Nuclear medicine is a branch of medicine that uses radioactive materials (radiopharmaceuticals) in the diagnosis and treatment of diseases.
Diagnostic methods study how a radionuclide chemically bounds itself to a molecule, by “mimicking” the metabolic activity and behaviours of a biological tissue/organ.
Targeted Radionuclide therapy (Metabolic Radiotherapy) occurs by using radiopharmaceuticals to target and destroy specific pathological tissues and hence largely save the healthy tissue. Pharmaceuticals use similar mechanisms as those of drugs used in the diagnostic field via a high linear energy transfer referred to as high LET (such as beta particles).
The imaging instruments (gamma camera and PET scanner) use scintillation crystals that shed light on the fixation areas of the radionuclide inside the body. These crystals emit light when they are struck by gamma rays which are emitted directly by the radiopharmaceutical that is injected into the patient’s body or generated by the electron-positron annihilation for PET scans (in this case one seeks to detect coincidence events, for further details see PET scans). The light is then converted to a digital signal for the computerized analysis. Certain areas of the body can also be investigated via the use of appropriate probes that can count the number of radiations with which they interact.
It is also possible to study the body in three-dimensional images via the use of tomographic scans.
Positron Emission Tomography (or PET) is a technique used in nuclear medicine and medical diagnostics to produce biological imaging (body imaging). The PET provides information of a physiological nature, unlike computed tomography (CT) or magnetic resonance imaging (MRI) that provide morphological information of the human body, PET provides actual physiological information.
The process begins by injecting into the patient a short half-lived radioactive tracer (radioactive isotope) that is chemically bonded to a metabolically active cell (vector). There is a time interval before the patient is positioned inside the scanner. During this time, the metabolically active cell (often glucose) reaches a certain concentration within the tissue to be analyzed. The short half-lived isotope decays, emitting a positron. After a pathway that can reach at most a few millimetres, the positron annihilates itself with an electron, producing a pair of 511 KeV gamma rays emitted in opposite directions. (back-to-back photons)
These photons are detected when they reach a scintillator in the scanning device, where they create a flash of light, detected by the photomultiplier tubes. A crucial point of the technique is the simultaneous detection of photon pairs: the photons that do not reach the detector in pairs, i.e. within a time interval of a few nanoseconds, are not taken into account. By measuring the position at which the photons hit the detector, one can reconstruct the hypothetical position of the body from which they were emitted, allowing the determination of the activity or chemical use inside the investigated body parts. The scanner uses the detection of the photon pairs to map the density of the isotope inside the body, in the form of cross-sectional images separated from each other by approximately 5 mm. The resulting map represents the tissues in which the molecule sample was mostly concentrated, and is read and interpreted by a specialist in nuclear medicine in order to reach a diagnosis and subsequent treatment.
PET scans are extensively used in clinical oncology (to have representations of the tumours and be able to search for metastases) and in cardiological and neurological research. Alternative diagnostic methods include the X-ray computed tomography (CT), magnetic resonance imaging (MRI), functional magnetic resonance imaging (FMRI), Ultrasound and single photon emission Computed Tomography.
In any case, unlike other scanning methods, such as CT and MRI that allow us to identify biological and anatomical alterations in the human body, PET scans are capable of detecting alterations at a molecular biological level that often precede the anatomical alteration. This is achieved through the use of molecular markers that show a different absorption rate depending on the affected tissue. PET scanning allows to visualize and quantify with reasonable precision the change in blood flow in various anatomic structures (by measuring the injected positron emitter’s concentration).
Positron emission tomography scans are often compared with computed tomography scans, providing anatomical, morphological and metabolic information (for the latter it is essentially how the tissue or organ have adapted and what they are doing). To cope with the technical and logistical difficulties resulting from the patient’s change of position, only the PET-CT scanner is used to perform the two tests and avoid potential inaccuracies. The scanner combines in a single gantry system both a PET scanner and a state-of-the art CT scanner which are controlled by one operating system. The PET-TC scanner has paved the way for substantial improvement in image accuracy and interpretability whilst significantly reducing examination times.
PET plays an increasingly important role in monitoring response to therapy, especially cancer therapy.
A limitation to the diffusion of PET is the cyclotrons’s cost for the production of radionuclides with short half-lives. Few hospitals and Universities can afford the purchase and maintenance of these expensive devices, and so most of the PET centres are supplied by external suppliers. This obstacle limits the use of clinical PET, mainly the use of 18F-labelled tracers that have a half-life of 110 minutes and can only be transported to a fair distance before being used. The 68Ga (obtainable thanks to a generator) also allows to obtain tracers in an easier way, while the 82Rb is sometimes used to study myocardial blood supply.
PET-CT imaging is used for:
PET CT SCAN
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