X-Ray optics for minimal radiation dose
When Wilhelm Röntgen discovered and investigated the X-ray in the late 19th century, he was one of the few pioneers in the field who routinely used protective lead shields. He may not have known the precise reasons why, but he suspected that this kind of radiation was not good for human health – and his suspicions were well founded. Despite this, X-rays help to restore health as they are one of the most powerful instruments in medical diagnostics and, in many cases, are crucial for identifying the right course of treatment. When it comes to achieving the best imaging with the lowest possible X-ray dose, lenses made by Italian company Optec are almost inevitably involved. Their aperture, focus, filters and zoom are moved by FAULHABER motors.
X-ray optics is somewhat different to the optics of visible light, the first difference being that X-rays are generated by an X-ray tube. This uses high voltage to make electrons collide with a metal target, which in turn releases high-energy, low-wavelength radiation. X-rays can penetrate most materials, but the denser the material, the more they are attenuated. This difference in penetration intensity is what we see on an X-ray image. Before the image can appear to the eye, the invisible X-rays have to be “converted” into the visible spectrum. Today, this is mostly done by flat panel detectors which produce digital images, similar to the optical sensors of common digital cameras.
A bulky past
Standard optical lenses, however, cannot be used to direct rays to the detector. As the refraction index of X-rays is very close to 1, optical glass barely changes their direction. This is why X-ray lenses were traditionally very bulky and difficult to handle, remembers Giuseppe Cilia, General Manager of Optec: “When our company was founded in 1985, two big lenses were necessary to transfer the X-ray image to the camera. They had a very long focal length, and the focus had to be adapted manually. They needed a clumsy lead aperture and a high level of radiation energy to produce acceptable pictures. And the patient, of course, was exposed to this radiation.” That year, Optec devised an optic relay, doubling radiation transmission through the lens and improving image quality while significantly reducing radiation levels at the same time. Now only one lens was needed instead of two.
This breakthrough formed the basis for Optec’s success in X-ray optics. About 70% of the lenses now used in medical radiography are manufactured by the company based in Parabiago, in the north-west of Milan’s metropolitan area. Today, these lenses are even smaller and optically more powerful. “The latest digital image processing techniques have markedly improved the perception of anatomical details, but, at the same time, impose new requirements on the performance of the X-ray imaging system as a whole,” says Giuseppe Cilia. “The dynamic range of the lens plays a crucial role here.”
In photography, the dynamic range describes the limits on the amount of luminance that can be captured, or, one could say, the spectrum between over- and underexposure. The greater this range, the more details doctors are able to see on X-ray images. The technology developed by Optec has allowed the dynamic range of the lenses to be extended tenfold, from 300:1 to 3000:1. Thanks to this optical flexibility, Optec’s compact lenses can be used for both high and low sensitivity capture. Fluoroscopy is an example of the latter and is used for real-time imaging during surgical interventions, e.g. during critical operations close to the spinal cord or on the heart. Since exposure to X-rays can last for many seconds or even some minutes here, the radiation dose must reduced to the absolute minimum.
“You can open the aperture of our lenses very wide to get a clear picture of the procedure,” explains Giuseppe Cilia. “When you need a static picture, for example of a knee joint exposition, this takes just a few milliseconds. However, the radiation dose can and must be higher to obtain a more detailed image. By combining aperture, focus and filters, we get a very high transmission and therefore expose patients to as little radiation as is technically possible. The resolution is also very high, close to the diffraction limit. With the FAULHABER motors moving the parts no manual handling or adjustment is necessary.”
Power for medicine and space
One of the biggest advantages of the lenses is their compact size which requires the motors to be very small, too. Optec uses 0816 SR Series DC-Micromotors for its X-ray lenses, featuring precious metal commutation, a diameter of just 8 mm and a length of 15.9 mm. A planetary gearhead from the 08/1 Series, also only 8 mm in diameter, transmits power to the optical mechanics. This combination delivers the power, speed and accuracy required for Optec’s X-ray optics. “We produce lenses of the highest quality for the most sophisticated applications. FAULHABER motors are the best fit for our products because they have the same levels of quality and engineering,” says the General Manager.
Apart from the motors, Optec purchases very little other than glass from external suppliers. From polishing and coating the optical lenses to assembling the units, all manufacturing steps are performed in Parabiago. There is no mass fabrication: depending on a client’s needs, Optec can even develop and produce one-off pieces. The product range includes short-wave infrared lenses, micro-lenses for endoscopes and lenses for optical applications in space. Giuseppe Cilia is especially proud that Optec will be the first company to supply a zoom lens to be mounted on a satellite. “The lens and the zoom’s motor need to work reliably for many years under the extreme conditions in space. At the same time, every gram counts and the equipment needs to be as light as possible. Our product was the one to meet all requirements.” As with all Optec lenses, the first zoom lens in space will also be powered by a FAULHABER motor.