What ist PET?
On the principle of examination PET does not differ from other imaging procedures in nuclear medicine. The patient is given a radiopharmaceutical and is examined in a specific retention time with the PET scanner. The special feature of PET emanates from the type of breakdown of the used radioisotopes (positrons), which in combination with the suitable ultra-sensitive examination device specifically for this (PET scanner), may present very small lesions.
PET scanners are nowadays combined with computer tomography equipment (PET/CT equipment) or magnetic resonance tomography equipment (PET/MRT equipment).The chemical elements which are used as radioisotopes allow the marking of substances, which do not deviate or only barely deviate from the compounds naturally occurring in the organism. Consequently it manages to present images of specific biochemical and physiological processes in the tissue.
Depending on the examination the patient is injected intravenously on a recliner or direct on the examination bed. Waiting time until the examination (retention time) must be in a relaxed state and muscular straining must be avoided. If the examination requires that the tracer is administered on the examination bed, the patient is urged to lie down in a relaxed state and without moving if possible for up to 60 minutes. Apart from that, the examination lasts between 15 and 30 minutes, with a retention time of 30 to 45 minutes additionally.
Interaction with other medication
Normally the patient may continue his medication as normal. Since however the [18F] FDG examination should normally be performed with low blood sugar levels, i.e. in fasting condition, it may be necessary for diabetics to stop oral antidiabetic medication. Insulin-dependent diabetics can inject themselves in the morning and reduce somewhat the number of bread units in the morning meal or stay fasting and adjust the insulin dose. Upon referral of a patient with Diabetes mellitus, we try to inform them of this and maybe have a consultation with a physician of the PET centre.
For a [18F] FDG-PET examination the patient is to have a basal blood sugar level and be well hydrated when the tracer is administered. To this end the person being examined is to fast for at least 5 hours before the examination but drink plenty of sugar-free drinks, e.g. mineral water. Physical efforts are to be avoided; even chewing gum can falsify examinations in the head-throat region.
The patient must under circumstances be able to tolerate lying down on an examination bed for quite a long time. In addition to giving any analgesics for back pain light sedation may also be necessary, since many patients tend to get claustrophobic in the scanner. Above all patients with restricted left-ventricular function must remember that lying down for a long time flat on your back can lead to considerable dyspnoea if you have not emptied your bladder before the examination. For PET examinations with other radiopharmaceuticals a different preparation may be necessary. We request a consultation on this. Due to the short-life span of the radiopharmaceutical it is absolutely vital that the patient turns up on time for the examination appointment.
The substance used for most PET examinations [18F] FDG is a glucose derivative and has no potential side effects whatsoever. In seldom individual cases substances are used, which indeed could cause major intolerance responses, but are used in such low quantities that their potential side effects are just as low as the radiopharmaceutical which is used in conventional nuclear medicine.
Due to the short half-life of the radionuclide used for PET the radiation load for patients and nursing staff on the wards is low.
Breastfeeding mothers should still feed once before the tracer is applied and after the examination, pump and dispose of the first lot of milk.
The special feature of the PET compared to other nuclear medicine imaging procedures lies in the fact that the used radiopharmaceuticals are either "natural" compounds, which the organism is not distinguished from, which it gives with food or synthesises itself, or marked medications, which are chemically identical with their non-radioactive sister compounds and are distributed in the same way in the body.
This enables an accurate assessment of the metabolism in the organism, e.g. with the question whether after chemo and radiation therapy a space occupying lesion contains living tumour cells or can be interpreted as only more than a scar. In cardiology, heart muscle tissue can be identified using PET, which profits from vessel enlarging measures such as dilation or bypass surgery; in neurology PET can, for example for seizure disorders that are not properly controlled with medicine, gauge the enlargement of the seizure focus and so define the target area of any surgical resections.