Does Using Isnull() Prevent The Optimizer From Using The Index For That Col.?

Hi all,
This table,

create table test (
t1 int,
t2 char(4) )

has about a few hundred rows. I have created a nonclustered index on column t1.

When I execute the following query, the query execution plan shows that the optimizer performs an index seek using the index on col. t1.

select * from test where t1 = 4

But, when I run the following query, using isnull(), the optimizer always performs a table scan.

select * from test where isnull(t1, 0) = 4

Could someone please explain why?

Thanks in advance,
Praveena