Image Processing Reference
In-Depth Information
19.1
INTRODUCTION
noninvasive quantitative assessment of cerebral hemodynamics is of
crucial importance for understanding brain functions in both normal and patho-
logical states. Positron emission tomography (PET) offers a powerful tool, e.g.,
one can measure cerebral blood flow (CBF) and cerebral blood volume (CBV)
by interpreting [
In vivo
O tracer activity images with suitable mathematical models
[1,2]. PET methods are the gold standard for CBF and CBV quantification, but
they have their own limitations, e.g., PET facilities are located only in specialized
clinical centers, radioactivity tracers are employed, and arterial sampling is
required. Recently, dynamic susceptibility contrast-enhanced magnetic resonance
imaging (DSC-MRI) has emerged as an alternative and clinically appealing tech-
nique in
15
O]H
2
assessment of cerebral hemodynamics. Briefly, in DSC-MRI, an
intravascularly distributed paramagnetic contrast agent is rapidly injected into a
peripheral vein. Once the bolus of the contrast agent reaches the region of interest,
a short blood relaxation time because of the paramagnetic label leads to a decline
in the MRI signal intensity acquired either by a spin-echo or gradient-echo method.
Despite the inherent complexity of susceptibility contrast mechanisms, a theory to
model the DSC-MRI information and several techniques to implement this theory
correctly have been developed during the last 20 yrs in order to allow quantitative
measures of CBF, CBV, and mean transit time (MTT).
The aim of this chapter is to review the theoretical fundamentals of the
quantification of DSC-MRI signals and to discuss relevant issues in obtaining
reliable estimates of CBF, CBV, and MTT.
in vivo
19.2
THEORY
The model used for quantification of DSC-MRI images is based on the principles of
tracer kinetics for nondiffusible tracers [3-5] and relies on the following assumptions:
1.
The contrast agent is completely nondiffusible.
2.
There is no recirculation of the contrast agent.
3.
The contrast agent is confined to the intravascular space. In other
words, the blood-brain barrier (BBB) is assumed to be intact; other-
wise, tracer leakage can occur.
4.
The system is in steady state during the experiment, i.e., the blood flow
is assumed to be constant. As a consequence, only a stationary flow can
be measured in a single experiment; however, flows that vary slowly
compared with the duration of the experiment are still quantifiable by a
series of consecutive experiments.
5.
The contrast agent dose must not appreciably perturb the system.
19.2.1
T
F
RANSPORT
UNCTION
Consider a bolus of amplitude q
0 in the
feeding vessel to the volume of interest (VOI) of tissue. The amount of
of a nondiffusible tracer at time t
=
0
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