  Whole slide imaging (WSI) has moved digital pathology closer to diagnostic
practice in recent years. Due to the inherent tissue topography variability,
accurate autofocusing remains a critical challenge for WSI and automated
microscopy systems. The traditional focus map surveying method is limited in
its ability to acquire a high degree of focus points while still maintaining
high throughput. Real-time approaches decouple image acquisition from focusing,
thus allowing for rapid scanning while maintaining continuous accurate focus.
This work reviews the traditional focus map approach and discusses the choice
of focus measure for focal plane determination. It also discusses various
real-time autofocusing approaches including reflective-based triangulation,
confocal pinhole detection, low-coherence interferometry, tilted sensor
approach, independent dual sensor scanning, beam splitter array, phase
detection, dual-LED illumination, and deep-learning approaches. The technical
concepts, merits, and limitations of these methods are explained and compared
to those of a traditional WSI system. This review may provide new insights for
the development of high-throughput automated microscopy imaging systems that
can be made broadly available and utilizable without loss of capacity.
