(µ) Micro is often used as a synonym for very small; micro is mathematically one millionth of a unit. For example a microgram (µg) is one millionth of a gram.

See also Kilogram.

Same as micrometer (µm), one millionth of a meter.

See Meter.

Microcalcifications in breast imaging are tiny white specks of calcium salts. Microcalcifications in clusters, seen on a mammogram, can be the only sign of ductal carcinoma in situ or early invasive cancer, or they can be associated with benign breast changes.

Grids are used in a test phantom to check the quality of x-ray images. Such a grid may consist for example, of gold structures on a silicon surface and a grid period length of 2 micrometer with space wide in between of 1 micrometer.
Filter grids are also used in x-ray imaging to reduce noise contributed by scatter.

A pixel is a picture element (pix, abbreviation of pictures + element). Tomographic images are composed of several pixels; the pixel size is determined by the used field of view and the number of elements in the display image matrix. The corresponding size of the pixel may be smaller than the actual spatial resolution.
Pixels do not have a fixed size; their diameters are generally measured in micrometers (microns). Although the pixel is not a unit of measurement itself, pixels are often used to measure the resolution (or sharpness) of images. As a hypothetical example, a 600 x 1000 pixel image has 4 times the pixel density and is thus 4 times sharper than a 300 x 500 pixel image, assuming the two images have the same physical size.