JPS62219074A - Method and device for restructuring picture - Google Patents

Abstract

PURPOSE:To obtain a picture with a few arch facts and picture quality suitable for a photographed part and a diagnosis purpose by obtaining the average of correction windows corresponding to samples of projection data, applying the arithmetic, correcting and processing a frequency component included in original projection data, then restructuring a picture. CONSTITUTION:To the measured projection data, a correction coefficient, the corrected projection data, and a correction window average are assumed as (g), (a), (h) and (w), respectively, the correction processing targeting h=g+aXw is applied, whereby the space frequency component included in the projection data is so controlled as to suit to each photographed part. If many arch facts appear due to the photographing a complexed part, the correction window processing is applied to the projection data with the correction coefficient (a)<0 given, and a high frequency band is suppressed to emphasize a low frequency part relatively. When constrast is emphasized, the correction window processing is applied to the projection data with the correction coefficient a>0 given, and a high frequency band is emphasized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an image reconstruction method and apparatus for obtaining a tomographic image of a subject by performing image reconstruction calculations on measured projection data.

[Background of the invention]

In devices (hereinafter referred to as CT devices) that obtain tomographic images by performing reconstruction operations on projection data obtained by measuring physical or physiological information from various directions within the subject, false images called artifacts appear. Sometimes.

As is well known, artifacts cause a decline in the diagnostic ability of obtained tomographic images (images). Furthermore, in low-contrast regions of tomographic images, it is difficult to distinguish between normal and abnormal tissues, and contrast enhancement processing is sometimes essential. As described above, reduction or elimination of artifacts and contrast enhancement processing have long been important clinical issues.

[Purpose of the invention]

The present invention was made in view of the above-mentioned circumstances.
An image reconstruction method and method capable of improving diagnostic ability by performing a controllable correction process on the frequency components of original projection data to obtain an image with few artifacts and a quality suitable for the imaging site or diagnostic purpose. The purpose is to provide equipment.

[Summary of the invention]

In reconstructing a tomographic image of a subject from measured projection data, the present invention calculates the average value of the correction window corresponding to each sample of the projection data, sets the original projection data to g, and sets the correction coefficient to α. , where w is the average value of the correction window and h is the projection data after correction.

The above object is achieved by performing the calculation h=g+αXw, correcting the frequency components of the original projection data g using the above α and W, and then reconstructing the image.

[Embodiments of the invention]

The present invention will be described in detail below with reference to the drawings.

Here, before explaining specific embodiments, the basic principle of the present invention will be described first.

In a CT apparatus, a false image called an artifact may appear in a reconstructed image. Such false images disturb the information inherent in the image and significantly reduce diagnostic ability. Many artifact components and noise components are contained in the high spatial frequency region (hereinafter, in the description of the present invention, spatial frequency is simply referred to as frequency) of projection data, and depending on the imaging site, the high frequency region (hereinafter referred to as high frequency region) If this is emphasized, spatial resolution and graininess will improve (increase in sharpness), but noise components will increase and artifacts will become more noticeable, which will actually reduce diagnostic ability. On the other hand, depending on the area being imaged, the contrast of the projection data may be weak and it may be difficult to distinguish between normal and abnormal tissues. Diagnosis ability can be improved by processing to emphasize high frequencies (contrast). In some cases. therefore,
It is necessary to be able to control the image quality at the reconstruction stage, and normally with the X141ACT device, several types of blur correction filters are prepared and the image quality can be controlled by selecting them at the time of reconstruction or by setting them for each area to be imaged. do.

The present invention provides projection data g for measured projection data.
, where α is the correction coefficient, h is the corrected projection data (hereinafter referred to as correction data), and W is the correction window average value, perform the correction process as follows (this is called correction window processing): h=g+α×w By this, the spatial frequency components of the projection data are controlled to be suitable for each imaging site.

Here, the average value of the correction window corresponds to each sample of the projection data, and refers to the average value (simple average or various weighted average) of the projection data for each sample N. The reason why the average value is taken is to blur the image so that low frequency components are not included. The number of samples N used for blurring is called the window size.

Therefore, the correction window processing refers to controlling the image quality using the size and type of the window and the correction coefficient α.

It is well known that in the case of X-ray CT devices, many artifacts appear when images are taken of areas with dense bones, such as the base of the skull or the vicinity of the ear ossicles. Therefore, as the first correction process of the present invention, it is conceivable to apply the above-described correction window process to the projection data with the correction coefficient α set to 0, thereby suppressing the high range and relatively emphasizing the low range. When the corrected data with high frequencies suppressed is reconstructed using reconstruction calculation processing such as the well-known convolution method or two-dimensional Fourier transform method, it is compared with the reconstructed image of the original projection data, and artifacts are reduced. You can obtain a clear image.

In addition, as the second correction process of the present invention, the contrast of the reconstructed image is weak depending on the organ, and when photographing such parts, it is essential to perform processing to enhance the contrast. come. Therefore, it is conceivable to apply the above-mentioned correction window processing to the projection data with a correction coefficient α>0 to emphasize the high frequency range. When the correction data with enhanced high frequencies is reconstructed as an image using reconstruction calculation processing such as the well-known convolution method or the two-dimensional Fourier transform method, the original projection data is compared with the reconstructed image.
Images with high contrast resolution can be obtained. Further, in these processes, the correction coefficient α may be changed according to the average value of the correction window.

As described above, the present invention performs correction window processing on projection data to control one frequency component, thereby making it possible to obtain an image with an image quality suitable for the imaged region and with improved diagnostic ability.

Next, specific embodiments of the present invention will be described with reference to FIGS. 1 to 5. Figure 1 shows a case where the present invention is applied to a third-generation X-ray CT system, in which a filtered pack projection method is used and a direct method in which the fan beam is directly back-projected is used as the reconstruction algorithm. FIG. 3 is a diagram showing the flow from X-ray exposure to image reconstruction in FIG. In FIG. 1, X-rays emitted from an X-ray tube 10 pass through a subject 12, are detected by an X-ray detector 11, and are captured as integral data along the beam path of the X-ray absorption coefficient. System (DAS: Data A
Quisition System) 13. The data from this D A S 13 is subjected to pre-processing 14 such as log correction, and then converted into original projection data (meaning data that has not been subjected to correction processing according to the present invention) 16
The image is input to the image processing device 21 as follows. Image processing device 2
1, the projection data 16 is subjected to the correction window processing 15 described above as h=g+α×w (1).

Next, the projection data subjected to the correction window processing is subjected to blur correction filter processing 17, and the obtained blur correction data 18
Reconstructed image data (tomographic image) by performing back projection processing 19 on
Get 20.

In the present invention, when performing the correction window processing 15, the size and type of the window are specified.

These values may be created in a reference table in a memory or magnetic disk (not shown), and then arbitrarily selected and read from the table, or may be set for each region to be imaged. The correction coefficient α is a high-frequency emphasis type correction in which the peak of the projection data 16 is strengthened when α>0, and a high-frequency suppression type correction in which the peak is suppressed by α〉o. Therefore, reconstruction is performed using α<O in areas where artifacts are noticeable, and α>0 in areas with low contrast. Similarly to the window size and type, α is also set in advance by creating a table or by setting it for each region to be imaged. However, when reconstructing with α〉O, if there is a region in the object 12 where the average value W of the correction window is large, sharp peaks on the projection data 16 will be further emphasized, making artifacts more noticeable. It may come. In this case, α is changed depending on the size of the average value W of the correction window. That is, when W is large, α may be made small, and when W is small, α may be made large. An example of this is shown in FIG. In FIG. 2, a line 22 indicates that when α is constant regardless of the size of W, a line 23 indicates that WO and wl are determined from the histogram of the original projection data. W. The case where the value is changed linearly between -1 is shown. Alternatively, α may be changed using an appropriate function.

In FIG. 3, projection data 16 of one sample (view), an example of which is indicated by line 30, is subjected to correction window processing (the average value W is
3) applied with α>0 is shown in line 31, and that applied with α<0 is shown in line 32. This window is size 3
The weighted average is calculated by .

A line 40 in FIG. 4 shows the modulation transfer function of the window shown in FIG. 3, and a line 41 in FIG.

Sample pitch (pitch between detector channels) of 1mm
Then, in line 41, when the modulation transfer function is 0.5, it is approximately 0.2098Rp/+w+w.

Taking a simple average with a size N in the correction window processing is equivalent to measuring with the detector 11 having a channel spacing of Nmm, assuming that the sample pitch of the original projection data 16 is 1.0 mm.

In the above embodiment, the window processing is performed using the projection data 16.
Although it was applied to all of the above, it is not limited to only this,
For example, a threshold value T may be provided, 1wl and the threshold value T may be compared, and correction may be performed only when the conditions are met. An example is shown in FIG. In other words, in FIG. 5, when the projection data 16 indicated by line 50 is subjected to the correction window processing 15 by setting α to O only when IWI becomes a value larger than T, the original projection data 16 is converted to g and the correction window indicated by line 52 is applied. Let w be the average value of , and h be the corrected projection data shown by the line 51 .

If Iwl≧T, h = g + a X w
-(2) When 0≦1wl h=g =4
3). In FIG. 5, W≧T only corresponds to channel n, and correction is applied only to the correction window corresponding to n. In the xmcT device, a substance with a high X-ray absorption coefficient such as a metal pin is used as the object 12.
If there is a sharp peak within the projection data 16, an artifact called a fine streak appears, but according to this example, a sharp peak in the projection data 16 can be extracted by T, and correction window processing is applied only to the area of that peak. As a result, a good image with reduced fine streak artifacts can be obtained.

In both of the embodiments described above, the correction window processing 15 is applied to the projection data 16 after the preprocessing 14, but it may also be applied to the blur correction data 18 after the filter processing 17.

Furthermore, the same effect can be obtained by performing the correction window processing 15 on each sample data of a channel having a predetermined width not only in the direction of the detector channel but also in the direction of the projection angle.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain an image with few artifacts and a quality suitable for the imaging site or the purpose of diagnosis, thereby improving diagnostic ability.

[Brief explanation of drawings]

Figure 1 shows the case where the present invention is applied to an X-ray CT device, and shows the flow from X-ray irradiation to image reconstruction. Figure 2 shows how the correction coefficient α is increased by 1 wl. Figure 3 is a diagram showing the difference in the effect of correction window processing depending on the positive and negative sign of α, Figure 4 is a diagram showing an example of Raido's modulation transfer function, FIG. 5 is an explanatory diagram of correction window processing using a threshold value. 10... X-ray tube, 11... X-ray detector, 12...
Subject, 15... Correction window processing, 16... Original projection data, 20... Reconstructed image data (tomographic image), 21... Image processing device. Patent Applicant Hitachi Medical Co., Ltd. Agent Patent Attorney Tadashi Akimoto Figure 1 Knob Figure 2

Claims (1)

[Claims] 1. In reconstructing a tomographic image of a subject from projection data obtained by measuring physical or physiological information inside the subject from each direction, each sample of the projection data is Find the average value of the corresponding correction window, and when the original projection data is g, the correction coefficient is α, the average value of the correction window is w, and the projection data after correction is h, perform the calculation h=g+α×w. , an image reconstruction method characterized in that the image is reconstructed after correcting the frequency components of the original projection data g using the above α and w. 2. Claim 1, characterized in that the correction process is performed only when the original projection data g or the average value w of the correction window is within a range defined by one or more set threshold values. Image reconstruction method described in section. 3. Image reconstruction according to claim 1 or 2, characterized in that the correction coefficient α is changed to increase in accordance with an increase in the original projection data g or the average value w of the correction window. Method. 4. When reconstructing a tomographic image of a subject from projection data obtained by measuring physical or physiological information inside the subject from each direction, the average of correction windows corresponding to individual samples of projection data Calculate the value, and when the original projection data is g, the correction coefficient is α, the average value of the correction window is w, and the projection data after correction is h, perform the calculation h=g+α×w, and use the above α and w. An image reconstruction device comprising means for reconstructing an image after correcting frequency components of original projection data g. 5. Claim 4, characterized in that the correction process is performed only when the original projection data g or the average value w of the correction window is within a range defined by one or more set threshold values. The image reconstruction device described in . 6. Image reconstruction according to claim 4 or 5, characterized in that the correction coefficient α is changed to increase in accordance with an increase in the original projection data g or the average value w of the correction window. Device.