Hybrid phantom for lung CT: Design and validation

Abstract

Background: CT lung imaging protocols need to be optimized. This claim is especially important due to the possible introduction of low-dose CT (LDCT) for lung cancer screening. Given the incorporation of non-linear reconstructions and post-processing, the use of phantoms that consider task-based evaluation is needed. This is also true for acceptance and continuous QC use.

Purpose: To present and validate a lung-CT hybrid phantom composed of two setups, one for task-based image quality metrics and the other anthropomorphic.

Methods: The task-based metrics setup was based on the well-known Mercury phantom and the anthropomorphic setup named Freddie (from Figure of Merit Performance evaluation of Detectability in Diagnostic CT Imaging Equipment) was designed with the same basic dimensions of the Mercury phantom, but including pieces and materials for mimicking chest structures, such as tracheobronchial tree and lung parenchyma. This setup allows the inclusion of pieces of different sizes to mimic ground-glass opacities, and sub-solid and solid lung nodules. The validation of the phantom adopted three methods: comparative evaluation of the attenuation properties and the corresponding Hounsfield Units (HU) values of the selected materials; image assessment according to five chest radiologists and eight non-radiologists' observations (reader study), and measurement of task-based metrics. Images of both setups were acquired using two clinical thorax protocols, both using automatic tube current modulation (TCM). Two x-ray filter combinations were adopted. The images were reconstructed using a deep learning-based algorithm.

Results: The agreement of nominal and observed HU values in the task-based setup was within 15%, except for three (TangoBlack+, VeroClear, and HIPS) of the materials employed in the phantom construction, at some beam energies. In the reader study, synthetic solid nodules printed in VeroClear received average Likert scores 4.0 (range 3.0-4.0) from radiologists and 3 (range 2.6-3.8) from non-radiologists, printed in TangoBlack+ received an average Likert score of 3.9 (range 3.8-4.2) from radiologists and 4.0 (range 3.8-4.4) from non-radiologists, while those printed in HIPS scored an average Likert of 3.8 (range 3.3-3.9) from radiologists and 3.3 (range 3.1-3.3) from non-radiologists. The synthetic ground-glass opacities (GGO) nodules manufactured in EVA received an average Likert score of 3.8 (range 2.8-4.6) from radiologists and 4.3 (range 3.6-4.8) from non-radiologists. The task-based setup demonstrated detectability index variations across protocols influenced by the dose levels, voltage, and x-ray beam filtration used.

Conclusions: The novelty of the proposed design is concentrated on the possibility of associating the response of the task-based setup (Mercury) with a patient-based setup (Freddie) in a unique phantom. This hybrid design enhances the potential to apply the detectability index for optimizing CT protocols in clinical scenarios.

Keywords: 3D printing; LDCT; anthropomorphic phantom; lung imaging.

FIGURE 1

Schematic drawings of the Freddie‐Mercury phantom. The left setup represents the Mercury phantom, considering the original geometric design. The right setup represents the Freddie option, with lung inserts, soft tissue compensation (in blue), vertebrae (in red), and lung equivalent foam (in light beige). To prevent movement of the structures, a loofah sponge was placed between the lung and soft tissue compensation (in green).

FIGURE 2

Mercury (physical) phantom in the sagittal (a) and (b) and axial planes Freddie (anthropomorphic) phantom in the sagittal (c) and axial planes (d), respectively.

FIGURE 3

Examples of segmentation of the lung tissue of interest: (a) tracheobronchial tree and (b) lung pleura.

FIGURE 4

Adjusted slice segmentations for each Freddie module, from largest to smallest.

FIGURE 5

Steps adopted on the 3D modeling of the Freddie setup of the developed phantom, from patient images to the model validation using CT images. CT, computed tomography.

FIGURE 6

Lung nodules located in the patient's lung CT images and corresponding segmentations are to be printed. CT, computed tomography.

FIGURE 7

Basic structures of the Freddie phantom modules. The blue structure presented in (a) is printed in ABS and filled using the ReH2O resin., The orange part printed in PETG in (a) corresponds to the thoracic column of the same patient. These structures are introduced into the phantom to optimize the TCM response. In (b), the result of introducing these structures can be observed in a phantom image. PETG, polyethylene terephthalate; TCM, tube current modulation.

FIGURE 8

Examples of images of synthetic nodules into the Freddie setup: (a) Two sub‐solid nodules allocated at the right lobule of section 3 (230 mm); (b) Two GGOs allocated at the right lobule of section 2 (300 mm); and (c) one solid nodule allocated at the left lobule of section 1 (370 mm). GGOs, ground‐glass opacities.

FIGURE 9

Measured and nominal HU values of the materials used for phantom construction. HU, Hounsfield units.

FIGURE 10

Examples of images of the synthetic lung nodules designed to simulate solid, sub‐solid, and GGOs imaged with the four clinical protocols described in Table 2. GGOs, ground‐glass opacities.

FIGURE 11

Results of the simplified reader evaluation of the materials for nodule characterization using a Likert scale. HIPS (HSG2, HSM, and HSP2), TangoBlack+ (TSG, TSM2, and TSP2)) and VeroClear (VSG2, VSM2, and VSP2) are materials used for mimicking solid and sub‐solid nodules, and EVA (E6, E10, and E11) was used for mimicking GGOs. Thirteen readers participated in the reading process: (a) 5 radiologists and (b) 8 non‐radiologists. GGOs, ground‐glass opacities; HIPS, high‐impact polystyrene.

FIGURE 12

The detectability index data of the 370 mm section of the Mercury phantom for the P1, P2, P3, and P4 protocols. (a) presents the detectability index evaluated using the high contrast inserts (PMMA, polyurethane, nylon), and (b) presents the detectability index determined using the low‐contrast inserts for different materials (polypropylene and HIPS). HIPS, high‐impact polystyrene; PMMA, polymethyl methacrylate.