Sir Stirling PrintsAbout the Daisy Appeal
PET-CT
What is a PET-CT?
PET-CT is a recent development combining two well established medical imaging technologies, Positron Emission Tomography (PET) and Computerised Tomography (CT), into a single scanner.
These two scanning techniques provide very different information about the body and the effects and extent of many diseases. Combining this information accurately results in a very powerful method of investigating a host of diseases including cancer, heart disease and many neurological conditions.
PET involves the injection of a small amount of a radioactive substance known as a radiopharmaceutical or tracer into a vein. (The dose of radiation is very small and the benefits from the scan far outweigh the risk from the radiation.) The commonest tracer used is a form of glucose known as FDG. Once injected it functions in the body just like normal glucose but is visible to the PET scanner. Diseased cells, for example cancer cells, use glucose differently to normal cells and this difference can be detected with PET. Abnormal areas are typically seen as "hot spots". The images provide information about the function and metabolism of normal and abnormal tissues but are of low spatial resolution and do not pinpoint the areas of abnormality with great accuracy nor do they show the body's anatomy in any detail.
CT is a long established technique, the first clinical CT scanner in the world having been installed at Atkinson Morley's Hospital in London in 1971. Modern CT scanners use an X Ray tube and detectors that rotate rapidly and continuously around the subject. The data is sent to powerful computers that generate very detailed and accurate images of the structure of the body and its organs.
PET-CT combines these two techniques in a single machine and the scans are done at the same sitting. The combination of the functional information from PET and the structural information from CT is invaluable in the clinical assessment of many diseases and has massive potential for research into these diseases and many others. It might be asked why the information from separate PET and CT scanners cannot simply be combined by modern computers. The answer is that such methods are simply not accurate enough and even minor inaccuracies could be very dangerous for the assessment and treatment of patients.
What is a Cyclotron and why does PET-CT need a Cyclotron?
As the name indicates, PET detects positrons. A positron is a relatively unusual particle, the anti-particle of the more commonly known electron. Radioactive substances injected into the body must by necessity have a short half-life i.e. the time for half of the radioactive atoms to decay into a stable (non-radioactive) form of the element.
A cyclotron is the machine required to produce radioisotopes that emit positrons. (It can do many other things too.) It uses very powerful alternating electromagnetic fields to accelerate subatomic particles towards the speed of light and then smashes them into a target. This results in a nuclear reaction that produces the unstable isotopes needed for PET.
The "F" in FDG stands for Fluorine-18, a positron emitting isotope of the gaseous element fluorine. F-18 has a half life of 110 minutes which is actually long for a positron emitter and is the reason why FDG is much the most commonly used PET radiopharmaceutical. Once produced by a cyclotron there is enough activity in a sample for it to be transported a reasonable distance from the cyclotron to a PET or PET-CT scanner and then be used to scan patients during a working day.
Although FDG is a very useful agent there is vast potential in PET and PET-CT using other radioisotopes. Carbon, oxygen and nitrogen all have isotopes that produce positrons. These can be combined with different molecules to make radiopharmaceuticals that can be used to study many different metabolic functions and abnormalities, opening exciting new windows into many brain and heart diseases as well as cancer. The drawback is that these other positron emitters have very short half lives. Carbon-11 has a half life of only 20 minutes and therefore can only be used if produced by a cyclotron in close proximity to the scanner. Oxygen-15 has a half life of 2 minutes!
That is where the second phase of the Daisy Appeal comes in. The charity is aiming to raise £4.5 million to purchase a cyclotron and a small laboratory and to fund the salaries of two to three highly specialised scientists required to run the cyclotron and synthesise the radiopharmaceuticals. The scanner and the clinical accommodation are being funded by the acute Hull & East Yorkshire Hospitals NHS Trust. By purchasing a cyclotron the charity will be providing a hugely exciting and important opportunity for the further development of PET-CT scanning in Hull and East Yorkshire, way beyond routine FDG tracer scanning, in both cancer and non-cancerous conditions. Patients in our sub-region will have access to these resources from the start. It is anticipated that once purchased the cyclotron will be self-funding, generating income from research programmes, clinical trial activity and commercial radioisotope production.
It represents a great opportunity for us to be at the cutting edge of a vital aspect of healthcare.
