Feature | Medical 3-D Printing | April 29, 2019 | By Steve Jeffery

The Development of Tungsten Collimators May Advance Medical Imaging

3-D print allows a new method to shape the extremely hard material

A 3-D printed tungsten X-ray system collimator. 3D printed, additive manufacturing for medical imaging.

A 3-D printed tungsten X-ray system collimator. The tungsten alloy powder is printed into the form desired and is laser fused so it can be machined and finished. Previously, making collimators from Tungsten was labor intensive because it required working with sheets of the metal to create the collimator matrix. 

Examples of complex 3-D tungsten shapes formed by additive manufacturing techniques.

Examples of complex 3-D tungsten shapes formed by additive manufacturing techniques.

In molecular radiotherapy (MRT) treatment of the thyroid, existing single photon emission computed tomography (SPECT) imaging systems are unable to accurately measure radiation absorbed by patients during and after treatment. As a consequence, only limited information regarding the success of radiotherapy treatment has been available. 

For the last six years, teams from the University of Liverpool’s Department of Physics and The Royal Marsden and Royal Liverpool University Hospitals have been working to develop an imaging system (known as DEPICT) that would better measure the absorbed radiation. The aim is to provide a more accurate treatment and diagnosis of patients. 

Central to any imaging scanner is a collimator. This device is used to narrow and align the beam of waves or particles so that they move toward a particular direction. Only rays that are traveling nearly parallel to the holes within the device will pass through them — any others will be absorbed by hitting its surface or the side of a hole. This ensures that rays are recorded in their proper place on the plate, producing a clear image.

Without a collimator, X-ray images will appear very blurred, making it non-diagnostic.

A critical part of the new imaging system is the use of a direct-digital conversion, cadmium-zinc-telluride (CZT) detector. This part is used in conjunction with a parallel hole collimator with an active area comprising an array of 0.6 mm holes. Traditionally made of lead, collimators on the market were not up to the task of supporting the new DEPICT system, because it required a very intricate design that was not possible to create from this relatively soft material. Researchers hit on the idea that high-density tungsten could be the ideal material if they could find a way shape it into the required collimator. 

Tungsten’s density is around 1.7 times that of lead traditionally used in collimators, but it is difficult to work with, because it has the highest boiling and melting points of any element known to man. It also means there is no other material in the world capable of holding tungsten as a molten liquid, so it cannot be cast into high precision shapes in the manner of iron, aluminum and other common metals. That is until now.

A group of experts in the U.K. recently began 3-D printing of intricate geometrical shapes from tungsten alloy powder as a new method to work with material. A team from Wolfmet, a company that makes components for automotive and aerospace, developed this highly specialized additive manufacturing metallurgy process, essentially fusing the tungsten alloy powder together. A high-powered laser is applied to fuse successive layers of pure tungsten powder until a complex component is built. Once fused, the powder is pressed into parts, sintered and then machined into the desired form.

The end material is an alloy, of 90-97 percent tungsten depending on the grade. This material retains the unique density and radiation shielding capabilities of pure tungsten however, for the first time, it can also be machined to tight tolerances.

The team at Wolfmet helped create the world’s first collimator of its kind for the DEPICT system. After undergoing recent trials, the group reported great success. With its excellent attenuation properties, a tungsten collimator significantly reduces septal penetration in comparison with the same collimator made from lead, resulting in much improved image quality.

“The final product shows great potential to enable better radiotherapy treatment monitoring,” said Samantha Colosimo, Ph.D., project manager, Optimization of Medical Accelerators (OMA) Project, University of Liverpool.

The team working on the project is hopeful that the DEPICT imaging system will be available commercially in the next three years.

The implications could be significant — the ability to individualize treatments is expected to reduce healthcare costs by improving efficacy and the efficiency.  Perhaps more important, it is hoped that this development will increase rates of successful cancer treatment, leading to improved quality of life and health for those patients.

Applications Outside of Healthcare 

The same principals of 3-D printing tungsten could mean huge advances in other sectors outside of healthcare, including the manufacture of various machinery and automotive.

Airport and cargo scanners used to examine the contents of transport containers could be upgraded in the same way using tungsten collimators. The scanners often have tungsten grids that screen out stray X-ray beams to give a more accurate image. These grids are currently built up manually from many individual tungsten sheets.

The application of the metallurgy process allows these complex parts to be manufactured as a single component. This results in a much shorter delivery time and eliminates a lot of costly hand assembly work.

The geometrical detail that can now be incorporated into the tungsten might mean that much smaller versions can be created, leading to improvements in handheld, lightweight scanners to produce incredibly accurate images.

Steve Jeffery is the business development manager for Wolfmet's 3-D printed tungsten components division. 

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