For conventional intra-cytoplasmic sperm injection (ICSI), spermatozoa are selected at a high magnification. However, the IMSI procedure allows a far better assessment, since spermatozoa are selected at a far higher magnification. IMSI may improve the chances of in vitro fertilization. IMSI is a real-time method where sperm are selected before the microinjection takes place. This is done using an inverted microscope that provides a much greater magnifying power (around 6000 times) than those that are normally used in reproductive laboratories (400 times) to carry out the ICSI.

With this microscope, biologists can see the internal morphology of sperm and discard those with abnormalities. Being able to select sperm without morphological alterations is believed to increase the chances of successful implantation of pre-embryo and decrease the likelihood of miscarriage.

The IMSI may improve the chances of fertilisation in vitro and can be used if in vitro fertilisation (IVF) has been tried without success.