<s>
Positron	B-Algorithm
emission	I-Algorithm
tomography	I-Algorithm
–	I-Algorithm
magnetic	I-Algorithm
resonance	I-Algorithm
imaging	I-Algorithm
(	O
PET	O
–	O
MRI	O
)	O
is	O
a	O
hybrid	O
imaging	O
technology	O
that	O
incorporates	O
magnetic	O
resonance	O
imaging	O
(	O
MRI	O
)	O
soft	O
tissue	O
morphological	O
imaging	O
and	O
positron	B-Application
emission	I-Application
tomography	I-Application
(	O
PET	O
)	O
functional	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Simultaneous	O
PET/MR	B-Algorithm
detection	O
was	O
first	O
demonstrated	O
in	O
1997	O
,	O
however	O
it	O
took	O
another	O
13	O
years	O
,	O
and	O
new	O
detector	O
technologies	O
,	O
for	O
clinical	O
systems	O
to	O
become	O
commercially	O
available	O
.	O
</s>
<s>
Presently	O
,	O
the	O
main	O
clinical	O
fields	O
of	O
PET-MRI	B-Algorithm
are	O
oncology	O
,	O
cardiology	O
,	O
neurology	O
,	O
and	O
neuroscience	O
.	O
</s>
<s>
Research	O
studies	O
are	O
actively	O
conducted	O
at	O
the	O
moment	O
to	O
understand	O
benefits	O
of	O
the	O
new	O
PET-MRI	B-Algorithm
diagnostic	O
method	O
.	O
</s>
<s>
The	O
technology	O
combines	O
the	O
exquisite	O
structural	O
and	O
functional	O
characterization	O
of	O
tissue	O
provided	O
by	O
MRI	O
with	O
the	O
extreme	O
sensitivity	O
of	O
PET	B-Application
imaging	I-Application
of	O
metabolism	O
and	O
tracking	O
of	O
uniquely	O
labeled	O
cell	O
types	O
or	O
cell	O
receptors	O
.	O
</s>
<s>
Several	O
companies	O
offer	O
clinical	O
and	O
pre-clinical	O
combined	O
PET-MR	B-Algorithm
system	O
;	O
clinical	O
systems	O
are	O
available	O
from	O
United	O
Imaging	O
,	O
Philips	O
,	O
Siemens	O
,	O
and	O
GE	O
.	O
</s>
<s>
The	O
first	O
two	O
clinical	O
whole	O
body	O
PET-MRI	B-Algorithm
systems	O
were	O
installed	O
by	O
Philips	O
at	O
Mount	O
Sinai	O
Medical	O
Centre	O
in	O
the	O
United	O
States	O
and	O
at	O
Geneva	O
University	O
Hospital	O
in	O
Switzerland	B-Protocol
,	O
in	O
2010	O
.	O
</s>
<s>
Siemens	O
was	O
the	O
first	O
company	O
to	O
offer	O
simultaneous	O
PET/MR	B-Algorithm
acquisitions	O
,	O
with	O
the	O
first	O
systems	O
installed	O
in	O
2010	O
based	O
on	O
avalanche	O
photodiode	O
detectors	O
.	O
</s>
<s>
Currently	O
Siemens	O
and	O
GE	O
are	O
the	O
only	O
companies	O
to	O
offer	O
a	O
fully	O
integrated	O
whole	O
body	O
and	O
simultaneous	O
acquisition	O
PET-MRI	B-Algorithm
system	O
.	O
</s>
<s>
The	O
GE	O
system	O
(	O
SIGNA	O
PET/MR	B-Algorithm
)	O
received	O
its	O
510K	O
&	O
CE	O
mark	O
in	O
2014	O
.	O
</s>
<s>
Currently	O
,	O
the	O
combination	O
of	O
positron	B-Application
emission	I-Application
tomography	I-Application
(	O
PET	O
)	O
and	O
magnetic	O
resonance	O
imaging	O
(	O
MRI	O
)	O
as	O
a	O
hybrid	O
imaging	O
modality	O
is	O
receiving	O
great	O
attention	O
not	O
only	O
in	O
its	O
emerging	O
clinical	O
applications	O
but	O
also	O
in	O
the	O
preclinical	O
field	O
.	O
</s>
<s>
Several	O
companies	O
offer	O
MR-compatible	O
preclinical	O
PET	B-Application
scanner	I-Application
inserts	O
for	O
use	O
in	O
the	O
bore	O
of	O
an	O
existing	O
MRI	O
,	O
enabling	O
simultaneous	O
PET/MR	B-Algorithm
image	O
acquisition	O
.	O
</s>
<s>
The	O
combination	O
of	O
PET	O
with	O
X-ray	O
computed	O
tomography	O
(	O
CT	O
)	O
is	O
the	O
more	O
established	O
PET	B-Application
imaging	I-Application
technology	O
.	O
</s>
<s>
With	O
both	O
PET-CT	O
and	O
PET-MR	B-Algorithm
the	O
intended	O
advantage	O
is	O
to	O
combine	O
functional	B-Algorithm
imaging	I-Algorithm
provided	O
by	O
PET	O
,	O
with	O
structural	O
(	O
anatomical	O
)	O
information	O
from	O
CT	O
or	O
MRI	O
.	O
</s>
<s>
Although	O
images	O
from	O
different	O
modalities	O
collected	O
at	O
different	O
scanning	O
sessions	O
can	O
be	O
overlaid	O
by	O
image	B-Algorithm
registration	I-Algorithm
,	O
a	O
simultaneous	O
acquisition	O
offers	O
better	O
alignment	O
of	O
images	O
and	O
direct	O
correlation	O
.	O
</s>
<s>
The	O
same	O
clinical	O
decisions	O
that	O
would	O
influence	O
the	O
choice	O
between	O
stand-alone	O
CT	O
or	O
MR	O
imaging	O
would	O
also	O
determine	O
areas	O
where	O
PET-CT	O
or	O
PET-MR	B-Algorithm
would	O
be	O
preferred	O
.	O
</s>
<s>
One	O
clear	O
advantage	O
of	O
PET-MR	B-Algorithm
compared	O
to	O
PET-CT	O
is	O
the	O
lower	O
total	O
ionising	O
radiation	O
dose	O
obtained	O
.	O
</s>
<s>
For	O
body	O
PET-CT	O
applications	O
,	O
the	O
CT	O
part	O
of	O
the	O
examination	O
constitutes	O
approximately	O
60-80	O
%	O
of	O
the	O
radiation	O
dose	O
,	O
with	O
the	O
remaining	O
radiation	O
dose	O
originating	O
from	O
the	O
PET	O
radiopharmaceutical	B-Algorithm
.	O
</s>
<s>
PET-MR	B-Algorithm
is	O
therefore	O
appealing	O
in	O
children	O
,	O
in	O
particularly	O
for	O
serial	O
follow-up	O
examinations	O
as	O
used	O
in	O
oncology	O
or	O
chronic	O
inflammatory	O
conditions	O
.	O
</s>
<s>
PET-MRI	B-Algorithm
systems	O
do	O
n't	O
offer	O
a	O
direct	O
way	O
to	O
obtain	O
attenuation	O
maps	O
,	O
unlike	O
stand-alone	O
PET	O
or	O
PET-CT	O
systems	O
.	O
</s>
<s>
One	O
method	O
uses	O
a	O
Dixon	O
MRI	B-Algorithm
sequence	I-Algorithm
,	O
and	O
segments	B-Algorithm
the	O
resultant	O
image	O
into	O
fat	O
and	O
water	O
,	O
with	O
pre-set	O
attenuation	O
factors	O
.	O
</s>
<s>
The	O
Dixon	O
method	O
can	O
be	O
combined	O
with	O
ultrashort	O
echo	O
time	O
(	O
UTE	O
)	O
sequences	O
to	O
better	O
identify	O
bone	O
and	O
increase	O
the	O
possible	O
classes	O
of	O
tissue	O
for	O
segmentation	B-Algorithm
.	O
</s>
<s>
the	O
head	O
)	O
,	O
segmentation	B-Algorithm
(	O
where	O
tissue	O
is	O
categorised	O
using	O
the	O
MRI	O
image	O
data	O
)	O
,	O
or	O
"	O
atlas	O
"	O
methods	O
can	O
be	O
used	O
.	O
</s>
<s>
While	O
some	O
of	O
these	O
use	O
an	O
atlas	O
technique	O
,	O
many	O
take	O
a	O
voxel	B-Algorithm
approach	O
where	O
actual	O
voxel	B-Algorithm
intensities	O
(	O
contrast	O
data	O
)	O
are	O
used	O
in	O
combination	O
with	O
machine	O
learning	O
(	O
trained	O
on	O
MR/CT	O
data	O
)	O
to	O
assign	O
electron	O
density	O
values	O
.	O
</s>
<s>
In	O
many	O
of	O
the	O
above	O
methods	O
,	O
MRI	B-Algorithm
artifacts	I-Algorithm
(	O
e.g.	O
</s>
