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| Positron Emission Tomography (PET) – the ultimate in imaging? | |||
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This article has
been adapted (with permission) from the Western Australian PET/Cyclotron
Centre Patient Information Sheet. Further information can be obtained
at: |
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| What is PET? | |||
This is
in contrast to CT, where the penetrating radiation originates outside
the body, and then passing through it. The PET image identifies both
physiological and chemical changes in the body. It primarily
distinguishes organ function rather than structure, by measuring the
organ’s metabolic activity. |
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| How is the radiopharmaceutical introduced into the body and how long does it take to get the PET scan done? | |||
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During
the procedure, patients are injected with a small quantity of
radiopharmaceutical (called a tracer) through a vein. Patients rest for
about 45 minutes and then are positioned on the PET scanner table. The
tracer distributes itself throughout the body, accumulating in specific
organs and in particular diseased tissues. |
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| How will PET scanning benefit patients? | |||
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PET is useful in the initial diagnosis of a number of tumours (especially the lymphomas, spread of cancers to lymph nodes) as well as in detection of cancer recurrence and in the monitoring of its treatment. It is also a useful diagnostic test for epilepsy, dementia and for assessing cardiac (heart) function. Compared to many current imaging modalities such as MRI and CT, PET detects disease earlier and more accurately. It is particularly useful in monitoring whether the applied treatments are successful. It can often detect improvement in, or failure of treatment before any other imaging test.
Currently, the most common imaging substance (tracer compound) is fluorine-18 fluorodeoxyglucose (18-FDG). In the future, novel and specific PET tracers, currently on the threshold of clinical application, will collectively be able to detect a wider range of diseases – particularly cancers.
PET is also important in drug research and development. Research using PET is being done to study how the brain works, in breast cancer, in kidney diseases and tumours such as malignant melanoma. For some patients, having a PET scan will mean they may not need to be subjected to more invasive and expensive tests. PET is unique as it shows the chemical and physiological changes related to metabolism. Therefore, PET may demonstrate changes long before CT or MRI. PET scanning also has the advantage of showing all the organ systems of the body with one image. It can be used to plan radiotherapy treatment more accurately. With
the recently developed combined imaging modality PET/CT, there is the
benefit of improving patient throughput by decreasing the time taken to
perform scans and also achieving a combination of the exquisite
anatomical information of CT with the important functional information
of PET. The fusion of images is done with computer software. |
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| Is PET safe? | |||
 There
are no known short or long-term side effects of a PET scan. Patients are
exposed to a small amount of radiation that is not considered in any way
an immediate threat to their health but one should be aware that the
dose received is many times that of a chest radiograph. The amount of
radiation exposure from PET scans is equivalent to a standard diagnostic
CT scan, such as is performed for diagnosis. However, pregnant or breast
feeding women should not have a PET scan without first discussing the
possibility with their doctor. A routine screening PET scan for
disease/cancer is not recommended in anyone in the absence of proper
indications/reasons for the scan.As in all investigations, the benefits of the examination should be weighed against the risks. There should be definite indications for a PET or PET/CT scan and the examination results must play a role in decision making in the management of the patient. |
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| How are PET tracers produced? | |||
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A PET scan requires that the patient be injected with a
small amount of a radiopharmaceutical (“tracer”). This
radiopharmaceutical is synthesised in a “hot cell”, in the exquisitely
controlled environment of a radiopharmaceutical laboratory. A key input
component in the synthesis of a radiopharmaceutical is a radioisotope,
which must be created from a nuclear reaction in a nearby cyclotron.
Positron emitting radionuclides have very short
half-lives. Therefore, in order for a centre to provide the PET
service, it must ideally be equipped with a cyclotron and a team of
medical physicists, computer experts, radiopharmacists, technicians and
clinicians. If a centre does not have a cyclotron, this radionuclide
will have to be transported to the centre. |
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| What is a cyclotron? | |||
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A cyclotron is an accelerator for atomic or sub-atomic
charged particles. As the particles are accelerated, they gain energy.
The cyclotron accelerates these particles along a spiral orbit through a
series of steps, so that a quantum of energy is gained per rotation. The
spiral motion is maintained by powerful magnets that are many times
stronger than the earth’s magnetic field. The energy for acceleration is
provided through a rapidly fluctuating (RF) voltage. The acceleration of
these particles occurs under high vacuum to minimise collisions with air
and other gases. For more on
the cyclotron: |
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| How is the radioisotope converted to a PET tracer? | |||
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The most common radioisotope used in PET is Fluorine-18
(18F), which has a radioactive half-life of just 110 minutes. This means
that the product produced by the cyclotron must be used immediately,
since it is worthless within about 3 half lives, or about 8 hours. Other
PET isotopes have even shorter half lives. The radioisotope can be
joined chemically with a molecule that has a specific affinity for a
particular biological function of the human body. The most common
example is where the molecule is a form of glucose, since glucose is
readily absorbed by metabolically active cells, in order to “fuel” their
metabolism. The most common radiopharmaceutical used worldwide in PET
imaging studies is 18F-labelled fluorodeoxyglucose (FDG). This
radiopharmaceutical is used extensively in tumour, brain and heart
imaging studies. Since the enviroment of the chemical synthesis is radioactive, much of the process of radiopharmaceuticals production must be automated, using components prepared previously under sterile conditions. To ensure that the final product is biologically sterile and toxin and pyrogen free, the production must take place in a “clean room” laboratory which must meet internationally defined standards. Staff who operate the apparatus are also required to wear protective clothing and to adhere to strict procedures of cleanliness. The final product is rapidly subjected to a battery of quality control (QC) tests, including pH, colour, radioisotope purity, and chemical purity. QC must be performed within about 20 minutes of synthesis. At every stage of production and validation, time is of the essence because of the short half lives of PET radioisotopes. |
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| What is the future of PET in medical diagnosis & management? | |||
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Many cancers can be spatially and functionally
quantitatively differentiated from surrounding healthy tissue because of
their abnormal glucose metabolism. In addition, there are around 500
other PET radiopharmaceuticals, each with different biological
affinities, and each providing a unique insight into human metabolism
and disease. Only about 10 of these currently have accepted clinical
applications. However, many more useful tracers will emerge in the next
decade. We expect to see serious diseases such as breast and prostate cancer, Alzheimer’s and other forms of dementia join the list of disorders already benefiting from the diagnostic insight of PET, and for which PET is an indispensable clinical management tool. Positron emission tomography, and in particular PET combined with CT in the form of a PET/CT scanner, and where the PET imaging component will harness a rapidly expanding range of medically important tracers, is by far the most exciting development in medical imaging for 20 years.
Further information can be obtained
at:
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Copyright © 2001-2008 College of Radiology, Academy of Medicine of Malaysia Last Updated: |
Positron Emission Tomography (PET) scan is performed by a PET scanner
that shares many engineering characteristics with the conventional
imaging modalities Computed Tomography (CT) or Magnetic Resonance
Imaging (MRI). Like the CT and MRI, the PET scanner produces an image by
using a computer to reconstruct the information obtained. However, the
similarity stops here. 