April 17, 2024

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Design and fabrication of 3D-printed patient-specific soft tissue and bone phantoms for CT imaging

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PixelPrint and 3D printing

The previously published PixelPrint algorithm was used to create G-code from CT image data to produce 3D-printed phantoms18. Briefly, density information was extracted from the clinical patient images to generate filament lines that varied in width according to the HU of individual pixels. These lines were uniformly spaced within each layer and perpendicular on adjacent layers. By adjusting the filament line widths pixel-by-pixel, volumetric filament per unit space, or infill ratio, was varied despite only using one type of filament. These different infill ratios then produced different attenuation in CT images due to the partial volume effect.

In this study, the filament lines were equally spaced at 0.5 mm. The width of the filament line changed at resolution of 0.167 mm. The minimum and maximum line widths were 0.2 and 0.5 mm, corresponding to the infill ratio ranging between 40 and 100%, respectively. Keeping a constant extrusion rate, the print head traveled at varying speeds based on the width of the extruded filament line. The slowest speed was 180 mm/min for the widest width of 0.5 mm, while the fastest was 450 mm/min for the smallest width of 0.2 mm. Each layer had a uniform height of 0.2 mm. The resulting volumetric rate of filament extrusion during the whole print remained constant at 18 mm3/min. To prevent overlapping of lines in consecutive layers with the same filament line direction, an offset of 0.167 mm (1/3 of the 0.5 mm line spacing) was introduced.

All phantoms were printed with Lulzbot TAZ 6 or Sidekick 747 (Fargo Additive Manufacturing Equipment 3D, LLC Fargo, ND, USA), paired with M175 v2 tool heads and 0.40 mm steel nozzles. Calcium-doped PLA filament (StoneFil, FormFutura, AM Nijmegen, the Netherlands) with a diameter 1.75 mm was utilized. This PLA-based filament is filled with powdered stone. Further details can be found on: The temperature of the nozzle was set at 200 °C and the bed was warmed to 50 °C to enhance adherence. Acceleration of the print head was to 500 mm/s2 and the threshold (jerk setting) was 8 mm/s.

Phantom design

Micro-CT phantom

Three cylindrical phantoms were designed and produced using PixelPrint filament lines to examine their stability and reproducibility. These filament lines constructed a matrix smaller than the typical resolution limit of clinical CT scanners. Three phantoms were printed with identical G-code instructions. These phantoms are 60 mm in length and 20 mm in diameter. Each of them consists of four sections with different but homogeneous infill ratios (100%, 70%, 50% and 30%). Calcium-doped PLA filament lines were printed at a spacing of 1 mm in all four sections but with corresponding line widths of 1.0, 0.7, 0.5, and 0.3 mm, respectively. A thin outer layer was added to the phantom for support, particularly for low infill ratio sections.

Calibration phantom

To compute the conversion between calcium-doped PLA filament infill ratios and HUs, a calibration phantom was designed. The phantom is a cylinder with a diameter of 10 cm and height of 1 cm. It consists of seven equally divided pie slice-shaped sections. Each section was printed at a fixed line spacing of 0.5 mm but with different filament line widths (0.2–0.5 mm), corresponding to seven infill ratios (40 -100%, with 10% intervals).

Cervical vertebrae phantom

Due to the retrospective nature of the study, the Institutional Review Board of University of Pennsylvania waived the need of obtaining informed consent. A cervical vertebrae phantom was created based on a patient image volume (10 × 10 × 10 cm3) that was acquired on a clinical CT scanner (Siemens SOMATOM Definition Edge, Siemens Healthcare GmbH, Erlangen, Germany) at a tube voltage of 120 kVp with a standard diagnostic protocol. Table 1 lists detailed acquisition and reconstruction parameters for the patient scan. The patient data consist of four cervical vertebrae (C4 to C7), including the trachea and esophagus. A circular region of interest with a diameter of 10 cm was cropped in axial slices to form the phantom. HUs were converted to infill ratios based on the calibration phantom.

Table 1 Acquisition parameters of CT image for phantom generation.

Knee phantom

A knee phantom was similarly generated using a patient scan on a clinical dual-layer CT scanner (IQon spectral CT, Philips Healthcare, the Netherlands) at a tube voltage of 120 kVp, as detailed in Table 1. A circular region of interest with a diameter of 10 cm was cropped from the axial slices of the patient’s left knee. HUs were then converted to infill ratios.

It’s important to note that all image data used in this study were collected retrospectively and anonymized. The imaging procedures were conducted as part of routine clinical practice. The parameter selections outlined in Table 1 were guided by the imaging protocols established within the department. The experimental protocol was approved by the institutional committee in University of Pennsylvania. All methods were performed in accordance with relevant guidelines and regulations.

Data acquisition

Three micro-CT phantoms were separately scanned on a commercial micro-CT (U-CT system, MILabs, CD Houten, the Netherlands) with a tube voltage of 50 kVp. In addition, these phantoms were also scanned on a clinical dual-layer CT system (IQon spectral CT, Philips Healthcare, the Netherlands) at a tube voltage of 120 kVp with a high-resolution protocol and a small field-of-view of 100 mm. Additional acquisition and reconstruction parameters of the two scans are listed in Table 2. The micro-CT scanning protocol adheres to the guidelines provided by the manufacturer for imaging objects characterized by these dimensions and feature scales. Micro-CT images were exported from the scanner and reprocessed with a multi-planar reconstruction algorithm (MPR) in an imaging post-processing software (Horos Project, Annapolis, MD, USA) to ensure filament lines were parallel to the axial plane.

Table 2 Scan protocols for the micro-CT phantom.

The calibration, cervical vertebrae, and the knee phantom were scanned inside the QRM chest phantom (Quality Assurance in Radiology and Medicine GmbH, Möhrendorf, Germany) with the clinical dual-layer CT system. The protocol parameters matched those of the original clinical examination of the patient, maintaining the same pixel spacing and slice thickness as indicated in Table 1. In the case of the cervical phantom, a 400 mg/ml QRM hydroxyapatite (HA) insert was also scanned along with the phantom to serve as a reference for bone mineral density. For both patient-based phantoms, additional high-dose scans were conducted using 1000 mAs while retaining the other scanning parameters unchanged. These high-exposure scans were included to reduce noise and facilitate comparisons of image quality.

Calibration and data analysis

For computing the conversion between HUs and infill ratios, mean and standard deviation HU values of seven areas were measured in the calibration phantom. Square regions of interest (ROI) of 19 × 19 pixel2 (13 × 13 mm2) were manually placed in each of the seven density regions within 10-mm-thick center of the phantom. A linear regression was computed, and the resulting Pearson’s correlation coefficient (r) was reported. All measurements were performed on a workstation with ImageJ (U. S. National Institutes of Health, and all analyses were computed with Python (Python Software Foundation, https://www.python.org/).

For the cervical vertebrae phantom and the knee phantom, CT images were exported from the scanner and registered to the original patient data (2D-wise) using the OpenCV Library (Open Source Computer Vision Library37, Mean and standard deviation in regions of interest for different tissue types were measured. Line profiles of the phantom scan were also compared with the original patient scan. Additionally, virtual monoenergetic images from 40 to 200 keV were extracted to quantify the spectral response of the bone regions within the patient-based phantoms.

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