Department of Radiology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637, USA.
Med Phys. 2009 Dec;36(12):5675-82. doi: 10.1118/1.3259712.
Temporal subtraction is used to detect the interval change in chest radiographs and aid radiologists in patient diagnosis. This method registers two temporally different images by geometrically warping the lung region, or "lung mask," of a previous radiographic image to align with the current image. The gray levels of every pixel in the current image are subtracted from the gray levels of the corresponding pixels in the warped previous image to form a temporal subtraction image. While temporal subtraction images effectively enhance areas of pathologic change, misregistration of the images can mislead radiologists by obscuring the interval change or by creating artifacts that mimic change. The purpose of this study was to investigate the utility of mutual information computed between two registered radiographic chest images as a metric for distinguishing between clinically acceptable and clinically unacceptable temporal subtraction images.
A radiologist subjectively rated the image quality of 138 temporal subtraction images using a 1 (poor) to 5 (excellent) scale. To objectively assess the registration accuracy depicted in the temporal subtraction images, which is the main factor that affects the quality of these images, mutual information was computed on the two constituent registered images prior to their subtraction to generate a temporal subtraction image. Mutual information measures the joint entropy of the current image and the warped previous image, yielding a higher value when the gray levels of spatially matched pixels in each image are consistent. Mutual information values were correlated with the radiologist's subjective ratings. To improve this correlation, mutual information was computed from a spatially limited lung mask, which was cropped from the bottom by 10%-60%. Additionally, the number of gray-level values used in the joint entropy histogram was varied. The ability of mutual information to predict the clinical acceptability of a temporal subtraction image was evaluated through receiver operating characteristic (ROC) analysis.
The mean correlation coefficient obtained between mutual information computed on constituent images and the subjective rating of temporal subtraction image quality was 0.785. ROC analysis yielded an average Az value of 0.852 for the ability of mutual information to distinguish between temporal subtraction images of clinically acceptable and clinically unacceptable quality.
The results of this study establish a relationship between mutual information and temporal subtraction registration accuracy and demonstrate the ability of mutual information to objectively indicate the presence of misregistration artifacts.
时间减影用于检测胸部 X 光片中的间隔变化，并帮助放射科医生进行患者诊断。该方法通过对先前放射图像的肺区域（即“肺掩模”）进行几何变形，将两个时间不同的图像进行配准。当前图像中每个像素的灰度值从变形后的先前图像中对应像素的灰度值中减去，形成时间减影图像。虽然时间减影图像有效地增强了病变区域，但图像配准不当会通过掩盖间隔变化或通过创建模仿变化的伪影来误导放射科医生。本研究的目的是研究在两个已注册的胸部放射图像之间计算互信息作为区分临床可接受和临床不可接受的时间减影图像的度量的效用。
一位放射科医生使用 1（差）到 5（优）的评分对 138 张时间减影图像的图像质量进行主观评分。为了客观评估时间减影图像中显示的注册准确性，这是影响这些图像质量的主要因素，在进行减影之前，在两个组成的注册图像上计算互信息以生成时间减影图像。互信息衡量当前图像和变形的先前图像的联合熵，当每个图像中空间匹配像素的灰度值一致时，会产生更高的值。互信息值与放射科医生的主观评分相关。为了提高这种相关性，从底部裁剪 10%-60%的空间有限的肺掩模中计算互信息。此外，还改变了联合熵直方图中使用的灰度值数量。通过受试者工作特征（ROC）分析评估互信息预测时间减影图像临床可接受性的能力。
在组成图像上计算的互信息与时间减影图像质量主观评分之间获得的平均相关系数为 0.785。ROC 分析得出，互信息区分临床可接受和临床不可接受质量的时间减影图像的平均 Az 值为 0.852。
本研究结果建立了互信息与时间减影注册准确性之间的关系，并证明了互信息客观指示配准伪影存在的能力。
