These are some of the common questions asked about the wavelet images created by Curator:
How does lossy wavelet compression compare with other supported compression types?
Can I configure certain stations/roles to view lossy images and others to view lossless images?
Q. If lossless wavelet images are comparable to lossless JPEG images, why keep the lossless JPEG images?
A. Although the display is identical, the lossless wavelet algorithm takes approximately three times as long to decompress as lossless JPEG. Keeping the original lossless JPEG images in the image cache allows users located within the hospital network to retrieve images faster than if they had only the wavelet version. However, on slower networks where bandwidth is a concern, an adaptive download format like wavelet is required for responsive image display. Lossless wavelet images are the equivalent of DICOM images and can be used for diagnosis.
Q. How does lossy wavelet compression compare with other supported compression types?
A. In a sample test using standard compression ratios, the following performance metrics were observed:
 | Note: Network bandwidth and adaptive download display also affect response times when displaying images. This test excluded these additional effects and concentrated only on decompression speed. |
| Factor | Lossy wavelet | Lossless JPEG | Lossy JPEG | Run Length Encoding (RLE) |
|---|---|---|---|---|
| Decompression speed (MB/sec) | 4.6 | 15.0 | 10.7 | 45.5 |
| Improvement over lossy wavelet | — | 3.3 | 2.3 | 9.9 |
Based on these numbers, the lossy wavelet compression ratio must be 3.3 times the lossless JPEG ratio to achieve similar decompression speeds.
Using the sample numbers and the typical compression ratios for lossless and lossy JPEG for color and grayscale data, the following table summarizes the lossy wavelet compression ratios that provide equivalent display performance:
| Format | Typical compression ratio | Equivalent lossy wavelet compression ratio |
|---|---|---|
| Lossless JPEG | 2.5:1 | 8.25:1 |
| Lossy JPEG color | 10:1 to 15:1 | 33:1 to 50:1 |
| Lossy JPEG grayscale | 30:1 | 99:1 |
Q. Can I configure certain stations/roles to view lossy images and others to view lossless images?
A. Yes. Using the Web Compression Manager, you can configure lossy wavelet compression in the web cache. You can then tailor stations and users requiring original image quality see the original lossless images and others to view the lossy web representations. Doing this has the following benefits:
Improved network utilization, as less image data is transmitted. All users benefit from speed improvements due to less image data flowing across the network.
Those viewing lossy wavelet representations get very good performance due to the small amount of data to transmit and decompress.
Q. Can I configure the system so that original DICOM images are shown for the current study, but priors are viewed as lossy wavelet?
A. You cannot directly configure IMPAX to show different representations based on study status; however, by configuring IMPAX with a large web cache and a small image cache and setting the user default in the List area to Show Original Images, you can achieve a similar effect.
If Show Original Images is enabled, images are retrieved using the following priority:
Display the original image if it is in the image cache.
If the image is not in the image cache, display the web representation from the web cache.
If the image is not in the web cache, ask the user if they want to retrieve the image from the archive.
Images retrieved from the archive are processed by Curator and the web representation is presented.
Because the image cache is small, the original image should still exist in the image cache, but the relevant priors will not. However, because the web cache is large, these images should still exist in the web cache and can be displayed without further delay. Therefore, the original images for the current study are displayed and the web representations of relevant priors are displayed.