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How to Open DICOM (.dcm) Files on Any Computer

By SimpleTools24Published 4 min read

To open a DICOM file, use a DICOM viewer: a program that understands the medical imaging format, decodes the image data and shows it with controls for brightness, contrast and scrolling through slices. Ordinary photo viewers can't open .dcm files. Free desktop viewers exist for Windows, Mac and Linux, and a browser-based DICOM viewer works on any device, including Chromebooks and tablets, without installing anything.

In this guide

A quick note before you start: viewing your own images is useful for curiosity, record-keeping or sharing with another clinic, but a general-purpose viewer isn't a diagnostic tool. Questions about what the images show belong with a qualified clinician.

What You Need to Know About DICOM Files First

A few facts explain most of the confusion people run into:

  • One scan is usually many files. A CT or MRI study often contains hundreds of files, one per slice, sometimes with no extension at all. You need to open them together to scroll through the series.
  • File names are often meaningless. Names like IM000123 or 1.2.840.113619... are normal. The viewer organises images using information stored inside each file.
  • Compression varies. DICOM files can contain uncompressed pixels or JPEG, JPEG 2000, RLE or other encodings. A viewer that lacks the right decoder will show an error for some files but not others.

What a DICOM file is explains the format in more depth.

Option 1: Open DICOM Files in the Browser

The quickest route on most devices is an online DICOM viewer:

  1. Choose the files. Select individual .dcm files, a whole folder, or a ZIP of the study. Selecting the folder is easiest when you have hundreds of slices.
  2. Let it organise them. Images are grouped into studies and series using their Study and Series Instance UIDs, then sorted by instance number.
  3. Browse the series. Scroll or use the slider to move through slices. Switch series from the list.
  4. Adjust the display. Drag to change window and level (brightness and contrast), or pick a CT preset for soft tissue, lung, bone or brain. DICOM window and level explains what those settings do.
  5. Measure and inspect. Length, angle and rectangle tools show distances and pixel statistics, and the tag panel lists the file's metadata.

The SimpleTools24 viewer decodes files in your browser, so the images aren't uploaded to a server. It handles up to about 3,000 files or 1.5 GB per session, because everything is held in your device's memory.

Option 2: Use a Desktop DICOM Viewer

If you work with scans regularly, a desktop viewer adds features like 3D reconstruction, comparison of studies and exports. Well-known free options include:

  • Weasis (Windows, Mac, Linux), an open-source Java-based viewer.
  • Horos (Mac), a free viewer based on the OsiriX codebase.
  • RadiAnt (Windows), which has a free trial and paid licence.
  • 3D Slicer (all platforms), a research platform with advanced tools.

Licensing terms change, so check each project's site before relying on one for work.

Option 3: Use the Viewer on the CD or USB Stick

Hospitals often hand out images on a disc or USB drive that includes its own viewer program. These usually run only on Windows and may not start on newer systems, or on a Mac at all. If the bundled viewer doesn't work, the images themselves are still on the disc. How to open DICOM files from a CD explains how to find and copy them.

Comparing the Options

DICOM viewing options compared
OptionInstall neededWorks on Chromebook / tabletBest for
Browser-based viewerNoYesQuick viewing, sharing screen, occasional use
Desktop viewerYesNo (mostly)Regular use, 3D, comparing studies
Viewer on the discNo (runs from disc)NoWindows PCs with an optical drive

When a DICOM File Won't Open

"Not a DICOM file"

The file might be a DICOMDIR index, a report in PDF form, or a non-image file from the disc. Standard DICOM files contain the letters DICM at byte 128; you can confirm this in a binary file viewer. Some very old files omit that header, which can also confuse viewers.

Some Images Show, Others Don't

That usually means a compression method the viewer can't decode. The SimpleTools24 viewer has verified support for uncompressed, JPEG baseline, JPEG 2000 lossless and RLE images; video encodings such as MPEG-2 or H.264 aren't supported. A desktop viewer may handle files a browser can't.

Slices Appear in the Wrong Order

Viewers sort by instance number or slice position. If files from different series were mixed into one folder, or identifiers are missing, sorting can go wrong. Load each series' files together.

The Image Is All White or All Black

The display window doesn't match the data. Reset the window/level, or choose a preset.

Frequently Asked Questions

Can I convert DICOM to JPG?

Most viewers can export the current image as PNG or JPEG. The export loses the metadata and the full range of pixel values, so keep the original files too.

Can I open a DICOM file on my phone?

Yes, with a browser-based viewer, though large studies need a fair amount of memory. A tablet is more comfortable for scrolling through slices.

Are DICOM files private?

They often contain your name, birth date and ID numbers in their metadata. Be careful where you upload or share them; see what patient information is stored in DICOM files.

Why are there so many files for one scan?

Each slice of a CT or MRI series is usually stored as its own file, so a single study can contain hundreds or thousands.

Summary

Use a DICOM viewer, not a photo app. A browser-based viewer is the fastest way to look at .dcm files on any device, desktop viewers suit regular use, and the disc's own viewer is a fallback on Windows.

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