Watermarking scheme proposes to enhance image security

The rapid spread of digital imaging and teleradiology raises the specter of malicious interception of and tampering with sensitive medical data. Now researchers in China have reported a lossless watermarking method capable of verifying the authenticity and integrity of medical images.

"The watermarked image may still be used for diagnostic purposes, since this scheme has the capability of not introducing any embedding-induced distortion in the region of interest of a medical image," said Xiaotao Guo, Ph.D., of the biomedical engineering department at Shanghai Jiaotong University.

Digital watermarking is an emerging technology that allows users to add hidden copyright notices or other verification messages to digital files. The watermarked image used in a medical environment can still conform to DICOM format.

Although a reversible watermark allows exact recovery of the original host image, it is still desirable to keep the embedding distortion to a minimum. One reason is that the ROI may be different for different applications.

"Ideally, we hope the difference between the watermarked image and the original image is imperceptible," Guo said.

Guo evaluated the embedding capacity under different distortion levels for a large number of medical images (J Digit Imaging 2007 Jul 10; [Epub ahead of print]).

"Experimental results indicate that this scheme achieves high embedding capacity with low levels of embedding-induced distortion," he said.

One limitation of this method is the lack of a way to automatically identify the ROI, Guo said. This now requires manual intervention, because the ROI may vary significantly for different organs or modalities. Manual intervention can be reduced, however, if the nature of a particular family of images is understood.

"If we know the host image is a head CT, then it is reasonable to deduce that the regions outside the skull are non-ROI," he said.

The healthcare industry has taken the security threat seriously. Not only has the federal government issued mandates for ensuring health data security in the form of the Health Insurance Portability and Accountability Act, but Part 15 of the DICOM Standard specifies security profiles and technical means to implement security policies (PS 3.15-2001).

Also, the Society for Computer Applications in Radiology (now the Society for Imaging Informatics in Medicine) issued two editions of a primer, Security Issues in the Digital Medical Enterprise, one in 2000 and an update in 2004.