Artificial insemination is a technique where semen is placed in the vagina, cervical canal, or uterus by means other than sexual intercourse, for the purpose of inducing a pregnancy. The sperm is introduced during the woman's most fertile period by one of two different donor processes. The first, known as husband insemination, is done when a mechanical difficulty prevents the husband from depositing the semen in his wife's vagina. Some of these difficulties include impotence, physical deformities, or paralysis. The second process is donor insemination. When infertility is the result of low or no sperm production by the husband, donor insemination uses the semen of a third party. Semen obtained from one donor may be used for many impregnations and can be kept indefinitely through freezing and storage at a sperm bank. Sperm banks house sperm from anonymous donors and from individuals that would like a sperm sample protected for future use. In cases where the husband has undergone radiation or chemotherapy that might damage the sperm, a couple may preserve a sperm sample to be used for later artificial insemination. They can also concentrate sperm by mechanical means to enhance sperm count.