Format guide
What is a DICOM file? The medical imaging format explained
DICOM (Digital Imaging and Communications in Medicine) is the international standard for storing and exchanging medical images. A DICOM file holds the image pixels plus a header with patient, study and scanner details — such as pixel spacing and slice position — which is what lets a viewer measure in millimetres and rebuild 3D volumes.
What is inside a DICOM file
- Header (data elements) — tags like Patient Name (0010,0010), Study Date, Modality, Pixel Spacing, Slice Thickness, Image Position.
- Pixel data — the image itself, uncompressed or compressed (JPEG, JPEG 2000, JPEG-LS, RLE…).
- Unique identifiers (UIDs) — link each image to its series and study.
Studies, series and images
A study is one examination (for example “CBCT maxilla, 12 March”). It contains one or more series — a CT might have a soft-tissue and a bone reconstruction; an MRI has one series per sequence (T1, T2, FLAIR…). Each series is a stack of images (slices). A CT series with 400 slices is often 400 separate files, sometimes with no file extension at all.
File extensions you will see
| What you see | What it is |
|---|---|
| .dcm / .dicom | A DICOM image file |
| Files with no extension (IM00001, 1.2.840…) | Usually DICOM too — very common on discs |
| DICOMDIR | An index of the disc: which files belong to which patient, study and series |
| .nii / .nii.gz | NIfTI, a research format for volumes — not DICOM, but Ossidex opens it |
How to open a DICOM file
On a computer you need a DICOM viewer; photo apps cannot read the format. On iPhone or iPad, install Ossidex, save the file to Files (or keep the link it came in) and import it. Step-by-step guide.
Privacy
Because the header contains patient details, share DICOM carefully. Ossidex can export an anonymized copy.
Common DICOM tags and why they matter
| Tag | Name | What a viewer uses it for |
|---|---|---|
| (0008,0060) | Modality | CT, MR, CR, DX, PX, IO… — picks presets and tools |
| (0020,000D) / (0020,000E) | Study / Series Instance UID | Groups files into studies and series |
| (0020,0032) / (0020,0037) | Image Position / Orientation | Orders slices and builds 3D volumes |
| (0028,0030) | Pixel Spacing | Converts pixels to millimetres for measurements |
| (0018,0050) | Slice Thickness | Reported with the series; affects MPR quality |
| (0028,1052) / (0028,1053) | Rescale Intercept / Slope | Turns stored values into Hounsfield units on CT |
| (0028,1050) / (0028,1051) | Window Center / Width | Default brightness and contrast |
Compression inside DICOM
The pixel data can be stored uncompressed or compressed with a transfer syntax: JPEG Baseline, JPEG Lossless, JPEG-LS, JPEG 2000, HTJ2K or RLE. Discs from dental CBCT units and imaging centres often use JPEG 2000 or JPEG Lossless to fit on a CD. Ossidex decodes all of these on the device; the full list is on the formats page.
When DICOM files are “broken”
Not every file that calls itself DICOM follows the standard. Some burning programs omit the 128-byte preamble, some exporters leave out UIDs, some put JPEG snapshots next to the images. Ossidex recognises DICOM by content rather than by file name and recovers these where the data allows.
Recent versions also handle the most common “it won’t open” cases from real discs: if you pick the disc’s Windows viewer (.exe) instead of the images, Ossidex suggests importing the whole folder; DICOM files without unique identifiers (UIDs) and JPEG images without a file extension are still imported; and helper files that come with the disc viewer no longer trigger a password prompt.
Frequently asked questions
Can I convert DICOM to JPG?
Yes — Ossidex exports the current view or a whole series as JPEG images or PDF. The JPEGs lose the medical metadata, so keep the original DICOM for your records.
Why does my disc have hundreds of files?
CT and MRI series usually store one slice per file. A viewer puts them back together.
Is DICOM the same as DICOMDIR?
No. DICOMDIR is an index file that points to the DICOM images on a disc. More on DICOMDIR.
Is DICOM only for radiology?
No. Dentistry (CBCT, panoramic and intraoral X-rays), cardiology, ophthalmology and veterinary imaging use DICOM too.
Can a DICOM file contain a PDF?
Yes — Encapsulated PDF objects hold reports. Ossidex keeps them with the study.
Sources
Related pages
DICOMDIR: what it is and how to open it on iPhone, iPad or PC
A DICOMDIR is the index file on a medical imaging disc. Learn what it contains, why it sometimes fails, and how to open the whole disc on iPhone or iPad.
DCM file viewer: what a .dcm file is and how to open it on any device
What a .dcm file is, which DCM viewer to use on Windows, Mac, a browser or iPad, why one file shows one image, and how to convert DCM to JPG safely.
Enhanced multi-frame DICOM (CT and MR) on iPhone and iPad
Enhanced CT and MR store a whole series in one multi-frame DICOM file. How Ossidex reads per-frame geometry, calibration and 4D series on iPhone and iPad.
NIfTI viewer for iPhone and iPad: open .nii and .nii.gz files
Open NIfTI-1 volumes (.nii, .nii.gz) on iPhone and iPad with Ossidex: correct orientation, MPR and 3D, label maps and segmentation masks as layers.
Open DICOM files on iPhone
Open .dcm and other DICOM files on iPhone from Mail, WhatsApp, Telegram, AirDrop or the Files app with Ossidex, a free DICOM viewer for iPhone and iPad.