Shedding light on the world within

AT first glance the mostly dark screen looks like an aerial view of a city at night. Multi-coloured clusters of light are scattered across the viewing area with particularly large clumps in the middle and at the top of the screen.

But as the radiologist explains, the large clumps of light are the patient's brain and heart.

As she turns down the contrast, many of the points of light disappear and the brain and heart clusters dim. It is the scattered, bright pinpricks of coloured light which remain visible on the screen which tell the doctor that the cancer patient being scanned by an advanced Positron Emission Tomography (PET) device is in very poor shape.

PET scanners are the next big thing in medical diagnostics. First there were basic X-ray machines. Then there were computer-aided X-ray machines. Then there were magnetic resonance imaging (MRI) scanners. Now there is PET.

Many European countries are investing heavily in equipping major hospitals with the new generation of PET scanners, and while there is an NHS plan to station PET scanners at key sites, this is not likely to happen much outside the South-East until early next year.

Currently, the private Woodlands Hospital on the edge of Darlington is the only place in the region where patients can be referred for PET scans. Because of a contractual agreement between primary care trusts in South Durham and the Tees Valley, NHS patients from those areas can be referred for scans. However, patients from North Durham, Tyneside and Wearside who need PET scans will still have to travel to an NHS hospital in London until a new service provided by a private PET scanner trailer gets under way early next year.

Back to the glittering lightshow.

The points of light indicate areas of the body where cells are more active.

The brain and the heart are the two main 'hotspots' for cellular activity but - unfortunately for Patient A - the other main source of cellular activity is cancer cells.

"Cancer cells have a faster metabolism than normal cells," says Linda Sorensen, the lead PET radiographer on the Lister In-Health mobile scanner parked next to the Woodlands Hospital. "When we scan patients we are looking for hotspots, areas where there is an increased uptake of the radioisotope we inject into the patient before the scan," says Linda. "There is a pattern of normal cell behaviour. We know the radioisotope is taken up in the heart and the brain. Every disease has a different signal. For instance, we can pick up diabetes pretty quickly."

While they are increasingly used in neurology and cardiology in Europe, in the UK PET scanners are mostly used to help cancer specialists manage their patients.

A PET scan - unlike any other scan - can clarify whether a primary cancer has spread and pinpoint exactly where the secondary cancer is located.

Depending on the results, a cancer specialist might recommend active treatment, or palliative treatment if nothing can be done.

For a cancer patient, the results of a PET scan could either lead to them being given the all-clear, help the cancer specialist decide the most appropriate treatment or result in them being spared surgery, radiotherapy or chemotherapy because the disease is inoperable.

Superficially, the PET body scanner looks similar to any other medical scanner and, in common with modern MRI and X-ray machines, it has a common element of CT, or computerised tomography, contained in a doughnut-shaped unit.

With tomography, a narrow beam and a detector move around a patient in a rotary fashion, producing an image of a precise 'slice' of tissue.

But in the case of a PET scanner, advanced technology means that the new machine works in a different way to anything which has gone before.

In the information provided to patients, the Woodlands Hospital website explains how PET scanners work.

The technology relies on the fact that every cell in the human body is powered by energy derived from sugar. Because diseased cells usually require more sugar than healthy cells, a way of identifying these cells has been developed by scientists.

An hour before a patient is given a PET scan they are injected with a small amount of glucose with has been made slightly radioactive. The substance injected - which is less than a fifth of teaspoon - contains glucose molecules which have been attached to radioactive isotopes which emit positrons, known to scientists as positron emission tomography isotopes.

Known as a radio tracer, the substance is taken up by the cells of the body.

During this process the positrons emitted by the injected substance interact with electrons within the body.

This interaction results in high gamma ray beams being produced which travel at precisely 180 degrees to each other. A ring of detectors inside the PET scanner picks up these rays and sophisticated computers process this information.

What is mind-boggling is that the injection results in millions of interactions throughout the body but the technology sorts this all out to produce an image on the screen which depicts the molecular activity of all the cells in the body.

This molecular scan can be overlaid on to a anatomical CT scan to produce a set of hybrid 3D images, allowing the skilled radiologist to pinpoint exactly where diseased cells are located.

The CT machine used by Lister In-Health rotates every half a second and is used to position and plan the PET scanning. Once the two machines are exactly aligned, the PET scan proper can begin.

"The injection we give to patients has a very short radioactive half-life of 110 minutes so we have a limited amount of time," says Linda. "It is mostly colourless saline with a tiny amount of radioactive glucose added and there is a one hour time delay before it takes effect."

"We have to get a very good medical history from the patient because, if someone has had treatment such as a biopsy or surgery, the body will try to repair itself and this activity will show up as a hotspot on the screen."

Because PET scans are still a novelty in the NHS - and because each scan involves injecting a radioactive substance into a patient - access is strictly controlled.

Moira Stokoe, head of radiography at the Woodlands, explains. "Every PET scan has to be individually approved by the licence holder who is based at a hospital in Nottingham. A consultant who wants a patient to be scanned has to discuss their request with a multi-disciplinary committee. Only then is a referral made. It is the same story if the patient is NHS or private."

Currently the Darlington PET scanner visits the Woodlands Hospital two or three times a week, scanning eight or nine patients a day. Apart from the Darlington site, the nearest PET scanner available to NHS patients in the North of England is at a hospital in Hull.

It is hoped that this will change early in 2008 when a mobile PET scanner trailer is due to spend three days a week at the Freeman Hospital in Newcastle and two days at the James Cook University Hospital in Middlesbrough.