You may opt out of an Advanced Imaging Technology (AIT) scans or walkthrough metal detectors, but understand that you are still subject to screening.  As an alternative, if you choose to forego the AIT screening, you will be subject to a pat-down search.

Information on AIT, including how you can prepare to pass smoothly through the checkpoint, can be found on this TSA page.
Pat-down procedures and frequently-asked questions are available on this TSA page.