Positron Emission Tomography (or PET) is a technique forimaging metabolic activity in the human subject. It is different from CT(X-ray computed tomography) and MRI (magnetic resonance imaging) which image anatomy. PET looks at function or physiology. Physiological changes occur before anatomical changes, so we may see changes in physiology before they have caused any structural damage. We can then avert such damage.
We inject very small amounts of radioactively labelledglucose or water or similar natural compounds that the human body uses.This glucose (or whatever) is taken up in metabolically active areas: in heart muscle for example, or in the language centers of the brain when you are talking (these are on the left side of your brain roughly midway between your forehead and the back of your ears).
The PET Scan, and its associated blood sampling, tellsus how metabolically active parts of our body are: for example we can locatethe lesions that cause epilepsy, we can find defects in heart muscle, wecan differentiate benign from malignant tumors, and we can gain a basicunderstanding of the healthy workings of the human brain when we think,imagine, visualize, speak, and so on, as well as the deficits and compensations that the human brain employs in various illnesses such as schizophrenia, depression, injury, and so on. In addition these studies are used clinicallyto better assess disease or injury and guide treatment.
The three PET Cameras currently in use at the Hospital of the University of Pennsylvania are :
- CPET Whole-Body Scanner- manufactured by UGM Medical Systems, Philadelphia
- GPET Brain Scanner-developed at University of Pennsylvania
- Allegro Whole-Body Scanner - manufactured by Philips Medical Systems
For more infrmations about our camers click here.
Clinically, a PET Scan can locate metabolic defects invivo (in the human body) often before they have caused structural damage,that is while they are still reversible. It can do this with high sensitivityand reliability. This is a great advantage in deciding treatment courses(the need or not for surgery or various drug treatments) and in diagnosingvarious conditions. For Research, PET offers the unique possibility of obtaining absolute physiological measures (eg blood flow in ml/100g/min,or glucose metabolism, oxygen utilization or neurotransmitter uptakes inthe brain). This, for the first time, offers the possibility of understandingthe basic processes in the human body /brain underlying health and disease.
Since PET accurately and sensitively measures functionor physiology in the human body it can detect problems before they causeirreversible damage to tissues. It may also detect response to treatmentearlier than many other techniques, such as CT or MRI scans.
A tiny amount of radiolabelled glucose is injected todetect active tumors in the body with very high sensitivity. In effectwe measure metabolism in the body: active tumors have a high metabolismand therefore a high demand for glucose. In this way malignant tumors maybe distinguished from benign tumors and hidden tumors may be found. Also,the response of tumors to treatment (surgery, chemotherapy, radiotherapy,immunotherapy or gene-therapy) may be seen and measured earlier by PETthan many other techniques. Successfully treated tumors are destroyed physiologicallylong before there is significant shrinkage of tumor volume. In this waystressful treatments can be assessed and completed earlier than might otherwisebe possible.
Blood flow and glucose metabolism in the muscles of theheart are accurately measured by PET. These measurements are made by imagingthe distribution of intravenously injected radiolabelled ammonia (bloodflow) and glucose (metabolism). Areas of low blood flow but normal metabolismin the heart muscle indicate heart muscle that may be repaired by restoringblood flow (repairing the appropriate coronary artery), while areas ofmatched low blood flow and metabolism may already be irreversibly damagedand so require other treatments. In addition blood flow in the heart maybe measured at rest and after exercise to detect partly obstructed arteriesin the heart responsible for angina.
Seizures or epilepsy often start from a single focus ofabnormal tissue in the brain. When the frequency and intensity of seizuresdo not respond well to medication, surgery may provide permanent relief.In any event, injection of a tiny amount of radiolabelled glucose is usedto image the metabolism of the brain by PET. If you were to have a seizurearound the time of glucose injection the seizure focus would be an areaof intense metabolism (and uptake). Between seizures the focus shows upas an area of decreased metabolism in the brain.
Brain trauma and neurodegenerative diseases often resultin subtle changes in brain anatomy and physiology which may have markedeffects on cognition and emotion. The physiological changes underlyingthese longer term effects of brain injury or neurodegenerative diseasescan frequently be demonstrated with glucose (18F-FDG) PET scans of thebrain, which are very sensitive and specific for these types of changes.
Bone injury and abnormalities detected with high sensitivityby PET imaging following intravenous injection of tiny amounts of labelledfluoride. Infection and inflammation sites may be located with high definitionby PET imaging following injection of labelled FDG (glucose).
PET radioisotopes are very short lived (2 mins to 2 hours) and very smallamounts are injected. In all cases very little or no radioactivity will remainin your body 10 mins to 6 hours after injection.
