<s>
In	O
physics	O
,	O
the	O
spin	B-Algorithm
–	I-Algorithm
spin	I-Algorithm
relaxation	I-Algorithm
is	O
the	O
mechanism	O
by	O
which	O
,	O
the	O
transverse	O
component	O
of	O
the	O
magnetization	O
vector	O
,	O
exponentially	O
decays	O
towards	O
its	O
equilibrium	O
value	O
in	O
nuclear	O
magnetic	O
resonance	O
(	O
NMR	O
)	O
and	O
magnetic	O
resonance	O
imaging	O
(	O
MRI	O
)	O
.	O
</s>
<s>
It	O
is	O
characterized	O
by	O
the	O
spin	B-Algorithm
–	I-Algorithm
spin	I-Algorithm
relaxation	I-Algorithm
time	O
,	O
known	O
as	O
2	O
,	O
a	O
time	O
constant	O
characterizing	O
the	O
signal	O
decay	O
.	O
</s>
<s>
It	O
is	O
named	O
in	O
contrast	O
to	O
1	O
,	O
the	O
spin	B-Algorithm
–	I-Algorithm
lattice	I-Algorithm
relaxation	I-Algorithm
time	I-Algorithm
.	O
</s>
<s>
2	O
relaxation	B-Algorithm
generally	O
proceeds	O
more	O
rapidly	O
than	O
1	O
recovery	O
,	O
and	O
different	O
samples	O
and	O
different	O
biological	O
tissues	O
have	O
different	O
2	O
.	O
</s>
<s>
For	O
example	O
,	O
fluids	O
have	O
the	O
longest	O
2	O
(	O
on	O
the	O
order	O
of	O
seconds	O
for	O
protons	B-Application
)	O
,	O
and	O
water	O
based	O
tissues	O
are	O
in	O
the	O
40	O
–	O
200ms	O
range	O
,	O
while	O
fat	O
based	O
tissues	O
are	O
in	O
the	O
10	O
–	O
100	O
ms	O
range	O
.	O
</s>
<s>
Unlike	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
,	O
considering	O
spin-spin	B-Algorithm
relaxation	I-Algorithm
using	O
only	O
a	O
single	O
isochromat	O
is	O
trivial	O
and	O
not	O
informative	O
.	O
</s>
<s>
Like	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
,	O
spin-spin	B-Algorithm
relaxation	I-Algorithm
can	O
be	O
studied	O
using	O
a	O
molecular	O
tumbling	O
autocorrelation	O
framework	O
.	O
</s>
<s>
The	O
relaxation	B-Algorithm
rate	O
experienced	O
by	O
a	O
spin	O
,	O
which	O
is	O
the	O
inverse	O
of	O
T2	O
,	O
is	O
proportional	O
to	O
a	O
spin	O
's	O
tumbling	O
energy	O
at	O
the	O
frequency	O
difference	O
between	O
one	O
spin	O
and	O
another	O
;	O
in	O
less	O
mathematical	O
terms	O
,	O
energy	O
is	O
transferred	O
between	O
two	O
spins	O
when	O
they	O
rotate	O
at	O
a	O
similar	O
frequency	O
to	O
their	O
beat	B-Application
frequency	I-Application
,	O
in	O
the	O
figure	O
at	O
right	O
.	O
</s>
<s>
In	O
that	O
the	O
beat	B-Application
frequency	I-Application
range	O
is	O
very	O
small	O
relative	O
to	O
the	O
average	B-Algorithm
rotation	I-Algorithm
rate	I-Algorithm
,	O
spin-spin	B-Algorithm
relaxation	I-Algorithm
is	O
not	O
heavily	O
dependent	O
on	O
magnetic	O
field	O
strength	O
.	O
</s>
<s>
This	O
directly	O
contrasts	O
with	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
,	O
which	O
occurs	O
at	O
tumbling	O
frequencies	O
equal	O
to	O
the	O
Larmor	O
frequency	O
.	O
</s>
<s>
Since	O
slower	O
tumbling	O
displaces	O
the	O
spectral	O
energy	O
at	O
high	O
tumbling	O
frequencies	O
to	O
lower	O
frequencies	O
,	O
the	O
relatively	O
low	O
beat	B-Application
frequency	I-Application
will	O
experience	O
a	O
monotonically	O
increasing	O
amount	O
of	O
energy	O
as	O
increases	O
,	O
decreasing	O
relaxation	B-Algorithm
time	O
.	O
</s>
<s>
It	O
is	O
worth	O
noting	O
again	O
that	O
fast	O
tumbling	O
spins	O
,	O
such	O
as	O
those	O
in	O
pure	O
water	O
,	O
have	O
similar	O
T1	B-Algorithm
and	O
T2	B-Algorithm
relaxation	I-Algorithm
times	O
,	O
while	O
slow	O
tumbling	O
spins	O
,	O
such	O
as	O
those	O
in	O
crystal	O
lattices	O
,	O
have	O
very	O
distinct	O
relaxation	B-Algorithm
times	O
.	O
</s>
<s>
In	O
order	O
to	O
reduce	O
the	O
amount	O
of	O
T1	B-Algorithm
information	O
and	O
therefore	O
contamination	O
in	O
the	O
image	O
,	O
excited	O
spins	O
are	O
allowed	O
to	O
return	O
to	O
near-equilibrium	O
on	O
a	O
T1	B-Algorithm
scale	O
before	O
being	O
excited	O
again	O
.	O
</s>
<s>
Pulse	O
sequences	O
other	O
than	O
the	O
conventional	O
spin	O
echo	O
can	O
also	O
be	O
used	O
to	O
measure	O
T2	O
;	O
gradient	O
echo	O
sequences	O
such	O
as	O
steady-state	B-Algorithm
free	I-Algorithm
precession	I-Algorithm
(	O
SSFP	O
)	O
and	O
multiple	O
spin	O
echo	O
sequences	O
can	O
be	O
used	O
to	O
accelerate	O
image	O
acquisition	O
or	O
inform	O
on	O
additional	O
parameters	O
.	O
</s>
