.DCM File Format

Introduction

DICOM (Digital Imaging and Communications in Medicine) is the standard for medical images and the network protocol that moves them between scanners, archives and viewing stations. It is published by NEMA and is free to read. A DICOM file holds one image or a series of frames from CT, MRI, X-ray, ultrasound and other machines, together with details of the patient, the study and the equipment.

Pixel values are often 12 or 16 bits per sample, and stand for measurements such as tissue density rather than display colours. Many DICOM files have no extension at all; .dcm is the usual one when there is.

Structure

A DICOM file starts with a 128-byte preamble, usually all zeros, followed by the four letters DICM. Then comes the file meta information (group 0002), always written as explicit VR little endian. It names the transfer syntax used for the rest of the file.

Everything after that is a list of data elements. Each element has a tag (a group and element number, two bytes each), a two-letter value representation such as PN for a person’s name or US for an unsigned short (in the explicit forms), a length and the value. The pixels sit in the element (7FE0,0010), either raw or compressed with JPEG, JPEG 2000, RLE or another codec named by the transfer syntax.

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