Scientists have developed an X-ray imaging method that could drastically improve the contrast of computed tomography (CT) scans whilst reducing the radiation dose deposited during the scan. The new method is based on the combination of the high contrast obtained by an X-ray technique known as grating interferometry with the three-dimensional capabilities of CT. It is also compatible with clinical CT apparatus, where an X-ray source and detector rotate continuously around the patient during the scan. The results are published in Proceedings of the National Academy of Sciences (PNAS) dated 4-8 June 2012.
The main author of the paper is Irene Zanette from the European Synchrotron Radiation Facility ESRF (Grenoble, FR) and Technical University of Munich TUM (DE), and the team also comprises scientists from the Paul Scherrer Institute PSI (Villigen, CH), the Karlsruhe Institute of Technology KIT (DE), and Synchrotron SOLEIL (Gif-sur-Yvette, FR).
The conventional way of producing X-ray images is to shine an X-ray beam on the investigated object and measure the transmitted intensity behind it. This is the method that W.C. R-ntgen developed in 1895, just after he discovered X-rays. To the present day, it is commonly used, for example, in hospitals and for security screening at airports. However, since this technique relies on variations in how the different constituents of an object absorb X-rays, it also has severe limitations notably in medical X-raying where cancerous and healthy soft tissue often do not show enough contrast to be clearly distinguished.
In the past years, a lot of effort has therefore been put into the development of new X-ray imaging techniques that do not rely solely on absorption but increase the contrast through the observation of other types of interaction between X-rays and matter.
Of these new methods, a very promising one is the so-called "X-ray grating interferometry", in which microstructures, gratings developed at PSI and KIT, serve as optical elements for X-rays. The setup for this contrast-enhancing technique is simple and compact, and it can be combined with computed tomography (CT) X-ray scanners to yield virtual slice images and full 3D information of an object. Over the past decade, grating interferometry has been constantly improved, with a focus on medical applications.
The team of scientists has now made an important step towards clinical implementation of this technique - a new measurement protocol called "sliding window" technique. "We wanted to shorten the gap between the potential offered by this extremely powerful technique and its application in the biomedical field. Our sliding window method reduces the dose and acquisition time and makes grating interferometry compatible with the continuous rotation of the gantry used in clinical CT", says Timm Weitkamp from Synchrotron SOLEIL.
Grating interferometry uses, in addition to information on absorption, measurements of X-ray phase changes to produce "differential phase contrast" images. Density differences of only 0.5 mg/cm3 can be discerned using grating-based phase contrast.