The Orthanc Ecosystem for Medical Imaging
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This paper reviews the components of Orthanc, a free and open-source, highly versatile ecosystem for medical imaging. At the core of the Orthanc ecosystem, the Orthanc server is a lightweight vendor neutral archive that provides PACS managers with a powerful environment to automate and optimize the imaging flows that are very specific to each hospital. The Orthanc server can be extended with plugins that provide solutions for teleradiology, digital pathology, or enterprise-ready databases. It is shown how software developers and research engineers can easily develop external software or Web portals dealing with medical images, with minimal knowledge of the DICOM standard, thanks to the advanced programming interface of the Orthanc server. The paper concludes by introducing the Stone of Orthanc, an innovative toolkit for the cross-platform rendering of medical images.
KeywordsDICOM Vendor neutral archive Teleradiology
Medical imaging has grown over years as an essential step for the diagnosis and treatment of many diseases in the clinical routine. By combining information arising from different imaging modalities (CT, PET, MRI, US…), patients benefit from extremely personalized, high-quality treatments . Consequently, hospitals are getting equipped with a steadily growing number of imaging devices, which in turn results in an explosion of the volume of medical images that are generated. The analysis of such an amount of data implies the development and deployment of many specialized clinical software dedicated to specific fields of expertise. Simultaneously, as healthcare is moving to a patient-centric approach , the seamless and secure diffusion of medical images over Internet becomes an important concern: Lifetime links with the personal EHR (Electronic Health Record), continuity of care (moves of the patient, rare diseases…), pooling of imaging devices between several hospitals, or remote second opinions must be taken into consideration.
Such concerns about the volume, the processing, and the diffusion of medical images are also present within research centers and university hospitals. Indeed, both pre-clinical and clinical research is increasingly interested by big-data algorithms applied to the (semi-)automatic analysis of multimodal medical images . Industrializing the software resulting from such a research is a complex task, as it must be able to smoothly integrate with the imaging infrastructure of hospitals. Similarly, sharing anonymized imaging datasets becomes crucial for educational purpose, for pharmaceutical research and as a support to open-science. All these factors explain why the data analysis of clinical images, the careful definition of the radiology workflow, as well as and the administration of the computer network of hospitals and research centers imply many technical challenges.
DICOM (Digital Imaging and Communications in Medicine) is the de facto standard for the encoding, management, and exchange of medical images . It was conceived to provide best interoperability across hardware designed by different manufacturers. Each DICOM file (known as an instance) can be thought of as an envelope that embeds the medical image together with the clinical meta-data associated with this image. Meta-data is essentially encoded as a hierarchy of key-value pairs that is similar in spirit to the more recent XML or JSON file formats. Thanks to the DICOM network protocol (one of the earliest examples of Web services), the DICOM instances can hopefully be shared between the modalities generating these medical images, the central PACS server (Picture Archiving and Communication System) archiving all these images in the long-term, and the computers displaying or processing the images.
Unfortunately, several practical problems are often encountered if dealing with the DICOM standard. Indeed, even though the DICOM standard enables hardware compatibility spanning almost the whole medical imaging scope, commercial PACS solutions are mostly focused on radiology workflow in hospital setups. This tends to exclude some specific specialties (such as radiotherapy, nuclear medicine, digital pathology, dentistry, neuro-imaging, quality control, or intraoperative imaging) from professional PACS support. In such specialties, it is common to deal with software that does not support DICOM networking and/or that is not endorsed by the PACS managers of the hospital for security reasons. In practice, this leads to the emergence of manual, parallel, sometimes unofficial workflow based upon the export of images to CD/DVD burners, USB sticks, or network drives. This manual approach should obviously be discouraged, as it breaks the traceability of clinical data, as it might lead to security breaches, and as it makes such process tedious and error-prone (any CT or MRI scan results in hundreds of files without meaningful filenames that must be handled as a whole).
Furthermore, despite its universality, the DICOM format and its associated network protocol are complex and require a long learning curve, both for imaging practitioners, for network administrators and for research engineers. Because DICOM was not designed to facilitate the software manipulation and analysis of imaging data, many companies or researchers creating new image analysis software encounter compatibility issues while integrating with other manufacturers. A typical example is that of image compression: Even though DICOM does include built-in support for lossless compression through JPEG-LS or JPEG-2000, many software dedicated to medical imaging does not support such compression algorithms, which often leads hospitals to simply choose to avoid any compression, putting a high load on their network infrastructure. As a reaction to bypass the complexity of DICOM, some medical specialisms have also developed their own file formats (notably NIfTI and BIDS for neuro-imaging, or STL-like 3D models for dentistry). Unfortunately, such competing formats necessitate seamless integration with DICOM to be useful in clinical setups, for which few software currently exists.
Summarizing, each and every hospital or research center in the world needs custom, ancillary software to add more flexibility to the star-shaped DICOM network topology revolving around their radiology-focused PACS server. This flexibility is needed to tackle specialized processes that are parallel to the baseline radiology, notably: Adding imaging buffers between clinical departments, quick and easy anonymization, supplying images from the PACS to specialized or research software, Web publication of images, automation through scripting (e.g., to prefetch images or to monitor doses), and exchanges between hospitals (or with pharmaceutical firms for clinical research). Currently, such deployments are often made by highly skilled PACS managers who learn on the job, writing custom applications on the top of advanced free and open-source toolkits such as GDCM  or DCMTK , with sparse sharing of the technical knowledge between hospitals.
This paper describes the free and open-source Orthanc ecosystem, whose development was triggered in 2012 by these observations . The long-term goal of the Orthanc project is to foster cooperation and knowledge sharing about medical imaging workflow in hospitals (including with vendors of imaging devices and data-science companies), to stimulate the development of new algorithms dedicated to multimodal medical imaging, as well as to improve interoperability in healthcare by providing a DICOM server that can be easily and quickly deployed at multiple places in hospitals as a complement to the main PACS server.
Small footprint: It can run on any kind of hardware platform, from Raspberry Pi to large-scale cloud infrastructures by way of virtual machines or desktop computers. It is even possible to run the Orthanc server from a USB stick.
Cross-platform: Entirely written in pure C++, it can be readily packaged for many operating systems. Binary packages for Microsoft Windows, Apple OS X, Docker, and several GNU/Linux or UNIX distributions (notably Debian, Fedora, and FreeBSD) are available.
Standalone: It comes bundled with its own database engine (SQLite), which avoids the burden of configuring a database server for simple use cases. Similarly, no external software or framework (such as Java or .NET) is needed to run the Orthanc server.
Compliant: Best DICOM interoperability is provided by embedding the well-known DCMTK toolkit . Emphasis is also put on the quality and the automated validation of its source code.
Thanks to this attention to the simplicity of packaging, deploying an Orthanc server is almost immediate: In most cases, it is sufficient to download the binaries and to adapt a handful of options (such as the DICOM AET— Application Entity Title— of the server) by editing the provided configuration file. Interfacing an Orthanc server with peripheral imaging modalities, including the PACS of the hospital, amounts to declaring them in the configuration file by providing their respective DICOM parameters (AET, IP address, and TCP port number).
Besides its built-in embedded DICOM server, the Orthanc server also comes bundled with an embedded HTTP server. This HTTP server exposes an embedded Web user interface that is known as Orthanc Explorer. This user interface allows the Orthanc users to browse and interact with the content of the DICOM store, from any client computer within the hospital network by using the default Web browser1. Obviously, the Orthanc server can be configured to use HTTPS and to assign passwords to users, in order to secure the access to the medical images through proper authentication and encryption.
Thanks to Orthanc Explorer, adding a cohort of Orthanc servers to the DICOM topology of the hospital can already facilitate the data management for clinical routine and medical research by providing easy-to-install, task-specific, fine-grained DICOM stores. Such servers can act as interconnection bridges between DICOM modalities, between medical departments, yet even between hospitals. It is for instance used at the University Hospital of Liège in order to provide medical physicists of the radiotherapy department with a backup access to the contours produced by the nuclear medicine department2, and with a research database that collects in-room images (CBCT and radiographs) produced by the treatment machines .
Lua scripting is very useful for basic auto-routing tasks. For more advanced scenarios of DICOM automation, the Orthanc server provides a full programmatic access to all its core features, under the form of a REST API [8, 9] that is served through its embedded HTTP server. Thanks to this REST API, external scripts or applications can drive the Orthanc server, independently of the programming language that is used to develop them (such as Python, C#, Java, Matlab…). By delegating the handling of the DICOM file format and network protocol to the Orthanc server, it is possible to speed up the development of new applications dedicated to medical imaging (may they be heavyweight, mobile or Web applications). Indeed, the Orthanc server essentially acts as a high-level bridge between on one hand, the DICOM standard, and on the other hand, software standards that are well-known to every modern developer (such as JSON, XML, PNG, or HTTP).
Research software has also been built using the REST API, with applications to the quantification of focal fibrosis in cardiac MRI, to the quality control of nuclear medicine modalities , to the comparison of delineation algorithms in oncology , or to the texture analysis of FDG PET/CT images .
It is very important to notice that the REST API of the Orthanc server can be used to securely transmit images between two remote sites over Internet, through the encrypted HTTPS protocol. This feature is used by hospitals in the Liège area (Belgium) to exchange their imaging data on a daily basis, particularly in the context of oncology. The Osimis S.A. company, spin-off of the University Hospital of Liège, was created alongside the Orthanc ecosystem in order to provide commercial services on the top of free and open-source project, notably to setup and maintain such inter-hospital links.
The plugin engine of Orthanc can also be used to replace the built-in SQLite database (that is suitable to store up to about 50,000 DICOM instances), by an enterprise-ready database (that can store a virtually unlimited number of DICOM instances). In that respect, the Orthanc project proposes a free and open-source plugin for PostgreSQL that has been reported to successfully manage at least 10TB of imaging data.
As depicted in Fig. 1, the Orthanc ecosystem also features a free and open-source plugin that proposes a reference implementation of DICOMweb, the emergent Web standard for medical imaging. It is also possible to serve DICOM worklists by developing a dedicated Orthanc plugin. The full list of Orthanc plugins is part of the Orthanc Book .
The DICOMizer, a standalone command-line tool that takes as input a non-DICOM whole-slide microscopic image, and that generates a compliant DICOM file. Because digital pathology images are extremely large (typically a size of 100,000×100,000 pixels in RGB colors), the DICOM standard specifies how to encode such images as a multiresolution pyramid, a technique that is commonly used in geospatial Web mapping systems (cf. Fig. 12).
A plugin for the Orthanc server that can serve such images over Internet using any Web browser. Screenshots of this plugin are provided in Fig. 13.
Stone of Orthanc
The last component of the Orthanc ecosystem of Fig. 1 that has not been discussed yet is the Stone of Orthanc. Stone of Orthanc is a building block for the rendering of 2D/3D medical images. It notably features support for MPR (multiplanar reconstruction of volume images), reslicing, layering (fusion of images), radiotherapy (rendering of RT-DOSE and RT-STRUCT), and accurate physical 3D world coordinates. Obviously, it can retrieve DICOM images from an Orthanc server through its REST API.
Stone of Orthanc takes the form of a lightweight, cross-platform C++ toolkit. As Stone of Orthanc is entirely written in pure C++ and is entirely standalone, it can easily be embedded into heavyweight software (C++ support comes out of the box, and bindings to Java and C# are in active development) or into native mobile applications (Android and iOS). Very importantly, the Stone of Orthanc is compatible with the emerging WebAssembly technology that can be used to run C++ applications in Web browsers without installing any browser extension . As a consequence, thanks to the Stone of Orthanc, it is possible to benefit from a single codebase to quickly write applications for displaying and/or analyzing medical images, and targeting any kind of platform (native, mobile or Web). Figure 14 shows some rendering produced by the Stone of Orthanc toolkit.
Summarizing the previous review, Orthanc is a rich, integrated, evolutive ecosystem spanning a large portion of the medical imaging scope. As a free and open-source project, Orthanc is supported by a vibrant worldwide community of users that happily share their experience about medical imaging in clinical or research contexts. Corporate services (such as support, custom development, integration, maintenance, training...) are also currently available from Osimis S.A., who works according to an open-source business model. Other service companies on the top of the Orthanc ecosystem will hopefully show up in the future.
The Orthanc server is similar to DCM4CHEE . Contrarily to Orthanc, however, DCM4CHEE gets into position as a full replacement to a large-scale PACS server, with advanced mechanisms to integrate with hospital information systems (HL7 messages, audit record repository, IHE cross-enterprise document sharing…). This contrasts with the Orthanc server that is rather developed as a complement to some large-scale radiology PACS in order to automate DICOM flows through scripting, or to facilitate the Web diffusion of images. It is possible that such enterprise-level features will be brought to Orthanc in the future, but they are not readily available at the time of writing. Also note that the Orthanc server can already act as a PACS for small-sized or medium-sized hospitals, community health centers or research centers, provided it is complemented by a certified DICOM viewer.
This paper has described the Orthanc ecosystem for medical imaging . Orthanc is used worldwide for applications such as the Web diffusion of (possibly anonymized) medical images, the automated exchange or routing of DICOM instances (both inside or outside of hospitals and research centers), the scientific research or industrial R&D about new imaging modalities or software, or the education of stakeholders (physicians or medical physicists).
At the core of this ecosystem lies the Orthanc server, a lightweight, novel, robust DICOM store. The Orthanc server features rich scripting capabilities (through its Lua engine and its REST API) and built-in compatibility with Web technologies, making it very versatile. The Orthanc server can be considered as a micro-service, in the sense of SOA (Service-Oriented Architectures): Higher-level external applications (such as Web portals) written in any language can handle DICOM by supervising an instance of Orthanc through its REST API. The Orthanc server is cross-platform and fully standalone, making its deployment quick and easy: In particular, numerous instances of the Orthanc server can be deployed inside one hospital to automate task-specific DICOM flows as a complement to the central PACS server.
Many plugins revolve around the Orthanc server. They can be used to reinforce the core features of Orthanc, by improving the user interface, by adding support for enterprise-ready databases (e.g., PostgreSQL), by interfacing with administrative servers of the hospital (e.g., supplying worklists from RIS through HL7 messages), by providing teleradiology solutions, or by implementing recent additions to the DICOM standard (such as DICOMweb or digital pathology support). The Orthanc ecosystem also features a lightweight, CPU-based rendering engine for medical images known as the Stone of Orthanc: This C++ toolkit is a building block to create heavyweight software, mobile applications (iOS or Android), or Web interfaces (through WebAssembly) that need to display or process medical images.
The Orthanc ecosystem is entirely free and open-source. The Orthanc server itself can be downloaded and used under the terms of the GPLv3 license. The official plugins are mostly released under the AGPLv3 license. The Orthanc ecosystem was designed to be as open and simple as possible, in order to cut down the DICOM learning curve by fostering compatibility between vendors and knowledge sharing about the DICOM standard, to the benefit of hospitals, research centers, public organizations, companies, or even general audience. Extensive technical documentation of the Orthanc ecosystem can be found in the Orthanc Book .
In general, proposing Web interfaces instead of heavyweight client applications is important for hospital setups, as the installation of third-party software is often prevented for security, licensing, and maintenance reasons.
Note that the Orthanc server implements disk space recycling: The oldest series of images can be automatically deleted when the disk usage becomes higher than a predefined threshold, which is very handy to deploy such transient DICOM buffers.
The author wishes to thank Prof. Paul Nagy and Prof. Steve Langer for their invitation to submit this paper, and to acknowledge the University Hospital of Liège for opening the code of the Orthanc server to the benefit of the medical imaging community.
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