In the French research team’s device, a cluster of scintillators is used for indirect X-ray detection. The scintillators (green) are loaded in the end of a conical optical horn antenna; when excited by X-rays (blue), they emit photons that are channeled into a fiber endoscope using an optical horn antenna. [Image: Miguel Angel Suarez, FEMTO-ST Institute]
A team of French researchers, using a novel combination of scintillation detectors and an optical horn antenna, has developed a nanoscale X-ray sensor that can be integrated into the end of medical endoscope (Opt. Lett., doi: 10.1364/OL.42.001361). The researchers believe that the new device, by allowing more refined, potentially nanoscale detection of X-ray delivery and dose, could “open up new avenues” in X-ray imaging and therapeutics.
A question of dose
Conventional radiotherapy for cancer is a comparatively crude affair, often delivering large doses of radiation to relatively small tumors or affected areas. The capability to measure and control X-ray dose in real time at the nanoscale through a fiber endoscope, and thus deliver highly targeted X-ray therapy, could make the therapy much more useful and flexible, delivering it where it is needed most—directly to the tumor, rather than to surrounding tissues.
Getting X-ray detection to work at the business end of a fiber endoscope, however, is a tricky proposition. The most common form of X-ray detection, indirect detection, uses scintillator or phosphor materials that absorb X-rays and give off longer-wavelength light, which in turn is picked up by an optical detector. But this scheme—though widely used in a variety of scientific, medical and industrial applications—doesn’t scale down well to the dimensions of a fiber endoscope.
In particular, scintillators tend to give off omnidirectional signals, rather than a directional signal that could couple well with the end of an optical fiber. As a result, even given the increasing sensitivity of modern optical sensors, it’s hard to get an indirect setup that will stuff enough photons from the scintillator into the fiber for a reliable dosimetry reading back in the lab.
Horn antenna
To address the problem, scientists from the Université de Bourgogne Franche-Comté and the Université de Aix Marseille, France, led by Thierry Grosjean, looked at amplifying the scintillator signal with a tiny optical antenna. And, while much of the action in optical antennas recently has revolved around plasmonic and other metasurfaces with specialized nanostructures (see OPN, June 2015 and January 2017), the French team explored a different optical-antenna concept borrowed from microwave electronics: the horn antenna.
A horn antenna is simply a flared waveguide that amplifies a dipolar signal and directs it into a linear waveguide, such as a coaxial cable. Calculations by Thierry and colleagues suggested that, by attaching a tiny horn antenna built at optical length scales to the end of a fiber, they could channel the otherwise omnidirectional cascade of photons from a scintillator in the same way.
To test the concept, the team fashioned a 38-µm-long conical polymer mircotip, with a radius of 1 µm at its apex, flaring to a diameter of 12 µm at its end, and coupled its wider end to a 125-µm-diameter single-mode fiber. They then loaded the narrow apex of the cone with a cluster of scintillator material, and coated the fiber-integrated cone with nanometer-thick layers of titanium and aluminum. The aluminum coating provided strong reflectivity at visible wavelengths and strong transparency for X-rays.
Ultracompact detectors
The combination of geometry and materials means that an X-ray signal will penetrate the end of the cone, causing the scintillation cluster to emit photons at a rate proportional to the X-ray dose. Those photons, in turn, are reflected and directed into the fiber by the horn antenna. In tests by the team, the sensor had a spatial resolution of 1 micron (the length scale of the antenna tip), which the researchers are working to hammer down to 100 nm.
The team believes that the new setup could constitute “a key step towards the widespread use of ultracompact X-ray detectors in a wide range of scientific, medical, and industrial domains.” This includes not just real-time imaging and dosimetry for targeted cancer radiation therapy, but other uses such as low-energy X-ray scanning microscopy and sensing at the nanoscale for industrial and materials applications.