3D printing in X-ray and gamma-ray imaging: A novel method for fabricating high-density imaging apertures

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Abstract

Advances in 3D rapid-prototyping printers, 3D modeling software, and casting techniques allow for cost-effective fabrication of custom components in gamma-ray and X-ray imaging systems. Applications extend to new fabrication methods for custom collimators, pinholes, calibration and resolution phantoms, mounting and shielding components, and imaging apertures. Details of the fabrication process for these components, specifically the 3D printing process, cold casting with a tungsten epoxy, and lost-wax casting in platinum are presented.

Introduction

In single-photon emission computed tomography (SPECT), a method for in vivo molecular imaging of laboratory animals and humans, radiolabeled molecules or a radiolabeled pharmaceutical are administered to the subject and migrate towards target receptors to form a distribution of radiotracer. Planar images of this distribution can be achieved using pinhole collimation and position-sensitive gamma-ray detectors. A pinhole imaging aperture typically consists of pinhole inserts made from platinum, gold, or tungsten and a supporting structure made from a high-density material, such as lead or tungsten, which provides shielding and reduces scattered gamma-ray photons. For 3D tomography, images from multiple views are acquired either by rotating the object/detector or by surrounding the object with multiple detectors and acquiring projection images simultaneously, as is the case in stationary SPECT [1], [2], [3], [4], [5]. In comparing these two acquisition methods, stationary SPECT has several key advantages. Radio-pharmaceuticals typically have a short half-life, e.g., 6 h for 99mTc. Because the projection images are acquired simultaneously, the administered dose to the subject can be lowered. The resolution of the tomographic reconstruction can be increased by changing the geometric parameters of the system, such as pinhole diameter. Another benefit of a stationary SPECT system is the capability for 4D imaging studies where the dynamic uptake of radio-pharmaceutical is monitored [6]. Additionally, motion artifacts and blur in the reconstruction, such as those due to respiration or heart motion, can be suppressed using gating.

The design and fabrication of imaging apertures used for stationary SPECT becomes expensive and difficult using traditional machining techniques when the imaging system has complex camera geometries. At the Center for Gamma-Ray Imaging (CGRI) recent designs of stationary SPECT systems having tens of detectors [1] and adaptive SPECT imaging systems where the pinhole/detector geometry dynamically changes [8] have prompted the need for a cost-effective fabrication method wherein complex imaging apertures can be produced for specific and adaptive imaging tasks. This has led to the development of a novel fabrication method where imaging apertures are readily produced using 3D rapid-prototyping printing technology, cold casting in high-density tungsten composites, and investment (lost-wax) casting in high-density materials such as platinum.

In this article we describe the process for fabricating pinhole imaging apertures in the context of an aperture developed for FastSPECT III, a stationary SPECT imager dedicated to rodent neurological studies [1]. Additionally, we have found 3D rapid-prototyping printing to be beneficial for other aspects of gamma-ray, SPECT, and X-ray CT imaging, and we present a few applications that may be beneficial to other imaging laboratories. These include printing resolution phantoms, calibration phantoms, and animal-imaging holders/beds.

Section snippets

3D printing

Advances in 3D printing technology have led to commercially available rapid-prototyping (RP) printers that produce highly detailed 3D parts. At CGRI, we utilize a 3D printer (Objet Geometries Ltd. [7], Connex350TM) that builds parts using a polymer-jetting technology, an additive manufacturing process where print heads (analogous to those used in inkjet printers) deposit thin layers of photopolymer that are then cured to build a part slice by slice. Printing involves first designing a virtual

Cold-casting tungsten composites

We have found that printing molds for cold-casting tungsten composite parts to be an effective, low-cost approach to aperture fabrication. The mold assemblies produced for the FastSPECT III imaging aperture and associated cast pieces are shown in Fig. 8. Multiple part copies were produced with the assembly to demonstrate the mold reusability. Lessons learned during this casting investigation include the importance of incorporating supporting structures to the assembly, e.g., outer rings or ribs

Additional applications

In addition to new aperture fabrication methods, 3D rapid prototyping printers can be used to print custom small-animal imaging beds with built-in anesthesia and respiration channels, Jaszczak-style SPECT and CT phantoms, and geometric calibration phantoms for X-ray CT, for example. Images of examples of these parts are shown in Fig. 16.

Discussion and summary

The rapid turn-around time from conception and design to part in hand makes 3D printing an attractive tool for system design, fabrication, and evaluation in SPECT and X-ray imaging applications.

We have found that, by using 3D rapid prototyping in conjunction with lost-wax and cold-casting techniques, we can fabricate complex aperture designs, cast pinholes for high-resolution SPECT imaging, and produce high-density shielding components for gamma-ray and X-ray applications. Additional benefits

Acknowledgments

The Center for Gamma-Ray Imaging is supported by NIBIB Grant P41-EB002035. The Objet Geometries Ltd., Connex350TM printer was funded in part by The University of Arizona, Technology Research Initiative Fund (TRIF). We would like to thank Dr. Michael Gehm and Wei Ren Ng at the Laboratory for Engineering Non-Traditional Sensors (LENS), University of Arizona, for assistance with the Objet Eden350TM rapid prototyping printer in aperture/pinhole development and fabrication.

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This work was supported by the National Institutes of Health under NIBIB Grant P41-EB002035 (Center for Gamma-Ray Imaging) and The University of Arizona, Technology Research Initiative Fund (TRIF).

1

http://www.gamma.radiology.arizona.edu

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