Compression shrinks digital mammograms down to practical size

Wavelet compression can shrink large, unwieldy digital mammography images by a factor of 100 without any loss in image quality, according to a German study presented at the ECR. This finding has tremendous potential benefits for productivity, transmission speed for teleradiology, and PACS storage costs.

The study, which involved a standard wavelet compression algorithm, indicates there is little difference in quality between a standard size original mammogram of 33 MB and one that has been compressed to as small as 325 KB, said Dr. Reinhard Loose, head of radiology at the Clinic Nuremberg.

Image size varies depending on the modality. Large image size is particularly problematic in mammography because some health authorities, such as the FDA, currently do not allow lossy compression. Studies showing equivalent image quality could help bolster the case in favor of lossy compression, which is permitted in other modalities.

In the Nuremberg study, 12 skilled radiologists with more than four years of experience in mammography each compared original and compressed digital mammography images for three patient groups: normal results, microcalcifications, and small carcinomas.

The radiologists rated image quality and ranked images for both computed radiography and flat-panel digital mammography. In cases where there was a loss in quality, they were asked if the image was still of diagnostic quality.

The compression rate threshold was set at 100 because researchers presumed loss of quality would occur somewhere beneath that level. But the results came as a surprise, Loose said.

"There was no statistically significant impact on quality or visible difference in images for compression rates up to 100," he said.

The researchers now plan to analyze the effect of increasing compression to the 200 level.

Even if radiologists played it safe by compressing images by a factor of just 30, they would be able to send an image in one minute over a standard ISDN line, Loose said.