X-ray attenuation of bone, soft and adipose tissue in CT from 70 to 140 kV and comparison with 3D printable additive manufacturing materials

Abstract

Additive manufacturing and 3D printing are widely used in medical imaging to produce phantoms for image quality optimization, imaging protocol definition, comparison of image quality between different imaging systems, dosimetry, and quality control. Anthropomorphic phantoms mimic tissues and contrasts in real patients with regard to X-ray attenuation, as well as dependence on X-ray spectra. If used with different X-ray energies, or to optimize the spectrum for a certain procedure, the energy dependence of the attenuation must replicate the corresponding energy dependence of the tissues mimicked, or at least be similar. In the latter case the materials’ Hounsfield values need to be known exactly to allow to correct contrast and contrast to noise ratios accordingly for different beam energies. Fresh bovine and porcine tissues including soft and adipose tissues, and hard tissues from soft spongious bone to cortical bone were scanned at different energies, and reference values of attenuation in Hounsfield units (HU) determined. Mathematical model equations describing CT number dependence on kV for bones of arbitrary density, and for adipose tissues are derived. These data can be used to select appropriate phantom constituents, compare CT values with arbitrary phantom materials, and calculate correction factors for phantoms consisting of materials with an energy dependence different to the tissues. Using data on a wide number of additive manufacturing and 3D printing materials, CT numbers and their energy dependence were compared to those of the tissues. Two commercially available printing filaments containing calcium carbonate powder imitate bone tissues with high accuracy at all kV values. Average adipose tissue can be duplicated by several off-the-shelf printing polymers. Since suitable printing materials typically exhibit a too high density for the desired attenuation of especially soft tissues, controlled density reduction by underfilling might improve tissue equivalence.

Leveraging 3D Bioprinting and Photon‐Counting Computed Tomography to Enable Noninvasive Quantitative Tracking of Multifunctional Tissue Engineered Constructs

“…[12,55,56] The 20% GelMA solution was used specifically for the Au and AuMA NP ink groups, where the 12% GelMA concentration was not sufficient to maintain adequate printability and crosslinking of constructs. The concentration of each contrast agent in GelMA was selected as the lowest value that would generate clinically relevant X-ray attenuation (200-300 HU [57] ). The 20% GelMA solution and LAP photoinitiator was used specifically for the Au and AuMA NP ink groups, following previously established methods to optimize the photocrosslinking of constructs containing sufficient Au concentrations for X-ray contrast.…”

“…Standard (calibration) curves were first generated for the molecular (Gd chelate and Iod, Figure 3A) and NP (Figure 3F) contrast agents. According to the obtained standard curves, the lowest concentration of contrast agent to achieve an X‐ray attenuation readily detected with preclinical scanners (200–300 HU [ 57 ] ) was chosen to investigate retention. Longitudinal studies for the molecular contrast agents demonstrated a quick (burst) release of reagents within a 1‐h (Gd) or 2‐h (Iod) window in static and dynamic (rocking) conditions (Figure 3B–I).…”

“…To print the constructs, bioinks were formulated at adequate contrast agent concentrations to ensure obtaining clinically relevant X‐ray attenuation for soft tissue imaging (>150 HU, based on the standard curves). [ 57 ] Specifically, the concentrations used were 42 (Iod), 50 (Gd), 20 (Au NP), and 22 m m (Gd 2 O 3 NPs).…”

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

“…With the growing popularity and accessibility of 3D printing technology, a variety of printing filaments are now available for printing human bone and soft tissue. Several studies have discussed materials for 3D-printed phantoms in CT [24]- [26]. Novel filament materials composed of hydroxyapatite and biocompatible, biodegradable polymers, such as CT-Bone (Xilloc Medical Int., Sittard-Geleen, the Netherlands), can be utilized for printing synthetic bone implants that rapidly induce bone regeneration and growth [41], [42].…”

“…By contrast, materials such as vinyl and PLA with stone (PLA/stone) can offer up to nearly 1000 HU at 96.9% infill ratio at tube voltage of 120 kVp, as they exhibit relatively higher X-ray absorption. Additionally, considering materials for spectral CT phantoms, high impact polystyrene (HIPS) based filaments may be suitable for mimicking CT numbers in applications where energy dependence is important [26], because they show similar spectral profiles as the human body. In this study, we employed StoneFil filament, one type of PLA/stone filament.…”