Keywords

1 Introduction

Since the development of confocal microscopy technology in 1957, a long journey has brought us to the analysis of digital images of hematoxylin and eosin, and artificial intelligence as applied to the field of histology. Progress in ex vivo confocal microscopy technology now allows immediate visualization of freshly excised tissue, in less than 3 min and at the bedside. This instant diagnosis has opened the door to significant progress in the field of dermatology and also in general pathology [1,2,3,4].

Not all the journey has been easy and represents more than 20 years of continuous progress. From a slow device with white and gray images divided into squares, to a fast device with real- time digital hematoxylin and eosin (H&E) images obtained by strip mosaicking, this road began with one modest first step (Fig. 16.1). Technology, as applied to ex vivo confocal microscopy, has steadily improved over the years. Furthermore, not only has the technology changed, but staining methods have also changed over time, along with methods of preparing the specimen to be scanned [4] (Table 16.1).

Fig. 16.1
figure 1

Progression of evCM microscopes. a VS2500 Gen3. b Diagnoptics prototype. c Nodular basal cell carcinoma image obtained by VS2500 Gen3 with reflectance laser. d Infiltrating basal cell carcinoma image obtained by VS2500 Diagnoptics prototype with fusion mode and strip mosaicking.

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Table 16.1 Evolution of Vivascope® devices
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2 Reflectance Confocal Microscopy in Ex Vivo Confocal Microscopy

The first published results in ex vivo confocal microscopy (evCM) for clinical applications were in 2001, by Milind Rajadhyaksha and Salvador González [5]. A similar technique was described in 1995, for in vivo confocal microscopy (ivCM) by Rajadhyaksha et al. [6] and was applied for the analysis of evCM images.

These images were obtained by two reflectance confocal microscopes. The authors stained the nuclei with acetic acid at 5% in order to enhance the contrast by whitening the skin component and compacting the chromatin. A laser of 1064 nm wavelength allowed deep visualization of the tissue, in spite of a lower quality resolution of the structures.

In 2003, a new commercialized device only for ex vivo purposes (VivaScope 2000s, Lucid Inc., NY, USA), with an 830 nm diode laser (a reflectance laser) and a 30X objective lens, was utilized to evaluate discarded tissue from the first stage of Mohs surgery operations on basal cell carcinomas and squamous cell carcinomas. Acetic acid at 5% was utilized as a stain contrast with better results than reported in previous studies [7]. A real-time image visualization for Mohs surgery was achieved.

In 2007, the same pioneer group which created the first confocal created a mosaic of 15 × 15 mm (equivalent to a 2X view of macroscopic image) in 9 min [8]. They also realized that the flattening of the tissue was fundamental to visualizing all the structures during the scanning. A flattening device consisting of an Agarose 3% gel disk, 2 mm thick was proposed for the first time. The gel disk was placed on top of the specimen inside a piston housing of polycarbonate. A piston compressed the gel and the specimen on the slide, achieving a uniformly flattened specimen. Visualization of the tissue was greatly improved.

Staining methods also changed in 2009 when a German group proposed the use of Citric acid 10% for 2 min, scanning with the VS2500, observing the images in reflectance mode [9].

3 Fluorescence Confocal Microscopy for Ex Vivo Confocal Microscopy

Another radical change occurred in 2009 when the use of a fluorescent laser (445 nm) was proposed for Mohs surgery [10]. The staining method also varied, and the use of acridine orange was introduced in the preparation of the sample. Only a fluorescent laser was utilized.

Since 2010, a fluorescence 488 nm laser has been used in most published studies, and the technology has not changed significantly. Although the device could employ a fluorescent 658 nm laser or a reflectance 830 nm laser, investigators have, in general, preferred using acridine orange and a continuous wave 488 nm laser. A great deal of evidence has been accumulated in favor of safety in the use of this device and technique [3, 10,11,12,13,14,15,16,17,18,19].

Simultaneously, another device called HistoLog scanR FCM® was developed which allowed the visualization of larger images than the other devices 8 × 8 cm. The final image of the scanned tissue can be seen in digital purple, using only a 488 nm laser [4] (Table 16.2).

Table 16.2 Comparison of evCM devices. Adapted from Malvehy et al. [4].
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In 2007, efficacy in the visualization of tumoral islands of basal cell carcinoma was compared between the three different lasers available at that time: two fluorescent lasers (633 and 656 nm) and a reflectance laser (830 nm). In addition, new contrast agents were proposed: methylene blue 1% and toluidine blue 1%.

4 Fusion Mode: Simultaneous Fluorescence + Reflectance

In 2016, the authors of this chapter analyzed different benign and malignant lesions using fluorescence and reflectance lasers, with a new staining protocol of acetic acid and acridine orange, using the VS2500 Gen2 (Figs. 16.2 and 16.3) [20].

Fig. 16.2.
figure 2

VS2500 Gen2 scanned image of a superficial BCC stained with Acridine orange 0.1 mmol L1 + acetic acid 50%. Different stromal reactions were observed in each laser. a Reflectance laser. b Fluorescence laser. c H&E [20]

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Fig. 16.3.
figure 3

Nuclear visualization with different stain protocols [30]. Nodular basal cell carcinoma using reflectance laser. Image obtained by VS2500 Diagnoptics prototype with fusion mode and strip mosaicking. a Acridine orange 0.1 mmol L1 + acetic acid 50%. b Acridine orange 0.1 mmol L1 [20]

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Since then, other European authors have also compared different visualization features in both lasers as applied to diverse skin lesions [21,22,23].

Regarding the flattening of the tissue, in 2017, another option appeared in an attempt to improve the representativeness of the sample. This system consisted of a plastic slide with grub screws that covered the specimen. Unfortunately, this method was time-consuming and rapid tissue visualization is essential [24].

The basis of the fusion mode 4th generation VivaScope device (the latest generation of confocal microscopy) consists in scanning with two different lasers (fluorescence and reflectance) at the same time, strip mosaicking and subsequently, the analysis of digital images of hematoxylin and eosin.

The first time that two different lasers, fluorescence (488 nm) and reflectance (830 nm), scanned at the same time was with a prototype developed in 2005 [25]. In this study, in vivo confocal images were correlated with ex vivo conventional microscopy images.

Another substantial change occurred in 2011 when the strip mosaicking in ratios of 10:1 instead of ∼1:1 was obtained and stitched together using novel software [26]. With this technology, an area of 10 × 10 mm was scanned in about 3 min—3 times as fast asthe previous method.

5 Fusion Mode with Strip Mosaicking and Digital H&E

In 2009, the third revolutionary technological advance was initiated: for the first time, confocal ex vivo digital images of H&E could be observed, simulating real histopathology slides. The visualization was rendered by two lasers (according to the described prototype where both fluorescent and reflectance lasers were used simultaneously) and using software to change the initial colors of the scanned images: the reflectance mosaic was digitally changed to pink and the fluorescence mosaic to purple. This was the last significant step to the technology that is available today together with the improvement of the software compared to the first method of digital H&E staining in evCM [27, 28].

A further step was taken in 2016 when the authors of this chapter started to work with a prototype in the context of a European consortium called Diagnoptics (Fig. 16.4). The device was a prototype that scanned simultaneously in reflectance and fluorescence modes and used the strip mosaicking software to integrate the captured images (Fig. 16.4) [29]. Differences between fluorescence, reflectance, and fusion laser mode and also a comparison between the different staining methods were described for basal cell carcinoma [20, 30].

Fig. 16.4.
figure 4

Diagnoptics prototype scanned image of a nodular BCC stained with Acridine orange 0.1 mmol L1 + acetic acid 50%. a Fluorescence laser. b Reflectance laser. c Fusion image (simultaneous fluorescence and reflectance lasers) and H&E. Histological features can be recognized: clefting (red asterisk), palisading (red triangle), nuclear pleomorphism and increased nuclear/cytoplasm ratio (gray asterisk), peri-tumoral stromal reaction (gray triangle), thickened collagen bundles (red star) [30]

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With a few changes suggested by the study group, the algorithm put forward by Gareau et al. [27] was integrated into the new version of the commercial machine and the VS 4th Gen appeared on the market in 2018 (Figs. 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10 and 16.11) [29,30,31,32,33].

Fig. 16.5.
figure 5

VivaScope® 2500 MM-Gen4 digital H&E scanned image of a nodular BCC, previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Histological features can be recognized: clefting (red asterisk), palisading (red triangle), thickened collagen bundles (red star) [4, 30, 36,37,38]

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Fig. 16.6.
figure 6

VivaScope® 2500 MM-Gen4 digital H&E scanned detail image of a nodular BCC previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Histological features can be recognized: clefting (red asterisk), palisading (red triangle), nuclear pleomorphism and increased nuclear/cytoplasm ratio (gray asterisk), peri-tumoral stromal reaction (gray triangle), thickened collagen bundles (red star) [4, 30, 36,37,38]

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Fig. 16.7.
figure 7

VivaScope® 2500 MM-Gen4 digital H&E scanned detail image of an infiltrating BCC previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Histological features can be recognized: clefting (red asterisk), palisading (red triangle), nuclear pleomorphism and increased nuclear/cytoplasm ratio (gray asterisk), peri-tumoral stromal reaction (gray triangle), thickened collagen bundles (red star) [4, 30, 36,37,38]

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Fig. 16.8.
figure 8

VivaScope® 2500 MM-Gen4 digital H&E scanned detail image of a micronodular BCC previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Histological features can be recognized: clefting (red asterisk), palisading (red triangle), nuclear pleomorphism and increased nuclear/cytoplasm ratio (gray asterisk), peri-tumoral stromal reaction (gray triangle), thickened collagen bundles (red star) [4, 30, 36,37,38]

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Fig. 16.9.
figure 9

VivaScope® 2500 MM-Gen4 digital H&E scanned image of an infiltrating SCC previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Histological features can be recognized. Epidermis (dark green arrow), DEJ (light green triangle), dermis (yellow asterisk) [4, 30, 36,37,38]

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Fig. 16.10.
figure 10

VivaScope® 2500 MM-Gen4 digital H&E scanned detail image of an infiltrating SCC previously prepared with Acridine orange 0.1 mmol L1  + acetic acid 50%. Full macroscopic histological features can be seen. Epidermis (dark green arrow), DEJ (light green triangle), dermis (yellow asterisk), keratin pearls (light gray arrow), parakeratosis (turquoise arrow), inflammatory cells (dark orange asterisk) [4, 30, 36,37,38]

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Fig. 16.11.
figure 11

Detail image of infiltrating SCC. Full microscopic histological features can be seen. Epidermis (dark green arrow), DEJ (light green triangle), dermis (yellow star), vessels (purple arrow), keratin pearls (light gray arrow), cellular atypia (black triangle), dyskeratotic cells (black asterisk), inflammatory cells (dark orange asterisk) [4, 36,37,38]

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6 New Staining Protocols

A new staining method was also suggested, combining the benefits of acetic acid and acridine orange staining (Table 16.3) (Figs. 16.3, 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10 and 16.11).

Table 16.3 Staining methods for ex vivo CM and application in dermatology. RCM: reflectance confocal microscopy; FCM: fluorescence confocal microscopy; M-CM: multimodal confocal microscopy. Adapted from Malvehy et al. [4].
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The next significant change was suggested by the authors regarding flattening methods [34] and steps were suggested to obtain a good image with this technique (also described in Chap. 3) [35]. In addition to mechanical flattening methods, recently, Sendin-Martin et al. [36] demonstrated a digital approach to flatten excised Mohs margin tissue for complete visualization of epidermal margin.

The greatest advantage of applying digital H&E is that the diagnostic criteria normally applied in different histological diagnoses can also be visualized and applied in the diagnosis with ex vivo confocal microscopy [4, 37,38,39]. This has previously been demonstrated by the authors of this chapter, and the feature visualization of the structures, only in fluorescence or reflectance mode, is now utilized to differentiate doubtful areas of the image of the scanned tissue [4, 37,38,39].

The new generation of ex vivo confocal microscopy has led to a revolution in the field of diagnosis and histology-driven treatment. New applications in other fields of general pathology have also quickly begun to appear [1, 2, 40] and pathologists have been able to increase their confidence in diagnosis using ex vivo confocal microscopy with digital hematoxylin and eosin.

7 Artificial Intelligence Color and Image Interpretation

Progress toward this method started through experimenting with artificial intelligence. Marc Combalia et al. proposed digital staining through deep learning [41], with digital colors almost equal to regular H&E that may have clinical applicability in the future (Fig. 16.12). As with digital pathology, AI will have a role in diagnosis in evCM, assessment of tumor margins, and in other applications. However, validation of machine learning needs to be done in this field to prove its clinical usability.

Fig. 16.12.
figure 12

VivaScope® 2500 M-Gen4 AI stained scanned detail image of normal skin previously prepared with Acridine orange 0.1 mmol L1 + acetic acid 50%. Microscopic histological features can be recognized. Epidermis (dark green arrow), DEJ (light green triangle), dermis (yellow asterisk) [40]

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To summarize, the technology for evCM has evolved steadily over recent years. The core technology has been miniaturized in different devices and new lasers have been developed, bringing fluorescence capability instead of only reflectance mode (830 nm). Fusion images of evRCM and evFCM and digital H&E staining render high-quality pathological images similar to standard digital pathology. Moreover, the most recent changes have been the incorporation of a macroscopic camera and also the implementation of strip mosaicking (V2500 Gen4). Furthermore, the first device (VS2000), built in 2003 from the prototype developed by Rajadhyaksha in 2001, had a 25X objective, and the latest generation uses 38X, increasing the resolution from previous models. Additionally, the field of view of the VS2000 was 750 µm and the maximum scan area was 10 × 10 mm compared to 30 × 3 0 mm.

The very first efforts made by Rajadhyaksha and González and their team in 2001 have, twenty years later, resulted in fast, efficient, revolutionary technology that is radically changing the traditional method of processing histological samples.