This section includes recommendations and important information to consider when scanning different types of specimens mounted on microscope slides.
Scanning parameters: optimization by specimen type
When configuring the imaging profile in NZ, it is important to define several parameters, including bjectives, scan area, split tissue setting, number and positions of focal points, and the number and spacing of Z-stack layers. The parameter settings depend on the specimen type and size.
Objectives: The NZ system uses a 20X objective with a precision optical coupler that allows scanning in two modes, 20X with a 0.46 μm/pixel resolution and 40X with 0.23 μm/pixel resolution. This feature has the advantage that the time of scanning in both modes is similar (i.e, scanning speed of a 15 x 15 μm area will take ~30s in 20X and 40X).
Specimens such as insects, wood tissue, and coal ball peels can be imaged using both modes, 20X and 40X, while 40X is preferred for microfossils, such as pollen, diatoms, and radiolarians.
Scan area: The size of the selected scan area depends on the type of sample or specimen being imaged.
For samples with microfossil specimens (pollen, spores, dinoflagellates, radiolarians, including diatoms and phytoliths), which have hundreds to thousands of individual specimens per slide, we recommend selecting one or two scan areas of approximately 20 mm x 20 mm, depending on the cover slip size. This is generally sufficient to image most specimens, while keeping file sizes manageable for analysis (< 30Gb), facilitating storage, management, and sharing of files among collaborators.
For larger specimens (>0.5 µm) such as cuticles, wood samples, fungi tissue, coal ball peels, and small invertebrates (e.g., ostracods and insects), the size of the scan area varies, based on the size and/or number of specimens present on the slide. We recommend avoiding scanning the edges of the cover slip as this can interfere with the scanner’s ability to detect and focus on the specimen.
Focus points: The focal points are initially set automatically when defining the profile settings. However, even when a specific number is selected, the NZ may automatically adjust the number of focus points based on material detection, and in those cases, these points can be manually modified or moved.
For microfossil slides with 400 mm2 selected scan areas, we recommend five focal points per subdivided scan area (see Split tissue ); for larger scan areas, nine points are recommended. However, autofocus success depends critically on the method of slide preparation. Automatically placed focal points typically produced sharp images if the microfossils were applied to the coverslip, which was then inverted and affixed to the slide with mounting medium of uniform thickness.
Manual focal point adjustment is commonly required for historical collections, where the mounting medium shows signs of degradation, such as coloration, or when the material was mixed with mounting medium and then applied to the slides. In these cases, the scanner’s automatic focal points often failed to detect material or lost focus because specimens were distributed unevenly across focal planes, and/or the mounting medium had deteriorated (dried and shattered), and the coverslip had deformed.
Manual focus is also recommended for slides in which the mounting medium has shattered, but the microfossils are preserved in good condition and can provide important information for biostratigraphy, or ecological and evolutionary inferences.
For larger specimens (>0.5 µm), the optimal number of focal points will vary based on the size of the specimen. For specimens that occupy the entire cover slip, using the “split tissue” function (see Steps section) is critical, as well as assigning five or nine focal points per subregion. For smaller fragments, such as cuticles, five points are generally sufficient, but the number may be adjusted according to the scan area.
Split tissue: For microfossils, as well as larger specimens, in which the scan area is larger than 10 mm x 10 mm, we recommend using the automated split function to divide the scanned area into several equal-sized subareas to increase the likelihood that most of the material will be in focus.
For slides with multiple large fragments or specimens (>0.5 µm), we recommend manually drawing scan areas around each specimen, rather than relying solely on automatic subregion detections (e.g., invertebrates and cuticle fragments). This allows the size of the scan area to vary based on the size of each specimen/fragment, improving focus.
Z-Stack: The Z-stack settings (number of layers and spacing between them) vary based on the specimen size and the details of morphological information that can be captured in the Z-space (see more in the associated manuscript). It is important to highlight that the number of layers selected is always an odd number because the middle layer is the central focal plane, which is set using the focus points, then the layers above (+) and below (-) are numbered accordingly. For example, if 15 layers are scanned, the microscope will image the middle layer plus seven above (layers +1 to +7) and seven below (layers -1 to -7).
For microfossils, the optimal Z-stack configuration depends on the depth of individual microfossils and how they are distributed along the Z-axis. For palynological slides (with pollen, diatoms, and radiolarians) from NHC, the Z-stack settings recomended is between 25 and 30 layers at 1 µm intervals because has proven effective in capturing not only the three-dimensional structure of individual microfossils but also most of the microfossils in the slide. For other slides in which all specimens are in a similar Z-plane because the concentrate was dried onto the underside of the cover slip before mounting, a Z-stack comprising 15 to 21 layers at 1 µm intervals may be sufficient to capture the full depth of the material along the Z-axis.
For larger specimens (>0.5 µm), the number of z-stack layers varies with the size of the specimen. Most samples of leaf cuticles, wood and fungi sections, and coal ball peels can be scanned with three to five planes spaced three to five µm apart on the Z-axis. This produces focused images in spite of undulations in the specimens. For small invertebrates (e.g., fleas, beetles, and ostracods), the focal plane layers can vary from 31 to 81, according to the depth of the specimens. The intervals between layers can vary within 3 to 4 µm intervals for insects, while for ostracods and smaller insects, such as white flies, 2 µm intervals between layers capture more morphological details.
The NZ can scan an area of 15 mm x 15 mm in approximately 30s, this in both 20X and 40X modes. The final NDPI file always records the total scanning time, as well as the timing of the three processes included within it, which are focusing, scanning, and writing. Focus time refers to the period during which the NZ reviews each focal point to locate the material to be scanned and to define the middle focal plane. This time depends on the total number of focal points per slide. Scanning time corresponds to the time the NZ spends scanning a single slide and depends on the selected scan area and the number of focal planes. Writing time refers to the period during which the software generates the final NDPI file.
This time does not include manual operation of the scanner, which is defined by the time required to load and unload slides, set the scanning profile, and replace slides. This manual time varies from approximately 5 to 20 minutes. This includes 5 to 10 minutes uploading the slides, setting the scanning profile, and reviewing the profile settings in each sample, especially the scanning area and focal points. The other additional time includes unloading the slides and closing the system, or uploading another set of slides and restarting the scanning process.
It is important to highlight that once the settings for the first slide are finalized, the scanning can begin, while the parameters of the remaining slides are reviewed and adjusted. Therefore, this manual effort does not delay the scanning process, as it can be performed in parallel with scanning, and only the time of uploading and unloading the slides can be added to the total scanning time provided by the NZ.