Mr imaging device

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to an MR imaging apparatus, and in particular, MRs that obtain one image of a desired region of a subject by synthesizing a plurality of images obtained for each surface coil using magnetic resonance signals obtained from a plurality of surface coils. Regarding imaging equipment.

【0002】

[Conventional technology]

Conventionally, it has been known that in an MR imaging apparatus, a plurality of surface coils are used, magnetic resonance data is collected by these, and a plurality of reconstructed images are combined to obtain one image. According to this, one image with a higher S / N than when one uniform coil is used and a wider area than when only one surface coil is used can be obtained.

【0003】

By the way, regarding this image composition method, for example, in Japanese Patent Application Laid-Open No. 2-500175, pixels at the same position in each image are used by using a weight distribution determined based on the high-frequency magnetic field distribution generated by each surface coil. Weighting is added for each.

【0004】

It is also known that the non-uniformity of the density of the image after synthesis caused by the non-uniform high-frequency magnetic field distribution of each surface coil is corrected by using the plurality of weight distributions (high-frequency magnetic field distribution). (Refer to the above-mentioned special table No. 2-500175).

【0005】

[Problems to be Solved by the Invention]

However, in the above-mentioned known technique, it is necessary to obtain the high-frequency magnetic field distribution generated by each surface coil in advance in order to obtain the weight distribution, and there is a problem that extra time is required. In addition, when the non-uniformity of the density of the image after composition is corrected, for example, in a space where the subject does not exist and at a position far from any surface coil, white noise that does not depend on the position from the surface coil is generated. There is also a problem that the image is emphasized and the image of the subject and the white noise are mixed and the image is difficult to see.

【0006】

In view of the above, the present invention obtains the weight distribution by a simple calculation from each image data itself reconstructed from the data obtained by each surface coil, thereby obtaining an extra imaging time and the like in order to obtain the weight distribution. It is an object of the present invention to provide an MR imaging apparatus which has been improved so as not to cause a problem.

【0007】

Another object of the present invention is to provide an improved MR imaging device so that a white noise component can be removed from such a composite image.

【0008】

[Means for solving problems]

In order to achieve the above object, in the MR imaging apparatus according to the present invention, a means for applying a static magnetic field and a gradient magnetic field pulse, and an RF transmitting means for irradiating a subject placed in the magnetic field with an RF pulse. , Image re-image using the receiving means for receiving the magnetic resonance signal from the subject via a plurality of surface coils arranged close to the subject to obtain data, and the data obtained from each of the above surface coils. An image reconstruction means for constructing and obtaining each image, a weight distribution calculation means for obtaining each weight distribution by filtering each of the obtained plurality of images, and the above-mentioned plurality using the weight distribution. It is characterized in that it is provided with an image synthesizing means for synthesizing one image by weighting and adding the image data of the above to each position.

【0009】

Further, each image is provided with a means for removing noise by smoothing the images while changing the degree of smoothing according to the corresponding weight distribution, and each image after the smoothing process is synthesized by the above-mentioned image synthesis means. It can also be configured to do so.

【0010】

[Action]

The weight distribution used when synthesizing a plurality of images reconstructed for each surface coil by weighting addition is obtained by filtering the images for each of those images. Then, the weight distribution thus obtained corresponds substantially well to the high frequency magnetic field distribution of each surface coil. Therefore, it is possible to obtain a weight distribution practically equivalent to that obtained by performing this without the need to perform a separate imaging sequence. Further, since this filtering process can be performed at high speed by a simple calculation, the time required to obtain a composite image can be shortened.

【0011】

Further, if the smoothing process is performed while changing the degree of smoothing according to the sensitivity of the surface coil, the noise of the image obtained for the surface coil can be removed. On the other hand, the above weight distribution corresponds to the sensitivity distribution of each surface coil. Therefore, the smoothing process of each image is performed while changing the degree of smoothing of each image using the weight distribution corresponding to the image of each surface coil, and the image after the smoothing process is obtained as described above. By synthesizing, it is possible to obtain one composite image in which noise is suppressed in a low-sensitivity portion far from any surface coil.

【0012】

[Example]

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, the subject 10 is placed in a static magnetic field generated by the static magnetic field magnet 11. A pulse-shaped exciting current is supplied from the drive device 21 to the gradient magnetic field generation coil 12, and a pulse-like gradient magnetic field is generated in each direction of the three orthogonal axes. The gradient magnetic field pulse generated by the gradient magnetic field generation coil 12 is superimposed on the static magnetic field in the space where the subject 10 is arranged. A plurality of surface coils 13 are arranged on the subject 10 so as to be close to the subject 10. In this embodiment, the surface coil 13 irradiates the subject 10 with an RF signal (radio frequency radio wave) in a pulsed manner, and functions as an antenna for receiving a magnetic resonance signal (radio wave) generated from the subject 10. To do.

【0013】

RF power is supplied to these surface coils 13 from the transmitter 22. In the transmitter 22, AM modulation of the RF signal is performed according to the waveform determined under the control of the sequence controller 24, and the envelope waveform of the RF signal becomes the waveform. The AM-modulated RF signal is sent from the transmitter 22 to the surface coil 13 to irradiate the pulsed RF signal.

【0014】

The sequence controller 24 determines the waveform of the RF pulse and the generation timing thereof in this way, and controls the drive device 21 to determine the waveform and the generation timing of the gradient magnetic field of each axis. That is, an RF excitation pulse, an RF refocus pulse, and a gradient magnetic field pulse for slice selection and a readout are generated as a gradient magnetic field pulse in each axial direction according to an imaging pulse sequence such as a field echo method or a spin echo method. Determine the respective waveforms and timings of the gradient magnetic field pulse and the gradient magnetic field pulse for phase encoding.

【0015】

The magnetic resonance signal generated in the subject 10 is received by each of the surface coils 13, sent to the receiver 23 for phase detection, and then sent to the data acquisition device 25 for A / D conversion to obtain digital data. .. This digital data is taken into the computer 26. The computer 26 is provided with an input device 27 such as a keyboard 27, a display device 28 for displaying an image, and a recording device such as a hard disk device (not shown).

【0016】

Further, in this embodiment, the image reconstruction device 31, the weight distribution calculation device 32, and the image composition device 33 are connected to the computer 26. When the data is taken into the computer 26 as described above, it is sent to the image reconstructing device 31 and the image reconstructing process is performed for each of the data obtained from each of the plurality of surface coils 13. This image reconstruction process is performed using, for example, a two-dimensional Fourier transform algorithm.

【0017】

When an image for each of the plurality of surface coils 13 is obtained in this way, this image is sent to the weight distribution calculation device 32. The weight distribution calculation device 32 performs the following calculation to obtain the weight distribution for each image. That is, as shown in FIG. 2, when the weight distribution of the image 41 is obtained, it is considerably large (for example, the vertical and horizontal sizes of the original image 41 are centered on the pixel of interest 42). Set window 43 (about 1/4). Then, the average value of the pixels included in the window 43 is set as the value of the pixel of interest 42.

【0018】

The pixel 42 of interest is moved one after another, and the window 43 is also moved at the same time to perform a two-dimensional moving average filter processing. As a result, two-dimensional data having the same size as the original image 41 can be obtained. This two-dimensional data is used as a weight distribution. Since the two-dimensional moving average filter processing in this case can be performed by a simple four-rule operation, it can be executed easily and at a very high speed. At the end of the image 41, it is possible to use the data of the opposite end by utilizing the folding back of the original image 41.

【0019】

The image reconstructing device 33 synthesizes a plurality of images obtained for each surface coil 13 by weighting and adding them using the weight distribution obtained for each of the images. That is, by multiplying the pixel data at the same position of those images by the weights of the corresponding positions of the respective weight distributions and then adding them, the plurality of images are combined to obtain one image. This weight distribution is the two-dimensional data obtained as described above, which is the global pixel value distribution of each of the original images, and is substantially proportional to the high-frequency magnetic field distribution of each surface coil 13. Therefore, by using this two-dimensional data for weighting addition in the image reconstruction device 33, it is possible to obtain practically exactly the same result as when the high-frequency magnetic field distribution of each surface coil 13 actually measured separately is used as the weight distribution. ..

【0020】

FIG. 3 shows a part of the second embodiment. In this embodiment, an image noise removing device 34 is added, and a process of removing noise of the original image is performed before image composition. Other configurations are the same as in FIG. That is, when the image reconstruction device 31 obtains an image for each of the surface coils 13, the weight distribution calculation device 32 calculates the weight distribution for each image, which is the same as in FIG. The image obtained for each of the surface coils 13 is sent to the image noise removing device 34 for processing before being sent to the image compositing device 33. This process is an image smoothing process, but the degree is different for each pixel. Depending on the sensitivity of the surface coil 13 at the position of each pixel, the smaller the sensitivity, the greater the degree of smoothing.

【0021】

The magnetic resonance signal from the subject 10 becomes small in the portion where the sensitivity of the surface coil 13 is low. Therefore, in such a low-sensitivity portion, the white noise component of the coil 13 that does not depend on the sensitivity becomes relatively large with respect to the signal. Therefore, if the smoothing process is performed by increasing the degree of smoothing of the image in such a portion, white noise can be effectively removed.

【0022】

As the data representing this sensitivity distribution, the weight distribution calculated by the above weight distribution calculation device 32 is used in this embodiment. Specifically, assuming that one weight distribution is obtained corresponding to one image, when the data of the pixel at a certain position (x, y) is Ixy and the weight is αxy, this weight αxy is Smoothing is performed when the threshold value is αt (for example, 10% of the maximum weight value) or less. This processing is performed when the pixel data of the processing result is Jxy. Jxy = (αxy / αt) Ixy + {(1-αxy) / αt} Iave Can be expressed as. here, 0 α xy α t <α max = 1 And αmax is the maximum value of the weight. Iave is an average value within a narrow area (for example, a range of 3 × 3 pixels) centered on the pixel at the position (x, y) of the image.

【0023】

Therefore, in this case, if one combined image is displayed by the display device 28, it is possible to observe an image having a high S / N ratio in which noise is suppressed in a low-sensitivity portion of each of the surface coils 13. Can be done. Therefore, it is possible to observe an image having a high S / N ratio, which is extremely easy to see, in a wide area, which facilitates medical diagnosis.

【0024】

Needless to say, the above description relates to examples and can be changed in various ways without departing from the spirit of the present invention. For example, in the weight distribution calculation device 32, the size of the window 43 used when performing the two-dimensional moving average filter processing is not limited to the above. Further, the smoothing process for noise removal in the image noise removing device 34 is not limited to the above example, and various methods can be adopted. Further, in the above, the image reconstruction device 31, the weight distribution calculation device 32, the image composition device 33, and the image noise removal device 34 are expressed as being configured by hardware separate from the computer 26. It is also possible to configure all the functions to be performed within the computer 26 (in software).

【0025】

[Effect of the invention]

As described above, according to the MR imaging apparatus of the present invention, the weight distribution used when synthesizing the images obtained for each of a plurality of surface coils can be obtained at high speed by a simple calculation using those images. You can ask. Therefore, it is possible to solve problems such as requiring an extra imaging time. In addition, by adding smoothing processing while changing the degree of smoothing using the weight distribution for noise removal, high S / N that suppresses noise in low-sensitivity parts far from any surface coil. A composite image of one ratio can be obtained. Moreover, this smoothing process can also be performed at high speed with a simple calculation.

[Simple explanation of drawings]

[Figure 1]

The block diagram which shows one Example of this invention.

[Figure 2]

The schematic diagram for demonstrating the two-dimensional moving average filter processing in a weight distribution calculation apparatus.

[Fig. 3]

The block diagram which shows a part of the 2nd Example.

[Explanation of symbols]

10 subjects 11 Static magnetic field magnet 12 Coil for generating gradient magnetic field 13 Surface coil 21 Drive 22 transmitter 23 receiver 24 Sequence controller 25 Data acquisition device 26 Computer 27 keyboard 28 Display device 31 Image reconstructor 32 Weight distribution calculation device 33 Image synthesizer 34 Image noise remover