A Comparison of JPEG and Wavelet Compression Applied to Computed Tomography Brain, Chest, and Abdomen Images
A Saffor, K Ng, A bin Ramli, D Dowsett
A Saffor, K Ng, A bin Ramli, D Dowsett. A Comparison of JPEG and Wavelet Compression Applied to Computed Tomography Brain, Chest, and Abdomen Images. The Internet Journal of Radiology. 2001 Volume 16 Number 1.
Background: A study of image compression is becoming more important since an uncompressed image requires a large amount of storage space and high transmission bandwidth. This paper focuses on the quantitative comparison of lossy compression methods applied to a variety of 8-bit Computed Tomography (CT) images.
Method: Joint Photographic Experts Group (JPEG) and Wavelet compression algorithms were used on a set of CT images, namely brain, chest, and abdomen. These algorithms were applied to each image to achieve maximum compression ratio (CR). Each compressed image was then decompressed and quantitative analysis was performed to compare each compressed-then-decompressed image with its corresponding original image. The Wavelet Compression Engine (standard edition 2.5), and JPEG Wizard (Version 1.1.7) were used in this study. The statistical indices computed were mean square error (MSE), signal-to-noise ratio (SNR), and peak signal-to-noise ratio (PSNR).
Results: Our results mostly agreed with other published studies, which show that Wavelet compression yields better compression quality at constant compressed file sizes compared with JPEG.
Conclusion: The degree of compression is dependent on anatomic structures and complexity of diagnostic information in the image so careful consideration must be given to the level of compression ratio before archiving clinical images otherwise essential information will be lost.
Image compression is fundamental to the efficient and cost-effective use of digital medical imaging technology and applications. There are two methods of image compression: lossless compression enables an exact reproduction of the original image from the compressed file. However, this scheme achieves relatively low compression rates of about 3:1. The second, lossy compression eliminates the redundant and high frequency data from an image, which is usually outside the range of human visual perception. This results in much higher compression ratios, typically 20:1 or greater, but with some data loss .
There are a number of techniques or compression algorithms for producing lossy or lossless images, it is quite important that the method used is based on an adopted medical imaging standard. Standards ensure simplified and long-term interoperability with other imaging devices; they also minimize the risk of diagnostic data being rendered obsolete over time . Digital Imaging and Communications in Medicine (DICOM) is a medical imaging standard for all imaging modalities. The DICOM standard supports RLE (Run Length Encoding) lossless compression, and JPEG lossy compression for both static and dynamic clips. RLE lossless compression is good for static images, typically achieving ratios of 3:1 for grayscale images . A variety of lossy compression techniques are available, some of them standardized e.g., JPEG has the advantage of being available as commercial products, but also has the disadvantage of creating block artifacts at respectable compression ratios, i.e., over 10:1. This is a consequence of its 8 x 8 discrete cosine transform (DCT) coding scheme .
Most current research efforts in lossy compression that appear promising involve the discrete wavelet transform (DWT), after the pioneering work by Dubieties . The reasons for this are that the DWT operates on the whole image as a single block thereby avoiding blocking artifacts typical in JPEG, while dynamically adjusting its spatial/frequency resolution to the appropriate level in various regions of the image. Wavelet compression methods appear to perform better than JPEG, particularly for large-matrix images such as radiographs using the dual pathology approach, compression ratios as high as 80:1. The goal of this paper is to compare Wavelet and JPEG lossy compression methods applied to a set of CT images using Wavelet Compression Engine (standard edition 2.5) available at , and JPEG wizard version (1.1.7) available at . The size of each image before compression is 512x512 x 8 bit.
Techniques commonly employed for image compression result in some degradation of the reconstructed image. A widely used measure of reconstructed image fidelity for an
Another quantitative measure is the peak signal-to-noise ratio (PSNR), based on the root mean square error of the reconstructed image. The formula for PSNR is given by
where RMSE, is calculated as
Values for these quantities were obtained using LuraWave Smart Compression software (version 1.7.1) available at .
Materials and Methods
By using the formulas in the previous section, some parameters were calculated for JPEG and Wavelet respectively. Signal-to-Noise-Ratio (SNR) measures are estimates of the quality of a reconstructed image compared with the original image. Mean square error (MSE) is the cumulative squared error between original and compressed image. Lower value of MSE means lesser error, and these values give higher peak signal-to-noise ratio (PSNR). Peak signal-to-noise ratio (PSNR) is another qualitative measure based on the root-mean-square-error of the reconstructed image. In our study we calculate the compression ratio, MSE, SNR, and PSNR for various sets of CT images: - 18,20,17 image sequence for brain, chest and abdomen respectively.
Results and Discussion
Figure 1 shows three different CT images, which are brain, chest, and abdomen. Tables 1, and 2, represent the results for compression ratio (CR), MSE, SNR, and PSNR for CT-brain by using JPEG and Wavelet compression. This result was also plotted in Fig. 2, 3, and 4. The results for chest and abdomen images are given in Tables 3 - 6 respectively. This result was also plotted in Fig. 5 - 10. The comparisons between the results are given in Table 7.
From the results of this study we conclude that the Wavelet compression can be used at higher compression ratios before information loss than JPEG for CT images. The Wavelet algorithm introduces less image errors, which yields higher PSNR for low bit rate. We have shown that in terms of image quality, the Wavelet algorithm is either equivalent or better than JPEG for these images.
Our results illustrate that we can achieve higher compression ratios for brain than chest and abdomen images and that the anatomical structure and its complexity have an effect on image compression. Furthermore we also observe that by using JPEG, for chest and abdomen images the PSNR values obtained were higher than those achieved by using Wavelet compression. For a lower compression ratio, JPEG yielded higher quality image than Wavelet compression. From the numerical values obtained we observe that for chest and abdomen images the PSNR is equal to 24dB for compression ratio up to 31:1by using JPEG, whereas for brain image the PSNR is equal to 22 to 34 dB for compression ratio between 52 to 240:1 by using Wavelet compression.
The degree of compression is dependent on anatomic structures and complexity of diagnostic information in the image so careful consideration must be given to the level of compression ratio before archiving clinical images otherwise essential information will be lost.