Guide
What is a DICOM file?
The file format behind every CT, MRI and X-ray, explained in plain language.
Last updated: October 2026
The short answer
A DICOM file is a medical image stored in DICOM (Digital Imaging and Communications in Medicine), the international standard for medical imaging, recognized as ISO 12052. Each file holds the image, such as one slice of a CT or MRI, plus details about the patient, the scan and the scanner. Files often end in .dcm, but many have no extension. A DICOMDIR file is the index on a scan CD.
You open DICOM files with a DICOM viewer, not a photo app. Because they contain personal details, treat them like any medical record.
What does DICOM stand for?
DICOM stands for Digital Imaging and Communications in Medicine. Per dicomstandard.org, it is "the international standard to transmit, store, retrieve, print, process, and display medical imaging information."
In practice, it is why a scan made on one company's machine can be opened on another company's computer, in any hospital. It covers both the file format and the way systems send scans to each other.
The standard is developed by the DICOM Standards Committee, whose members include manufacturers, professional societies and government agencies. NEMA is the DICOM Secretariat, and MITA provides secretariat services on its behalf. The standard is updated several times a year and is free to read online.
What's inside a DICOM file?
An image, plus a header of labeled details called tags. The tags travel with the image wherever it goes.
Pixel data
The image itself, usually at a higher depth of gray than a normal photo, so a radiologist can bring out bone, lung or soft tissue.
Patient details
Name, ID, date of birth and sex, written into the file. This is why a DICOM file is personal health data.
Study and series details
Which hospital, which scanner, the date, the body part and the type of scan, plus codes that tie each slice to its study.
Technical details
Pixel spacing, slice thickness and position, so a viewer can measure accurately and rebuild slices in other directions.
What are .dcm files and DICOMDIR?
.dcm is the usual ending for a DICOM file. It is optional. On scan CDs, files often have short names like IM000001 and no extension at all. They are still DICOM files, and a DICOM viewer opens them.
DICOMDIR is the index file on a CD, DVD or USB drive of scans. It lists each patient, study, series and image and where each file sits. It holds no images, but viewers use it to load a whole disc in the right order. The standard covers this in Part 10, "Media Storage and File Format for Media Interchange".
Holding a disc right now? Follow our step-by-step guide on how to open a CT or MRI scan CD.
Why isn't a JPEG the same as a DICOM file?
A JPEG is a snapshot. A DICOM study is the full scan, with the data a radiologist needs.
| Feature | DICOM | JPEG |
|---|---|---|
| Brightness and contrast | Adjustable after the scan (window/level) to show bone, lung or soft tissue from the same data | Fixed when the picture was saved |
| Number of images | A full CT or MRI series: often hundreds of slices you can scroll | Usually a few chosen pictures |
| Measurements | Real-world sizes, because pixel spacing is stored | No reliable scale |
| Other views | Can rebuild slices in other planes and 3D | Not possible |
| Patient details | Stored inside the file | Only if burned into the picture |
| Opens in | A DICOM viewer | Any photo app |
This is why doctors and second-opinion services ask for the DICOM files, not photos of the screen or the printed film.
How do I open a DICOM file?
With a DICOM viewer. There are two kinds.
Desktop viewers
Installed on your computer. Best for files on a CD or pen drive. Free options include Weasis (Windows, Mac, Linux), MicroDicom (Windows, free for non-commercial use) and Horos (Mac).
Web viewers
Run in a browser with nothing to install. Hospitals and scan centers often use them for patient portals and share links. Use the one your hospital gives you, not a random website.
5C's radiologists read in RadDream, 5C's web DICOM viewer, which runs in the browser and comes with 5C's platform for hospitals. For a fuller comparison of options, see our guide to the best DICOM viewers.
Viewer details are from each maker's website as of 26 October 2026: Weasis, MicroDicom, Horos.
Is a DICOM file private, and can it be anonymized?
A DICOM file usually carries the patient's name, ID and date of birth inside it. Treat it like a medical record. Share it only with doctors or services you trust, and don't upload it to unknown websites.
Anonymizing (de-identifying) means removing or replacing the identifying tags. The DICOM standard sets out profiles for doing this. Two cautions: renaming a file removes nothing, and some images have names or dates burned into the pixels, which tag-cleaning alone does not remove. Hospitals sharing scans for research or teaching should use dedicated de-identification software.
How do hospitals store DICOM files?
In a PACS (picture archiving and communication system). Scanners send each study to the PACS in DICOM format. Radiologists open it from there in a viewer, and the hospital keeps it for future comparison.
Read our PACS guide for hospitals, compare the best PACS software in India, or see 5C SuperPACS, 5C's cloud PACS with a browser viewer.
5C SecondRead
Have your DICOM files and want a specialist to look again?
Upload the CT, MRI, X-ray or mammogram you already have. A super-specialist radiologist re-reads it independently and you get a signed, written report to discuss with your doctor. No new scan, no radiation.
Get a specialist second opinionFor hospitals and diagnostic centers: see RadDream, 5C's web DICOM viewer.
Frequently asked questions
What is a DICOM file?
A DICOM file is a medical image saved in DICOM (Digital Imaging and Communications in Medicine), the international standard for storing and sharing medical imaging. Each file holds an image, such as one slice of a CT or MRI, together with details about the patient, the scan and the scanner. CT, MRI, X-ray, mammography and other scans are stored this way in hospitals around the world.
What is a .dcm file?
A .dcm file is a DICOM file. The .dcm ending is common but optional: files on scan CDs often have no extension at all, just a short name of letters and numbers. Both are opened the same way, with a DICOM viewer.
What is DICOMDIR?
DICOMDIR is the index file on a DICOM CD, DVD or USB drive. It lists the patients, studies, series and images on the disc and where each file is, so a viewer can load the scan in the right order. It contains no images itself. Opening the DICOMDIR file in a viewer is usually the quickest way to open a whole disc.
How do I open a DICOM file?
Use a DICOM viewer. On Windows, free options include Weasis and MicroDicom (free for non-commercial use). On a Mac, Horos and Weasis are free and open source. If your scan came on a CD, the disc may include its own viewer, usually for Windows. If your hospital has a patient portal, it may let you view your images in a web browser.
Can I convert a DICOM file to JPEG?
Most DICOM viewers can export a picture as JPEG or PNG, which is handy for showing one image to family. But a JPEG keeps only what was on screen at that moment. It loses the other slices, the adjustable brightness and the measurements, so doctors need the original DICOM files, not JPEGs.
Does a DICOM file contain my personal information?
Usually, yes. A DICOM file normally stores your name, patient ID, date of birth and details of the hospital and scan alongside the image. Treat it like any medical record: only share it with doctors or services you trust, and don't upload it to unknown websites.
What does it mean to anonymize a DICOM file?
Anonymizing (de-identifying) a DICOM file means removing or replacing the details that identify the patient, such as name, ID and date of birth. The DICOM standard describes how to do this. Text can also be burned into the image pixels themselves, which needs separate handling, so anonymization should be done with proper software, not by renaming files.
Who maintains the DICOM standard?
The DICOM Standards Committee develops and maintains it. Its members include equipment manufacturers, professional societies, government agencies and others. NEMA is the DICOM Secretariat, with MITA providing secretariat services on its behalf. DICOM is also recognized as ISO 12052, and the full standard is free to read at dicomstandard.org.
Source for the standard: dicomstandard.org, read 26 October 2026.
