If you really think your boss wouldn't be up for a bit of negotiating, for the sake of an hour and a half I'd be faking car trouble, medical problem, whatever, if it is this important to you and you aren't willing to change the appointment.

FWIW (and I know you are excited and want it done asap) the scan is usually performed between 18-20 weeks so if you are only 18 weeks you have some time up your sleeve.

I really don't see why they couldn't do the scan and then afterwards you could run back and get the referral and bring it back to them and they could then tabulate your results, but I guess that is neither here nor there now.