I have spent most of my research life using microscopes to examine biological structures, and so I was interested when the Blackrock Clinic introduced a new form of medical imaging to Ireland last month, writes William Reville.
Positron emission tomography (PET) is a leading technology that has revolutionised the diagnosis and treatment of cancer, heart disease and neurological disorders in the US and elsewhere.
PET scanning allows us to see inside the body without the need to physically invade the body. Historically, the first non-invasive medical imaging used X-rays. X-rays, discovered in 1895 by Wilhelm Roentgen, are invisible rays of electromagnetic radiation that readily pass through the body but are partially absorbed by denser structures such as bone.
X-rays act on photographic film as visible light does, i.e. they render the emulsion developable. If you shine X-rays through the body and then allow them to fall on a photographic plate they outline a shadowgraph of the inside of the body, with denser structures standing out as lighter areas.
Roentgen immediately recognised the medical usefulness of his discovery. The first X-ray picture (radiograph) published was of his wife's hand, showing the internal bone structure.
A sophisticated, high-resolution form of X-radiography, called computed axial tomography (CAT scanning), was invented in 1972. This fires thin streams of X-rays through the body from several directions and builds up a computer image of a "slice" through the person.
Another technique called magnetic resonance imaging (MRI) does much the same thing, using magnetic and radio wave fields. The term tomography refers to any of a number of techniques used to obtain an image of a section through the human body.
Conventional X-radiography, CAT scanning and MRI scanning are pretty much limited to imaging the anatomy of the structures inside the body. PET scanning, on the other hand, images biochemical activity. Local biochemical changes occur much earlier than anatomical changes, and PET can detect the presence of a disease not yet visible by X-ray, CAT or MRI scan.
How does PET scanning work? A radioactive chemical that mimics a natural substance used by the body is administered to the patient. The radioactive tag on the chemical is a positron-emitting atom. A positron is the anti-matter equivalent of an electron. It carries a positive electrical charge, whereas the electron is negatively charged.
When the positron encounters an ordinary electron, both are annihilated in a burst of energy. This emerges as two gamma rays, each moving in opposite directions, and they leave the patient's body to be detected by the PET scanner. The information is fed into a computer to produce a picture of a slice through the body.
The body is a biochemical machine. Every disease has its origin in altered biochemistry, and PET scanning allows us to see this in the living body. Consider a specific example. All body cells use glucose as an energy source. However, cancer cells burn glucose at faster-than-normal rates. This allows malignant tumours to be detected, even at very early stages, by PET scanning.
A special form of glucose tagged with positron-emitting fluorine and called Fluoro-deoxy-glucose (FDG), is administered to the patient. FDG is sufficiently similar to ordinary glucose to be taken up by cells as normal. Malignant tumours now stand out as "hot-spots" against a quieter background in the PET scan image.
The body function to be studied by PET determines which radioactive chemical is used. PET is the most accurate test to reveal or rule out coronary artery disease. It can also quickly indicate when heart failure symptoms are present, whether by-pass surgery or transplantation is called for.PET scans of the brain can show whether Parkinson's disease is present and can often recognise incipient Alzheimer's disease several years before the physician can confirm the diagnosis.
The positron-emitting radioactive elements (F-18, C-11, O-15, N-13) are artificially created in a machine called a cyclotron. All the positron-emitting elements used in PET must be manufactured close to the PET scanner. Consequently, as well as buying the PET scanner (at a cost of €1.3 million), the Blackrock Clinic established a new company (Molecular Imaging Institute Ltd) to produce the short-lived radioactive elements and FDG.
If you attend for a PET scan, you receive an injection of FDG and rest for one hour to allow it to distribute throughout your body. You then lie on a scanning couch which carries you stepwise through a large ring-like detector. A whole body scan takes 45 minutes to one hour. The only discomfort you suffer is the injection prick.
In addition to the cost of the PET Scanner the Blackrock Clinic spent €1 million on building costs, so having a PET scan is expensive, but prices are similar to those paid by our European neighbours. A whole body scan costs €2,400, a heart scan €1,700 and a brain scan €1,300. Private medical insurance cover is available. Public hospitals are now sending patients for PET scans, and these account for half the patients scanned.
William Reville, is associate professor of biochemistry and director of microscopy at UCC