JP2650973B2 - Tomographic imaging device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an in-vivo tomographic imaging apparatus using a magnetic resonance (MR) phenomenon, and more particularly to improving the quality of an image captured at twice the speed. The present invention relates to a tomographic imaging apparatus to be operated.

[Conventional technology]

In general, in the MR imaging method, a plurality of measurement data observed in a time domain is subjected to a two-dimensional Fourier transform and converted to a frequency domain to obtain an image. Hereinafter, the former region will be referred to as a measurement space, and the latter region will be referred to as an image space.

Since time is required for MR imaging, various speed-up methods have been proposed. As one kind of effective speeding-up method, there is a method of halving the measured data and doubling the shooting speed as compared with the conventional method.

The principle is that if the measurement is ideal, the measurement data is point-symmetric with respect to the origin of the measurement space, so after measuring the upper half data of the space, the remaining lower data is
It is intended to be estimated from the upper half data. For this type of technology, see, for example, “Radio Logi, November (19
86) pages 527 to 531 (Radiology, May, 1986, pp. 527-53)
1) ".

However, in practice, if there is distortion of the apparatus, a symmetry relationship between the measurement data is not established, and large image quality deterioration occurs. This is because phase distortion occurs on an image due to device distortion, and various methods for correcting this phase distortion have been proposed. For example, SPIE, vol. 593 (1985), pp. 6-13 (SPIE, vol. 593, 1985, pp. 6)
−13), Book of Abstract Society
Of Magnetic Resonance in Medicine, Sixties Any Meeting and Exhibition, August 17-21, (1987) 455, 804
Page, page 809 (Book of Abstracts Society of Magnetic
Resonance in Medicine Sixth Annual Meeting and Ex
hibition, August, 17-21, 1987, p455, p804, p809).

Each method includes a step of estimating a phase distortion map, a step of estimating unknown data using the map, and a step of reproducing an image. The estimation of the phase distortion map is
Each method is common. That is, since the phase distortion due to the apparatus distortion changes smoothly, the phase distortion of the image obtained by performing the Fourier transform on the central data of the measurement signal indicating the low frequency component of the image is used as the estimated value.

Although the contents of the step of estimating unknown data differ from each method, in each case only the unknown data portion is estimated, and the measured data portion and the estimated data portion are combined and used as data for image reproduction.

Regarding the steps of image reproduction, all data are subjected to two-dimensional Fourier transform by any method, and distortion correction using a phase map is performed as necessary.

[Problems to be solved by the invention]

The prior art described above has a problem that image quality degradation occurs on an image due to the fact that the estimated phase map is different from the true phase map.

That is, although the phase map is smooth, since only the data in the low-frequency portion of the frequency domain is cut out and used, a site lobe is generated, which slightly differs from the true phase map. Therefore, a method of reducing side lobes by using a filter is often used, but cannot be completely removed. Due to these side lobes, stripes also occur on the image, which causes deterioration in image quality. In particular, this phenomenon becomes remarkable when a uniform phantom is photographed.

SUMMARY OF THE INVENTION An object of the present invention is to remove a fringe caused by an incomplete phase map from approximately half of the measurement data to reproduce a high-quality image.

[Means for solving the problem]

The above object is achieved by performing the following steps.

(1) The measurement data and the estimated data obtained from the estimated phase map of the image are used in a superimposed manner, an averaging is performed, and the connection is made so that the connection is smooth, thereby achieving continuity.

 Further, it is effective to use the following process (2) in combination.

(2) When creating a phase by cutting out measurement data, a filter is applied in the phase encoding direction to cut out the data.

The following processing may be performed instead of the processing of (2).

(2 ') On the created estimated phase map, local smoothing processing is performed only in the range where the object exists, in the phase encoding direction.

In other words, the configuration is as follows. A magnetic field generating means for generating a magnetic field, a receiving means for receiving measurement data generated from the subject according to the generated magnetic field, and estimating an image phase map of an unmeasured part from a part of the received measurement data. On the estimated image phase map, by performing local smoothing processing according to the characteristics of the subject in the phase encoding direction, the data of the unmeasured portion is estimated, and the area of the measured data is estimated as the area of the measured data. Processing means for performing weighted averaging in an overlapping area of the data area.

[Action]

If the partial data obtained in the measurement space and the phase map of the image are estimated and the data estimated from the phase are simply connected, as shown in FIG. 5, the estimated data is not always complete. , It becomes discontinuous with the measured data. Experiments have shown that the frequency portion obtained by Fourier transforming this discontinuous portion causes horizontal stripes in the image and is a major cause of deterioration in image quality.

Therefore, the measurement data area is also estimated, and the averaging is performed by using the measurement data and the estimated data in a superimposed manner. As a result, the measured data and the estimated data are smoothly connected, and the stripes appearing on the image do not occur.

Further, when the following filter processing is performed in combination with the above processing, further effects are obtained.

On the phase map in which the image is estimated, a local phase smoothing process is performed in the phase encoding direction in a range where the object exists. That is, the rectangular function shown in FIG.
A convolution operation is performed using 403 as a weight. The width of this rectangle is set to the stripe period or half thereof. As a result, the phase becomes smoother and the estimated phase becomes more accurate. The reason for removing the background portion is to suppress the influence of noise.

Instead of the adaptive local smoothing filter described above, at the time of cutting out on the measurement data, the si as shown in FIGS. 4 (a) and 4 (b) is used.
It is also effective to use the nc function 401 or the Hanning function 402.In the case of the sinc function 401, only the part 404 that first passes through the phase of the zero point from the origin is used, and the range is matched with the measurement data extraction range, and other parts are used. Is zero.

Using such a function as a filter reduces the discontinuity in the measurement data, reduces the number of stripes on the image,
The effect of the averaging process on the measured data works more effectively.

〔Example〕

 Hereinafter, the present invention will be described in detail based on examples.

FIG. 2 is a block diagram of an embodiment of the present invention. 201 is a static magnetic field generation system for generating a uniform static magnetic field, 202 is a transmission system for generating a high-frequency pulse for exciting spin, and 203 is independent of the magnetic field strength in x, y, and z directions. A gradient magnetic field generation system that can change the intensity linearly, 204 receives an electromagnetic wave generated from the subject, and a reception system that performs A / D conversion after detection, 205 is based on measurement data from the reception system. In addition, a processing device for performing various operations necessary for image reproduction, 206 is a CRT for displaying a reproduction result, 207 is a pulse sequence file for storing a control procedure of operation of each system in the above configuration, and 208 is a pulse sequence file. A sequence control unit that controls the operation timing of each device based on the sequence file 207.

An implementation procedure of the present invention in the above configuration will be described below.

Hereinafter, as typical examples, there are 512 sampling points in the reading direction (horizontal direction), and the phase encoding direction (vertical direction).
Consider the case of 256 lines. The measurement data range 301 is a half + α, with α being 32, and 160 lines. Therefore, the central data range 303 has 64 lines. 64
The phase map is estimated from the line.

Embodiment 1. Step A01: In the data in the measurement space of FIG. 3, Fourier transform is performed in the direction of 160 times for the measurement data range 301 for each line.

Step A02: Using the data obtained in the above A01 as a target, data 303 equivalent to 64 lines in the central part of the measurement space is cut out in the vertical direction by applying a sinc function shown in 401 (FIG. 4) as a filter.

Step A03: For the data obtained in the above step, zeros are set around the data, and Fourier transform of 256 points in the vertical direction is performed 512 times, which is the number of sampling points, as 256 points of data.

Step A04: The image obtained in the above step is a complex number, but a phase component is obtained for each point to obtain an estimated phase map.

Step A05: The non-measured data range 302 and a part of the measured data in FIG. 3 are estimated from the phase map obtained from the above steps and the measured data.

The estimating method is described in detail in, for example, application number p62-042556.

Step A06: Connect the data obtained in the above step and the measurement data. At that time, the function shown in 101 (FIG. 1) is applied to the measurement data as a filter. If this filter is a (i), Becomes In other words, the overlapping portion 304 of the measured data and the estimated data is a quadratic function.

The estimated data is multiplied by a function indicated by 102 as a filter. This filter is set to be (1-a (i)).

Step A07: An image is reproduced from the data.

Embodiment 2. Steps A01, A02, and A0 of the first embodiment.
3, Step A04, Step A05, Step A06, Step A07
Perform the same operation as.

Where the sinc function filter in step A02 is
Change to Hanning function filter 402 (FIG. 4). Then, the quadratic function in step A06 is one of general n-order functions.

Here, the measured data is f (i), and the estimated data is g
(I), and the connected data h (i) becomes h (i) = a (i) f (i) + (1-a (i)) g (i).

Here, the filter a (i) is And n1.

Embodiment 3 Step B01: In the data in the measurement space of FIG. 3, a Fourier transform is performed in the horizontal direction 160 times for each line.

Step B02: With respect to the data obtained in the above B01, data corresponding to 64 lines in the central part of the measurement space is simply cut out in the horizontal direction.

Step B03: For the data obtained in the above step,
Zero points around and 256 points data, 512 times vertically, 256 points
Performs a point Fourier transform.

Step B04: The image obtained in the above step becomes a complex number. On this image, only the area where the object exists,
That is, the background portion is removed, and 403 (4th
A moving average is obtained by applying a rectangular function as shown in FIG. Here, the number of smoothing points is eight.

Step B05: Although the image obtained in the above step is a complex number, a phase component is obtained for each point to obtain an estimated phase map.

Step B06: The phase map given in the above step,
From the measured data, an unmeasured data portion 303 and a part of the measured data in FIG. 3 are estimated.

Step B07: The data obtained in the above step is connected to the measurement data in FIG. At that time, the measurement data includes
Apply as a function filter shown in 101.

If this filter is a (i), Becomes In other words, the overlapping portion 304 of the measured data and the estimated data is a quadratic function.

A function indicated by 102 is applied to the estimated data as a filter. This filter is set to be (1-a (i)).

Steps B01, B02, B03, B04, B05, B06, and B06, which are the same as those of the third embodiment, are also performed on data obtained by high-speed imaging with large phase distortion.
By performing the operation of B07, a high-quality image can be obtained.

〔The invention's effect〕

According to the present invention, by removing stripes generated on an image reproduced from partial data, deterioration of image quality is reduced,
A higher quality image can be obtained.

Therefore, the photographing time can be doubled, and there is an effect of improving the photographing throughput.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a weighting function for performing averaging between measured data and estimated data, FIG. 2 is a block diagram of an imaging processing apparatus used in an embodiment of the present invention, and FIG. FIG. 4 is an illustration of a function for extracting measurement data used in phase estimation, and FIG. 5 is an explanatory diagram showing a discontinuity between the measurement data and the estimation data. It is.

Continued on the front page (72) Inventor Akira Maeda 1099 Ozenji Temple, Aso-ku, Kawasaki City, Kanagawa Prefecture Inside Hitachi, Ltd.System Development Laboratory Co., Ltd. (72) Inventor Hideaki Koizumi 882 Ma, Katsuta-shi, Ibaraki Pref. Naka Plant, Hitachi, Ltd. In-house (56) References JP-A-61-272644 (JP, A)

Claims (2)

(57) [Claims] 1. A magnetic field generating means for generating a magnetic field, a receiving means for receiving measurement data generated from a subject according to the generated magnetic field, and an image phase of a part of the received measurement data to an unmeasured part By estimating the map, on the estimated image phase map, by performing local smoothing processing according to the characteristics of the subject in the phase encoding direction, the data of the unmeasured portion is estimated, A tomographic imaging apparatus comprising: processing means for performing weighted averaging in a region overlapping with a region of the estimated data. 2. The tomographic imaging apparatus according to claim 1, wherein the processing unit normalizes using a portion that first passes a zero point phase from an origin in the phase encoding direction.
The tomographic imaging apparatus according to claim 1, wherein weighted averaging is performed by estimating a phase using a sinc function.