Uterine fibroid. Coloured magnetic resonance imaging (MRI) scan of a sagittal (sideways) section through a woman's pelvic region showing a large fibroid (orange) in the uterus. A fibroid is a benign tumour of fibrous and muscular tissue. Most do not require treatment, but some can cause bleeding, pain and other disorders. Large fibroids may need to be surgically removed. MRI uses radio waves and a powerful magnet to produce slice images through the body. The base of the spine (turquoise, upper centre) is also seen.