Basic principles and technical aspects

Basic principles and technical aspects
positron emission tomography ( pet ) is that the most selective
and sensitive ( picomolar to nanomolar vary ) imaging technique
on behalf of measuring molecular pathways and that interactions in
vivo. positron-emitting isotopes are radioactive variants of
elements naturally occurring in organic molecules and is
incorporated while not changing the chemical and biological
characteristics of the labeled molecule. they decay by emission
of a positron, that is that the subatomic, positively charged, antiparticle
of the negatively charged electron. the positron will
annihilate with an electron and produce 2 511-kev photons,
emitted in opposite directions. the detection of varied of
these annihilations by the detector rings of the pet camera
generates high-resolution pictures ( five to ten mm ) indicating the
sites of tracer accumulation within the body.
the foremost frequently used tracer in pet oncology is that the
glucose analogue eighteen f-fluorodeoxyglucose ( fdg ). its use is
based mostly on the increased glycolysis of cancer cells compared
with normal tissues. this increased glycolysis is linked to
a rise in glucose membrane transporters and upregulation
of the principal enzymes that management the glycolytic
pathway. one fdg uptake is, though, not specific for cancer
cells, and that increased fdg uptake is additionally seen in some
i nflammatory conditions, the foremost common cause of falsepositive
fdg–pet findings. two the ability to perform wholebody
imaging among one examination makes pet an ideal
technique for cancer staging. in clinical oncology, the fdg
uptake is typically quantified because the standardized uptake value
( suv ; i. e., the ratio of the activity in tissue per unit volume
to the injected dose per patient body weight ). in vitro studies
demonstrated that the quantity of fdg uptake in tumor
tissue is mainly connected to the quantity of viable cancer cells 3
and also their proliferation capability. four thus, suv changes
on fdg-pet is used to evaluate treatment efficacy,
as a result of tumor cell kill results in a very proportional reduction
of the fdg s ignal. five furthermore, the correlation between
fdg uptake and proliferation capability allows in vivo evaluation
of tumor aggressiveness.
interpretation of pet scans is hampered by the shortage
of anatomical detail, that makes it typically difficult
to correctly localize hot spots or differentiate tumor tissue
from benign structures with physiologically high fdg uptake
( e. g., muscle, brown fat, gut ). thus, pet always
has to be interpreted in conjunction with anatomical images
like computed tomography ( ct ). tries to align
or coregister ct and pet information sets acquired on separate
machines with fusion software are usually merely successful
within the brain, whereas within the remainder of the body differences
in patient setup present a challenge to the software
approaches. recently, integrated pet/ct systems were introduced,
that enable acquisition of pet and ct information in
an equivalent session while not changing the patients’ position.
since the installation of the primary clinical pet/ct in 2001,
the technology has gained widespread use and all new pet
scanners installed nowadays are integrated pet/ct machines.
another advantage of pet/ct is that the possibility to use the
ct part for attenuation correction of the pet images
and cut
back the scan time substantially ( − ;50% ), but
specific artifacts will typically be a problem. six an example
are the errors in localization of lesions caused by breathing
and also the distinction in scan time to acquire a pet ( minutes )
and ct image ( seconds ). this could result in incorrect anatomical
localization of lesions close to the diaphragm on the
attenuation-corrected images ( fig. twenty seven. one ). high- density objects
( dental fillings, chemotherapy ports, barium contrast )
will result in an overestimation of tracer uptake, thereby
manufacturing false-positive pet findings. thus, non–
attenuation-corrected pet images, that don't manifest
these errors, ought to invariably be reviewed in parallel to recognize
these artifacts.
though fdg has created the manner for pet in clinical oncology,
many alternative radiopharmaceuticals is used to study
processes like blood flow ( h two fifteen o ), hypoxia ( eighteen f-miso ),
dna synthesis ( eighteen f-fluorothymidine flt ), and somatostatin
receptor expression ( ga-68 dotatoc ). certainly, with the
rapid development of molecular-targeted treatments, noninvasive
assessment of metabolic processes are going to become increasingly
vital to assess efficacy of those medication. though, in
the succeeding text, pet invariably refers to fdg-pet, unless
otherwise specified. it summarizes the commonplace indications,
alternative applications, and that innovative use of this imaging technique