SQL NOT IN statement Data is not retrieved Using Index

I have simple query :

SELECT * FROM MH.APPOINTMENT WHERE DOCTOR_INC_ID = 1391791151

When I examine the execution plan, I see that the data is retrieved using index

However the following query :

SELECT * FROM MH.APPOINTMENT WHERE DOCTOR_INC_ID NOT IN (1391791151)

does not benefit from our index. We are using Oracle 11g Release2. Any suggestions is welcome. Thanks