<s>
The	O
physics	B-Algorithm
of	I-Algorithm
magnetic	I-Algorithm
resonance	I-Algorithm
imaging	I-Algorithm
(	O
MRI	O
)	O
concerns	O
fundamental	O
physical	O
considerations	O
of	O
MRI	O
techniques	O
and	O
technological	O
aspects	O
of	O
MRI	O
devices	O
.	O
</s>
<s>
MRI	O
is	O
a	O
medical	B-Application
imaging	I-Application
technique	I-Application
mostly	O
used	O
in	O
radiology	O
and	O
nuclear	O
medicine	O
in	O
order	O
to	O
investigate	O
the	O
anatomy	O
and	O
physiology	O
of	O
the	O
body	O
,	O
and	O
to	O
detect	O
pathologies	O
including	O
tumors	O
,	O
inflammation	O
,	O
neurological	O
conditions	O
such	O
as	O
stroke	O
,	O
disorders	O
of	O
muscles	O
and	O
joints	O
,	O
and	O
abnormalities	O
in	O
the	O
heart	O
and	O
blood	O
vessels	O
among	O
others	O
.	O
</s>
<s>
Contrast	B-Algorithm
agents	I-Algorithm
may	O
be	O
injected	O
intravenously	O
or	O
into	O
a	O
joint	O
to	O
enhance	O
the	O
image	O
and	O
facilitate	O
diagnosis	O
.	O
</s>
<s>
Unlike	O
CT	O
and	O
X-ray	B-Library
,	O
MRI	O
uses	O
no	O
ionizing	O
radiation	O
and	O
is	O
,	O
therefore	O
,	O
a	O
safe	O
procedure	O
suitable	O
for	O
diagnosis	O
in	O
children	O
and	O
repeated	O
runs	O
.	O
</s>
<s>
By	O
varying	O
the	O
parameters	O
of	O
the	O
pulse	O
sequence	O
,	O
different	O
contrasts	O
may	O
be	O
generated	O
between	O
tissues	O
based	O
on	O
the	O
relaxation	B-Algorithm
properties	O
of	O
the	O
hydrogen	O
atoms	O
therein	O
.	O
</s>
<s>
When	O
inside	O
the	O
magnetic	O
field	O
(	O
B0	O
)	O
of	O
the	O
scanner	O
,	O
the	O
magnetic	O
moments	O
of	O
the	O
protons	B-Application
align	O
to	O
be	O
either	O
parallel	O
or	O
anti-parallel	O
to	O
the	O
direction	O
of	O
the	O
field	O
.	O
</s>
<s>
While	O
each	O
individual	O
proton	B-Application
can	O
only	O
have	O
one	O
of	O
two	O
alignments	O
,	O
the	O
collection	O
of	O
protons	B-Application
appear	O
to	O
behave	O
as	O
though	O
they	O
can	O
have	O
any	O
alignment	O
.	O
</s>
<s>
Most	O
protons	B-Application
align	O
parallel	O
to	O
B0	O
as	O
this	O
is	O
a	O
lower	O
energy	O
state	O
.	O
</s>
<s>
A	O
radio	O
frequency	O
pulse	O
is	O
then	O
applied	O
,	O
which	O
can	O
excite	O
protons	B-Application
from	O
parallel	O
to	O
anti-parallel	O
alignment	O
,	O
only	O
the	O
latter	O
are	O
relevant	O
to	O
the	O
rest	O
of	O
the	O
discussion	O
.	O
</s>
<s>
In	O
response	O
to	O
the	O
force	O
bringing	O
them	O
back	O
to	O
their	O
equilibrium	O
orientation	O
,	O
the	O
protons	B-Application
undergo	O
a	O
rotating	O
motion	O
(	O
precession	O
)	O
,	O
much	O
like	O
a	O
spun	O
wheel	O
under	O
the	O
effect	O
of	O
gravity	O
.	O
</s>
<s>
The	O
protons	B-Application
will	O
return	O
to	O
the	O
low	O
energy	O
state	O
by	O
the	O
process	O
of	O
spin-lattice	B-Algorithm
relaxation	I-Algorithm
.	O
</s>
<s>
The	O
frequency	O
at	O
which	O
a	O
proton	B-Application
or	O
group	O
of	O
protons	B-Application
in	O
a	O
voxel	B-Algorithm
resonates	O
depends	O
on	O
the	O
strength	O
of	O
the	O
local	O
magnetic	O
field	O
around	O
the	O
proton	B-Application
or	O
group	O
of	O
protons	B-Application
,	O
a	O
stronger	O
field	O
corresponds	O
to	O
a	O
larger	O
energy	O
difference	O
and	O
higher	O
frequency	O
photons	B-Application
.	O
</s>
<s>
By	O
applying	O
additional	O
magnetic	O
fields	O
(	O
gradients	O
)	O
that	O
vary	O
linearly	O
over	O
space	O
,	O
specific	O
slices	O
to	O
be	O
imaged	O
can	O
be	O
selected	O
,	O
and	O
an	O
image	O
is	O
obtained	O
by	O
taking	O
the	O
2-D	O
Fourier	B-Algorithm
transform	I-Algorithm
of	O
the	O
spatial	O
frequencies	O
of	O
the	O
signal	O
(	O
k-space	B-Algorithm
)	O
.	O
</s>
<s>
Certain	O
nuclei	O
such	O
as	O
1H	O
(	O
protons	B-Application
)	O
,	O
2H	O
,	O
3He	O
,	O
23Na	O
or	O
31P	O
,	O
have	O
a	O
non	O
–	O
zero	O
spin	O
and	O
therefore	O
a	O
magnetic	O
moment	O
.	O
</s>
<s>
Nuclei	O
such	O
as	O
12C	O
have	O
no	O
unpaired	O
neutrons	O
or	O
protons	B-Application
,	O
and	O
no	O
net	O
spin	O
;	O
however	O
,	O
the	O
isotope	O
13C	O
does	O
.	O
</s>
<s>
Protons	B-Application
align	O
in	O
two	O
energy	O
eigenstates	O
(	O
the	O
Zeeman	O
effect	O
)	O
:	O
one	O
low-energy	O
and	O
one	O
high-energy	O
,	O
which	O
are	O
separated	O
by	O
a	O
very	O
small	O
splitting	O
energy	O
.	O
</s>
<s>
Quantum	O
mechanics	O
is	O
required	O
to	O
accurately	O
model	O
the	O
behaviour	O
of	O
a	O
single	O
proton	B-Application
.	O
</s>
<s>
However	O
,	O
classical	O
mechanics	O
can	O
be	O
used	O
to	O
describe	O
the	O
behaviour	O
of	O
an	O
ensemble	O
of	O
protons	B-Application
adequately	O
.	O
</s>
<s>
As	O
with	O
other	O
spin	O
particles	O
,	O
whenever	O
the	O
spin	O
of	O
a	O
single	O
proton	B-Application
is	O
measured	O
it	O
can	O
only	O
have	O
one	O
of	O
two	O
results	O
commonly	O
called	O
parallel	O
and	O
anti-parallel	O
.	O
</s>
<s>
When	O
we	O
discuss	O
the	O
state	O
of	O
a	O
proton	B-Application
or	O
protons	B-Application
we	O
are	O
referring	O
to	O
the	O
wave	O
function	O
of	O
that	O
proton	B-Application
which	O
is	O
a	O
linear	O
combination	O
of	O
the	O
parallel	O
and	O
anti-parallel	O
states	O
.	O
</s>
<s>
In	O
the	O
presence	O
of	O
the	O
magnetic	O
field	O
,	O
B0	O
,	O
the	O
protons	B-Application
will	O
appear	O
to	O
precess	O
at	O
the	O
Larmor	O
frequency	O
determined	O
by	O
the	O
particle	O
's	O
gyro-magnetic	O
ratio	O
and	O
the	O
strength	O
of	O
the	O
field	O
.	O
</s>
<s>
The	O
static	O
fields	O
used	O
most	O
commonly	O
in	O
MRI	O
cause	O
precession	O
which	O
corresponds	O
to	O
a	O
radiofrequency	O
(	O
RF	O
)	O
photon	B-Application
.	O
</s>
<s>
The	O
net	O
longitudinal	O
magnetization	O
in	O
thermodynamic	O
equilibrium	O
is	O
due	O
to	O
a	O
tiny	O
excess	O
of	O
protons	B-Application
in	O
the	O
lower	O
energy	O
state	O
.	O
</s>
<s>
The	O
protons	B-Application
will	O
come	O
into	O
phase	O
with	O
the	O
RF	O
pulse	O
and	O
therefore	O
each	O
other	O
.	O
</s>
<s>
The	O
recovery	O
of	O
longitudinal	O
magnetization	O
is	O
called	O
longitudinal	O
or	O
T1	B-Algorithm
relaxation	I-Algorithm
and	O
occurs	O
exponentially	O
with	O
a	O
time	O
constant	O
T1	B-Algorithm
.	O
</s>
<s>
The	O
loss	O
of	O
phase	O
coherence	O
in	O
the	O
transverse	O
plane	O
is	O
called	O
transverse	O
or	O
T2	O
relaxation	B-Algorithm
.	O
</s>
<s>
T1	B-Algorithm
is	O
thus	O
associated	O
with	O
the	O
enthalpy	O
of	O
the	O
spin	O
system	O
,	O
or	O
the	O
number	O
of	O
nuclei	O
with	O
parallel	O
versus	O
anti-parallel	O
spin	O
.	O
</s>
<s>
T2	O
on	O
the	O
other	O
hand	O
is	O
associated	O
with	O
the	O
entropy	B-Algorithm
of	O
the	O
system	O
,	O
or	O
the	O
number	O
of	O
nuclei	O
in	O
phase	O
.	O
</s>
<s>
The	O
time	O
constant	O
for	O
the	O
observed	O
decay	O
of	O
the	O
FID	O
is	O
called	O
the	O
T	O
relaxation	B-Algorithm
time	O
,	O
and	O
is	O
always	O
shorter	O
than	O
T2	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
the	O
longitudinal	O
magnetization	O
starts	O
to	O
recover	O
exponentially	O
with	O
a	O
time	O
constant	O
T1	B-Algorithm
which	O
is	O
much	O
larger	O
than	O
T2	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
Usually	O
,	O
these	O
field	O
gradients	O
are	O
modulated	O
to	O
sweep	O
across	O
the	O
region	O
to	O
be	O
scanned	O
,	O
and	O
it	O
is	O
the	O
almost	O
infinite	B-Application
variety	O
of	O
RF	O
and	O
gradient	O
pulse	O
sequences	O
that	O
gives	O
MRI	O
its	O
versatility	O
.	O
</s>
<s>
The	O
whole	O
process	O
can	O
be	O
repeated	O
when	O
some	O
T1-relaxation	O
has	O
occurred	O
and	O
the	O
thermal	O
equilibrium	O
of	O
the	O
spins	O
has	O
been	O
more	O
or	O
less	O
restored	O
.	O
</s>
<s>
Typically	O
,	O
in	O
soft	O
tissues	O
T1	B-Algorithm
is	O
around	O
one	O
second	O
while	O
T2	O
and	O
T	O
are	O
a	O
few	O
tens	O
of	O
milliseconds	O
.	O
</s>
<s>
MRI	B-Algorithm
contrast	I-Algorithm
agents	I-Algorithm
,	O
such	O
as	O
those	O
containing	O
Gadolinium(III )	O
work	O
by	O
altering	O
(	O
shortening	O
)	O
the	O
relaxation	B-Algorithm
parameters	O
,	O
especially	O
T1	B-Algorithm
.	O
</s>
<s>
Although	O
each	O
of	O
these	O
schemes	O
is	O
occasionally	O
used	O
in	O
specialist	O
applications	O
,	O
the	O
majority	O
of	O
MR	O
Images	O
today	O
are	O
created	O
either	O
by	O
the	O
two-dimensional	O
Fourier	B-Algorithm
transform	I-Algorithm
(	O
2DFT	O
)	O
technique	O
with	O
slice	O
selection	O
,	O
or	O
by	O
the	O
three-dimensional	O
Fourier	B-Algorithm
transform	I-Algorithm
(	O
3DFT	O
)	O
technique	O
.	O
</s>
<s>
Multiplexed-EPI	O
is	O
even	O
faster	O
,	O
e.g.	O
,	O
for	O
whole	O
brain	O
functional	B-Algorithm
MRI	I-Algorithm
(	O
fMRI	B-Algorithm
)	O
or	O
diffusion	B-Algorithm
MRI	I-Algorithm
.	O
</s>
<s>
This	O
depends	O
upon	O
the	O
relative	O
density	O
of	O
excited	O
nuclei	O
(	O
usually	O
water	O
protons	B-Application
)	O
,	O
on	O
differences	O
in	O
relaxation	B-Algorithm
times	O
(	O
T1	B-Algorithm
,	O
T2	O
,	O
and	O
T	O
)	O
of	O
those	O
nuclei	O
after	O
the	O
pulse	O
sequence	O
,	O
and	O
often	O
on	O
other	O
parameters	O
discussed	O
under	O
specialized	O
MR	O
scans	O
.	O
</s>
<s>
In	O
the	O
brain	O
,	O
T1-weighting	O
causes	O
the	O
nerve	O
connections	O
of	O
white	O
matter	O
to	O
appear	O
white	O
,	O
and	O
the	O
congregations	O
of	O
neurons	O
of	O
gray	B-Device
matter	I-Device
to	O
appear	O
gray	O
,	O
while	O
cerebrospinal	O
fluid	O
(	O
CSF	O
)	O
appears	O
dark	O
.	O
</s>
<s>
The	O
contrast	O
of	O
white	O
matter	O
,	O
gray	B-Device
matter	I-Device
and	O
cerebrospinal	O
fluid	O
is	O
reversed	O
using	O
T2	O
or	O
T	O
imaging	O
,	O
whereas	O
proton-density-weighted	O
imaging	O
provides	O
little	O
contrast	O
in	O
healthy	O
subjects	O
.	O
</s>
<s>
Additionally	O
,	O
functional	O
parameters	O
such	O
as	O
cerebral	O
blood	O
flow	O
(	O
CBF	O
)	O
,	O
cerebral	O
blood	O
volume	O
(	O
CBV	O
)	O
or	O
blood	O
oxygenation	O
can	O
affect	O
T1	B-Algorithm
,	O
T2	O
,	O
and	O
T	O
and	O
so	O
can	O
be	O
encoded	O
with	O
suitable	O
pulse	O
sequences	O
.	O
</s>
<s>
In	O
some	O
situations	O
it	O
is	O
not	O
possible	O
to	O
generate	O
enough	O
image	O
contrast	O
to	O
adequately	O
show	O
the	O
anatomy	O
or	O
pathology	O
of	O
interest	O
by	O
adjusting	O
the	O
imaging	O
parameters	O
alone	O
,	O
in	O
which	O
case	O
a	O
contrast	B-Algorithm
agent	I-Algorithm
may	O
be	O
administered	O
.	O
</s>
<s>
However	O
,	O
most	O
contrast	B-Algorithm
agents	I-Algorithm
used	I-Algorithm
in	I-Algorithm
MRI	I-Algorithm
are	O
selected	O
for	O
their	O
specific	O
magnetic	O
properties	O
.	O
</s>
<s>
Most	O
commonly	O
,	O
a	O
paramagnetic	O
contrast	B-Algorithm
agent	I-Algorithm
(	O
usually	O
a	O
gadolinium	O
compound	O
)	O
is	O
given	O
.	O
</s>
<s>
Gadolinium-enhanced	O
tissues	O
and	O
fluids	O
appear	O
extremely	O
bright	O
on	O
T1-weighted	O
images	O
.	O
</s>
<s>
There	O
have	O
been	O
concerns	O
raised	O
recently	O
regarding	O
the	O
toxicity	O
of	O
gadolinium-based	O
contrast	B-Algorithm
agents	I-Algorithm
and	O
their	O
impact	O
on	O
persons	O
with	O
impaired	O
kidney	O
function	O
.	O
</s>
<s>
More	O
recently	O
,	O
superparamagnetic	O
contrast	B-Algorithm
agents	I-Algorithm
,	O
e.g.	O
,	O
iron	O
oxide	O
nanoparticles	O
,	O
have	O
become	O
available	O
.	O
</s>
<s>
In	O
1983	O
,	O
Ljunggren	O
and	O
Twieg	O
independently	O
introduced	O
the	O
k-space	B-Algorithm
formalism	O
,	O
a	O
technique	O
that	O
proved	O
invaluable	O
in	O
unifying	O
different	O
MR	O
imaging	O
techniques	O
.	O
</s>
<s>
They	O
showed	O
that	O
the	O
demodulated	O
MR	O
signal	O
S(t )	O
generated	O
by	O
the	O
interaction	O
between	O
an	O
ensemble	O
of	O
freely	O
precessing	O
nuclear	O
spins	O
in	O
the	O
presence	O
of	O
a	O
linear	O
magnetic	O
field	O
gradient	O
G	O
and	O
a	O
receiver-coil	O
equals	O
the	O
Fourier	B-Algorithm
transform	I-Algorithm
of	O
the	O
effective	O
spin	O
density	O
,	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
as	O
time	O
progresses	O
the	O
signal	O
traces	O
out	O
a	O
trajectory	O
in	O
k-space	B-Algorithm
with	O
the	O
velocity	O
vector	O
of	O
the	O
trajectory	O
proportional	O
to	O
the	O
vector	O
of	O
the	O
applied	O
magnetic	O
field	O
gradient	O
.	O
</s>
<s>
By	O
the	O
term	O
effective	O
spin	O
density	O
we	O
mean	O
the	O
true	O
spin	O
density	O
corrected	O
for	O
the	O
effects	O
of	O
T1	B-Algorithm
preparation	O
,	O
T2	O
decay	O
,	O
dephasing	O
due	O
to	O
field	O
inhomogeneity	O
,	O
flow	O
,	O
diffusion	O
,	O
etc	O
.	O
</s>
<s>
From	O
the	O
basic	O
k-space	B-Algorithm
formula	O
,	O
it	O
follows	O
immediately	O
that	O
we	O
reconstruct	O
an	O
image	O
by	O
taking	O
the	O
inverse	O
Fourier	B-Algorithm
transform	I-Algorithm
of	O
the	O
sampled	O
data	O
,	O
viz	O
.	O
</s>
<s>
Using	O
the	O
k-space	B-Algorithm
formalism	O
,	O
a	O
number	O
of	O
seemingly	O
complex	O
ideas	O
became	O
simple	O
.	O
</s>
<s>
In	O
a	O
standard	O
spin	O
echo	O
or	O
gradient	O
echo	O
scan	O
,	O
where	O
the	O
readout	O
(	O
or	O
view	O
)	O
gradient	O
is	O
constant	O
(	O
e.g.	O
,	O
G	O
)	O
,	O
a	O
single	O
line	O
of	O
k-space	B-Algorithm
is	O
scanned	O
per	O
RF	O
excitation	O
.	O
</s>
<s>
When	O
a	O
non-zero	O
phase-encoding	O
pulse	O
is	O
added	O
in	O
between	O
the	O
RF	O
excitation	O
and	O
the	O
commencement	O
of	O
the	O
readout	O
gradient	O
,	O
this	O
line	O
moves	O
up	O
or	O
down	O
in	O
k-space	B-Algorithm
,	O
i.e.	O
,	O
we	O
scan	O
the	O
line	O
ky	O
=	O
constant	O
.	O
</s>
<s>
The	O
k-space	B-Algorithm
formalism	O
also	O
makes	O
it	O
very	O
easy	O
to	O
compare	O
different	O
scanning	O
techniques	O
.	O
</s>
<s>
In	O
single-shot	O
EPI	O
,	O
all	O
of	O
k-space	B-Algorithm
is	O
scanned	O
in	O
a	O
single	O
shot	O
,	O
following	O
either	O
a	O
sinusoidal	O
or	O
zig-zag	O
trajectory	O
.	O
</s>
<s>
Since	O
alternating	O
lines	O
of	O
k-space	B-Algorithm
are	O
scanned	O
in	O
opposite	O
directions	O
,	O
this	O
must	O
be	O
taken	O
into	O
account	O
in	O
the	O
reconstruction	O
.	O
</s>
<s>
Multi-shot	O
EPI	O
and	O
fast	O
spin	O
echo	O
techniques	O
acquire	O
only	O
part	O
of	O
k-space	B-Algorithm
per	O
excitation	O
.	O
</s>
<s>
In	O
each	O
shot	O
,	O
a	O
different	O
interleaved	O
segment	O
is	O
acquired	O
,	O
and	O
the	O
shots	O
are	O
repeated	O
until	O
k-space	B-Algorithm
is	O
sufficiently	O
well-covered	O
.	O
</s>
<s>
Since	O
the	O
data	O
at	O
the	O
center	O
of	O
k-space	B-Algorithm
represent	O
lower	O
spatial	O
frequencies	O
than	O
the	O
data	O
at	O
the	O
edges	O
of	O
k-space	B-Algorithm
,	O
the	O
TE	O
value	O
for	O
the	O
center	O
of	O
k-space	B-Algorithm
determines	O
the	O
image	O
's	O
T2	O
contrast	O
.	O
</s>
<s>
The	O
importance	O
of	O
the	O
center	O
of	O
k-space	B-Algorithm
in	O
determining	O
image	O
contrast	O
can	O
be	O
exploited	O
in	O
more	O
advanced	O
imaging	O
techniques	O
.	O
</s>
<s>
One	O
such	O
technique	O
is	O
spiral	O
acquisition	O
—	O
a	O
rotating	O
magnetic	O
field	O
gradient	O
is	O
applied	O
,	O
causing	O
the	O
trajectory	O
in	O
k-space	B-Algorithm
to	O
spiral	O
out	O
from	O
the	O
center	O
to	O
the	O
edge	O
.	O
</s>
<s>
contrast	B-Algorithm
to	I-Algorithm
noise	I-Algorithm
ratio	I-Algorithm
(	O
CNR	B-Algorithm
)	O
when	O
compared	O
to	O
conventional	O
zig-zag	O
acquisitions	O
,	O
especially	O
in	O
the	O
presence	O
of	O
rapid	O
movement	O
.	O
</s>
<s>
Since	O
and	O
are	O
conjugate	O
variables	O
(	O
with	O
respect	O
to	O
the	O
Fourier	B-Algorithm
transform	I-Algorithm
)	O
we	O
can	O
use	O
the	O
Nyquist	O
theorem	O
to	O
show	O
that	O
a	O
step	O
in	O
k-space	B-Algorithm
determines	O
the	O
field	O
of	O
view	O
of	O
the	O
image	O
(	O
maximum	O
frequency	O
that	O
is	O
correctly	O
sampled	O
)	O
and	O
the	O
maximum	O
value	O
of	O
k	O
sampled	O
determines	O
the	O
resolution	O
;	O
i.e.	O
,	O
</s>
<s>
In	O
the	O
timing	B-Application
diagram	I-Application
,	O
the	O
horizontal	O
axis	O
represents	O
time	O
.	O
</s>
<s>
A	O
shaped	O
pulse	O
(	O
shown	O
here	O
with	O
a	O
sinc	B-Algorithm
modulation	O
)	O
causes	O
a	O
90°	O
nutation	O
of	O
longitudinal	O
nuclear	O
magnetization	O
within	O
a	O
slab	O
,	O
or	O
slice	O
,	O
creating	O
transverse	O
magnetization	O
.	O
</s>
<s>
After	O
the	O
two-dimensional	O
matrix	O
(	O
typical	O
dimension	O
between	O
128×128	O
and	O
512×512	O
)	O
has	O
been	O
acquired	O
,	O
producing	O
the	O
so-called	O
k-space	B-Algorithm
data	O
,	O
a	O
two-dimensional	O
inverse	O
Fourier	B-Algorithm
transform	I-Algorithm
is	O
performed	O
to	O
provide	O
the	O
familiar	O
MR	O
image	O
.	O
</s>
<s>
Either	O
the	O
magnitude	O
or	O
phase	O
of	O
the	O
Fourier	B-Algorithm
transform	I-Algorithm
can	O
be	O
taken	O
,	O
the	O
former	O
being	O
far	O
more	O
common	O
.	O
</s>
<s>
Resistive	O
electromagnet	O
:	O
A	O
solenoid	B-Algorithm
wound	O
from	O
copper	O
wire	O
is	O
an	O
alternative	O
to	O
a	O
permanent	O
magnet	O
.	O
</s>
<s>
Gradient	O
coils	O
are	O
used	O
to	O
spatially	O
encode	O
the	O
positions	O
of	O
protons	B-Application
by	O
varying	O
the	O
magnetic	O
field	O
linearly	O
across	O
the	O
imaging	O
volume	O
.	O
</s>
<s>
The	O
radio	O
frequency	O
(	O
RF	O
)	O
transmission	O
system	O
consists	O
of	O
an	O
RF	O
synthesizer	O
,	O
power	O
amplifier	O
and	O
transmitting	B-Algorithm
coil	I-Algorithm
.	O
</s>
<s>
The	O
RF	O
frequency	O
electromagnetic	O
field	O
produced	O
in	O
the	O
"	O
transmitting	B-Algorithm
coil	I-Algorithm
"	O
is	O
a	O
magnetic	O
near-field	B-Algorithm
with	O
very	O
little	O
associated	O
changing	O
electric	O
field	O
component	O
(	O
such	O
as	O
all	O
conventional	O
radio	O
wave	O
transmissions	O
have	O
)	O
.	O
</s>
<s>
Thus	O
,	O
the	O
transmitting	B-Algorithm
coil	I-Algorithm
is	O
a	O
good	O
EM	O
field	O
transmitter	O
at	O
radio	O
frequency	O
,	O
but	O
a	O
poor	O
EM	O
radiation	O
transmitter	O
at	O
radio	O
frequency	O
.	O
</s>
<s>
The	O
RF	O
electromagnetic	O
radiation	O
produced	O
by	O
nuclear	O
relaxation	B-Algorithm
inside	O
the	O
subject	O
is	O
true	O
EM	O
radiation	O
(	O
radio	O
waves	O
)	O
,	O
and	O
these	O
leave	O
the	O
subject	O
as	O
RF	O
radiation	O
,	O
but	O
they	O
are	O
of	O
such	O
low	O
power	O
as	O
to	O
also	O
not	O
cause	O
appreciable	O
RF	O
interference	O
that	O
can	O
be	O
picked	O
up	O
by	O
nearby	O
radio	O
tuners	O
(	O
in	O
addition	O
,	O
MRI	O
scanners	O
are	O
generally	O
situated	O
in	O
metal	O
mesh	O
lined	O
rooms	O
which	O
act	O
as	O
Faraday	O
cages	O
.	O
)	O
</s>
