Virtual microscopy stands for acquiring and handling digitized versions of microscope glass slides, which are in turn commonly referred to as "virtual slides" or "whole-slide images" (WSIs). Virtual slides are montage images consisting of up to thousands of high resolution digital micrographs (image tiles), as illustrated in Figure 1. Several types of computer-controlled motorised microscopes and dedicated slide scanners can be used for image acquisition (such as dotSlide, Mirax, and ScanScope). This video-clip shows the scanning procedure using a motorised microscope and the Surveyor software (by Objective Imaging Inc.). Stitching of the virtual slide image tiles into a montage image can be done, for example, by using our previously developed LargeMontage software.
Figure 1. The image tile-based acquisition of a virtual slide.
Since microscope specimens are often up to 20×30 mm in size, a virtual slide can contain up to 40 gigabytes of uncompressed image data (scanning resolution 0.2-0.5 μm per pixel). The amount of data increases further if scanning is done at a higher magnification or if several focus layers (along Z-axis) are scanned (e.g., in cytopathology). Due to the large size of virtual slides, all viewing systems described to date apply the on-demand principle: that is, only a user-requested area (with a desired resolution) of the virtual slide is decoded and displayed (Figure 2). Likewise, the use of lossy image compression is imperative. Lossy compression can yield a 10 to 30-fold compression ratio compared to lossless compression, without affecting the diagnostic properties of a virtual slide. Thus, virtual slides require an image format that is based on an effective lossy image compression algorithm, as well as one that provides sophisticated random access techniques.
Figure 2. Displaying and decoding only a user-requested area of a virtual slide.
Virtual microscopy has already turned out very useful for teaching in medical, dental and veterinary schools, and is also useful for clinical and research applications. Pathologists world-wide can access image servers containing virtual slides of extremely rare histopathologic or cytopathologic entities. Virtual microscopy is currently applied in Finland in pathology and cell biology education, various slide seminars as well as in quality control programs in clinical cytology and hematology laboratories.