JPH03224540A - MR device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

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

The present invention relates to an MR (magnetic resonance) apparatus, and particularly to an MR apparatus that can correct images when using a surface coil.

[Conventional technology]

2. Description of the Related Art Conventionally, images have been obtained using surface coils in MR apparatuses. However, surface coils have the characteristic that their sensitivity attenuates depending on the distance from them (in sagittal section imaging of vertebrae and spinal cord), signals from fat areas near the coil are received too strongly. Therefore, the fat part shines too white on the image, making it difficult to interpret the vertebrae and chain parts.To avoid this, conventional methods collect data after suppressing the signal from the fat part. Alternatively, the obtained image is subjected to two-dimensional Fourier transform and the low frequency components are used as the sensitivity distribution of the coil to normalize the original image.

[Problems to be Solved by the Invention] However, although suppressing signals from fat areas solves the problem of fat areas shining on images, it is difficult to obtain Mizuko fat images necessary for normal diagnosis. Rarena(
Moreover, the problem of image unevenness caused by non-uniform sensitivity of the surface coil occurring in areas other than fat areas remains unsolved. In addition, performing two-dimensional Fourier transform on the image and normalizing it using its low-frequency components is a common method for correcting non-uniform sensitivity of coils, but there is a problem that it takes a long time to process and If a structure that generates a frequency component exists in the subject tissue itself, there is a problem in that even that structure is corrected, and an abnormality may occur in the image after the correction process. An object of the present invention is to provide an MR apparatus that can easily correct image unevenness due to non-uniform sensitivity when using a surface coil to obtain an image that is easy to diagnose. [Means for Solving the Problems] In order to achieve the above object, according to the present invention, in an MR apparatus that obtains an image using a surface coil, the distance from the coil of the image obtained using the surface coil is means for determining an average pixel value according to this distance, means for determining a sensitivity curve in the distance direction from the coil from the average pixel value according to this distance, means for determining a function approximating this sensitivity curve, and an inverse of the function. It is characterized by comprising means for applying a function to the image.

[For production]

On an image obtained using a surface coil, find the average pixel value according to the distance from the coil, then find the sensitivity curve in the direction of distance from the coil, and find a function that approximates that sensitivity curve. The function will represent the sensitivity distribution of the coil versus distance from the coil. Therefore, if the inverse function of this function is applied to the original image above, it is possible to correct the spatial sensitivity non-uniformity, suppress the areas where the sensitivity is too high and the signal is too large, and diagnose It is possible to obtain an image that is easy to view. Furthermore, since the processing is simple, it can be easily realized without taking much time.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the overall schematic configuration of an MR apparatus according to an embodiment of the present invention. In FIG. 1, a subject 1 is placed in a space where a static magnetic field magnet 2 and a gradient magnetic field coil 3 are superimposed on a static magnetic field and a gradient magnetic field. A gradient magnetic field power source 32 controlled by a gradient magnetic field control device 31 is connected to the gradient magnetic field coil 3, and a gradient magnetic field having a predetermined waveform is formed. On the other hand, subject 1
A surface coil 4 is placed near the coil 4.
Then, a high frequency signal having a predetermined waveform envelope generated by the high frequency control device 41 is sent via the high frequency amplifier 42, and the nuclear spins of the subject 1 are excited. The NMR signal generated thereafter is received by the surface coil 4 and sent to the receiving device 52 via the preamplifier 51. This receiving device 52 includes a detection circuit using a quadrature phase detection method, and samples and A/D converts the signal after the detection 4, and sends the signal to the host computer 6. The host computer 6 performs processing such as Fourier transform to create an image. The sequence controller 7 controls the gradient magnetic field control device 31, the high frequency control device 41, and the receiving device 52 in cooperation with the host computer 6. Assume that an image of a lumbar vertebrae region is obtained as shown in FIG. 2 using an MR apparatus having such a configuration. The surface coil 81 is placed almost parallel to the Y-axis direction of the image, and the fat 82 is parallel to and close to this surface coil 81.
The signal from this fat 82 is very large, and the signal from the vertebrae 83, which is far from the surface coil 81, is small.As a result, the pixel value of the fat 82 part is abnormally high, causing that part to shine white. It becomes difficult to read the image of the vertebrae 83. Therefore, the following processing is performed within the host computer 6. First, create an image as shown in Figure 2 I (x
, y), and then calculate the average of pixel values equidistant from the surface coil 81. In this case, using the fact that the surface coil 81 is parallel to the Y axis of the screen, the average pixel value P(X) can be calculated as is the matrix size). This P (x) is expressed as shown in Figure 3. By this processing, an X-direction distribution of average pixel values can be obtained that substantially ignores the magnitude of the signal due to the tissue itself within the subject 1. Here, all pixel values are averaged in the Y direction, but to save time, the average may be calculated for about ten areas in the center of the Y direction. Next, a threshold value Po corresponding to the boundary of the subject 1 is introduced, and the subsequent processing is performed for objects exceeding this threshold value. This threshold value PO is normally set to a value of about 100 to 300, so that subsequent processing can be performed more accurately. An approximate function is obtained for the limited portion of the curve P (x) by threshold processing. Typically, the linear interval number F as shown in Fig. 3 is calculated using the least squares method.
Find (x). Of course, it may be approximated by a higher-order function. After obtaining the numerical number F(X) that approximates the sensitivity distribution in the X direction in this way, its inverse function is applied to the original image I (x, y). For example, let the center of the image in the X direction be x=0, and F (
When x)=Ax+B, 9I' (x, y)=(B
Image I after correction by /(Ax+B))-1(x,y)
' Find (x, y). The reason why B is multiplied in this equation is to make the pixel value at the center of the image (X20) match the pixel value of the original image. By applying the inverse function in this way, the average pixel value X-direction distribution curve P (x) is corrected as P' (x) as shown in FIG. This suppresses abnormally high pixel values, resulting in an image that is easier to view. Note that the part far from the surface coil 81, that is, the second
In the left end part of the image in the figure and the no-signal part in the subject 1,
By applying the inverse function of the function that approximates the sensitivity distribution as described above, only the noise will be enhanced, so it is preferable not to correct it. Therefore, for example, the inverse function may not be applied to the left half of the image, or at the same time, pixel values smaller than a certain value may be regarded as noise and the inverse function may not be applied. Furthermore, in the above description, the host computer 6 performs processing such as calculating the average pixel value and determining the approximation function, but it is of course possible to perform the processing using dedicated hardware.

【Effect of the invention】

According to the MR device of the present invention, it is possible to easily correct the spatial sensitivity non-uniformity when using a surface coil, and it is possible to suppress the pixel value of images of fat areas and easily create images that are easy to see for diagnosis. Obtainable.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an MR apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing an example of an image obtained in the above embodiment, and FIG. It is a graph which shows the distance direction distribution from a surface coil. 1... Subject, 2... Static magnetic field magnet, 3...
・Gradient magnetic field coil, 31...Gradient magnetic field control device, 3
2... Gradient magnetic field power supply, 4... Surface coil,
41... High frequency control device, 42... High frequency amplifier,
51... Preamplifier, 52... Receiving device, 6...
Host computer, 7... Sequence controller, 81... Surface coil, 82... Fat, 83
...is...

Claims (1)

[Claims] (1) In an MR device that obtains an image using a surface coil, a means for determining an average pixel value of the image obtained using the surface coil according to a distance from the coil, and an average pixel value according to this distance. An MR apparatus comprising means for determining a sensitivity curve in the distance direction from the coil, means for determining a function approximating the sensitivity curve, and means for applying an inverse function of the function to the image.