Scientists create new form of medical imaging

Purdue University scientists developing a new imaging technology have created the world’s first ‘visual fly-throughs’ of a living tumour.

The technique, which uses lasers, holograms and detectors, offers promise for a new kind of medical imaging based on light instead of tissue-damaging X-rays, said David Nolte, a professor of physics at Purdue.

His research team recently used the new technique, called optical coherence imaging, to take a video of the insides of a cancerous rat tumour.

‘This is the first time that anybody has ever done a holographic fly-through of a living tumour,’ Nolte said.

The tumour was not viewed while inside a rat, but was cultured and kept alive in a nutrient medium.

Critical to optical coherence imaging is a semiconductor holographic film developed by the team. Many other imaging technologies require that specimens, such as tumours, be specially prepared and cut into pieces for examination, killing the tissue.

The new imaging technique is made possible by lasers and special ‘dynamic holographic films,’ which have been developed in Nolte’s lab. When two crossing laser beams are shined onto the film, holographic images are created.

‘These holograms adjust to the changing light conditions and the changing information that is carried on the laser beams,’ said Nolte. ‘All of that coherent information is stored from the light so that it looks 3-D. It looks as if it were coming from the original object.’

The film is combined with a series of lenses and mirrors, acting as a filter that rejects ordinary, ‘scattered light’ and lets through only the ‘coherent’ laser light needed to produce the images.

‘I can take a flashlight in the dark, put it up against my hand, and my whole hand will glow red,’ Nolte said. ‘But I can’t see any bones at all, even though the bones are just below the skin.’

The reason is because most of the light is said to be scattered, meaning it is a jumble of numerous separate light paths that do not move straight through the hand.

‘The emerging light is like ripples produced by a whole handful of rocks thrown at the same time into a pond,’ Nolte said. ‘The ripples run into each other, producing a chaotic mix of irregularly spaced waves.’

However, by using lasers, it is possible to find light paths that do go straight through an object.

Lasers shine ‘coherent light’ that could be likened to the ripples created by only a single rock thrown into a pond. The ripples, or light waves from a laser, are spaced regularly apart and move in unison.

But most light detectors, including the human eye and conventional video cameras, do not detect coherent light; a limitation that can be overcome with the holographic film as it is said to be sensitive to coherent light.

The holographic film in this case is made of alternating layers of gallium arsenide and aluminium gallium arsenide. These materials, semiconductors similar to those used to make lasers for CD players, form a 200-layer film. Each layer is 8 nanometers thick. Forcing electrons to move through layers so thin reportedly boosts the optical properties of the film and makes it more efficient at detecting coherent light.

The Purdue team used the technique to take video images inside a tiny tumour, a form of cancer called osteogenic sarcoma, which afflicts bones and connective tissues.

Optical coherence imaging offers numerous possible applications, including diagnostic imaging for medicine and industry. The method might allow scientists to study how live tumours behave in real time, even how they react to experimental drugs.