JPH03143426A - Apparatus for picking up mr image - Google Patents

Abstract

PURPOSE:To remove an influence of a subject, s body motion by comparatively simple constitutions and with no elongation of echo time so as to reduce motion artifacts by detecting a stretch-contraction of the subject due to body motion and by installing a device for making a gradient of inclined magnetic field varied in that stretch-contraction direction according to the detected stretch- contraction values. CONSTITUTION:A body surface position detector 33 detects a body surface of subject 11 by measuring a time from an emission of e.g. ultrasonic wave to its return. lt is set so as to detect the body surface position of subject's belly and detects that the subject's belly moves up and down accompanied by respiration. That is to say, it detects a movement in vertical direction of the belly of subject 11 lying in a bed 12 on the back. The detected information of body surface position is sent to a measurement control device 34, a stretch- contraction value in vertical direction of the belly of subject 11 lying on the back is found out and a gradient of inclined magnetic field in X-direction is made to vary according to that stretch-contraction value.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an MR imaging device that obtains in-vivo image information using nuclear magnetic resonance (NMR).

[Conventional technology]

An MR imaging device usually forms an image by capturing electromagnetic waves generated from hydrogen atoms by a nuclear magnetic resonance phenomenon. The electromagnetic waves generated by one excitation are integral information from the region of interest, and the position cannot be specified as is, so position information is encoded into frequency or phase using a gradient magnetic field. That is, since there is a relationship ω=γH between the magnetic field strength H and the frequency ω (γ is the gyromagnetic ratio), position information is obtained using this relationship. Specifically, for example, when the matrix of the image to be reconstructed is 256x256, two gradient magnetic fields whose magnetic field strengths are gradient in orthogonal directions are used, one as a gradient magnetic field for frequency encoding and the other as a gradient for phase encoding. The magnetic field was excited 256 times while changing the gradient of the phase encoding gradient magnetic field in 256 ways, and the NMR signals received each time were sampled at 256 points to obtain 256 x 256 data. Reconstruct the image by performing a dimensional Fourier transform. However, if the subject moves while excitation and reception are performed multiple times in this way, the same part moves to a different location within the gradient magnetic field, resulting in a different magnetic field strength and giving different position information. This will adversely affect the reconstructed image. In this way, motion artifacts occur due to body movements of the subject mainly based on respiratory movements. Therefore, in order to reduce this motion artifact, the following measures have been considered. First, we measure periodic body movements based on the subject's breathing movements,
A breathing gate method is known in which data is collected only around the same phase of the periodic motion. Additionally, an abdominal band is wrapped around the abdomen of the subject to prevent body movement due to breathing. In addition, methods for adding waveforms to correct movement to the gradient magnetic field waveforms used in the spin echo method and field echo method, or changing the order of phase encoding, which affects image quality when there is large movement from the reference position. There are also known methods for collecting data on high-frequency components with little frequency.

[Problem to be solved by the invention]

However, all conventional methods have problems. That is, in the respiratory gate method, it takes a very long time to complete the entire data collection. The method of using an abdominal band compresses the abdomen of the subject and places a heavy burden on the subject, so it may not be usable depending on the subject. Furthermore, with the method of adding a correction waveform to the gradient magnetic field waveform, the two'coe time increases and T1-weighted imaging cannot be performed.In addition, the method of changing the order of phase encoding simply makes the effect of body movement more noticeable. This invention does not attempt to eliminate the effect of body motion itself.In this way, in the conventional technology, instead of reducing motion artifacts, the problem of wrinkling occurs somewhere.This invention is based on the data An MR imaging device that does not increase the acquisition time, does not place any burden on the patient, does not extend the echo time, and can eliminate the influence of body movement itself and reduce motion artifacts with a relatively simple configuration. The purpose is to provide.

[Means for Solving the Problem] In order to achieve the above object, an MR according to the present invention. The imaging device includes a means for detecting expansion and contraction of the subject due to body movement;
It is characterized in that it is provided with means for changing the gradient of the gradient magnetic field in the direction of expansion and contraction in accordance with the detected expansion and contraction value. [Operation] The expansion and contraction of the subject due to body movement is detected, and the gradient of the gradient magnetic field in the direction of expansion and contraction is changed according to the detected expansion and contraction value. Then, the size of the field of view (data collection range) in the direction of expansion and contraction is changed according to the expansion and contraction, and the ratio between the size of the field of view in the direction of expansion and contraction and the size of the subject remains unchanged regardless of body movement. It will always remain constant. In other words, even if the subject expands or contracts, a gradient magnetic field of the same intensity will be applied to the same part of the subject. In this way, the subject can be made to appear not to be moving with respect to the visual field, and motion artifacts can be removed.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. In an MR imaging apparatus according to an embodiment of the present invention, as shown in FIG. 1, a subject 11 lies on a bed 12 and is introduced into a static magnetic field formed by a main magnet 21. Inside the upper magnet 1-21, there are three orthogonal
A gradient magnetic field coil 22 and an RF coil 23 are arranged to generate three gradient magnetic fields whose magnetic field strengths are gradient in the axial (X, Y, Z) directions. In addition, a body surface position detection @"jr3B is fixed to the inner garden of the gradient magnetic field coil 22. The gradient magnetic field coil 22 is connected to a gradient magnetic field power supply 31, and a t current of a predetermined waveform for generating a gradient magnetic field is sent. The RF coil 23 is connected to the RF transmitting/receiving device 32, irradiates the subject 11 with an RF multiplied signal to excite it, and receives the NMRq signal generated within the subject 11.
The F-fold signal transmission/reception and the waveform of the gradient magnetic field are controlled by the measurement control device 34, and the data obtained from the received NMR signal is sent to the image reconstruction device 35, where it is subjected to image reconstruction processing such as two-dimensional Fourier transformation. The reconstructed image is sent to the image display device 36 and displayed, or is stored in an external storage device (not shown). Computer 37 performs overall control. The body surface position detector 33 described above detects the position of the subject's body surface by, for example, emitting ultrasonic waves and measuring the time it takes for the ultrasound to return. It is attached to detect the body surface position of the abdomen of No. 11, and detects the rise and fall of the abdomen with breathing movements. That is, the movement of the abdomen of the subject 11 lying face up on the bed 12 in the vertical direction (X direction) is detected. Information on the body surface position detected in this way is sent to the measurement control device 34, and the expansion and contraction value in the height direction (X direction) of the abdomen of the subject 11 who is lying face up while breathing is determined. , the gradient of the X-direction gradient magnetic field is changed according to the expansion/contraction value. This embodiment aims to reduce artifacts caused by expansion and contraction of the abdomen in the height direction (X direction) due to breathing when an image is taken in a cross section that crosses the abdomen of the subject 11 at right angles to the body axis. First, the center of the X-direction gradient magnetic field is selected at a stationary part of the subject 11, for example, near the vertebrae. Next, the first excitation reception sequence is performed when the patient 11 takes the first exhalation and the abdomen is at its lowest position. Then, the second and third
, . . . are repeated for a predetermined repetition time and a predetermined number of times according to the matrix size. The abdominal height detected in the first excitation/reception sequence (more precisely, the distance from the vertebrae to the upper end of the abdomen) is L x l as shown in Figure 2A, and in the second and 3 In the second and fourth times, L x 2, L x 3, L x 4 ', and L x l is the minimum, so the JTI constant controller 34 changes the gradient of the X-direction gradient magnetic field for the second and third times. , 4th time' is 〈L8,'LKz
Ha, < L −4,” L x3) times, (L xi,
/L×4) times. That is, as shown in FIG. 2B, the gradient of the X-direction gradient magnetic field is changed so that the magnetic field strength at the upper end of the abdomen is always the same. By altering the gradient of the X-direction gradient magnetic field in this way, the field of view (data collection time) in the X-direction can be changed as shown in Figure 2C.
Dl, ,D Body movements can be eliminated. In other words, even if the height (width in the X direction) of the subject's abdominal section changes as shown in Figures 3A and B, the strength of the magnetic field that the upper end of the abdomen receives due to the gradient magnetic field in the X direction will change as shown in Figure 3[]C. (In order to avoid damage, the gradient of the X-direction gradient Ia field is changed from a solid line to a dotted line, so the position y that is encoded according to the gradient of this heavenly direction I14 magnetic field) Therefore, position information in the X direction can be obtained assuming that the body does not move in the X direction. Therefore, motion artifacts in the image related to body movement in the X direction can be removed. Note that this means that the frequency in the X direction is It is relatively easy to understand when the encoding direction is ゛, but the same holds true even when the X direction is the phase encoding direction. This is because at that time, the gradient of the gradient magnetic field in the X direction becomes large, and when it reaches the abdomen or higher, the gradient can be reduced.In other words, this causes the gradient magnetic field to become 31 can be used efficiently.When such considerations are not necessary, a large number of repeated sequences can be combined at any desired timing. [Effects of the Invention] ] According to the MR imaging device of the present invention, it is relatively simple to detect the expansion and contraction of the subject due to body movement and change the gradient (If) of the gradient magnetic field in the direction of expansion and contraction according to the detected expansion and contraction value. With this configuration, it is possible to remove the influence of body movement itself and reduce motion artifacts.Also, the inclination of the gradient magnet 1 is changed in each of the repeated sequences many times. from,
Data collection time is not increased, and echo time is also likely to be extended. Moreover, it does not impose any burden on the examinee.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of one embodiment of the present invention;
Figures A, B, and C show the cross-sectional shape, magnetic field, and visual field of the subject in each sequence, respectively.
2 is a diagram showing the relationship between the cross-sectional shape and the gradient of a gradient magnetic field during inhalation and exhalation. 11... Subject, ]2... Bed, 21 - Main magnet 1-122... Gradient magnetic field coil, 23... RF
Coil, 31・Gradient magnetic field power supply, 32・RF transmitter/receiver,
33... Body surface position detector, 34... Measurement control device, 35... Image reconstruction device, 36... Image display device, 37... Computer.

Claims (1)

[Claims] (1) An MR imaging device characterized by having means for detecting expansion and contraction of a subject due to body movement, and means for changing the gradient of a gradient magnetic field in the direction of expansion and contraction according to the detected expansion and contraction value.