Eliminating computed tomography imaging artifacts through 3D printed radiotherapy head supports

Abstract

Purpose: The geometry of an immobilization device such as a headrest can cause undesired computed tomography (CT) artifacts that may affect both volume definition and dosimetry in radiotherapy of the brain. The purpose of this work was to reduce CT artifacts caused by a standard hard plastic hollow radiotherapy headrest. This was to be achieved through design and prototyping of a custom-made head support.

Methods: A series of CT scans were acquired of both a water phantom and an anthropomorphic head phantom which were resting on custom-made three-dimensional (3D) printed supports. All custom-made supports were made of polylactic acid (PLA) plastic filament and printed by fused deposition modeling (FDM) 3D printing technology. Initial designs were studied with a water phantom using a simplified support with straight and curved shapes both at the edges and as infill patterns. Imaging of a 3D printed clinical prototype was then compared to our standard headrest using an anthropomorphic head phantom.

Results: The presence of dark streaks inside both phantoms was seen on the CT images for headrests involving supports with straight shapes at the edges or as infill patterns. Such artifacts were ascribed to the exponential edge-gradient effect (EEGE). No such artifact was observed when the support was designed with a combination of curved edges and infill patterns.

Conclusion: When developing immobilization accessories for use in CT scanners, more attention could be paid to artifact attenuating design elements. This work illustrates the usefulness of 3D printing in prototyping radiotherapy accessories and solving concrete clinical problems.

Keywords: 3D printing; CT artifact; Head CT scan; Immobilization.

Fig. 1

Simplified support models and infill patterns for three‐dimensional (3D) printing. (a) The support with straight edges (left) and curved edges (right) are both sculpted from a cube of dimensions (40 × 40 × 40) mm³. (b) Infill patterns, such as triangles (left) and gyroid (right), are required for 3D printing.

Fig. 2

Technical drawing of the artifact‐free headrest. The isometric, front, side, and top views show the headrest model of maximum dimensions (280 × 175 × 88) mm³.

Fig. 3

Phantoms with their supports. (a) The water phantom rests on four variations of the simplified support. The head phantom lies on (b) the clinical headrest and (c) the custom‐made headrest.

Fig. 4

Computed tomography images of the water phantom on the four variations of a simplified support. The axial and sagittal views are centered on the support, while the location of the coronal view is indicated by the dashed line. The window center is 50 HU and the window width is 220 HU. The four variants of the support consist of the combination of the shape of side walls and infill patterns, such as (a) straight and triangles, (b) straight and gyroid, (c) curved and triangles, and (d) curved and gyroid.

Fig. 5

Computed tomography images of the head phantom on two headrests models. The top row shows the data obtained with the clinical headrest and the bottom row with the custom‐made one. The window center is 50 HU and the window width is 220 HU. Computed tomography volumes are registered on an acquisition without headrest, also used to generate the difference maps with a greyscale bar expressed in HU. The arrows point to the artifacts introduced by the presence of the headrest, visible on the axial and coronal views.