An insight into digital pathology

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After 25 years of being conjoined with my microscope and reliant on glass slides for histopathological and cytological diagnoses, I was recently faced with embracing digital pathology. This was a concept that, I must admit, came with a little scepticism and trepidation on my part. However, in this ‘post-COVID lockdown’ world, there have been a lot of challenges facing our industry, not the least being the difficulty in training, employing, and retaining veterinarians and those in related specialities. 

The shift toward flexible and part-time working hours and working remotely have also contributed to the complexity of providing a consistent and full service to our patients and clients. In diagnostic veterinary pathology, digital pathology (not artificial intelligence) has somewhat helped to mitigate some of these issues. Digital pathology refers to the use of scanners to produce images of glass histopathological and cytological slides. These images can then be reviewed by a pathologist using a computer terminal rather than a microscope. This technology is already relatively well-established and is becoming widely utilised. But while it continues to improve, there still remains some limitations.

Firstly, it is important to note that digital pathology does not do away with the need to produce high-quality cytological smears and histopathological slides. The quality of the scans are always directly limited by the quality of the slides or smears, and a bad quality slide will be just as poorly-diagnostic on glass as it is in the scanned image.

The slides that are produced in-clinic or in-lab must be barcoded, stained and cover slipped. They are then placed in a state-of-the-art automated slide scanner that scans the slides, typically at a 40x magnification. The time taken to scan these slides is variable and dependant on the amount and size of the tissue on the slide (approximately 20-50 slides per hour). This can be an important limiting factor when you consider that some cases may require 10 or more slides. 

Most scanners have an automated function to identify the diagnostic tissue, and focus on this. Alternatively, the area of interest may be manually selected. The glass slides are then filed and can be retrieved if microscopic review is required. The digital images are collated (using barcode identification) and stored for review by a pathologist. 

These digital files are very large and a single slide scan can generate a file size ranging from approximately 0.45GB to 3.5GB. Understandably, data storage is an important consideration with regard to capacity, accessibility, cost, privacy and security. As a result, although the glass slides may be archived for several years, the digital files are usually only stored for weeks or months. 

The digital slides are now available for review by a pathologist anywhere and anytime. However, there is requirement for minimum internet speed (generally greater than 15-20 Mb/s) and large high-resolution computer monitors for routine diagnostic work.

The slides are reviewed utilising calibrated software, an image management system (IMS) that allows for zoom function, slide rotation, slide navigation and has a host of annotation functions. The latter includes the ability to place circles, arrows, rulers, text and measure surface area. 

Figure 1: Photo of a laboratory slide scanner with a capacity of 300 slides. This is linked to an image management system (IMS) where the images can be viewed by the pathologist. Figure 2: A low magnification image of a mast cell tumour with a 2.37mm2 box placed for mitotic count, and deep and circumferential tissue margins measured using the calibrated ruler. Figure 3: Simultaneous review of two linked slides is possible. In this example a 2 x H&E-stained section on the left is linked to a 2x ZN-stained section image on the right to facilitate identification of mycobacterial organisms within a granuloma. Figure 4: An image of the same ZN-stained section at 40x magnification. Pink mycobacterial organism clusters are indicated by an arrow and circle. The diameter of the granuloma is measured (dotted line). Figure 5: A low-magnification image of a cutaneous malignant melanoma. Numerous mitoses are counted in the box to the left of the image and tissue margins are measured (right of image).

Furthermore, there are very useful screen capture functions for quickly and easily taking photomicrographs and there is the ability to share slide images in real time with colleagues (although this requires having the same IMS software). Slides can also be easily reviewed, at the click of a mouse, while discussing a case with a veterinarian on the phone. 

Multiple slides can be viewed simultaneously, and separate slides can be linked or overlaid so that similar areas can be compared. This function is particularly useful when evaluating special stains and immunohistochemistry in conjunction with the H&E-stained slides. 

The zoom and annotation functions are particularly useful in evaluating tissue margins in tumour pathology and for mitotic counts. Mitotic counts are taken from within a 2.37mm2 box and this equates to the traditional 10 HPF (40x) microscopic fields.  

Counting the percentage of positive cells in some immunohistochemical stained slides (e.g. Ki67 indexes) are also accommodated by this software without the need for separate manual counters.

There are, of course, currently some limitations to digital pathology. As mentioned above, there is requirement at the pathologist’s end for high-speed internet, the ability to store or download large files and the need for one or more large high-resolution monitors. Additionally, the image resolution may limit the ability to identify some very small structures (e.g. bacterial organisms) and assess tissue structures for refractile/birerefringent qualities. The digital images are by definition strictly two dimensional whereas microscopic evaluations of glass slides allow for focusing in different planes. Inspection of tissues at magnifications greater than 40x requires oil immersion and this cannot be done on scanned slides. Fortunately, in these circumstances, the glass slide always remains available to the pathologist for microscopic review where it is deemed necessary. 

By nature of their precision and sensitivity, the scanners can also be prone to technical and mechanical malfunction—such as lens contamination with wet coverslip mounting media, which may contribute to scanner downtime and require reverting to the use of glass slides. 

The costs (typically borne by the laboratory) of scanners, technicians, scanner maintenance, digital storage, computer software and hardware are also significant and may be the limiting factor for some small laboratories. Regardless, this is outweighed by the ability to utilise pathologists working remotely and helps with staff retention. 

Digital pathology also provides the ability to offer second opinions on cases and can be utilised in training pathologists, both locally and remotely. Interesting cases can be flagged and stored in personal files for studies or publication, and experts can be consulted on cases even if they are overseas.

Digital pathology is here to stay and although there remains some limitations, this utility continues to make great improvements, and has become a routine method used by pathologists around the world. I’m no longer sceptical.


Dr Michael Rozmanec BVSc MVSc MACVSc DipACVP MRCVS

digital pathology

QML Vetnostics

Dr Michael Rozmanec is a diagnostic veterinary pathologist working at QML-Vetnostics in Brisbane. He graduated from Sydney University in 1994 and initially worked in general practice in Newcastle and Brisbane. 

Dr Rozmanec completed an internship in veterinary pathology and gained his master’s degree from the University of Queensland before returning to Sydney to work in diagnostic pathology. 

In 2005 he completed a residency in anatomic pathology at Cornell University and then become a Diplomate of the American College of Veterinary Pathology. 

A few years later he travelled to the UK with his family where he co-owned a small independent diagnostic laboratory for eight years. 

Dr Rozmanec recently returned to Australia to be nearer to family and enjoy the sun again.

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