Comparison of Interpolative versus Full-Frame Cosine Transform Image Compression of Digital Chest Radiographs

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Abstract
Pixel subsampling is a simple and straight-forward method for digital image compression. Adjacent pixels in chest radiographs are highly correlated. Therefore, one may reduce the heavy burden of image data storage by subsampling pixels and reconstructing by interpolation whenever viewing is required. We performed a study on compression of chest radiographs using: (1) pixel subsampling followed by interpolation; and (2) a bit-allocation technique based on the full-frame discrete cosine transform (DCT). To obtain a compression ratio of 16:1, we subsample one pixel from each 4x4 pixel matrix. Bilinear and cubic spline interpolation are the two methods used in this study since they are simple and smooth interpolation methods. The more complex process of full-frame DCT yields optimal performance when the correlation of adjacent pixels are high [6]. By adjusting two quantization parameters in the full-frame DCT method, one can also achieve a 16:1 compression ratio. Our evaluation of pixel subsampling versus full-frame DCT uses: (1) statistical comparison of root mean-square errors (RMSE); (2) comparison of local structure fidelity; and (3) comparison of differences in subtraction images (original minus reconstructed).

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