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Iris

The iris is the thin, circular, pigmented structure in the anterior segment of the eye that surrounds the pupil and regulates the amount of light entering the retina. It is easily visible through the cornea as the colored part of the eye. The iris consists of smooth muscle fibers that control pupil size, responding reflexively to light and accommodation.

Synonyms for Iris

  • Coloboma iridis (if referring to iris defects)

  • Uveal diaphragm (broader context including ciliary body and choroid)

  • Eye diaphragm

  • Coloured part of the eye

Function of the Iris

  • Regulates the diameter and size of the pupil

  • Controls the amount of light reaching the retina by constriction (miosis) or dilation (mydriasis) of the pupil

  • Contributes to the eye's aesthetic appearance (eye color)

  • Acts as a barrier between anterior and posterior chambers of the eye

  • Assists in the aqueous humor dynamics

MRI Appearance of the Iris

  • T2-weighted images:

    • The iris appears as a thin, hypointense (dark) structure compared to the hyperintense signal of intraocular fluid.

    • Fine delineation possible due to the contrast between iris and aqueous/vitreous humor.

  • T1-weighted images:

    • The iris appears as a thin, isointense to slightly hypointense band relative to surrounding fluid.

    • Less contrast than T2; more difficult to delineate from ciliary body unless high-resolution is used.

  • STIR (Short Tau Inversion Recovery) sequences:

    • The iris remains hypointense due to suppression of fat and high contrast with intraocular fluids.

    • Useful in identifying pathological processes such as edema or infiltration if present.

CT Appearance of the Iris

  • The iris is usually not well visualized on conventional CT due to its thinness and similar density to surrounding aqueous humor.

MRI images

Iris mri sagittal image