Analysis: Intruder alert

Military technology is being used in the form of a radar that can identify patients at risk of a stroke.

Ever since troy fell victim to Greek soldiers concealed in a wooden horse, armies have been well aware that knowing your enemy's location can play a decisive part in determining the outcome of a battle. Now some rather more high-tech solutions to tracking opposition forces and weapons are finding new applications in the fight against diseases that kill millions every year, by spotting intruders in the human body.

In Britain, medical physicists at the University of Leicester believe signal-processing techniques used in military radar systems could help cut deaths from stroke by improving early diagnosis. The same approach could provide a more effective way of monitoring patients as they recover.

The breakthrough would address the third most common cause of death and the most common cause of adult disability in the UK. A quarter of strokes are the result of blood clots or other foreign bodies known as emboli blocking small blood vessels in the brain. These obstructions can originate from a number of sources such as the heart, or from plaques in arteries in the head or neck due to vascular disease.

The method was developed by Leicester researcher Joanne Cowe, a graduate in electrical and electronic engineering who before embarking on her PhD worked in the Radar Systems Division of AMS Ltd, now part of BAE Systems.

Cowe investigated how radar could improve the operation of medical ultrasound devices, focusing on ways of detecting obstructions in blood vessels and revealing the extent to which useful information can be obtained by processing an ultrasound signal. Her research has involved processing transcranial Doppler ultrasound (TCD) radio-frequency signals, to develop new techniques for the detection of cerebral emboli and to improve the axial resolution obtainable from TCD systems.

TCD is commonly used to detect emboli by measuring how fast blood flows through the carotid arteries or the arteries at the base of the brain. It can also show blood clots in leg veins that could break loose and block blood flow to the lungs. A handheld transducer passed lightly over the skin analyses the echo from sound waves and the way their pitch is affected as they bounce off moving solid objects such as blood cells. An image of the flow of blood through the vessels can be obtained by processing the resulting signal.

Cowe's research has involved investigating new ways of identifying embolic signals using the radio-frequency signal from a TCD system instead of the demodulated Doppler signal. Using a technique derived from military radar systems, she has been able to increase the bandwidth of the transmitted pulse and improve the axial resolution of Doppler ultrasound.

In a conventional system, bandwidth can be increased by decreasing the transmitted pulse length. To achieve an adequate signal to noise ratio, however, pressure amplitude over the duration of the pulse has to be increased proportionally. This could have an adverse effect on system
safety, in particular on the mechanical index (MI), which measures the likelihood and severity of non-thermal biological effects.

Instead, Cowe has used a technique known as coded excitation, paired with matched filtering and pulse compression, to improve axial resolution. This provides a resolution equivalent to using a non-coded, shorter pulse at the same time as improving signal to noise ratio and without affecting safety.

"Radar systems tend to use coded excitation slightly differently, in that they will increase the length of the transmitted pulse to transmit more energy and then use pulse compression to obtain the resolution they could achieve using a shorter, non-coded pulse," she explains.

Cowe says many people have considered the use of coded-excitation in medical imaging ultrasound systems, but its use in Doppler systems is still very much in its infancy. "One benefit is the improved vessel localisation, resulting from the ability to use smaller range gate sizes. Using the pulse-compressed RF signal also provides improved visualisation of emboli depths and paths, thus enhancing their detectability."

Heat therapy

Patients at risk of a stroke aren't the only ones set to benefit from technology developed in the defence and security industry. The latest clinical trials in the United States of a breast cancer treatment that takes advantage of heat therapy derived from radar research suggest it can significantly increase the effectiveness of chemotherapy.

Large tumours treated with a combination of chemotherapy and a focused microwave heat treatment shrank nearly 50 per cent more than tumours treated with chemotherapy alone.

Dr Alan J Fenn of MIT's Lincoln Laboratory adapted the thermotherapy treatment from a system that used focused microwaves to detect missiles and block out interfering enemy signals. "It's a very simple idea that can be applied to the treatment of many different cancers, including breast cancer," Fenn said.

Patients received two microwave heat treatments along with four rounds of chemotherapy before surgery. The goal was to shrink tumours sufficiently to enable a breast-conserving lumpectomy procedure instead of a more invasive mastectomy. Microwaves delivered by two applicators placed near the breast kill the cancerous tissue while preserving normal breast tissue by targeting tumour cells that contain high amounts of both water and ions. When the microwave energy passes through the tumour, the water molecules begin to vibrate and generate heat through friction. This process eventually elevates the cancer cells to at least 42°C in most cases, killing them.

"It appears that heating the tumours drastically increased the effectiveness of the chemotherapy," said Dr William Dooley, director of surgical oncology at the University of Oklahoma and the principal investigator of the study. "The tumours shrank faster and died faster using the additional microwave hyperthermia on top of the chemotherapy."

Identifying tissues

Back in the UK, scientists at the University of Manchester are working on an X-ray technique that as well as detecting hidden explosives and drugs could help treat cancers more effectively. Known as tomographic energy dispersive diffraction imaging, or TEDDI, it harnesses all the wavelengths present in an X-ray beam to create probing 3D pictures.

Unlike existing methods, TEDDI allows detailed images to be created with one very simple scanning motion, reducing the time required from hours to a few minutes and eliminating the problem of radiation damage, which makes it difficult to distinguish between normal and abnormal tissue types.

"Current imaging systems such as spiral CAT scanners don't use all the information contained in the X-ray beam," explained Professor Robert Cernik of Manchester's School of Materials. "We use all the wavelengths present to give a colour X-ray image. This extra information can be used to fingerprint the material present at each point in a 3D image."

"The TEDDI method is highly applicable to biomaterials, with the possibility of specific tissue identification in humans or identifying explosives, cocaine or heroin in freight. It could also be used in aerospace engineering, to establish whether the alloys in a weld have too much strain."

To develop the technology, Professor Cernik and his team have had to overcome two major technological challenges. The first was to produce pixellated spectroscopy-grade energy-sensitive detectors. This was carried out in collaboration with Rutherford Appleton Laboratory, Oxford and Daresbury Laboratory, Cheshire.

The second challenge was to build a device known as a 2D collimator, which filters and directs streams of scattered X-rays. The collimator device needed to have a high aspect ratio of 6000:1, meaning that it its length-to-width ratio is greater than that of the Channel Tunnel. This device was built using a laser drilling method in collaboration with the University of Cambridge.

Of course, what determines whether new techniques end up being used in hospitals is their affordability. Cowe believes that improving detection of emboli and vascular disease using ultrasound will not only reduce stroke death and disability rates but could also generate big savings in the £2.3bn the NHS currently spends on treatment every year.

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