It depends on the context. Generally, it is uncommon for a professional email to contain an informal greeting, especially when the greeting is multiple sentences long, as you suggested. If you do decide to include it, you should keep it short and to-the-point.
Dr. Lastname, // Use whichever greeting you commonly use with this person.
It has been awhile since we last spoke. I was wondering if {intent of email}. I hope all is going well with {process}.
Thanks,
However, if there is a relationship established that isn't strictly professional, say, you work with them for 1 year and leave the company or class, then is more common to include an informal greeting at the beginning.
Hello Dr. LastName,
It has been awhile since we last spoke. How are things going with {class|business process}? I was wondering if {intent of email}. Thank you!
Regards,
If it is someone you see frequently, it is best to keep that informal conversation to small talk between business meetings or worktime.
Dr. Lastname,
Can you email me a copy of the memo from yesterday's meeting?
Thanks,
Lastly, remember to check your spelling and grammar.