My dentist told me that tooth #18 needs a root canal;she put temp fillings on; is it possible to get invisalign without doing anything to that tooth?

You can: You can do anything you want and you might even get some Orthodontist to do it although a reputable one won't. However if you don't get it done expect SEVERE pain one day. Can one paint their house inside with a hole in the roof? Of course. Is that a smart thing to do? No!

Not recommended: Sorry to hear that. Getting root canal can be upsetting. However, you have to have your cavities, gums and root canals fixed, before you get any orthodontic treatment started. Your orthodontists will have you sign a paper when you get your treatment saying that you promise to keep regular checkups and cleanings with your regular dentist.

I guess it is...: ... but why? If root canal therapy was needed during your Invisalign treatment, the shape of that tooth would change, the aligners would not fit correctly, and you would incur additional fees for the Invisalign. Restore your mouth correctly.. you'll be glad you did. I think you would have a hard time finding anyone to treat you cosmetically before all necessary work was completed.

Orthodontic Tx: is the treatment not just to preserve the dentition by correcting the malocclusion, establish the integrity of the bone and protect the teeth from trauma during function. The end product is a healthy beautiful!Teeth should have no cavitation to maintain the structure, and the periodontal condition should be under control before the teeth is subjected to move in an ideal relationship and level...

Need root canal: You still would need the root canal as the tooth is probably infected and without treatment will cause pain. I would get the root canal first, then start your Invisalign treatment.

No : No reputable Orthodontic Specialist will treat you unless and until all infections are brought under control first. Untreated infected tooth can damage adjacent test, bone, gums, can cause damage to other organ systems, and can even be life threatening. Please have any and all infections eliminated before starting tooth movement.