I have to take a MIBG scan and they want me to discontinue taking Amitriptilyne (Elavil) for six weeks. Why do I have to wait so long?

They are trying to locate the Pheochromocytoma(s) and I am taking Elavil as a sleep aid. I don’t see the connection. I have googled and they mention a problem, but I dont understand. The Doctors at the James hospital didn’t specify a step-down, rather they wanted me to stop all at once. It is getting pretty uncomfortable as I have been taking it for some 8 years. Withdrawal symptoms are pretty troublesome. The Chemist at the Drug Store says it should clear my body in 2 weeks. Why 6 for the tests?