As recent means for obtaining an image of an object (especially, an image inside a human body) by X-ray irradiation, the spatial distribution of X-ray intensities is directly converted into an electrical signal using a large X-ray sensor panel, the electrical signal is converted into a digital value by analog-to-digital (A/D) conversion, the digital value is input into a computer to form a digital image, and the digital image is used for saving, an image process, and observation.
In order to sense a chest image of a human body at one time, a sensor panel having a size of about 40 cm×40 cm is brought into nearly contact with the human body and is irradiated with X-rays from a direction opposite of the sensor panel to the human body, and the intensity distribution of X-ray transmitted through the human body is acquired by the sensor panel. In order to sense the detailed structure of the human body, a pixel resolution as high as 0.1 to 0.2 mm2 is required by the sensor and one image consists of 2000×2000 to 4000×4000 pixels. Thus, the amount of image data becomes very large.
As a method for quickly and stably reading image information of a large X-ray sensor panel, basically, the following two methods are used.
(1) One large sensor panel is formed by combining relatively small partial sensor panels, similar to arrangement of tiles. The individual partial sensor panels are driven in parallel to obtain an A/D convert images.
(2) When a single large sensor panel is used, the sensor is divisionally driven in small portions, and independent amplifiers and A/D converters are connected to these portions to acquire data, so as to attain high-speed image data capture or to shorten the data wiring length on the sensor panel.
That is, it is difficult for a single system to quickly and stably acquire data since the size of the sensor panel as well as the image data size are large.
When a plurality of systems are used in place of a single system to obtain an image by part, the characteristics of the amplifiers, A/D converters, and the like, used in the systems, for processing respective image signals vary independently due to environmental change, aging, and so on.
FIG. 13 shows a system arrangement for capturing a normal X-ray image. In this case, one sensor panel is divided into four regions 1a to 1d, which are driven independently. Independent amplifiers 2a to 2d amplify electrical signals output from the regions 1a to 1d with gains, A/D converters 3a to 3d convert the output signals from the amplifiers 2a to 2d into digital values, and independent DMA controllers 4a to 4d store partial image data in parallel into a frame memory 5. A line 15 is a bus line of this system, and a central processing unit (CPU) 9 sequentially executes programs stored in a program memory 16 to process data via the bus 15 in this computer system. The frame memory 5 is a dual-port memory from which image data is read out as the CPU 9 controls the read address.
The image sensing sequence is as follows. An irradiation controller 12 for an X-ray bulb controls an X-ray generator 13 (bulb) to emit X-rays toward an object (human body) 14. In synchronism with the X-ray irradiation, panel drivers (not shown) drive the sensor panel regions 1a to 1d (to sequentially drive internal switching transistors) to output electrical signals corresponding to pixels, thus storing an image in the frame memory 5 via the amplifiers 2a to 2d, A/D converters 3a to 3d, and DMA controllers 4a to 4d. 
Reference numeral 6 denotes a frame memory. Similar operation as described above is made without emitting any X-rays in order to capture a fixed pattern representing an offset in the frame memory 5, and that pattern is stored in the frame memory 6. Reference numeral 8 denotes a memory which pre-stores gain variation information of each pixel of the sensor panel regions 1a to 1d. This information is normally obtained by irradiating the sensor with X-rays without any object, and capturing that image. The fixed pattern is removed from the captured image, and the resultant image is converted into a logarithmic value.
Reference numeral 20 denotes a look up table (LUT) used for converting pixel data from which the fixed pattern stored in the frame memory 6 is subtracted by a subtractor 24 into a logarithmic value, and outputs the logarithmic value. A subtractor 23 subtracts gain variation data held in the memory 8 from the image data converted into the logarithmic value. A memory 18 pre-stores the positions of pixels that cannot be corrected by using gain variation data (pixels themselves are defective and no data are obtained therefrom), and a defect correction unit 19 corrects pixel data output from the subtractor 23 by interpolating data at the defective pixel positions stored in the memory 18 from surrounding non-defective pixel values. The corrected data is converted into an analog video signal again, and the converted signal is displayed on a monitor 21. In this method, data can be processed while capturing image data, and a plurality of images can be successively processed. Hence, an X-ray moving image that displays the motion of an object can be displayed.
A moving image is often saved as a file in a storage device 11 such as a magnetic storage device, large-capacity nonvolatile storage device, or the like, or is often output to an external display device, recording device, or storage device via an interface (not shown).
In this case, since partial image data obtained by independently driving the sensor panel regions 1a to 1d are normalized using the fixed pattern held in the frame memory 6 and the gain variation pattern held in the memory 8, the observer does not notice that an image obtained by compositing partial image data is made up of images captured for respective regions.
In the aforementioned basic operation, the gain variation data for respective pixels held in the memory 8 are obtained by irradiating the sensor with X-rays without any object, and is difficult to obtain for each image sensing in a normal medical facility. The data is sensed, e.g., once per day. Also, the fixed pattern held in the frame memory 6 is obtained at a time very close to the image sensing time but not at the same time. The capture time difference between the correction data held in the frame memory 6 and memory 8 and image data obtained by sensing an object corresponds to environmental differences (temperature, humidity, and the like) upon capturing those data, and the characteristics of the partial panels, amplifiers, and the like may change. In this case, different characteristics appear in respective partial images, and a clear boundary exists between respective partial images.
The present inventors proposed a method of solving such problem, i.e., making the boundary inconspicuous by extracting components having features that continue in the boundary direction near the boundary and removing these feature components near the boundary (Japanese Patent Laid-Open No. 2000-132663).
This method is very effective when partial images suffer relatively small variations, and implements correction that does not require correction over the entire image by smoothing only the neighborhood of the boundary. However, variations among partial images are often too large to absorb unnaturalness as a whole by only partial correction, and a measure against such case is required. In the above method, when important image information happens to be present at a boundary position and along a boundary, that image information may be damaged by correction. Further, since correction is made after the entire image is obtained, it is difficult to attain a moving image process in nearly real time.