PET Imaging

Positron Emission Tomography (PET) Imaging is an established diagnostic tool used in Nuclear Medicine to obtain clinical information from the patient.  An extension of this imaging modality is the PET-CT hybrid. This system consists of two scanning machines, namely the PET scanner and an X-ray computed tomography (CT) scanner.  These modern medical modalities occupy a central position in the technological apex of Nuclear Medicine and occupy a prominent role in theranostics.

PET-FECH-scanPET-FECH scan indicating prostate cancer metastases

The second hybrid is PET combined with magnetic resonance imaging (MRI). This powerful imaging tool can obtain clinical images and provide corresponding information about the diagnosis of many disease states.  These hybrid scanning machines can be used to show the effect of current and new therapies.  PET scanning can detect the processes by which certain cancers utilise sugar molecules; while MRI provides structural details of tissues within a tumour or in brain disease. The advantage of using PET-MRI over PET-CT is the two-fold reduction in radiation dose given to the patient.  PET-MRI is a valuable imaging tool, especially in the areas of oncology, neuropsychiatric and heart diseases.

Hybrid-scans-of-a-brain-tumourHybrid scans of a brain tumour

Consequently, PET-CT has the capability to evaluate diseases through a simultaneous functional approach, including morphostructural analysis. The benefits of PET scanning is the potential for early diagnosis of the disease state, leading to a favourable prognosis and appropriate therapy towards the patient. Today, the most used PET radiotracer is [18F]fluorodeoxyglucose (FDG) and this is of a central role in oncology.

PET-FDG-scan-of-ovarian-cancerPET-FDG scan of ovarian cancer

Vital information can be acquired by using [18F]FDG and several other radiopharmaceuticals. These include fluorine-18 florbetapir (Amyvid), fluorine-18 florbetaben (Neuraceq), fluorine-j18 flutemetamol (Vizamyl), fluorine-18 ethylcholine (FECH) amongst others with the overall aim to evaluate biological processes. PET imaging is creating vast opportunities in the area of molecular imaging and provides a platform for a potential revolution in clinical diagnostics especially for oncology, neurology and cardiology.

PET-FDG-scan-of-sarcoidosisPET-FDG scan of sarcoidosis

History of PET Imaging

PET imaging has been the focus of research in the human body since the 1970s.  This impressive technological platform has allowed for a prominent clinical role in diagnostic, staging and monitoring of disease in patients by the 1990s. The pioneering work commenced by Brownell and Sweet at Massachusetts General Hospital in 1953 allowed for the first positron detector to study human brain function.  Several milestone discoveries in the clinical application of PET imaging have been reached by the following scientists which include Kuhl, Pogossian, Wolf, Sokoloff, Phelps, Di Chiro, Alavi and Wagner.

In the 1990s, Wagner demonstrated that the glucose analogue, FDG could be utilised in various PET studies.  Preliminary applications of clinical interest were carried out in the studies of brain disease, cancer and dementia.

In the late 1980s, technological advances permitted a faster and more accurate whole-body examination acquired by using PET-FDG and this started to become an important clinical tool in oncology.  The central role of PET-FDG was for diagnosing, staging and re-staging of several types of cancer in patients on a daily basis.

PET-FDG-scan-of-tuberculosisPET-FDG scan of tuberculosis

An important milestone came in 1997 when the FDA gave approval for the utilisation of PET as an imaging tool in the area of diagnostics. Subsequently, PET-FDG became an established imaging modality in the clinical assessment of many neoplasms, finding a role also in non-malignant diseases such as dementia, myocardial ischaemia, inflammation and infection.

Principle of PET Imaging

PET imaging works by the detection of radiation inside the human body.  The radiation generated by the decay of radiotracers containing an unstable radionuclide.  In the decay process, a positron (β+) and neutrino are simultaneously emitted.  The positron is an anti-matter electron (β) with identical mass to an electron having a positive charge.  In the decay process, the ejected positron loses kinetic energy by colliding with the surrounding atoms.  This action suddenly causes the positron to come to rest.  These positrons have characteristic energies peaking at 0.63 MeV and have a very short range within the tissue.

positron-electron-annihilation

Positron-electron (e+ – e) annihilation

The positron hits an electron (β) resulting in an annihilation reaction which results in energy being produced.  This energy forms two ‘annihilation’ photons (511 keV), from the point of (β+ – β) interaction in opposite directions. The annihilated photons are measured using the principles of coincidence PET detection of gamma rays.

The gamma rays leave the patient’s body and interact with the scintillation crystals such as BGO (bismuth germinate), LSO (lutetium oxyorthosilicate), GSO (cerium-doped gadolinium silicate), LYSO (cerium-doped lutetium yttrium orthosilicate) and the photomultiplier tubes in the detectors.  These crystals act as transducers by converting the gamma rays into ‘light’ photons.

The photons are then converted into electrical signals that are registered by the tomography electronics.  The information is then processed to form a complex 3-D real-time image, for example, the brain or a whole body scan.