.DCM File Format – Pixel data

Image pixel elements

All planes share the same rows, columns, bits allocated, bits stored, high bit, pixel representation and aspect ratio. To turn a stored value into a measurement, many objects give Rescale Intercept (0028,1052) and Rescale Slope (0028,1053): output = slope × stored + intercept (Hounsfield units in CT). Window Center (0028,1050) and Window Width (0028,1051) suggest a display range.

Photometric interpretations

Native pixel data

With an uncompressed transfer syntax the value of (7FE0,0010) is the pixels themselves, with an explicit length.

• Each sample sits in a cell of Bits Allocated bits. Its data occupies bits High Bit − Bits Stored + 1 to High Bit; other bits may hold anything, so a reader masks them (and sign-extends for Pixel Representation 1).
• Samples run left to right, rows top to bottom, then frame after frame with no padding between frames. One padding byte at the very end keeps the length even.
• Little endian syntaxes store 16-bit cells least significant byte first. The VR is OW, or OB when Bits Allocated is 8 or less in an explicit VR syntax.
• With Bits Allocated 1, pixels are packed eight to a byte, and a frame may start in the middle of a byte.

The size is Rows × Columns × Samples per Pixel × Number of Frames × Bits Allocated / 8 bytes, rounded up to even, except for YBR_FULL_422, where two pixels share one pair of colour samples.

Encapsulated pixel data

With a compressed transfer syntax, (7FE0,0010) is OB with undefined length, and its value is a run of items:

Encapsulation is only used for top-level pixel data, never for float pixel data, and the data set is always explicit VR little endian. A reader walks the item lengths rather than searching for the delimiter.

E0 7F 10 00  4F 42  00 00  FF FF FF FF      (7FE0,0010) OB, undefined length
FE FF 00 E0  00 00 00 00                    empty Basic Offset Table
FE FF 00 E0  2A 3B 00 00  FF D8 FF …        fragment 1: 15146 bytes of JPEG
FE FF DD E0  00 00 00 00                    sequence delimitation

RLE Lossless

Transfer syntax 1.2.840.10008.1.2.5 uses DICOM’s own run-length coding (PS3.5 Annex G). Each frame is one fragment, made of a 64-byte header and up to 15 segments.

Offsets are little endian and count from the start of the header. Each sample is padded with zero bits at the top to whole bytes, and the bytes are split into segments: the most significant bytes of all pixels first. A 16-bit greyscale image has two segments (high bytes, then low bytes); an 8-bit RGB image three (red, green, blue). Each segment is coded on its own:

Runs never cross the end of a row, and each segment is padded to an even length with a zero byte.