Biomedical Engineering Reference
In-Depth Information
(FDG and its phosphorylated form FDG-6-PO
4
). The compartments of a tracer
kinetic model are linked by a set of parameters called
rate constants
,
k
i
, which
represent the rates at which the radiotracer in one compartment is transported
to the connected compartments. More precisely, these rate constant parameters
represent specific physiologic or biochemical processes (e.g. flow or transport
across physical spaces, or rates of transformation from one chemical form to
the other in a chemical entity) within the biologic system. For the FDG model as
shown in Fig. 2.9, the three compartments represent (from left) vascular space
for FDG, tissue space for free FDG, and tissue space for FDG-6-phosphate (FDG-
6-P). The rate constants describe the movement of FDG between compartments:
k
1
and
k
2
for the forward and backward transport of FDG across the blood-brain
barrier,
k
3
for the phosphorylation of FDG to FDG-6-P, and
k
4
for the dephos-
phorylation of FDG-6-P back to FDG.
The aim of modeling is to interpret the fate of the administered radiotracer
quantitatively in terms of the standard parameters in the compartmental model.
In conjunction with knowledge of the transport and metabolism of the radio-
tracer, it is possible to relate the rate constants to physiologic parameters of in-
terest. Figure 2.10 summarizes the key steps in physiologic parameter estimation
Acquired arterial
blood samples
(input function)
Plasma/Blood
TAC
Compartmental
model fitting
Physiological
parameters
Weighted
integration
Injected
or inhaled
tracer
Compartment
model
Graphical
techniques
Tissue TAC
e.g. rCBF,
rCMRGlc,
Vd, BP, etc.
Spectral
analysis
Acquired
PET images
(output function)
Figure 2.10:
(Color slide) Quantitative physiological parameter estimation with
PET includes radiotracer administration, data acquisition with a PET scanner,
measurement of tracer plasma concentration, a suitable mathematical model
and a parameter estimation method to estimate the physiological parameter of
interest.
