DICOM viewers: online, open source and diagnostic. Which one, and when

The three kinds of DICOM viewer

DICOM is the file format and network standard every CT and MRI scanner uses, so a DICOM viewer is simply software that can read those files and display them. The differences between viewers are about purpose, not format.

The rest of this guide covers each kind, what the display standards require, and how a teleradiology network such as Raydiac uses a web viewer for primary reads without pretending a phone is a workstation. For background on where a viewer sits among PACS, RIS and DICOM, see the guide PACS, RIS, DICOM and HL7 explained.

Free and online DICOM viewers

A free desktop viewer is the right tool for a patient who has been given a CD, a referring doctor who wants to look at the images alongside the report, or a student. Commonly used examples are RadiAnt and MicroDicom on Windows and Horos and OsiriX on macOS; none of these is an endorsement, and features and licences change, so check the current terms before use. Most CDs from Indian imaging centres also include a small viewer that starts automatically on a Windows PC.

Online viewers, where you drag a DICOM file into a website, are a different matter. A DICOM header carries the patient’s name, date of birth, sex, patient ID, the referring doctor and the institution. Uploading it to a site you cannot identify sends all of that to an unknown operator, possibly outside India. Under the DPDP Act 2023 the site operator becomes a data fiduciary for that data and owes the safeguards in Rule 6 of the DPDP Rules 2025¹⁰, but a patient has no practical way to enforce that against an anonymous website. For a hospital or clinic, uploading patient studies to a third-party site without a processor contract is a compliance failure, not a convenience. The DPDP guide for hospitals covers the obligations.

Web viewers and OHIF

A web viewer runs in a standard browser and loads images from a server, so there is nothing to install and nothing to keep updated on the reading computer. The server holds the study and controls who may open it. This is how remote reporting works in practice: the radiologist opens a link, and the full study streams into the browser.

The best-known open-source web viewer is OHIF Viewer, maintained by the Open Health Imaging Foundation and released under the MIT licence; version 3.13 was released in July 2026¹. OHIF loads studies over DICOMweb, supports OpenID Connect for login, hanging protocols, multiplanar reconstruction and multimodal fusion, and renders with GPU acceleration and multi-threaded decoding². Version 3.13 added a “smart scrollbar” that shows which slices have already downloaded so the reader can jump to any loaded slice rather than wait for the whole series¹, which matters on Indian hospital links. OHIF v3 is embedded in other applications through an iframe with a messaging bridge rather than as a script tag³, which is why many teleradiology platforms, Raydiac included, present OHIF inside their own worklist.

What “full study in the browser” means

It means every series and every slice the scanner produced is available to the reader, at original resolution, with the DICOM header intact, not a set of exported JPEGs or a video. The viewer fetches slices on demand, decodes them in the browser, and lets the reader scroll, window and reconstruct. Progressive encodings such as HTJ2K can render a first image in a fraction of the time older codecs need on a mobile link; DICOM Working Group 4 reported a first render in 66 milliseconds against 4,586 milliseconds for JPEG-LS on a 4G connection⁹, although the PACS must serve that encoding for the gain to apply.

Per-study tokened links

A shared PACS login gives everyone access to everything. The better pattern is a token minted for one study, one user and a limited time. The Orthanc Authorization plugin, for example, issues per-resource tokens with expiry and filters DICOMweb queries so a viewer only sees what the token permits⁸. A referring doctor gets a link to one patient’s study; a radiologist gets access to the studies assigned to them; both expire.

Diagnostic reading and displays

The viewer software is only half of a diagnostic setup. The other half is the display and the room, and this is where standards exist. The ACR–AAPM–SIIM Technical Standard for Electronic Practice of Medical Imaging sets the following for diagnostic displays⁴:

A phone in a lit ward, or a glossy consumer laptop by a window, does not meet these conditions, and no viewer software can compensate. That is why phones are for reviewing key images, acknowledging critical results and answering queries, not for primary reads. The ACR’s teleradiology white paper makes the same point from the other direction: the standard of care for a remote read is the same as for an on-site read, which includes the display¹¹. Radiologists considering remote work will find the workstation requirements in the guide to teleradiology jobs in India.

Windowing, MPR, measurements and priors

Four functions separate a viewer that can display a scan from one that a radiologist can report on.

• Windowing. CT pixel values span thousands of Hounsfield units; a screen shows a few hundred grey levels. Windowing selects the range to display (brain, bone, lung, soft tissue) and everything outside it clips to black or white. A viewer needs presets and a fast manual adjustment.
• Multiplanar reconstruction (MPR). CT and many MRI sequences are acquired as a stack of thin slices, so the viewer can rebuild coronal, sagittal or oblique planes from the axial data. Reading a spine or a vessel without MPR is slow and error-prone.
• Measurements. Distance, area, angle and region-of-interest density, saved with the image so the report can quote them and the next reader can find them. Stored as DICOM Structured Reports, measurements become data rather than free text.
• Priors side by side. The previous study of the same patient, linked slice by slice, so a change is measured rather than remembered. The ACR white paper describes reads without prior images and clinical context as lower value and suggests they be labelled preliminary¹¹.

Open-source viewers compared

Licences are stated as published by each project at the time of writing; verify before deploying, particularly for GPL-family software embedded in a commercial product.

How Raydiac uses OHIF

Raydiac’s radiologists read every CT and MRI study in an OHIF viewer opened from the Raydiac worklist in a standard browser. The study is served from Raydiac’s own Orthanc-based DICOM store in India over DICOMweb, with a token issued per study and per reader, so nobody holds a general PACS login. The reader sees the full study with all series, the pseudonym (for example RDX-26-000418), age, sex, clinical history and the clinical question, and priors from the same site side by side. Measurements made in the viewer flow into the structured report.

The display is the radiologist’s responsibility and Raydiac treats it as one: panel radiologists are asked to read on a display that meets the ACR–AAPM–SIIM guidance, and the mobile app is limited to alerts, critical-result acknowledgements and roster management, never to primary reads. Referring doctors at the hospital receive the signed PDF report through the site portal. The platform page describes the viewer, worklist and report editor together.