Educational Guide

DICOM Format Explained: What Every Doctor Should Know

Updated April 2026 · 7 min read

DICOM (Digital Imaging and Communications in Medicine) is the universal standard for medical imaging. Every CT scanner, MRI machine, X-ray system, and ultrasound device speaks DICOM. Understanding how it works helps you manage patient imaging data more effectively.

What is DICOM?

DICOM is both a file format and a network protocol. It was developed by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) in the 1980s, and has become the global standard for medical imaging.

Unlike JPEG or PNG, a DICOM file contains not just the image but also metadata: patient name, study date, imaging parameters, slice position, window/level presets, and hundreds of other attributes called “DICOM tags.”

DICOM File Structure

Every DICOM file follows this hierarchy:

Patient

└ Study (one exam session)

└ Series (one sequence/protocol)

└ Instance (one image/slice)

A single CT study might contain 500+ instances (slices), organized into one or more series. Each .dcm file represents one instance.

Transfer Syntaxes

DICOM supports multiple compression methods called “transfer syntaxes.” The most common ones:

Implicit VR Little Endian: Default, uncompressed. Largest file size but universal support.

Explicit VR Little Endian: Uncompressed with explicit value representation. Most common.

JPEG Baseline: Lossy compression. Smaller files, minimal quality loss.

JPEG 2000: Both lossy and lossless. Best compression ratio. Common in modern systems.

JPEG-LS: Near-lossless compression. Good balance of size and quality.

RLE Lossless: Run-length encoding. Lossless but limited compression.

MedScan supports all of these transfer syntaxes, built with DCMTK 3.6.8 and OpenJPEG 2.5.0.

Common DICOM Modalities

CTComputed Tomography

MRMagnetic Resonance

DXDigital Radiography

USUltrasound

PXPanoramic X-Ray

IOIntraoral X-Ray

PTPET Scan

NMNuclear Medicine

XCExternal Camera

How to Get DICOM Files

DICOM CD/DVD: Most imaging centers provide a CD with DICOM files and a basic viewer.
Patient Portal: Many hospitals offer DICOM download through online portals.
PACS Export: Radiologists can export studies from PACS to USB or network share.
Cloud PACS: Services like Google Cloud Healthcare or AWS HealthImaging provide DICOM access via DICOMweb.

Viewing DICOM on Mobile

Traditionally, DICOM viewing required desktop software (OsiriX, RadiAnt, Horos) or a PACS workstation. Modern mobile DICOM viewers like MedScan now offer the same capabilities on iPhone and iPad — with the added advantage of portability, offline access, and AI-powered analysis.

MedScan opens .dcm files, ZIP archives, and complete DICOM CD folder structures. It automatically groups files by patient and study, detects the modality, and applies appropriate viewing presets.

DICOM Privacy Considerations

DICOM files contain Protected Health Information (PHI) including patient name, date of birth, and medical record numbers. When sharing DICOM files, always use DICOM anonymization tools to remove PHI. When viewing on a mobile device, ensure the app processes data locally — avoid apps that upload DICOM files to cloud servers.