Do you NEED a Ceph Xray for your Invisalign Patient?

Ultimately, you'll need to consult your dental practice act in your state or country for the answer on this question.

For me, I only take cephs if there's a benefit from the cephs. I take cephs on growing kids who need AP or vertical changes and on adults where we are contemplating making drastic AP or vertical changes. 

If you are just starting orthodontics in your practice and you don't want to invest in a ceph arm or module, you can always refer out for the ceph. There are many oral and maxillofacial radiologists in urban areas who often have free-standing xray only practices. https://www.aaomr.org/find-an-omr2#/

If you don't have a center near you, you can also consider a dental school. 

Lastly, find a friend nearby who you can outsource the xray too, who will be professional and not poach your patient.