We can distinguish three types of telepathology: static telepathology, dynamic telepathology, virtual slide telepathology. In this paper, we analyze the last two for describing how we can implement a "hybrid telepathology system" that takes advantage of the peculiarities of each one. The dynamic telepathology allows exploration, in real time, of the whole slide surface thanks to a robotized remote controlled microscope; this also permits us to change magnification, focus and digitize the interesting portion. The disadvantages regard the necessary presence of a technician for positioning the desired slide within the microscope and the discontinuous digitization in terms of area and magnification. In virtual slide telepathology, the slide is completely digitized and stored in a repository; this permits a single or multiple user consultation, in every time and without human intervention. The virtual slide allows the exploration of the whole slide surface with different magnification. There are many types of devices that can be used for slide digitization and the right option depends on workload and necessary scan rate . Virtual slide telepathology has a a negligible disadvantage : it is not possible to focus the areas that were not correctly acquired. During the scan process, the scanning device takes into account a finite point number for the focusing procedure; each point is characterized by its own focal plane. When the sample surface is irregular, the device uses proprietary algorithms to calculate an "in focus surface" above the slide; this allows an average good focus but locally a less accurate one. During this treatment, it can be noted that this problem is directly correlated with sample analysis type and grows with increasing histological or cytological sample surface irregularity. Another characteristic to take into account is the virtual slide file size; this depends principally on the following parameters: the resolution, the compression ratio, the compression algorithm and the color depth. Considering the resolution used for the histological and cytological slide scan (0,5 μm/pixel – 20×), the scan area maximum dimension (9 cm2), an adequate compression ratio (15:1) and an appropriate compression algorithm (JPEG2000) it can be possible to obtain averagely a 500 MB file even if the non compressed file is approximately 7.5 GB [3, 4]. This file size allows, at present, a selected virtual slide storage for scientific archive creation but doesn't permit complete archiving of all slides; in fact, in case of complete archiving the storage device costs would be much too expensive. Certainly, in the future, the cost/byte reduction for storage and an international agreement about the standard for this storage will permit all slides complete archiving, deleting the conservation necessity. This will lead to many advantages regarding clinical data availability and accessibility. In the end, the necessity of an adequate guaranteed data communication bandwidth for sharing virtual slide must be considered. This problem can be solved thanks to the use of an Image Server and the specific image format (image pyramid); this offers the possibility of selective visualization of images in terms of resolution and portion of interest, without the necessity of a complete transfer of virtual slide from Image Server to a local PC. The complete focusing of the whole slide surface is therefore the real problem to solve, while the archiving and the consultation of virtual slides are effectively faced thanks to scalable and high performance storage devices and dedicated Image Server.
There are mainly two types of second opinions: second opinions in real time on frozen section during surgery (intraoperative consultation) and second opinions on histological or cytological slides which require a complex interpretation. Each type of second opinion has specific characteristics that are directly correlated with previous discussion. In the first second opinion type the analyses are conducted on thick sample or with irregular surface (this arises from the methods through which the samples are obtained and the liquid amount contained into the tissue to analyze); for this reason the automatic scanner device (virtual slide telepathology) can't focus correctly on the whole slide surface but with a motorized microscope (dynamic telepathology) it is possible to focalize each sample surface point. Therefore, the dynamic telepathology represents an important tool for second opinion during intraoperative consultation, especially for the presence on the territory of two hospitals with surgery but without a resident pathologist. Regarding histological and cytological slides, the tissue samples are treated for obtaining thin sections; this deletes the trouble regarding focus due to irregular surface, as previously described. In this case the use of virtual slide telepathology is favorable because it allows the visualization of the morphological picture at any time, remotely with a customary PC, without human intervention. This allows one to obtain a second opinion on complex cases from an expert of personal choice and to perform an external quality control.
Based on previous considerations and for obtaining an effective telepathology system, the best choice is a "hybrid system" composed of motorized microscopes, with remote control, and a scanner for slide digitization, in order to achieve the best characteristics from each system without respective disadvantages. This choice has been applied to Rovigo province health structures, in particular Rovigo, Adria and Trecenta hospitals. For storage purposes it has been used a NAS (Network Attached Storage) device (50 TB capacity – 1 Gb/s transfer rate). The previously described system is completely integrated with the CPOE (Computerized Physician Order Entry) based Hospital information System.
Thanks to complete slides digitization and the use of Image Server with high computational performances, it will be possible to apply filters to acquired images or to apply algorithms for calculating interesting quantities (e.g.: the cellular membrane distribution and continuity). These techniques adequately developed, tested and standardized will be the base for Computer Aided Diagnosis (CAD) introduction.