How PECM Could Help Manufacture X-Ray Machines
Learn how pulsed electrochemical machining (PECM) works here.
Note: be aware of the distinction between ‘cathode/anode’ in both radiology and PECM.
Currently, the machinability of refractory metals via PECM is extremely limited, and is a subject of active research by Voxel Innovations. Machinability of the components mentioned in this article is purely hypothetical. Contact us for more details on our current capabilities.
Cover photo credit: LAP Laser
Radiographic and radiotherapeutic technology continues to play a vital role for both medical and non-medical applications. The growing prevalence of chronic disorders and geriatric patients in both developed and developing countries, as well as the intersection of medical technology with digital software, are all propelling new demand for medical radiography and radiotherapy. Non-medical applications of radiography are also steadily growing—for example, advanced airport security scanners and gamma-ray imaging of cargo containers.
Although radiology is a broad term encompassing a variety of technologies (X-rays, CT scanners, mammograms), some general principles remain. At its core, radiographic equipment utilizes a form of radiation to non-intrusively scan the inside of an object, while radiotherapy purposely directs radioactive particles on the patient. Temperature in the thousands of degrees is necessary to both generate and, ultimately, guide these particles to their destination. Refractory metals such as Molybdenum, Rhenium and Tungsten have superior resistance to heat and wear, making them ideal for some components in these devices.
While the very nature of radiology requires these unique materials for radiation shielding, particle guidance, and high-temperature resistivity, material choice is further limited when considering the radiological application itself. For example, an element of higher atomic number produces improved imaging resolution of thicker subjects (such as analyzing a broken femur compared to breast tissue for a mammogram). Extreme limitations of material choice force radiographic equipment manufacturers to utilize materials that many engineers/machinists have little to no experience with. Ultimately, the unique geometries and material composition of these components presents an extremely difficult challenge for manufacturers to machine them in a cost-effective way.
Non-conventional machining methods should be considered by radiological equipment manufacturers to improve production economics on key refractory components. In the future, Pulsed Electrochemical Machining (PECM) may enable improved designs and higher-volume production for refractory parts such as x-ray anode targets, collimators, and rotating anode holders as PECM capability is further developed.
This article will both explain the manufacturing challenges associated with key radiological components and how PECM could potentially improve manufacturing economics here.
Let’s review the roles and importance of some specific components and exactly why their materials and geometries present manufacturability challenges.
Many radiotherapeutic devices utilize a multileaf collimator (MLC) to shape a radiation particle beam to the specific geometry of the cancerous area. This is ideal to minimize unnecessary radiation on healthy tissue. MLCs are a grouping of flat tungsten ‘leaves’ that move horizontally to achieve the desired geometry. MLCs generally contain between 40 and 120 leaves, and each leaf can take around 24 hours to machine, usually via wire EDM. As PECM technology develops, production time could be reduced by utilizing a single PECM cathode to machine multiple leaves simultaneously, as PECM can achieve <10 μm (0.0004 in.) repeatability under the right circumstances.
While leaf geometries differ between manufacturers (rounded versus flat edges, for example), the minimization of between-leaf radiation leakage is always paramount for manufacturers. The general geometry of the part plays a significant role in leakage reduction but surface quality, notably between leaves, is equally vital. Voxel is currently capable of surface finishing of .005-.4 μm Ra of certain materials, and is actively developing technology to apply this superior surface quality to components made of refractory metals.
For imaging technology, collimators direct radiation emitted by the subject onto a detector via small holes. Many are composed of lead, however lead does not always produce adequate results; most lead collimators let less than 1% of photons through, sometimes resulting in inadequate image resolution. Some manufacturers are additively manufacturing tungsten alloys in place of lead, thereby improving imaging capabilities while others are using advanced casting techniques. As PECM technology develops, it could potentially machine collimator holes by itself, or as a postprocessing method for additively manufactured collimators by adding high-resolution features and reducing minimum wall thickness, improving the part’s capabilities.
For x-rays, the anode target is a beveled disc composed of a high temperature material such as tungsten, tungsten-rhenium alloy, or molybdenum. Extreme heat is created when electrons interact with the material of the anode target. Most anode targets in radiological equipment rotate to dissipate heat. As previously mentioned, the material utilized in this component is one of the primary variables that determines the intensity and resolution of the x-ray.. Both the geometry and surface quality of this component are correlated with its lifetime, as constant heating and cooling subjects the anode disk to crazing. For conventional x-rays, this disc has a 7mm thickness and 50-100mm diameter. A beveled disc composed of an exotic material with a central hole is a fairly complex geometry that may be well-suited to PECM in the future.
Mo Anode Holder
The previously mentioned anode disk must be mounted on a rotating refractory metal stem, in order to reduce heat flow backwards that could adversely affect the bearings (usually copper) rotating the anode upwards of 10,000 rpm. While its geometry is not necessarily complex, radiological equipment manufacturers must machine somewhat high volumes of these parts, as this is an essential component of x-rays, CT scanners, and other radiological equipment, therefore the anode holder may be a good application for PECM.
Miniaturization in Radiology
Radiological equipment manufacturers are not excluded from the growing pressure on manufacturers to produce lighter and smaller parts while simultaneously lowering operating costs. The advent of handheld, portable equipment has allowed new applications of radiological technology—for example, by reducing inspection times at airports, ‘non-destructive’ material testing of wind turbines, and improving accessibility of medical radiology in developing countries. Although the principals of x-ray imaging have not changed, the size of components and availability of advanced materials has changed, introducing new challenges for manufacturers to miniaturize parts to meet demand. As part sizes decrease, manufacturers must be acutely aware of processes that may alter the geometry of the final product. The intensity of tool vibration may affect machining accuracy, and heat-affected zones may alter the geometries of areas sensitive to thermal distortion (such as thin walls). PECM is capable of thin-walled features down to 10-100 micrometers in thickness.
Voxel Innovations actively develops pulsed electrochemical machining (PECM), a non-thermal, non-contact material removal process capable of small features, superfinished surfaces, and high repeatability for metallic parts. Our latest research is focused on developing techniques for processing refractory metals so that the benefits and value of PECM can be expanded to other industries such as radiology and radiotherapy.
Learn more about our process and its applicability for aerospace and medical device manufacturers.
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