How is Positron Emission Tomography (PET) Used?

What is Positron Emission Tomography (PET)?
Positron Emission Tomography (PET) is a relatively new medical imaging technique. PET is the most sophisticated nuclear medicine technique that produces 3D images or pictures of functional processes in the human organism. The Positron Emission Tomography (PET) system is designed to selectively detect pairs of gamma photons emitted, as a result of positron and electron collision. The positrons are emitted by radionuclide, which is injected into the body on a biologically active molecule.

The Positron Emission Tomography (PET) produces 3D images of the tracer concentration in various parts of the body. The acquired nuclear data is processed by the PET computing system and used for the construction of the 3D images.

How is Positron Emission Tomography (PET) Used?
Recently, the Positron Emission Tomography (PET) system, coupled with a CT scan, is performed on the patient during the same session, using the same machine. The CT images are matched with the PET scans. This technique improves the anatomic precision of the displayed images and provides a means for the nuclear count correction.

When a patient is scanned, a short-lived radioactive tracer is injected. The tracer is chemically incorporated into a biologically active molecule. There is a waiting period while the active molecule becomes concentrated in tissues being studied, then the subject is placed in the imaging scanner.

The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour. During the scan, a record of tissue concentration is made, as the tracer decays.

Positron can be emitted from radioactive nuclei which have too many neutrons for stability. Positrons do not last for very long in matter, since they quickly encounter an electron, resulting in a process called annihilation. In this process, the positron and electron vanish and their energy is converted into two gamma-rays which are emitted at roughly 180º degrees to each other. The emission is often referred to as two back-to-back gamma-rays and they each have a discrete energy of 0.51 MeV.

What are the PET Reconstruction Techniques?
A technique similar to the reconstruction of computed tomography (CT) and single-photon emission computed tomography data is more commonly used, although the data set collected in PET is much poorer than CT, so reconstruction techniques are more difficult.

Using statistics collected from tens-of-thousands of coincidence events, a set of simultaneous equations for the total activity of each parcel of tissue can be solved by a number of techniques, and thus, a map of radioactivities as a function of location for parcels or bits of tissue may be constructed and plotted. The resulting map shows the tissues in which the molecular tracer has become concentrated, and can be interpreted by a nuclear medicine physician or radiologist in the context of the patient's diagnosis and treatment plan.

How Positron Emission Tomography (PET) is Used with CT
PET scans are increasingly read alongside CT or magnetic resonance imaging (MRI) scans, the combination giving both anatomic and metabolic information. Because Positron Emission Tomography (PET) imaging is most useful in combination with anatomical imaging, such as CT, modern PET scanners are now available with an integrated CT scanner.

Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more precisely registered, so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images. This is very useful in showing detailed views of moving organs or structures with higher anatomical variation.
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