An X-ray of the patella is a safe and painless test to visualise the knee cap using small amount of radiation. Patella is a circular-triangular bone which articulates with the lower end of the femur (thigh bone) and covers the knee joint. The image is recorded on a special x ray film. The X-ray image is black and white. Dense structures such as the bones, appear white and softer body tissues, such as the skin and muscles appear darker. The actual exposure time to radiation is usually less than a second, although the overall test may take 15-20 minutes.