Proton therapy is similar to conventional radiotherapy but, in principle, has a finite range that spares surrounding healthy tissue. To make the most of that finite range, however, requires high confidence in the position of the dose peak. Proton imaging is thought to be the best way of mapping the insides of a patient in order to locate that dose peak, but it suffers from poor spatial resolution as the protons scatter off various atomic nuclei en route to the detector.

Algorithms already exist to enhance the resolution of proton imaging, but they can only "learn" from a large set of radiographic images taken beforehand. "To acquire all these projections is a long and arduous process that delivers extra dose to the patient," explains Charles-Antoine Collins-Fekete of Laval University in Québec, Canada. "The innovation of this work is that we maximize the spatial resolution for each projection completely independently."

Collins-Fekete and colleagues, who are based at Laval and other institutions in the US and Belgium, developed their tool by exploiting the fact that protons are more likely to scatter through some tissues than others. Once a preliminary map of tissue has been delivered by proton imaging, therefore, it is possible to estimate statistically the likely route that the protons must have taken in order to produce it. Then, it is possible to retrieve the image that would have been acquired, had the scattering not taken place. "The idea of using a path estimate to optimize proton radiography independently is a new one," says Collins-Fekete.

In computer simulations, Collins-Fekete and colleagues tested their statistical tool – which they call a "maximum likelihood least squares estimator" – on three different tissue phantoms. Adopting a widely used metric for the quantification of spatial resolution in imagery, known as MTF10%, the researchers found that they could improve the resolution of a bare proton image with their tool by 65%. The resolution boost was the same as that delivered by existing methods, even though it required no prior radiographic imagery.

"The real advantage is that you can acquire a single [radiographic image] and improve it independently, which was impossible earlier," says Collins-Fekete. "That means proton radiography can directly be used for many applications, such as accurate positioning of the patient before the treatment and monitoring the patient's potential physiology change."

Although the tests currently reported were computer simulations, the researchers already have preliminary results from real detectors. "The first results follow closely our prediction," says Collins-Fekete. "This makes us quite confident that the technique could be brought to a clinical environment without too much delay."

Related articles in PMB
A maximum likelihood method for high resolution proton radiography/proton CT
Charles-Antoine Collins-Fekete et al Phys. Med. Biol. 61 8232
Monte Carlo comparison of x-ray and proton CT for range calculations of proton therapy beams
N Arbor et al Phys. Med. Biol. 60 7585
Developing a phenomenological model of the proton trajectory within a heterogeneous medium required for proton imaging
Charles-Antoine Collins Fekete et al Phys. Med. Biol. 60 5071

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