<s>
During	O
nuclear	O
magnetic	O
resonance	O
observations	O
,	O
spin	B-Algorithm
–	I-Algorithm
lattice	I-Algorithm
relaxation	I-Algorithm
is	O
the	O
mechanism	O
by	O
which	O
the	O
longitudinal	O
component	O
of	O
the	O
total	O
nuclear	O
magnetic	O
moment	O
vector	O
(	O
parallel	O
to	O
the	O
constant	O
magnetic	O
field	O
)	O
exponentially	O
relaxes	O
from	O
a	O
higher	O
energy	O
,	O
non-equilibrium	O
state	O
to	O
thermodynamic	O
equilibrium	O
with	O
its	O
surroundings	O
(	O
the	O
"	O
lattice	O
"	O
)	O
.	O
</s>
<s>
It	O
is	O
characterized	O
by	O
the	O
spin	B-Algorithm
–	I-Algorithm
lattice	I-Algorithm
relaxation	I-Algorithm
time	I-Algorithm
,	O
a	O
time	O
constant	O
known	O
as	O
T1	B-Algorithm
.	O
</s>
<s>
There	O
is	O
a	O
different	O
parameter	O
,	O
T2	O
,	O
the	O
spin-spin	B-Algorithm
relaxation	I-Algorithm
time	I-Algorithm
,	O
which	O
concerns	O
the	O
exponential	O
relaxation	B-Algorithm
of	O
the	O
transverse	O
component	O
of	O
the	O
nuclear	O
magnetization	O
vector	O
(	O
to	O
the	O
external	O
magnetic	O
field	O
)	O
.	O
</s>
<s>
Measuring	O
the	O
variation	O
of	O
T1	B-Algorithm
and	O
T2	O
in	O
different	O
materials	O
is	O
the	O
basis	O
for	O
some	O
magnetic	O
resonance	O
imaging	O
techniques	O
.	O
</s>
<s>
The	O
name	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
refers	O
to	O
the	O
process	O
in	O
which	O
the	O
spins	O
give	O
the	O
energy	O
they	O
obtained	O
from	O
the	O
RF	O
pulse	O
back	O
to	O
the	O
surrounding	O
lattice	O
,	O
thereby	O
restoring	O
their	O
equilibrium	O
state	O
.	O
</s>
<s>
The	O
relaxation	B-Algorithm
time	O
,	O
T1	B-Algorithm
(	O
the	O
average	O
lifetime	O
of	O
nuclei	O
in	O
the	O
higher	O
energy	O
state	O
)	O
is	O
dependent	O
on	O
the	O
gyromagnetic	O
ratio	O
of	O
the	O
nucleus	O
and	O
the	O
mobility	O
of	O
the	O
lattice	O
.	O
</s>
<s>
Different	O
tissues	O
have	O
different	O
T1	B-Algorithm
values	O
.	O
</s>
<s>
For	O
example	O
,	O
fluids	O
have	O
long	O
T1s	B-Algorithm
(	O
1500-2000	O
ms	O
)	O
,	O
and	O
water-based	O
tissues	O
are	O
in	O
the	O
400-1200	O
ms	O
range	O
,	O
while	O
fat	O
based	O
tissues	O
are	O
in	O
the	O
shorter	O
100-150	O
ms	O
range	O
.	O
</s>
<s>
The	O
presence	O
of	O
strongly	O
magnetic	O
ions	O
or	O
particles	O
(	O
e.g.	O
,	O
ferromagnetic	O
or	O
paramagnetic	O
)	O
also	O
strongly	O
alter	O
T1	B-Algorithm
values	O
and	O
are	O
widely	O
used	O
as	O
MRI	B-Algorithm
contrast	I-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
The	O
magnetization	O
of	O
the	O
proton	O
ensemble	O
goes	O
back	O
to	O
its	O
equilibrium	O
value	O
with	O
an	O
exponential	O
curve	O
characterized	O
by	O
a	O
time	O
constant	O
T1	B-Algorithm
(	O
see	O
Relaxation	B-Algorithm
(	O
NMR	O
)	O
)	O
.	O
</s>
<s>
T1	B-Algorithm
weighted	O
images	O
can	O
be	O
obtained	O
by	O
setting	O
short	O
repetition	O
time	O
(	O
TR	O
)	O
such	O
as	O
<	O
750	O
ms	O
and	O
echo	O
time	O
(	O
TE	O
)	O
such	O
as	O
<	O
40	O
ms	O
in	O
conventional	O
spin	O
echo	O
sequences	O
,	O
while	O
in	O
Gradient	O
Echo	O
Sequences	O
they	O
can	O
be	O
obtained	O
by	O
using	O
flip	O
angles	O
of	O
larger	O
than	O
50o	O
while	O
setting	O
TE	O
values	O
to	O
less	O
than	O
15	O
ms.	O
</s>
<s>
T1	B-Algorithm
is	O
significantly	O
different	O
between	O
grey	O
matter	O
and	O
white	O
matter	O
and	O
is	O
used	O
when	O
undertaking	O
brain	O
scans	O
.	O
</s>
<s>
A	O
strong	O
T1	B-Algorithm
contrast	O
is	O
present	O
between	O
fluid	O
and	O
more	O
solid	O
anatomical	O
structures	O
,	O
making	O
T1	B-Algorithm
contrast	O
suitable	O
for	O
morphological	O
assessment	O
of	O
the	O
normal	O
or	O
pathological	O
anatomy	O
,	O
e.g.	O
,	O
for	O
musculoskeletal	O
applications	O
.	O
</s>
<s>
Spin	B-Algorithm
–	I-Algorithm
lattice	I-Algorithm
relaxation	I-Algorithm
in	I-Algorithm
the	I-Algorithm
rotating	I-Algorithm
frame	I-Algorithm
is	O
the	O
mechanism	O
by	O
which	O
Mxy	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
of	O
zero	O
,	O
under	O
the	O
influence	O
of	O
a	O
radio	O
frequency	O
(	O
RF	O
)	O
field	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
lattice	I-Algorithm
relaxation	I-Algorithm
time	I-Algorithm
constant	O
in	O
the	O
rotating	O
frame	O
,	O
T1ρ	O
.	O
</s>
<s>
It	O
is	O
named	O
in	O
contrast	O
to	O
T1	B-Algorithm
,	O
the	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
time	I-Algorithm
.	O
</s>
<s>
T1ρ	O
MRI	O
is	O
an	O
alternative	O
to	O
conventional	O
T1	B-Algorithm
and	O
T2	O
MRI	O
by	O
its	O
use	O
of	O
a	O
long-duration	O
,	O
low-power	O
radio	O
frequency	O
referred	O
to	O
as	O
spin-lock	O
(	O
SL	O
)	O
pulse	O
applied	O
to	O
the	O
magnetization	O
in	O
the	O
transverse	O
plane	O
.	O
</s>
<s>
The	O
spin-locked	O
magnetization	O
will	O
relax	O
with	O
a	O
time	O
constant	O
T1ρ	O
,	O
which	O
is	O
the	O
time	O
it	O
takes	O
for	O
the	O
magnetic	O
resonance	O
signal	O
to	O
reach	O
37%	O
(	O
1/e	O
)	O
of	O
its	O
initial	O
value	O
,	O
.	O
</s>
<s>
T1ρ	O
can	O
be	O
quantified	O
(	O
relaxometry	O
)	O
by	O
curve	B-Algorithm
fitting	I-Algorithm
the	O
signal	O
expression	O
above	O
as	O
a	O
function	O
of	O
the	O
duration	O
of	O
the	O
spin-lock	O
pulse	O
while	O
the	O
amplitude	O
of	O
spin-lock	O
pulse	O
( γB1	O
~	O
0.1-few	O
kHz	O
)	O
is	O
fixed	O
.	O
</s>
<s>
Quantitative	O
T1ρ	O
MRI	O
relaxation	B-Algorithm
maps	O
reflect	O
the	O
biochemical	O
composition	O
of	O
tissues	O
.	O
</s>
<s>
T1ρ	O
MRI	O
has	O
been	O
used	O
to	O
image	O
tissues	O
such	O
as	O
cartilage	O
,	O
intervertebral	O
discs	O
,	O
brain	O
,	O
and	O
heart	O
,	O
as	O
well	O
as	O
certain	O
types	O
of	O
cancers	O
.	O
</s>
