X-rays can now tell us about soft tissues

Japanese researchers have figured out a way to use X-rays to tell doctors about soft tissues like skin and internal organs — not just dense materials like bones — in a similar way to how ultrasound or magnetic resonance imaging (MRI) work, but with much greater resolution. Their study, published in the journal Applied Physics Express, should allow healthcare professionals to identify much smaller and deeper tissue problems, such as lesions, than they can with other methods.

Ultrasound and MRI are both examples of elastography — a non-invasive method of medical imaging that investigates the stiffness and elasticity of soft tissue. Ultrasound uses sound waves with frequencies higher than what humans can hear, and works by sending ‘shear waves’ through us — the sort of waves that occur when you whip a rope up and down quickly. Shear waves travel faster through stiffer tissue than through softer tissue. Since cancerous tumours, lesions from cirrhosis of the liver and hardened arteries are stiffer than the surrounding healthy tissue, identifying where the waves pass through tissue more slowly, clinicians are able to spot these stiffer tissues.

MRI works in a related fashion, but via the use of very strong magnets to force protons in the body to align with a magnetic field. How long it takes those protons to make this move tells us a similar story about stiff or hard tissues.

Now, researchers have developed a technique to do much the same with X-rays instead. The advantage of this is that X-rays can provide much greater resolution than ultrasound — on the order of tens of micrometres (millionths of a metre) instead of millimetres.

“This greater precision doesn’t just mean identification of much smaller or deeper lesions, but importantly for patients, because smaller lesions can be newer ones, potentially also much earlier on in a disease or condition,” said lead researcher Wataru Yashiro, an associate professor at Tohoku University.

Although previous studies have suggested X-ray elastography is possible in principle, this is believed to be the first time that any real-world visualisation of stiffness using the concept has been demonstrated. The next step is to further develop the technique to produce 3D visualisations, and ultimately the researchers want to manufacture X-ray elastography medical diagnostic equipment.

Image caption: Maps of stiffness (storage modulus) in uniform-concentration sample (left) and sample with harder inclusion (right) (sample: polyacrylamide gel). It can be seen that harder inclusion is clearly visible in spite that its concentration is only slightly different from the surrounding matrix. Note that such a slight difference cannot be discerned by typical X-ray radiography for medical diagnostics. Image ©Wataru Yashiro, the Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University.

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