If your doctor has ever given you a CD or USB drive with your imaging results, the files on it almost certainly have a .dcm extension. You try to double-click one and nothing happens — or your computer asks you which program to use. This guide explains what a DICOM file actually is and what you can do with it.
The short answer
DICOM (Digital Imaging and Communications in Medicine) is the universal file format for medical images. CT scans, MRIs, X-rays, ultrasounds, mammograms, and PET scans are all stored as DICOM files. It has been the international standard since 1993 and is used in virtually every hospital and radiology practice worldwide.
A .dcm file is not just a picture. It is a container that bundles together:
- The actual image pixel data
- A rich metadata header with hundreds of attributes
- Optionally, multiple image frames (slices or a video sequence)
What's inside the metadata header?
This is where DICOM differs most from ordinary image formats like JPG or PNG. The header contains structured tags defined by the DICOM standard. A single file might include:
| Tag category | Examples |
|---|---|
| Patient | Name, date of birth, sex, patient ID |
| Study | Study date, time, description, accession number |
| Series | Modality (CT/MR/XR), body part, series number |
| Equipment | Manufacturer, model, institution name |
| Image | Rows, columns, pixel spacing, bit depth, window/level |
These tags are what allow a PACS (Picture Archiving and Communication System) at a hospital to automatically route images to the right patient record without any manual entry.
PHI: the privacy concern
Because the header often contains patient name, date of birth, and institution details, a DICOM file is considered Protected Health Information (PHI) under regulations like HIPAA (US) and GDPR (EU). This matters when you want to share a scan:
- Sharing a raw
.dcmfile with a colleague may expose patient identity. - Uploading to a cloud converter carries the same risk.
- Stripping the metadata before sharing — or using a client-side converter that never uploads — is the safe approach.
Multi-frame DICOM
A single DICOM file can contain hundreds of image frames. A CT chest series might be one .dcm file with 300 axial slices. An ultrasound might contain a short video loop. DICOM viewers let you scroll through these frames; basic image converters will show only the first frame unless they have specific multi-frame support.
Window / Level: why the image looks different
CT images can have up to 65,536 shades of grey. A standard monitor can only display about 256. The Window/Level (also called Window Width/Center) setting controls which slice of that range is mapped to the visible grey scale. This is why the same CT scan can look completely different depending on whether you are viewing it in "lung window," "bone window," or "soft tissue window."
When you convert a DICOM to JPG or PNG, you are applying a specific window to compress the full range into a visible image. The 365tools DICOM Converter lets you choose the window preset or set it manually so you get the most diagnostically useful view.
How to open a DICOM file
There are three main approaches:
- Dedicated DICOM viewer — RadiAnt (Windows), OsiriX/Horos (Mac), 3D Slicer (cross-platform). Best for clinical review; supports all tags and multi-frame.
- Browser-based converter — The 365tools DICOM Converter converts .dcm files to JPG, PNG, or WebP entirely in your browser, with no upload. Good for quickly sharing or viewing a scan.
- Scripting — Python libraries like
pydicomlet you read, modify, and write DICOM files programmatically. This is the approach used in research and AI pipelines.
Need to convert a DICOM file right now?
The 365tools DICOM Converter is free, runs in your browser, and never uploads your medical files. Supports Window/Level controls, multi-frame, and PHI metadata stripping.
Open DICOM Converter →Frequently asked questions
What does DICOM stand for?
DICOM stands for Digital Imaging and Communications in Medicine. It is the international standard for transmitting, storing, and displaying medical imaging data.
What programs can open a .dcm file?
Dedicated DICOM viewers like RadiAnt, OsiriX, and Horos can open .dcm files. You can also use a browser-based converter like the 365tools DICOM Converter, which requires no installation and never uploads your files.
Does a DICOM file contain patient information?
Yes. DICOM files include a header with metadata tags that typically contain patient name, date of birth, study date, institution name, and physician name. This is called PHI (Protected Health Information). Before sharing a DICOM image, you should strip this metadata.
How is a DICOM file different from a regular image?
A DICOM file is not just an image — it is a container that bundles the pixel data with a rich metadata header containing hundreds of attributes. It can also store multi-frame sequences (video-like series of slices) in a single file.
Can I convert a DICOM file to JPG or PNG?
Yes. You can convert .dcm files to JPG, PNG, or WebP using the free 365tools DICOM Converter. It runs entirely in your browser, so your medical images are never uploaded to a server.