Image Processing Reference
In-Depth Information
19.3
PRACTICE
19.3.1
F
DSC-MRI S
T
C
ROM
IGNAL
TO
RACER
ONCENTRATION
In DSC-MRI, the amount of contrast agent present within a voxel locally perturbs
the total magnetic field, thus decreasing relaxation time constants and influencing
the detected T2*-weighted signal, S(t), from the voxel, as follows:
St
()
=
Se
−
∆
RtT
E
*
()
⋅
(19.21)
2
0
where:
•S
=
S(0) is the signal value from water protons at time t
=
0, when no
0
contrast agent is yet present.
•
is the change in transverse relaxation rate, i.e.,
the difference between water proton T2*-relaxation rate R
∆
Rt
()
=
Rt
()
−
R
2
0
( )
*
*
*
2
2
*
(t)
=
1/T2*(t) and its value at t
=
0 R
*
(0).
•T
is the echo time, i.e., a time parameter specific to the particular
gradient-echo sequence adopted.
E
Within frequently used low-dosage ranges of contrast agents at common field
strengths
, a linear relationship between the change in transverse relaxation
rate and tracer concentration C
B
0
(t) within the voxel can be reasonably assumed
VOI
[9-13] to be
Ct
()
=κ
∆
2
Rt
*
()
(19.22)
VOI
VOI
in which
is an unknown proportionality constant depending on the tissue,
the contrast agent, the field strength, and the pulse sequence. From Equation 19.21
and Equation 19.22, one can derive
κ
VOI
κ
St
S
()
VOI
Ct
()
=−
ln
(19.23)
VOI
T
E
0
which is the fundamental equation of DSC-MRI, relating the tracer concentration
profile within a voxel to the measured signal produced by the perturbed water
protons spin-
system. Equation 19.21 is used to convert both arterial as well as
tissue DSC-MRI-measured signals. Because of the complexity of the relaxation
mechanism underlying the DSC-MRI signal generation and the consequent dif-
ficulty in retrieving the correct
∫
κ
value for each voxel, the same proportionality
VOI
constant (
) is usually assumed for both tissue and arterial concentration.
However, this assumption can affect a correct quantification of CBF, CBV, and
MTT [14,15].
κ
=
κ
VOI
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