I do this all the time. If I have had a particularly productive session or T really helped me in something that she did, I send an email or a text along the lines of, "thanks for today, it was great progress for me" or "just wanted to say thank you for today, it really helped me see things more clearly" or something like that and I might add that I think it will be good to work on again in the next session. I would send it if that's how you feel, I'm sure it will be positively received.