Epilepsy Awareness Program - Positron Emission Tomography (PET)
What is a "PET scan?
A PET scan analyzes the amount of glucose, or sugar uptake by the brain. One would inject in a glucose substitute that has a label on it, and then take images of the brain during a period of time shortly after the seizure. The PET scan allows us to look at regions of the brain that are either hypermetabolic i.e. using a lot of sugar, or parts of the brain that are hypometabolic i.e. using very little sugar. It turns out that for Temporal Lobe Epilepsy, the temporal lobe shows hypometabolism.
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PET is a minimally invasive method of nuclear medicine imaging that uses short-lived radiopharmaceuticals to detect and assess perfusion and metabolic activity in various organ systems. It provides information about function and metabolism that is complementary to the structural information provided by traditional imaging techniques such as radiology (Prvulovich & Bomanji 1998).
The majority of radioactive isotopes used in nuclear medical imaging release energy as single gamma rays (photons) as they decay. Conventional nuclear medical imaging is based upon the detection of these photons using stationary single or multi-headed gamma cameras that produce two-dimensional images. Tomographic techniques such as
single photon emission (computed) tomography (SPE(C)T) can use rotating camera heads to acquire imaging data in a 360° circle around the patient, from which multiple cross-sectional images are reconstructed (Flynn & Adams 1996).
PET is a related technology that uses radioisotopes which decay by emitting a positively charged electron (positron) from the nucleus. The positron annihilates a negatively charged electron, resulting in two high-energy photons (511 kilo-electron-volts, keV) that travel at 180 degrees to each other. The high-energy photon is subject to less absorption or scatter by tissue than the comparatively lower-energy photons released during conventional nuclear medicine imaging, and is detected in coincidence with its pair, which usually results in superior image quality (Lewellen, Miyaoka, & Swan 1999).
A positron camera is typically arranged in a ring around the patient and produces crosssectional tomographic images. Traditional PET scanners come in full-ring and more recently partial-ring models.
Robert and Milne (1999) described four
ways of describing PET scanners:
• traditional full-ring PET systems;
• partial-ring rotating PET scanners;
• coincidence imaging with modified gamma camera technology;
• high-energy collimator imaging of 511 keV photons with modified gamma camera technology.
Studies reported in the main body of this submission have used traditional full-ring PET scanners. Results from these systems can be further subclassified according to whether or not they acquire data in two or three dimensions and the mode (if any) of attenuation correction.
Earlier scanners have a relatively low resolution and would be expected to yield a high positive predictive value and a lower negative predictive value. Overall accuracy with such scanners would be expected to be lower than more recent scanners. All modern scanners use attenuation correction obtained through maps generated by the attenuation of gamma rays from either single photon or positron-emitting radionuclides, or X-rays in PET/CT scanners.
Charged particle accelerators (eg generators and cyclotrons) produce the radiopharmaceuticals used for PET scanning. The particles principally used include oxygen (15O), nitrogen (13N), carbon (11C) or fluorine (18F). This review is restricted to an examination of the radionuclide 18F-FDG (2-[18F]fluoro-2-deoxy-D-glucose), which is a
radiolabelled analogue of glucose. This radionuclide is useful for imaging in neurology, and epilepsy specifically, because an epileptogenic seizure focus tends to display decreased glucose utilisation interictally, and 18F-FDG can be used to examine the decreased metabolism of potentially epileptogenic areas compared with nonepileptogenic
areas (Immonen et al 2004).
Dual-modality PET/CT scans have recently been introduced. Such systems allow for PET emission data to be routinely corrected for photon attenuation by CT, thus producing noiseless attenuation correction and reduced scanning times when compared with PET (Carney & Townsend 2003). The reduction in scanning time is reported to be in the order of 25-30% (Schulthess 2003). An additional benefit is the production of detailed anatomical information by CT which is absent on PET scans. These “hardware” coregistered images can aid in diagnostic interpretation. The coregistration of PET and CT images in a single system is reported to be more time-efficient and provide a higher level of image quality than software coregistration of images (Schulthess 2003).
Patients are asked to fast for five to six hours on the day of their scan. Upon arrival, FDG is injected via an intravenous cannula placed in the patient’s arm. After the injection of FDG the patient waits up to an hour before undergoing the scan. This allows the FDG tracer to accumulate in normal brain areas. Abnormal brain areas
(corresponding to epileptogenic seizure foci) have reduced FDG accumulation (hypometabolism). The patient is then scanned. The scan takes between fifteen and thirty minutes to complete (Dussault et al 2001).
PET Intended purpose
PET is intended to image the functional/metabolic activity of neurological structures and aid in the diagnosis and treatment planning for medically refractory epilepsy.
*- You should not eat or drink anything four to six hours prior to the exam.
*- You should wear comfortable clothing.
*- You shoudl take all prescribed medication(s) as usual unless instructed otherwise by the physician.
During a PET scan
*- Before the scan, a small amount of radioactive tracer will be injected or inhaled. The tracer is a compound, such as sugar, "labelled" with a short-lived radioisotope.
*- Following the injection, you will be asked to rest for approximately 30-45 minutes while the substance reaches the brain.
*- The technologist will place you on a scanner table.
*- Head will be in a special head rest and immobilised using foam blocks or a special mould shaped for you.
*- It is important for you to lie completely still.
*- The scanner table will slowly pass through the PET scanner. Your head will be inside the large, doughnut-shaped machine. The scanner detects the radioactive material to produce a computer image of your brain.
*- Scanning time is approximately 1-2 hours.
After the PET test
*- There are no after effects from the injection or the PET imaging.
*- You can resume normal activity immediately after the PET scan is completed.
*- The PET scan will be reviewed by a radiologist or nuclear medicine technologist who will report to your physician. The physician will then make a follow-up appointment to discuss the results with you.
PET is a noninvasive and safe diagnostic procedure. Safety issues are primarily discussed in terms of the safety of the positron-emitting radiopharmaceutical, rather than the safety of the procedure as a whole. .
The United States Pharmacopoeia drug information for FDG also indicates that there are no known adverse effects associated with the use of FDG. In addition, radiotracers are generally used in microgram quantities, and as such the incidence of adverse reactions to very small amounts of labelled molecules is likely to continue to be low.
Patients undergoing a PET scan will be exposed to a certain amount of ionising radiation. It has been estimated that the radiation dose in a patient undergoing a FDGPET scan is on par with that received during a diagnostic CT scan.
See Also: Epilepsy Health Corner
See Also: Neurophysiology Health Corner
Back to: Epilepsy Awareness Program
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