The MHD file extension stands for MetaImage Header. It is a text-based metadata file format used primarily in medical imaging and scientific visualization to describe volumetric data. An MHD file does not contain the actual pixel or voxel data itself; instead, it contains tags and values that define the properties of the image, such as dimensions, spacing, origin, and data type. It points to a companion raw data file, typically with an .mha or .raw extension, which contains the binary image data. This format was developed as part of the Insight Segmentation and Registration Toolkit (ITK) and is widely used in research and clinical settings for storing CT, MRI, and ultrasound scans. Because the header is human-readable, it allows researchers to easily inspect and modify metadata without altering the underlying binary data. It supports multi-dimensional data, including 2D, 3D, and even 4D time-series images, making it a versatile choice for complex medical datasets. It is often preferred in academic environments because of its simplicity and compatibility with various open-source processing libraries.