Whole Slide Imaging (WSI) systems are high-throughput automated microscopes for application in digital pathology. Optimum image quality of the final digital tissues slides is made possible by safeguarding the optical quality of the WSI system. We developed a simple and cost-effective method to test and monitor the optical quality in the manufacturing phase and during the operational lifetime of the WSI system.
An optical imaging system like a WSI system is characterized by its Optical Transfer Function (OTF), which is a measure for the signal level in the optical image as a function of the spatial scale of the imaged detail. In our method (
Shakeri 2015a) this OTF is extracted from a set of through focus images of a custom made target slide (see top panel A). An image analysis algorithm is used to find the OTF from the measured steepness of the black-to-white edges of different orientations. This OTF is subsequently used to extract the quantitative level of the type of optical errors in the WSI system, the so-called primary optical aberrations. In particular, the best focus line (focus level of maximum OTF as a function of spatial scale of image detail) can be used to assess the level of astigmatism, field curvature and spherical aberration. The phase of the OTF (measuring the shift of the center of the black-to-white edges with respect to their native position) at this best focus level (see bottom panel B) is used to measure the amount of coma in the WSI system.
The value for the primary aberrations that are measured to be present in the WSI system appear to depend asymmetrically on the position in the field of view (to the left or right of the imaged field on the camera). Top panel A shows the best focus as a function of field position for black-to-white edges of different orientations. The curves are shifted and tilted and cross at two points (the “nodes”). These asymmetries can be characterized using the so-called Nodal Aberration Theory, in which the asymmetries are linked to specific mechanical misalignments in the optical system. We found out (
Shakeri 2015b) that this framework applies well to WSI systems, and, moreover, that it can be used to monitor specific misalignments (tilt an decenter) of optical components such as objective lenses and tube lenses. Bottom panel B shows the “astigmatism” (the difference in best focus of black-to-white edges of mutually perpendicular orientation) as a function of the position within the field of view and the effect of misalignments thereon. The relation between field dependent optical aberrations and specific misalignments opens up the potential to use this quantitative characterization for tracking down and removing root causes of sub-optimal image quality.