navigation echo has been proposed (“High-speed interleaved echo planar imaging using a navigator: high-resolution anatomical and functional images at 4 Tesla”). RESONANCE IN MEDI
CINE, 35: 895-902, June 1996, Seong-Gi Kim et a
l). In this method, when the imaging sequence is repeated, for example, an echo signal with a phase encoding amount of 0 is generated separately from the original measurement echo (NMR signal) at each repetition, and this is used as a navigation echo. From the change in the phase of the echo, the change in the phase of the signal due to the body motion generated during the repetition is estimated and corrected.

[0005]

However, the imaging method using the navigation echo may not be able to cope with a local body movement in the imaging field of view. For example, FIG.
When a multiple coil 1000 including a plurality of receiving coils is used as shown in FIG. 0 (a), it is effective when the body motion 1021 is uniform among the sensitivity distributions of the respective receiving coils. If there are different movements 1022 to 1024 depending on the tissue or structure 1012 of the subject 1011 as shown in b), it cannot be dealt with. Such local movement is likely to occur in soft tissue such as the abdominal region. In the abdomen, the movement 1025 may differ between the dorsal side and the abdominal side of the subject due to the influence of respiration and the like as shown in FIG.

Accordingly, the present invention provides an MRI apparatus having a function of additionally generating and acquiring a navigation echo, which is capable of correcting body movement even when there is local body movement. It is an object of the present invention to provide an MRI apparatus capable of performing accurate imaging of such a soft tissue.

[0007]

In order to achieve the above object, an MRI apparatus according to the present invention comprises: means for applying a high-frequency magnetic field and a gradient magnetic field to a space where a subject is placed according to a predetermined pulse sequence; In an MRI apparatus including a receiving coil for receiving an NMR signal and a means for processing the NMR signal received by the receiving coil and reconstructing an image, a multiple coil including a plurality of small RF receiving coils is used as the receiving coil. In addition, the image reconstruction means is a small RF
A means for correcting the body movement of the subject is provided for each receiving coil,
The signals corrected by the respective correction means are combined to obtain an image in which the body motion is corrected.

In the MRI apparatus according to the present invention, the image reconstructing means includes a body movement correcting means for each of the small RF receiving coils constituting the multiple coils, and each of the sets of echo signals acquired by each of the small RF receiving coils is provided for each of the small RF receiving coils. After the appropriate body motion is corrected, signal synthesis is performed to obtain an image, so that the local motion of the subject can be corrected with high accuracy.

As the body movement correction, a body movement correction using a navigation echo can be adopted. In this case, the body movement correction means receives the navigation echo additionally generated in the small RF reception in the repetition of the pulse sequence. It consists of a means for acquiring each coil and a means for correcting the body movement of the subject with a set of the navigation echo and the echo signal acquired by the same small RF receiving coil.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an MRI apparatus according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a typical MRI to which the present invention is applied.
FIG. 2 is a diagram illustrating a configuration of an apparatus. This MRI apparatus is used for the subject 401
A magnet 402 that generates a uniform static magnetic field in the space where
A gradient magnetic field coil 403 for generating a gradient magnetic field in this space;
An irradiation coil 404 that generates a radio frequency (RF) magnetic field in this space
And a receiving coil that detects the NMR signal generated by the subject 401
405. The subject is carried into the space while lying on the bed 412.

The gradient magnetic field coil 403 is constituted by gradient magnetic field coils in three directions of X, Y and Z, and generates a gradient magnetic field in accordance with a signal from the gradient magnetic field power supply 409. Irradiation coil 40
4 generates a high-frequency magnetic field according to the signal of the RF transmission unit 410.

The receiving coil 405 is a small-sized R having a relatively high sensitivity.
A receive-only RF coil that arranges multiple F coils and combines the signals received by each small RF coil to expand the field of view and increase the sensitivity while maintaining the high sensitivity of the small RF coil. Known multiple coils, such as a combination of the same type of small RF coils or a combination of different shapes and types, can be employed. Further, depending on the direction of the static magnetic field, it is used for a vertical magnetic field or a horizontal magnetic field.

A signal received by each of the small RF coils of the multiple coil is detected by a signal detection unit 406, processed by a signal processing unit (image reconstructing unit) 407, and converted into a combined image signal.

FIG. 2 shows the details of the signal detection unit 406 and the signal processing unit 407 of the multiple coil 601 composed of four small RF coils 602. Each small RF coil 602 is connected via a preamplifier 603 to a signal detection unit 406 having an A / D converter and a quadrature detection circuit 604 for each coil. A signal detected by each small RF coil is converted into a two-series digital signal by a signal detection unit 406 and transmitted to a signal processing unit 407. The signal processing unit 407 includes means for performing signal processing such as phase correction on these digital signals, image reconstruction means (Fourier transform means) 605, and means 606 for synthesizing image signals. And constructs a signal. The signal processing unit 407 includes a body motion correcting unit using a navigation echo described later as a signal processing unit. The synthesized image is displayed on the display unit 408.

The control unit 11 controls these gradient magnetic field power supplies 409, RF
The transmission unit 410 and the signal detection unit 406 are controlled in accordance with an imaging sequence called a pulse sequence. In the present invention, a control for additionally generating and acquiring a navigation echo when acquiring an NMR signal required for image reconstruction is added to the pulse sequence.

FIG. 3 shows an example of a pulse sequence including generation of a navigation echo. This pulse sequence is a sequence based on the spin echo method, in which an RF magnetic field 203 for exciting nuclear spins constituting the tissue of the subject and an RF magnetic field 204 for reversing the excited magnetization, a gradient magnetic field 205 for selecting slices, After the application at the same time as 206, a gradient magnetic field 207 for imparting phase encoding to the signal is applied, and a gradient magnetic field 208 for inverting the polarity is applied.
The R signal is measured as a spin echo signal 210. The illustrated sequence 211 is repeated at a repetition time 202TR while changing the intensity of the phase encoding gradient magnetic field, and the number of echo signals 210 required for image reconstruction is obtained.

A step 304 for generating and acquiring a navigation echo is inserted between the reversal RF magnetic field 204 and the phase encoding gradient magnetic field 207. In this step 304, a reading gradient magnetic field 301 whose polarity is determined is applied and the sampling time is increased. At 302, a navigation echo 303 is measured. This navigation echo is a signal having no phase encoding and no phase encoding amount. In the illustrated example, one navigation echo is acquired for each repetition time TR. However, in the case of a sequence with a short repetition time, one navigation echo may be acquired for every several repetitions. For example, a navigation echo for a plurality of axes may be acquired within the time TR.

Next, a procedure for correcting an echo signal 210 (hereinafter referred to as a main measurement echo) measured for an image using such a navigation echo in the MRI apparatus of the present invention will be described with reference to FIG. .

The main measurement echo measured by the repetition of the pulse sequence is amplified for each small RF receiving coil as shown in a block 101, and is transmitted to the signal processing unit 407 after A / D conversion and quadrature detection. . On the other hand, for the navigation echo measured at the same time as the main measurement echo,
As shown in a block 102, the signal is amplified for each small RF receiving coil, and after A / D conversion and quadrature detection, is transmitted to the signal processing unit 407. The signal processing unit 407 performs correction using a combination of a main measurement echo and a navigation echo measured for each small RF receiving coil by the correction unit 103 provided for each small RF receiving coil.

In the body motion correction using the navigation echo, one of the navigation echoes acquired for each repetition time TR, for example, the first navigation echo is used as a reference, and the navigation echo measured within a certain repetition time and the reference navigation echo are used. Based on the phase shift, the main measurement echo measured within the repetition time is corrected.

The processing method by the correction means 103 includes correction in a hybrid space and correction in a real space.
The embodiment is shown in FIGS. In each figure, the navigation echo 701 and the main measurement echo signal 702 are a pair measured by the same small RF coil.

In the embodiment shown in FIG. 5, the acquired navigation echo 701 and the main measurement echo signal 702 are each subjected to one-dimensional Fourier transform 703 in the axial direction in which the navigation echo was acquired. For example, in the pulse sequence shown in FIG. 3, a navigation echo is generated by reversing the gradient magnetic field 301 in the reading direction, and the axis direction in which the navigation echo is obtained is the reading direction.

The phase difference between the signal obtained by subjecting the navigation echo to one-dimensional Fourier transform and the signal obtained by subjecting the reference navigation echo to one-dimensional Fourier change is obtained, and the navigation correction 704 of the main measurement echo is performed in the hybrid space based on the phase difference. This is body movement correction in the axial direction from which the navigation echo was acquired. Next, a corrected image signal 706 is obtained by performing one-dimensional Fourier transform on an axis different from that of the first one-dimensional Fourier transform 703.

In the embodiment shown in FIG. 6, the phase difference between the measured navigation echo 701 and the reference navigation echo is obtained, and based on this phase difference, the main measurement echo 702 is subjected to navigation correction 704 in k-space. After that, the main measurement echo 702 is subjected to a two-dimensional Fourier transform (707) to obtain a corrected image signal 706.

In the third embodiment shown in FIG. 7, the navigation echo 701 is subjected to a one-dimensional Fourier transform (703) in the axial direction from which the navigation echo was acquired, and similarly, the position change from the reference navigation after the one-dimensional Fourier transform is performed. , The amount of phase change is calculated from the correlation. The navigation correction 704 is performed on the main measurement echo 702 using the calculated phase change amount. Thereafter, a two-dimensional Fourier transform (707) of the main measurement echo is performed to obtain a corrected image signal 706.

The navigation correction 704 of these embodiments
In order to convert the phase difference of the navigation echo into the phase difference of the main measurement echo, there is a method of removing a phase change of a signal by a complex difference, for example.

The correction processing by the signal processing means 407 is as follows.
Any of the above processes may be used, but the process shown in FIG. 6 is advantageous in that the Fourier transform process of the navigation echo and the process of obtaining the phase correlation from the positional relationship are unnecessary, and that the body motion can be corrected in pixel units. It is.

After performing such correction processing 103 for each small RF coil, the image signal 706 obtained for each coil is obtained.
Are combined to reconstruct one image. In the synthesis, the signal may simply be added, but “multiple coils for wide field and high sensitivity of head and neck MRI” (MEDICAL IMAGING TEC
HNOLOGY, Vol.15, No.6, November 1997), it is preferable that the image signal from each small RF coil is weighted by the sensitivity distribution of the small RF coil. The weighting of the sensitivity distribution is expressed by the following equation.

[0030]

(Equation 1) (Where, en (i, j) is the image signal of the nth small coil, W
n (i, j) represents the sensitivity distribution of the nth small coil, and S (i, j) represents the synthesized image signal. By performing such processing, a high S / N image can be obtained even with a multiple coil combining small coils having different sensitivity distributions. The image obtained in this way during shooting is shown in FIG. Even if there are local movements of the subject 1022 to 1024, the images are obtained by synthesizing the signals after correcting these movements with the respective RF coils 6021 to 6024, so that the image is free from the influence of the movements. . Of course, even when there is an overall motion 1021 as shown in FIG. 10A, an image in which the motion is corrected can be obtained. Further, in the abdomen photographed image as shown in FIG. 10 (c), even if the movement is different between the back side and the abdomen side of the subject, the influence of the movement can be eliminated in the image.

In the above embodiment, the signal processing unit 407 performs the navigation correction based on one of the navigation echoes measured at each repetition of the pulse sequence. In the case of accompanying imaging, it is possible to acquire a navigation echo by this pre-scan and perform navigation correction based on the navigation echo. Such an embodiment is shown in FIG.

The prescan is a measurement of an echo signal which is executed prior to the main measurement in order to correct a change generated in the main measurement echo due to disturbance or the like. In this embodiment, the navigation echo 7011 and the navigation echo 7011 are used during the prescan 111. An echo signal 7021 is acquired by each small RF coil 602. Next, the pulse sequence of the main measurement is executed, and in this embodiment,
The navigation echo 7012 and the main measurement echo 7022 are acquired by the small RF coils 602, respectively.

The signal processing unit is provided for each small RF coil, for the navigation echo 7011 measured by the same small RF coil.
The navigation correction 113 of the main measurement echo 7022 is performed using the In this case, the phase difference of the navigation echo 7012 is determined based on the navigation echo 7011 acquired in the prescan 111, and the corresponding main measurement echo 7022 is determined.
Is corrected. This correction method may be any of the embodiments shown in FIGS. Next, the navigation-corrected signal is corrected 114 using the echo signal 7011 acquired in the pre-scan 111. This correction 114
For example, phase correction and amplitude correction can be performed in the measurement space or in the hybrid space.

As described above, the signal correction 114 is performed for each small RF coil.
Is performed, and if the correction is correction in the measurement space or the hybrid space, the necessary signal 706 is obtained after the necessary Fourier transform.
Get. These are combined as described above to obtain an image. Also in this case, after performing navigation correction with the signal acquired by each small RF coil, signal synthesis is performed, so that even if the tissue to be photographed moves locally, the effect of the movement can be eliminated in the image. .

The present invention can take various forms other than the contents disclosed in the above embodiments. For example, in the above embodiment, the SE sequence is described as the pulse sequence, but the present invention may be applied to an FSE sequence or an EPI sequence. Also, the case where the navigation echo in the readout direction is applied as a function to additionally generate and acquire the navigation echo in the present invention has been described. However, the orbital navigation echo, the slice encoding direction in three-dimensional imaging, and the phase in two-dimensional imaging It is also possible to perform the correction of the present invention using the navigation echo in the encoding direction. Further, a navigation echo may be applied to two or more axes. Further, navigation correction in a measurement space or a hybrid space may be used in combination.

[0036]

According to the present invention, a plurality of small-sized receiving coils are used.
By employing a multiple coil composed of RF coils and providing the signal processing unit with means for performing navigation correction for each small RF coil, local movement of the subject can be accurately corrected.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a signal detection unit and a signal processing unit of a multiple coil.

FIG. 3 is a diagram showing a pulse sequence to which generation of a navigation echo is added.

FIG. 4 is a view showing processing in a signal processing unit of the MRI apparatus of the present invention.

FIG. 5 is a diagram showing an embodiment of navigation correction in a signal processing unit.

FIG. 6 is a diagram showing another embodiment of the navigation correction in the signal processing unit.

FIG. 7 is a diagram showing another embodiment of the navigation correction in the signal processing unit.

FIG. 8 is a diagram illustrating the effect of the MRI apparatus of the present invention.

FIG. 9 is a diagram showing another embodiment of the navigation correction in the signal processing unit.

FIG. 10 is a diagram illustrating a problem in navigation correction.

[Explanation of symbols]

 402: Static magnetic field magnet 403: Gradient magnetic field coil 404: Irradiation coil 405: Receiving coil 406: Signal detector 407 ····· Signal processing unit (image reconstruction means) 408 ····· Display unit 602 ······· Small RF receiving coil

Claims (1)

[Claims] 1. A means for applying a high-frequency magnetic field and a gradient magnetic field to a space in which a subject is placed according to a predetermined pulse sequence, a receiving coil for receiving the nuclear magnetic resonance signal, and a signal received by the receiving coil. In the nuclear magnetic resonance imaging apparatus having a means for processing a nuclear magnetic resonance signal and reconstructing an image, while using a multiple coil composed of a plurality of small RF receiving coils as the receiving coil, the image reconstructing means is A nuclear magnetic resonance imaging apparatus comprising, for each small RF receiving coil, means for correcting the body movement of the subject, and synthesizing the signals corrected by the respective correcting means to obtain an image in which the body movement is corrected. .