What is the DICOM file format?
DICOM is a medical image file format used for storing medical image data such as CT scans, MRI, ultrasound, and X-ray images. Each DICOM file embeds patient and study metadata directly alongside the pixel data, so clinical context travels with the image. .dicom files are commonly encountered under the shorter .dcm extension - both are identical DICOM Part 10 files. On hospital imaging CDs, files are often stored with no extension at all, catalogued by a DICOMDIR index file.
Individual DICOM slices range from around 30 KB to 30 MB each; a complete CT or MRI study - hundreds of slices - can reach several gigabytes. It is a high-definition imaging format with well-defined standards supported by sophisticated imaging equipment worldwide. The standard is very strict but deliberately allows freedom in how optional and custom features are implemented through private tags and extension modules. Some implementations of those extensions may not be portable between different viewers or PACS systems.
DICOM encodes pixel data at 12-16 bits per pixel in grayscale - far beyond the 8 bits of a standard JPEG - which enables the windowing and leveling controls clinicians use to examine tissue contrast. Color modalities such as ultrasound use 24-bit RGB. The file begins with a 128-byte preamble followed by the DICM magic signature at byte offset 128. Pixel data compression is optional and may use JPEG, JPEG 2000, JPEG-LS, or RLE transfer syntaxes.
Security & safety
RISK: MEDIUMA DICOM file itself is passive image data and won't run code, so opening a scan from your own hospital is safe. The real concern is PRIVACY: DICOM embeds identifiable patient data (full name, date of birth, ID, sometimes address) in the metadata. Before sharing a scan publicly, in a forum, or for research, use a viewer's anonymize/de-identify function to strip those tags. Only open DICOM files obtained from a legitimate medical source.
Format details
in a nutshellPrograms that open DICOM files
Windows4 apps
3D Slicer
Open-source For 3D reconstruction of CT/MRI volumes; load the series via the DICOM module.
MicroDicom DICOM Viewer
Free Install, then File > Open (or open a whole folder/DICOMDIR from the CD). Free for personal/non-commercial use; actively updated.
RadiAnt DICOM Viewer
Paid Fast, polished viewer with a free trial; drag the folder in. Popular with clinicians.
Weasis
Open-source Cross-platform Java viewer; import the DICOM folder. Used clinically by hospitals. 
macOS2 apps
Horos
Open-source Free macOS-native DICOM viewer (OsiriX fork); import the folder/disc. The standard free Mac choice. 
Linux3 apps
OHIF Viewer (online)
Open-source Browser-based DICOM viewer for uploading/viewing a study without installing software (verify privacy before uploading medical data). Technical details
deep spec| File signature | DICM (4 ASCII bytes) at byte offset 128, preceded by a 128-byte preamble; some legacy DICOM files omit the preamble and signature entirely |
| Byte order | Little-endian or big-endian, declared by the Transfer Syntax UID in the file meta header; Explicit VR Little Endian is the default |
| Container structure | 128-byte preamble + DICM marker + file meta header (SOP Class and Transfer Syntax UIDs) + data set of group/element tagged elements ending with Pixel Data tag 7FE0,0010 |
| Pixel data compression | Uncompressed raw, JPEG, JPEG 2000, JPEG-LS, RLE Lossless, or HEVC/JPEG XL - selected by the Transfer Syntax UID declared in the file meta header |
| Bit depth | Typically 12-16 bits per pixel grayscale (monochrome); 24-bit RGB or YBR_FULL for color modalities such as ultrasound and endoscopy |
| Maximum image dimensions | Rows and Columns fields are 16-bit unsigned integers - up to 65,535 × 65,535 pixels per frame |
| Multi-frame support | A single DICOM file can store many frames (cine loops, multi-slice objects, multi-echo MRI sequences) |
| Embedded patient metadata | Patient name and ID, study and series UIDs, modality, acquisition parameters, and institution data are stored as tagged data elements alongside pixel data |
| MIME type | application/dicom |
| DICOMDIR | An optional companion DICOMDIR file (no extension) indexes all DICOM files in a media set (CD, DVD, USB) for portable interchange |
| Security | No built-in per-file encryption; optional Digital Signatures (PS3.15) and TLS are used at the PACS and network layer |
| Typical file size | 30 KB - 30 MB per image slice; a full CT or MRI study (hundreds of slices) commonly reaches 100 MB to several GB |
| Standard body | NEMA (National Electrical Manufacturers Association) and the DICOM Standards Committee, with continuously revised PS3.x parts |
| SOP Class UID | Each DICOM object carries a SOP Class UID identifying its image type - CT Image, MR Image, Digital X-Ray, Ultrasound, Secondary Capture, etc. |
| Released | 1993 (DICOM 3.0); predecessor ACR-NEMA 1985 |
| Latest version | DICOM standard is continuously revised (PS3.x, multiple releases per year); file format Part 10 unchanged |
| Open standard | Yes · royalty-free |
| Specification | www.dicomstandard.org |
DICOM conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about DICOM files.
