What is PET-CT

Positron emission tomography (PET) uses a small amount of radioactive material called, a radiotracer, to form an image of the metabolic state of your cells.  The radiotracer is injected intravenously and is taken up by the cells of your body. The PET camera can pick up the radiotracer from your body and convert the information into an image. It is a molecular image. When it is combined with a CT (computed tomography) it is called a PET/CT.

 

PET/CT is a dual-modality. (PET + CT) This means that we image the cellular/metabolic function (how does the cell WORK) AND the anatomy (what does it LOOK like) of the human body in one investigation.

 

The PET/CT scan has a specific place in the management of your disease and cannot replace a CT, MRI or other scan. One scan is not better than the other. Some information can only be seen on a PET/CT scan, but other information can only be seen on a MRI scan. Your referring doctor will know when a PET/CT scan is the best imaging modality.

 

The advantage of PET is that it may detect the early onset of disease before it is evident on other imaging tests. For example a PET scan can distinguish between dead scar tissue and active tumour tissues. The PET scan can evaluate if your current treatment regime (eg chemotherapy) is effective.

 

It is a whole-body imaging modality and we can therefore look at the entire body with one investigation. Routinely the scans are done from “eyes to thighs”, but in certain diseases we do a total-body scan where we scan the patient from head to toe (eg melanoma).

The most commonly used tracer is F-18 FDG (Fluoro-deoxyglucose). Essentially it is a radioactive sugar that we inject intravenously. It has no side-effects. It is taken up into the cells according to how “metabolically active” the cells are. Most cancerous cells tend to be more “metabolically active” or aggressive and therefore will take up  more of the tracer than surrounding normal tissue cells. The doctor can see where active tumour cells are located.

 

As soon as the cell starts to break down the radioactive sugar – it realizes that the sugar is not “real” and the radioactive sugar is pushed out of the cell, carried off to the kidneys and eventually the bladder –  this is how all the radio activity is flushed from your system. A few hours after the injection there will be almost nothing left in your body.

 

Although this study is mostly used in oncology,  many other indications do exist. An example of this is cells with infection and inflammation. These cells also takes up this tracer and we can also use this tracer to diagnose (and in some cases monitor response to treatment) in these conditions.