Run-length encoding (RLE) is a simple form of lossless data compression.  While other compression methods can achieve better compression ratios than RLE, RLE is both easy to implement and efficient to execute.

When RLE is applied to an image, identical pixels in a sequence are rendered as a single value and a count that indicates how often that value occurs. In the context of DICOM, run-length encoding is primarily used for single-frame ultrasound images.