Bringing Digital Pathology to Life
for Rapid Entire Slide Capture
Line scanning is a significant advancement in rapid, high resolution virtual microscopy and makes it practical to create seamless digital slides of an entire microscope slide with superior image quality and speed. Line Scanning versus Image TilingImage tiling involves the capture of multiple small regions of a microscope slide using a traditional CCD camera. Image tiles are stitched together (aligned) to create a large contiguous digital image (mosaic) of an entire slide. Image tiling systems can take hours to capture and align the thousands of tiles that are required to create a contiguous image. Line scanning is a superior methodology to image tiling. ScanScope scanners do not use a fixed-area camera to capture thousands of individual image tiles but instead employ linear-array detectors in conjunction with specialized motion control components, a microscope objective lens and customized optics. As a result, ScanScope scanners efficiently capture a small number of contiguous overlapping image stripes. Whereas image tiling is inherently stop-and-go, ScanScope scanners continuously move microscope slides during the acquisition of imagery data. This ability to capture imagery data while the sample is moving is a key reason why line scanning is ideally suited for rapid slide digitization. The table below illustrates the number of image tiles and image stripes required to digitize different areas of a slide. Approximately 28,000 image tiles must be captured and aligned to create a seamless digital slide of a 30 x 20 mm area of a slide at a scanning resolution of 0.25 µm/pixel (40x). In contrast, only 60 image stripes must be captured and aligned using the line scanning method employed by the ScanScope. This example illustrates one of the fundamental advantages of line scanning: the capture and alignment of a small number of image stripes is dramatically more efficient than the capture of thousands of image tiles. 
Table highlighting the number of image stripes versus the number of image tiles required to digitize different areas of a slide. The ScanScope also supports scanning 1k x L image stripes, in which case number of image stripes should be doubled.
Inherently Superior Image QualityIn addition to rapid slide digitization, the ScanScope achieves superior image quality. Reasons include: Optimal focus. Automatic focus adjustments are made rapidly to compensate for subtle changes in the topology of a specimen. ScanScope scanners adjust focus from one scan line to the next, approximately 1000 times more frequently than image tiling systems. The focus in an image tiling system is inherently limited to one focus position for each individual image tile. This limitation is most noticeable at high resolution when it is not possible to maintain adequate focus over an entire image tile. 
(Left) Optical aberrations in an image tiling system are 2-dimensional (circularly symmetric). (Right) In contrast, line scanning is free from optical aberrations along the scanning axis.
No optical aberrations along the scanning axis. ScanScope image stripes are free from optical aberrations along the scanning axis. The line being scanned is always exactly on the optical axis of the objective lens. In contrast, image tiling systems suffer from 2-dimensional optical aberrations. Seamless images. Aided by near-perfect image quality along the scanning axis (and relatively few image stripes), ScanScope scanners align image stripes while scanning. When a scan is completed a seamless digital slide can be viewed immediately. In contrast, image tiling systems are not able to create seamless images while scanning. This is because the inherent 2-dimensional optical aberrations and the large numbers of tiles prohibit alignment without complex image alignment software. 24-bit color at each digital slide pixel (without interpolation). The ScanScope linear-array detector comprises individual red, green and blue detectors. The benefit is 24-bit color data that is sampled at each digital slide pixel. In contrast, the CCD cameras employed in image tiling systems measure only 8 bits of one color at a pixel. 24-bit color data is created by interpolating between 8-bit data measured at non-adjacent pixels (e.g., using a Bayer mask). Interpolation results in a loss of resolution and in unnecessarily large images. Three chip CCD cameras can overcome the disadvantages of interpolation, but also result in a loss of resolution due to practical difficulties in aligning the three color-specific focal plane arrays. 
Left: Line scanning with ScanScope captures 24-bit color imagery data (8 bits per pixel) at each pixel. Right: Tiling systems based on Bayer mask CCDs interpolate between non-adjacent pixels, which results in loss of resolution and unnecessarily large images.
The ScanScope slide scanning method is simple, reliable, and fast, making for a system that is practical and cost effective. To learn more about the technical challenges involved in digitizing an entire glass slide at diagnostic resolution, download our white paper about the technical factors to consider in virtual microscopy. |
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