What is PET/CT?
PET/CT Overview
PET/CT is now clinically well-established, with
hundreds of systems installed worldwide. As its name suggests,
PET/CT combines two methods of imaging to create a powerful new
weapon in the fight against disease. Current PET/CT systems consist
of a single, long-bore gantry with the PET and CT systems adjacent
to one another. The CT scanner component of the systems tends to be
existing models that are available as separate systems, with the
same image quality and radiation dose characteristics as standalone
CT units. The patient usually undergoes the CT part of the
examination first, and then the couch is moved further into the
gantry to perform the PET scan
PET is an imaging modality that can provide functional,
metabolic, and chemical information complementing more
conventional, structural imaging studies. PET essentially records
the concentration of positron emitting radioisotopes in a 3-D
volume by obtaining external measurements of the radiation emitted
by these isotopes.
A positron is a positively charged electron that is emitted from
the nucleus of a radionuclide. Once emitted this positron travels
several millimeters (in human tissue in clinical cases) until it
meets a free electron and a so-called mutual annihilation event
takes place leading to the release of two "annihilation" photons
(two gamma rays). These photons are emitted in a 180 degree
opposite direction to one another.
A ring of detectors surrounds the patient and when the two 511 Kev
gamma rays are simultaneously recorded by opposing detectors, an
annihilation event is recorded as having taken place along a line
connecting the two detectors. PET therefore uses the principle of
annihilation coincidence detection. Gamma rays from annihilations
occurring outside the line that links the two opposite detectors
would only interact with one of these two detectors and not satisfy
the coincidence principle. This event then would not be
registered.
Over the past 60 years, many studies have shown that tumors tend
to have an increased rate of glucose utilization with respect to
normal tissue. At Cobalt Health we use the radionuclide FDG or
fluoro-2-deoxyglucose. This is the most commonly utilised
radionuclide in clinical PET within the UK, and is a glucose
analogue that is avidly accrued by tumor cells in the same way as
glucose. By labeling this compound with Fluorine-18 it allows the
recording of glucose accumulation as a function of the
positron emitter (i.e. to measure positron emission as a reflection
of glucose accumulation).
Normally, once glucose enters into a cell (via a transporter
enzyme) it is phosphorylated by an enzyme called hexokinase and
then enters directly into either the glycolytic or glycogenic
pathway. Similarly, FDG once intracellular, undergoes the
phosphorylation step but is subsequently unable to continue into
the usual glucose metabolic pathways and is essentially trapped in
the cell as FDG-phosphate.
PET is both a medical and research tool. It is used heavily in
clinical oncology (medical imaging of cancer and
the search for metastatic disease), and for clinical diagnosis of
certain diffuse brain diseases such as those causing various types
of dementias.
In addition to its diagnostic benefits, PET/CT has the potential
to substantially impact treatment plans for cancer patients:
- Radiation Oncologists can use PET/CT's combined information
about metabolic activity and anatomical location to better tailor
radiation fields-reaching all the cancer cells while avoiding
damage to the surrounding healthy tissue.
- Surgeons can use PET/CT to pinpoint exactly where to perform a
biopsy or surgical excision.
