<s>
Spillover	B-Algorithm
effect	O
can	O
be	O
defined	O
as	O
an	O
apparent	O
gain	O
in	O
activity	O
for	O
small	O
objects	O
or	O
regions	O
,	O
as	O
opposed	O
to	O
the	O
partial	B-Algorithm
volume	I-Algorithm
effect	I-Algorithm
.	O
</s>
<s>
It	O
occurs	O
often	O
in	O
biological	O
imaging	O
modalities	O
such	O
as	O
positron	B-Application
emission	I-Application
tomography	I-Application
(	O
PET	O
)	O
and	O
single-photon	O
emission	O
computed	O
tomography	O
(	O
SPECT	O
)	O
because	O
of	O
their	O
limited	O
spatial	O
resolution	O
.	O
</s>
<s>
Although	O
partial	B-Algorithm
volume	I-Algorithm
effect	I-Algorithm
and	O
spillover	B-Algorithm
refer	O
to	O
essentially	O
the	O
same	O
physical	O
problem	O
,	O
it	O
is	O
important	O
to	O
distinguish	O
the	O
outcome	O
of	O
these	O
two	O
different	O
effects	O
.	O
</s>
<s>
For	O
partial	B-Algorithm
volume	I-Algorithm
effect	I-Algorithm
,	O
the	O
apparent	O
loss	O
of	O
activity	O
in	O
the	O
object	O
is	O
distributed	O
across	O
adjacent	O
voxels	B-Algorithm
,	O
which	O
are	O
considered	O
outside	O
the	O
object	O
,	O
resulting	O
in	O
increase	O
in	O
activity	O
in	O
these	O
voxels	B-Algorithm
.	O
</s>
<s>
This	O
increase	O
in	O
activity	O
is	O
referred	O
to	O
as	O
spillover	B-Algorithm
,	O
whereas	O
loss	O
in	O
activity	O
is	O
referred	O
to	O
as	O
partial	B-Algorithm
volume	I-Algorithm
loss	O
.	O
</s>
