Image processing apparatus for processing x-ray image, radiation imaging system, image processing method, and storage medium

Abstract

An image forming apparatus includes a conversion acquisition unit configured to acquire a gradation conversion characteristic based on an X-ray image captured by an X-ray sensor, a storage unit configured to store a pixel value range in which a signal-to-noise ratio is higher than a threshold value according to a characteristic of the X-ray sensor, a determination unit configured to determine whether a pixel value range in which gradation is widened according to the gradation conversion characteristic is included in the stored pixel value range, and a correction unit configured to correct the gradation conversion characteristic depending on a result of determination by the determination unit.

1 . An image processing apparatus comprising:
 a conversion acquisition unit configured to acquire a gradation conversion characteristic based on an X-ray image captured by an X-ray sensor;   a storage unit configured to store a pixel value range in which a signal-to-noise ratio is higher than a threshold value according to a characteristic of the X-ray sensor;   a determination unit configured to determine whether a pixel value range in which gradation is widened according to the gradation conversion characteristic is included in the stored pixel value range; and   a correction unit configured to correct the gradation conversion characteristic depending on a result of determination by the determination unit.   
     
     
         2 . The image processing apparatus according to  claim 1 , wherein the correction unit corrects the gradation conversion characteristic so that the pixel value range in which gradation is widened is included in the stored pixel value range. 
     
     
         3 . The image processing apparatus according to  claim 2 , wherein, if it is determined that the pixel value range in which gradation is widened is included in the stored pixel value range, the correction unit acquires the gradation conversion characteristic so that the pixel value range in which gradation is widened by gradation conversion before the correction is included in the stored pixel value range and a gradation of a pixel value range smaller than the stored pixel value range is narrowed. 
     
     
         4 . The image processing apparatus according to  claim 1 , further comprising:
 a setting unit configured to set, based on an image captured by an image sensor, a pixel value range to be emphasized in the image; and   a characteristic acquisition unit configured to acquire a pixel value range corresponding to a dose at which a signal-to-noise ratio is higher than the threshold value for the image obtained from the image sensor and to store the acquired pixel value range in the storage unit,   wherein, if the determination unit determines that the pixel value range in which gradation is widened is not included in the stored pixel value range, the correction unit shifts the pixel value range in which gradation is widened to be included in the stored pixel value range.   
     
     
         5 . The image processing apparatus according to  claim 1 , wherein the correction unit shifts the pixel value range in which gradation is widened by a part of the pixel value range that is not included in the stored pixel value range. 
     
     
         6 . The image processing apparatus according to  claim 1 , wherein, if the pixel value range in which gradation is widened is included in the stored pixel value range, the correction unit subjects the image to processing for widening the gradation of the pixel value range, and, if the pixel value range in which gradation is widened is not included in the stored pixel value range, the correction unit shifts the pixel value range. 
     
     
         7 . The image processing apparatus according to  claim 4 , wherein the characteristic acquisition unit acquires a pixel value range in which a signal-to-noise ratio is higher than the threshold value for the image obtained by the image sensor and in which linearity of an output of a signal component of the image is kept. 
     
     
         8 . The image processing apparatus according to  claim 7 , wherein, if the pixel value range to be emphasized cannot fall within the pixel value range acquired by the characteristic acquisition unit even if the pixel value range to be emphasized is shifted, the correction unit shifts the pixel value range to be emphasized by giving priority to falling within the pixel value range in which the signal-to-noise ratio is higher than the threshold value, rather than falling within the pixel value range in which the linearity is kept. 
     
     
         9 . The image processing apparatus according to  claim 4 , further comprising an output unit configured to output, if the pixel value range acquired by the characteristic acquisition unit does not include a part of the pixel value range to be emphasized, a warning. 
     
     
         10 . The image processing apparatus according to  claim 4 , further comprising a generation unit configured to generate a gradation conversion function based on a region of interest in the image captured by the image sensor,
 wherein the setting unit sets a pixel value range in which a slope of the generated gradation conversion function is larger than a threshold value as a pixel value range in which a contrast of the image is emphasized.   
     
     
         11 . The image processing apparatus according to  claim 1 , further comprising
 an ROI setting unit configured to set a region of interest according to one of a fixed two-dimensional area in the image and an imaging region of a subject.   
     
     
         12 . The image processing apparatus according to  claim 4 , further comprising a control unit configured to control the correction unit to cause, if the pixel value range to be emphasized is included in the pixel value range acquired by the characteristic acquisition unit, the correction unit to perform processing for emphasizing the pixel value range to be emphasized, and not to subject, if the pixel value range to be emphasized is not included in the pixel value range acquired by the characteristic acquisition unit, the image to contrast emphasis processing. 
     
     
         13 . An image processing apparatus comprising:
 an acquisition unit configured to acquire a reference pixel value for gradation conversion based on an X-ray image captured by an X-ray sensor;   a storage unit configured to store a pixel value range in which a signal-to-noise ratio is higher than a threshold value according to a characteristic of the X-ray sensor;   a first determination unit configured to determine whether the reference pixel value is included in the stored pixel value range; and   a second determination unit configured to determine a gradation conversion characteristic depending on a result of determination by the first determination unit.   
     
     
         14 . The image processing apparatus according to  claim 13 , wherein the acquisition unit acquires a statistical value in a subject area in the X-ray image as the reference pixel value,
 wherein, if it is determined that the reference pixel value is included in the stored pixel value range, the second determination unit determines the gradation conversion characteristic so that the center of a pixel value range in which gradation is widened by the gradation conversion becomes the reference pixel value, and   wherein, when it is not determined that the reference pixel value is included in the stored pixel value range, the second determination unit determines the gradation conversion characteristic so that a particular pixel value included in the stored pixel value range becomes the center of the pixel value range in which gradation is widened by the gradation conversion.   
     
     
         15 . The image processing apparatus according to  claim 13 , wherein the second determination unit suppresses noise for the X-ray image obtained when a dose of X-rays radiated onto a subject is a dose by narrowing gradation in a subject area in the X-ray image. 
     
     
         16 . A radiation imaging system comprising:
 the image processing apparatus according to  claim 1 ;   a radiation source;   an image sensor configured to detect X-rays emitted by the radiation source; and   a display unit configured to display an image obtained by the image sensor and processed by the image processing apparatus.   
     
     
         17 . An image processing method comprising:
 setting, based on an image captured by an image sensor, a pixel value range to be emphasized in the image;   acquiring a pixel value range corresponding to a dose at which a signal-to-noise ratio is higher than a threshold value for the image obtained from the image sensor;   determining whether the pixel value range to be emphasized is included in a pixel value range in which a characteristic of the image sensor is in a predetermined range; and   performing, if it is determined that the pixel value range to be emphasized is not included in the pixel value range in which the characteristic of the image sensor is in the predetermined range, correction so that the pixel value range to be emphasized is included in the acquired pixel value range.   
     
     
         18 . A non-transitory computer-readable storage medium storing an instruction to cause a computer to execute the image processing method according to  claim 17 .