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Positron Emission Tomography (PET) Explained

19 July, 2011 | PET-CT

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.

So, if we administer a positron-emitting radiopharmaceutical to a patient, an emitted positron can annihilate with a nearby electron and two gamma-rays will be released in opposite directions. These gamma-rays can be detected using a ring of radiation detectors encircling the patient and tomographic images can be generated using a computer system. The detectors are typically specialized scintillation devices which are optimized for detection of the 0.51 MeV gamma-rays. This ring of detectors, associated apparatus and computers system are called a Positron Emission Tomography (PET) Scanner.

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.

The radioisotopes used for Positron Emission Tomography (PET) scanning include C-11, N-13, O-15 and F-18. These isotopes are usually produced using a cyclotron. In addition these isotopes have relatively short half lives. PET scanning therefore needs a cyclotron and associated radiopharmaceutical production facilities located close by.

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.

What are the Limitations of PET?

Limitations to the widespread use of Positron Emission Tomography (PET) arise from the high costs of cyclotrons needed to produce the short-lived radionuclides for PET scanning and the need for specially adapted on-site chemical synthesis apparatus to produce the radiopharmaceuticals. Few hospitals and universities are capable of maintaining a cyclotron.

This limitation restricts clinical PET primarily to use of tracers labeled with fluorine-18, which has a half-life of 110 minutes and can be transported a reasonable distance before use, or to rubidium-82′ which can be created in a portable generator and  is used for myocardial perfusion studies. Nevertheless, in recent years a few on-site cyclotrons with integrated shielding and hot labs have begun to accompany PET units to remote hospitals. The presence of the small on-site cyclotron promises to expand in the future, as the cyclotrons shrink in response to the high cost of isotope transportation to remote PET machines.

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