What is a DICOM (.dcm) file and what is inside it?

A .dcm file is a DICOM file, the standard format for storing medical images such as CT, MRI and X-ray. It holds the image itself together with information about who it belongs to and how it was taken. One file is usually one image, or one short image sequence.

What DICOM stands for

DICOM means Digital Imaging and Communications in Medicine. It is both a file format and a set of rules that let scanners, hospital archives and viewing software exchange images. That is why a scan from one hospital can usually be opened by software from a different company.

The .dcm ending is common but not guaranteed. Many scan discs store DICOM files with no extension at all, or with names like IM0001. The content decides whether a file is DICOM, not the name.

How the file is laid out

A DICOM file has a clear structure:

1. A 128-byte preamble. Usually empty, and software ignores it.
2. The marker DICM. These four letters sit right after the preamble, at byte 128. They are how software recognises a DICOM file.
3. Tags. A long list of labelled values describing the image.
4. Pixel data. The actual image, stored as numbers for each pixel.

You can check the marker yourself with a hex viewer, but for everyday use you never need to. A viewer does it for you.

What the tags contain

Tags are the file’s built-in labels. Each one has a number and a value. They are grouped by what they describe:

• Patient: name, ID, birth date and sex
• Study: the date of the examination, a description, and an identifier for the whole visit
• Series: which sequence or scan run the image belongs to
• Image: the position in the series, the pixel size, the image dimensions and how the pixels should be displayed

Because of this, a DICOM file carries personal information inside it. That is worth remembering before you share one. See DICOM tags and metadata for a closer look at the individual fields.

Study, series and image

DICOM organises scans in three levels:

• A study is one examination, for example a head scan on a given day.
• A series is one run within that study, such as one MRI sequence or one set of CT slices.
• An image is a single picture within a series.

One CT can easily be several hundred files, because each slice is its own file. A single series is a stack of these slices that a viewer lets you scroll through. Some files, such as an ultrasound or a heart recording, hold many frames in a single file and play as a short loop. See multi-frame and cine files.

Why the image looks different from a photo

A photo stores colour for each pixel. A medical image usually stores a single grey value per pixel, and that range can be much wider than a normal screen can show. A viewer picks part of the range to display, which is why brightness and contrast settings matter. See window and level explained.

The pixels can also be stored in different ways. Some files are uncompressed, and others use compression such as JPEG, JPEG 2000 or RLE. A viewer has to support the compression used in the file.

What a .dcm file is not

It is not a picture you can safely edit and send as a report. It is also not a diagnosis. The file is a record of what the scanner captured, along with labels. What it means for a person’s health is for the treating doctor to say.

How to open one

A regular photo app will not open it. You need a DICOM viewer, which exists for computers, for browsers and for phones. Radix DICOM opens single .dcm files and whole patient folders on Android, works offline and does not upload images. For options on a computer, see how to open a .dcm file without installing hospital software.