Biomedical Engineering Reference
In-Depth Information
parameters of interest. It has been demonstrated that myocardial blood flow
can be studied by dynamic SPECT imaging of
99
m
Tc-teboroxime [109, 110]. An-
other interesting application of dynamic cardiac SPECT is the measurement
of perfusion and the distribution volume in the heart using
201
Tl [111, 112].
Other applications include brain imaging with
99m
Tc and
123
I flow agents
and some neuroreceptor studies, such as [
123
I]iomazenil for benzodiazepine
receptors [115], [
123
I]iododexetimide for cholinergic muscarinic neurorecep-
tors [114], and [
123
I]iodo-A-85380 for central neuronal nicotinic acetylcholine
receptors [115, 116], etc., both in human and nonhuman primates.
While PET and SPECT allow absolute measurements of radioactivity con-
centration in tissue and have the capability to relate the
in vivo
measurements
to physiological functions, there are a number of limiting factors which can im-
pact on their ability to produce reliable physiologic parameters. In particular, the
small tissue volume, limitation on the total amount of activity that can be admin-
istered, physical artifacts, and low sensitivity (particularly for SPECT) result in
measurements with high noise levels. The measurement noise (and mechanical
rotation of the detectors in the case of SPECT) has imposed an upper limit on
the fastest sampling rate which is
>
1 sec/sample for PET and
>
5 sec/sample
for SPECT. Furthermore, the short half-life of the radiopharmaceuticals used
for PET and SPECT and the need for the patient to remain still on the scan-
ner bed usually limit duration of the PET and SPECT experiments to relatively
short periods of time, which vary from minutes to hours rather than days.
Slow physiologic processes are therefore difficult to be reliably estimated with
both PET and SPECT. All these factors limit the number of parameters which
can be reliably estimated from PET or SPECT measurement and complicated
compartmental models used for analysis must be simplified. Huang
et al.
pro-
vide detailed guidelines on kinetic model development for PET applications and
these principles are equally applicable to SPECT tracer studies.
One intrinsic limitation of PET imaging is the presence of physiological sites
of tracer accumulation (secretion or excretion). It is sometimes difficult to differ-
entiate real pathology from an unusual pattern of physiologic accumulation. In
contrast, some organs of the body, for instance, the pancreas, uterus, and ovary,
do not have significant physiological uptake. Spatially differentiation among
these organs is sometimes difficult, particularly when they are closely related
to one another. In this respect, structural imaging modalities such as X-ray CT
can serve as an excellent anatomical roadmap for the functional PET images.
