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Objective and Subjective Evaluation of Compressed Computed Tomography (CT) Images

机译：压缩计算机断层扫描（CT）图像的主观和主观评估

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Background: Techniques commonly employed for image data compression result in some degradation of the reconstructed images. Evaluation of the quality of medical image compression remains an important issue. In our study, we evaluated the diagnostic quality of the compressed Computed Tomography (CT) images using both objective and subjective tests. Methods: Three different CT images namely brain, chest, and abdomen were compressed and reconstructed by using Wavelet Compression Engine software (standard edition 2.5) for different compression ratios. Both objective and subjective methods were evaluated for 9 reconstructed images. Thirteen observers from Department of Radiology at the University of Malaya Medical Center (UMMC) carried out the subjective test. Results: The Peak-Signal-to-Noise-Ratio (PSNR), which represents the quality of reconstructed images obtained, was between 57 to 36 dB and 52 to 34 dB for chest and abdomen images respectively, whereas for brain it was 57 to 40 dB. All these images were compressed until 30:1 compression ratio. Conclusion: By using subjective test, compression ratio of 30:1 is acceptable for diagnosis chest and abdomen images, and 20:1 for brain images. Introduction Evaluation of the quality of medical image compression remains an important issue, by both objective and subjective means. Image quality has two implications: fidelity and intelligibility. The former describes how the reconstructed image differs from the original one, with mean-square-error (MSE) as a typical example, and the latter shows the ability through which the image can offer information to people, with classification-accuracy. Both are fundamental in measuring image quality. It must be pointed out that fidelity is not always objective and intelligibility is not always subjective. Whether an objective measure on image quality is efficient or not depends strongly on its accordance with subjective measure [1]. Most methods for compressing data have been evaluated on the basis of minimizing an objective distortion measure such as MSE at a given level of data compression [4,7]. However, a lower MSE does not always mean better quality in the compressed image because MSE is not necessarily a subjective measure of the quality. Physiological, anatomical, and psychophysical aspects have been used to study visual perception [2,6]. It has been shown that the human visual perception system is sensitive to changes in luminance rather than the absolute luminance values themselves, and that perception is most sensitive to mid-frequencies and less sensitive to high frequencies in the image [2]. In this paper, we attempt to evaluate both objective and subjective methods for an acceptable degree of the reconstructed CT images for different compression levels. Materials and Methods Methods for image quality evaluation can be classified as objective and subjective measures. By objective measures some statistical indices are calculated to indicate the reconstructed image quality and by subjective measure viewers read images directly to determine their quality. Objective measure A widely used measure of reconstructed image for an N x M size image is the mean square error (MSE) as given by [5].
机译：背景技术：通常用于图像数据压缩的技术导致重构图像的某些退化。医学图像压缩质量的评估仍然是一个重要的问题。在我们的研究中，我们使用客观和主观测试评估了压缩计算机断层扫描（CT）图像的诊断质量。方法：使用Wavelet Compression Engine软件（标准版2.5）针对不同的压缩率，压缩并重建三个不同的CT图像，即大脑，胸部和腹部。客观和主观方法都针对9幅重建图像进行了评估。马来亚大学医学中心放射学系的十三名观察员进行了主观测试。结果：代表所获得的重建图像质量的峰值信噪比（PSNR）在胸部和腹部图像之间分别为57至36 dB和52至34 dB之间，而对于大脑，其为57至60 dB。 40分贝。将所有这些图像压缩到30：1的压缩比。结论：通过主观测试，诊断胸部和腹部图像的压缩比为30：1，对于大脑图像的压缩比为20：1。简介无论是通过客观还是主观的方法，对医学图像压缩质量的评估仍然是一个重要的问题。图像质量有两个含义：保真度和清晰度。前者以均方误差（MSE）为例，描述了重建图像与原始图像的区别，后者显示了图像能够通过分类准确度向人们提供信息的能力。两者都是衡量图像质量的基础。必须指出，忠诚并不总是客观的，可懂度也不总是主观的。图像质量的客观度量是否有效，很大程度上取决于其与主观度量的一致性[1]。在给定的数据压缩级别上，基于最小化客观失真度量（例如MSE），已经评估了大多数压缩数据的方法[4,7]。但是，较低的MSE并不总是意味着压缩图像的质量更高，因为MSE不一定是质量的主观衡量指标。生理，解剖和心理方面已用于研究视觉感知[2,6]。已经表明，人类视觉感知系统对亮度的变化敏感，而不是对绝对亮度值本身敏感，并且感知对图像中的频率最敏感，对图像的高频最不敏感[2]。在本文中，我们尝试评估在不同压缩水平下可接受程度的重建CT图像的客观和主观方法。材料和方法图像质量评估的方法可分为客观和主观措施。通过客观测量，可以计算一些统计指标以指示重建的图像质量，而通过主观测量，观看者可以直接读取图像以确定其质量。客观度量对于N x M尺寸图像，重建图像的一种广泛使用的度量是均方误差（MSE），由[5]给出。

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