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Positron emission Tomography

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Positron emission Tomography

Postby dipjyoti » Wed Sep 27, 2006 9:57 am


To conduct the scan, a short-lived radioactive tracer isotope which decays by emitting a positron, and which has been chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (most commonly fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour) becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner.

As the short-lived isotope decays (110 minute half-life), it emits a positron. After travelling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation photons moving in nearly opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes. The technique depends on simultaneous or coincident detection of the pair of photons: photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored.

The most significant fraction of electron-positron decays result in two 511 keV photons being emitted at almost 180 degrees to each other, hence it is possible to localise their source along a straight line of coincidence (also called formally the "line of response" or LOR). In practice the LOR has a finite width as the emitted photons are not exactly 180 degrees apart. If the recovery time of detectors was in the picosecond range rather than the 10's of nanosecond range, it would be possible in theory to calculate the single point on the LOR at which an annihilation event originated, by measuring the "time of flight" of the two photons. However, at present, this is not possible due to detector limitations. Instead, a technique very like the reconstruction of CT and SPECT data is used. Using statistics collected from tens-of-thousands of coincidence events, a set of simultaneous equations for the total activity of each parcel of tissue along many LORs can be solved, 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 probe has become concentrated, and can be interpreted by nuclear medicine physician or radiologist in the context of the patient's diagnosis and treatment plan.

PET scans are increasingly read alongside CT scans or MRI scans, the combination giving both anatomic and metabolic information (i.e., what the structure is, and what it is doing biochemically). Because PET imaging is most useful in combination with anatomical imaging, such as CT, modern PET scanners are now available with integrated high-end multi-detector-row CT scanners. Because the two scans can be performed simultaneously, not only is time saved, but the two sets of images are precisely registered so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images. This is less helpful for the brain, but very useful in showing detailed views of moving organs or structures with higher amounts of anatomical variation, such as commonly occur in the abdomen.
PET is both a medical and research tool. It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), for clinical diagnosis of brain diseases such as dementias. PET is also an important research tool to map human brain and heart function.

PET is also used in pre-clinical studies using animals, where it allows repeated investigations into the same subjects. This is particularly valuable in cancer research, as it results in an increase in the statistical quality of the data (subjects can act as their own control) and very substantially reduces the numbers of animals required for a given study.

Alternative methods of scanning include x-ray computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI), ultrasound and single photon emission computed tomography (SPECT).

While some imaging scans such as CT and MRI isolate organic anatomic changes in the body, PET scanners, like SPECT and fMRI scanners are capable of detecting areas of molecular biology detail (even prior to anatomic change). The PET scanner does this via the use of radiolabelled molecular probes that have different rates of uptake, depending on the type and function of tissue involved. The changing of regional blood flow in various anatomic structures (as a measure of the injected positron emitter) can be visualized and relatively quantified with a PET scan.

PET imaging is best performed using a dedicated PET scanner. However, it is possible to acquire PET images using a conventional dual-head gamma camera fitted with a coincidence detector. The quality of gamma-camera PET is considerably lower, and acquisition is slower. However, for institutions with low demand for PET, this may allow on-site imaging, instead of referring patients to another center, or relying on a visit by a mobile scanner.

Radionuclides used in PET scanning are typically isotopes with short half lives such as 11C (~20 min), 13N (~10 min), 15O (~2 min), and 18F (~110 min). Due to their short half lives, the radionuclides must be produced in a cyclotron at or near the site of the PET scanner. These radionuclides are incorporated into compounds normally used by the body such as glucose, water or ammonia and then injected into the body to trace where they become distributed. Such labelled compounds are known as radiotracers.

PET as a technique for scientific investigation in humans is limited by the need for clearance by ethics committees to inject radioactive material into participants, and also by the fact that it is not advisable to subject any one participant to too many scans. In neurological research, this limitation can be partly overcome by the use of short-lived radionuclides that result in a lower radiation dose. PET also has an expanding role in the assessment of response to therapy, and in particular cancer therapy (e.g. Young et al. 1999), where the risk to the patient from lack of knowledge about disease progress, is much greater than the risk from the test radiation.

A further limitation arises from the high costs of cyclotrons needed to produce the short-lived radionuclides for PET scanning (for example 18F). Few hospitals and universities are capable of maintaining such systems, and most clinical PET is supported by third-party suppliers of radio-tracers which can supply many sites simultaneously. This limitation restricts clinical PET primarily to the use of tracers labelled with 18F, which has a half life of 110 minutes and can be transported a reasonable distance before use, or to 82Rb, which can be created in a portable generator and is used for myocardial perfusion studies.

Because the half-life of 18F is about two hours, the prepared doses decay significantly during the working day. If the FDG is delivered to the scanning suite in the morning, the specific activity falls during the day, and a larger volume of "pharmaceutical solution" (in this case, a mixture of FDG and its decay product which is ordinary glucose labeled with 18O) must be injected into later patients to deliver the same radiopharmaceutical (FDG) dose.


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