Introduction
Digital pathology Whole Slide Images (WSI) are gigapixel sized images, that need to be created in less than a minute on a fast whole slide scanner such as the Philips Ultra Fast Scanner.
Additionally a suitable image format for WSI needs to support fast zooming and panning. Currently the most common image format used for storing whole slide images is the TIFF image format with the JPEG image compression method. While this satisfies the speed requirements both for image compression as well as for fast image decompression in web applications, the JPEG compression standard is a consumer grade compression method, and not optimal for medical grade images. In particular the decimation of the color resolution and the low dynamic range of 8 bit per channel and the limitation to three color channels is problematic.
Additionally JPEG is not an hierarchical image compression method, meaning that it is not possible to quickly retrieve a lower resolution representation of a subregion of an image. This problem is typically overcome by storing multiple lower resolution versions of the image in the same TIFF file - often referred to as the pyramid TIFF format; this however results in a 30% larger image file at the same image quality level, or in a lower image quality at the same file size. The limitations of the now 24 year old JPEG standard are well known, and in the year 2000 the JPEG group created JPEG 2000 to supersede the JPEG standard. JPEG 2000 takes away most if not all limitations in JPEG that make it less suitable for medical grade images: it supports high dynamic range images (up to 32bit per channel), it does not force a color resolution decimation, and it supports multiple color channels. In addition, because it is a wavelet based compression method it is inherently a hierarchical image format which allows for fast retrieval of lower resolution versions of the original image without the need for storing the redundant pyramid of lower resolution versions of the image as required by the JPEG TIFF image format.
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