Additive manufacturing of high density tungsten collimators

Objectives The construction of complex tungsten collimators with a high number of pinholes or loftholes becomes very expensive with the current available techniques (drilling, milling or electric discharge machining). All these techniques are subtractive techniques; the process starts with a solid plate and the holes are made by removing material. The main factor determining the cost is the production time and the amount of required material. The production time is related to the amount of holes and the complexity of the holes. Other materials like gold or platinum that are easier to machine are very expensive, alternative techniques like cold casting with tungsten epoxy result in a lower density (close to lead, 11g/cm^3). This results in transmission of high energy peaks in the emission spectrum through the plate and to a relative high amount of pinhole edge penetration. We have used a novel technique for collimator construction, called metal additive manufacturing. The collimator is build up in layers using selective laser melting of tungsten powders.

Methods A collimator design with 20 loftholes with 0.5 mm pinhole opening was designed in CAD software and the resulting STL file was sent to the rapid prototyping company. Within 2 weeks the collimator (16 mm thick and 48x48 mm transverse size) was produced. The density of the collimator was determined by determining the volume (in the CAD program) and measuring the weight. The collimator was attached to a compact high resolution SPECT detector (SPECTatress) and a point source has been measured.

Results The predicted accuracy of this rapid prototyping technique is +/- 50 micron. The density of the material is 16.9g/cm^3. A point source located 2cm in front of the collimator is seen by all apertures.

Conclusions Metal additive manufacturing allows fast prototype collimator design of very complex shapes. The density is close to tungsten (87%). The cost of this technique is only related to the amount of material needed.

Research Support Roel Van Holen is supported by a postdoctoral fellowship of the Research Foundation Flanders (FWO)