Bone-like Materials Research Articles

A new constitutive relation to describe the response of bones

Trabecular bone, a solid that has a heterogeneous porous structure, demonstrates nonlinear stress–strain relationship, even within the small strain region, when subject to stresses. It also exhibits different responses when subject to tension and compression. This study presents the development of an implicit constitutive relation between the stress and the linearized strain specifically tailored for trabecular bone-like materials. The structure of the constitutive relation requires the solution of the balance of linear momentum and the constitutive relations simultaneously, and in view of this, a two-field mixed finite element model capable of solving general boundary value problems governed by a system of coupled equations is proposed. We investigate the effects of nonlinearity and heterogeneity in a dogbone-shaped sample. Our study is able to capture the significant nonlinear characteristics of the response of the trabecular bone undergoing small strains in experiments, in both tension and compression, very well.

Effect of the Electric Field on the Biomineralization of Collagen.

Collagen/hydroxyapatite hybrids are promising biomimetic materials that can replace or temporarily substitute bone tissues. The process of biomineralization was carried out through a double diffusion system. The methodological principle consisted in applying an electric field on the incubation medium to promote the opposite migration of ions into collagen membranes to form hydroxyapatite (HA) on the collagen membrane. Two physically separated solutions were used for the incubation medium, one rich in phosphate ions and the other in calcium ions, and their effects were evaluated against the traditional mineralization in Simulated Body Fluid (SBF). Pre-polarization of the organic membranes and the effect of incubation time on the biomineralization process were also assessed by FTIR and Raman spectroscopies.Our results demonstrated that the membrane pre-polarization significantly accelerated the mineralization process on collagen. On the other side, it was found that the application of the electric field influenced the collagen structure and its interactions with the mineral phase. The increment of the mineralization degree enhanced the photoluminescence properties of the collagen/HA materials, while the conductivity and the dielectric constant were reduced. These results might provide a useful approach for future applications in manufacturing biomimetic bone-like materials.

A biomimetic well-aligned silk fibroin/calcium phosphate composite by electrospinning and in-situ mineralization

• Well-aligned silk-fibroin nanofibers are prepared via the electrospinning technique. • Calcium phosphate is introduced along the fibers by in-situ mineralization. • The silk fibroin/calcium phosphate composite holds excellent mechanical properties. • The job offers a general routine for preparing the bone-like biomedical composite. In this study, well-aligned silk fibroin nanofibers are prepared to simulate anisotropic bone extracellular matrix via an optimized process of the electrospinning technique. Then calcium phosphate is introduced along the fibers by in-situ mineralization to prepare a biomimetic composite. After hot pressing, the silk fibroin-based calcium phosphate composite holds not only bone-like composition and structure but also excellent mechanical performance. The elastic modulus and bending strength of silk fibroin scaffold is close to that of the subchondral bone. This work provides a general routine for preparing the bone-like biomedical materials.

3D Printed Bone-like Materials for Delivering Natural Medicine

Some of the greatest advances in medical history have revolved around the creation of new materials that can replace damaged tissues in the body. Today, many researchers focus on creating materials that can replace damaged bone tissue, which often cannot heal naturally. Dr Susmita Bose and her team at Washington State University have been researching ways to engineer exciting new materials that mimic the structure of natural bone, allowing us to live happier, healthier, and longer lives.

Oriented Crystallization of Hydroxyapatite in Self-Assembled Peptide Fibrils as a Bonelike Material.

Controlling oriented crystallization is key to producing bonelike composite materials with a well-organized structure. However, producing this type of composite material using synthetic biopolymers as scaffolds is challenging. Inspired by the molecular structure of collagen-I, a collagenlike peptide─(Pro-Hyp-Gly)10 (POG10)─was designed to produce self-assembled fibrils that resemble the structure of collagen-I fibrils. In addition, the oriented mineralization of HAP crystals is formed in the fibrils that reproduces a bonelike material similar to collagen-I fibril mineralization. Unlike collagen-I fibrils, POG10 fibrils do not contain gap spaces. The molecular simulation results indicate that in addition to space confinement, the molecular field generated by POG10 can also confine the orientation of HAP, enriching our understanding of physical confinement and shedding light on the design of synthetic biopolymer scaffolds for bonelike material fabrication.

Three-dimensional biomechanical modeling of cylindrical bone-like porous materials subject to acoustic waves

This paper describes a three-dimensional (3D) analytical solution for the acoustic response of cancellous bone-like porous materials saturated with a viscous fluid. The effect of dynamic tortuosity, especially in clinically relevant ultrasound frequency ranges, is considered to investigate the effect of viscous interaction between the fluid and solid phases. The solution includes the effects of both fast and slow longitudinal waves as well as transverse waves propagating through the medium. The scattering operators and radial displacements are derived in terms of ultrasonic waveforms by applying the Helmholtz decomposition. The effect of different porosities, wall thickness ratios, and frequencies of incident waves on the radial displacement and scattering operators are investigated by considering various incident wave angles at forward and sideward directions. The results demonstrate that the incident wave angle has a significant effect on the radial displacement and scattering operators regardless of the porosity, wall thickness ratio, and viscosity of pore fluid. Furthermore, the distribution pattern of the radial displacement and scattering operators in relatively low frequency ranges is almost symmetric while asymmetric in relatively high frequency ranges. It is shown that the bone characterization using ultrasonic techniques is not only based on the mineral density, as used currently by electromagnetic wave-based tools, but also other biomechanical factors such as the porosity, viscosity of pore fluid, and wall thickness ratio of a cancellous bone structure. Also, the pattern of the reflected pressure can be an indicator of the state of a cancellous bone (healthy versus osteoporosis).

Interaction of therapeutic 12C ions with bone-like targets: physical characterization and dosimetric effect at material interfaces

Carbon therapy is a promising treatment option for cancer. The physical and biological properties of carbon ions can theoretically allow for the delivery of curative doses to the tumor, while simultaneously limiting risks of toxicity to adjacent healthy structures. The treatment effectiveness can be further improved by decreasing the uncertainties stemming from several sources, including the modeling of tissue heterogeneity. Current treatment plans employ density-based conversion methods to translate patient-specific anatomy into a water system, where dose distribution is calculated. This approach neglects differences in nuclear interactions stemming from the elemental composition of each tissue. In this work, we investigated the interaction of therapeutic carbon ions with bone-like materials. The study concentrated on nuclear interactions and included attenuation curves of 200 and 400 AMeV beams in different types of bones, as well as kinetic energy spectra of all charged fragments produced up to 29 degrees from the beam direction. The comparison between measurements and calculations of the treatment planning system TRiP98 indicated that bone tissue causes less fragmentation of carbon ions than water. Overall, hydrogen and helium particles were found to be the most abundant species, while heavier fragments were mostly detected within 5 degrees from the beam direction. We also investigated how the presence of a soft tissue-bone interface could affect the depth-dose profile. The results revealed a dose spike in the transition region, that extended from the entry channel to the target volume. The findings of this work indicated that the tissue-to-water conversion method based only on density considerations can result in dose inaccuracies. Tissue heterogeneity regions containing bones can potentially produce dose spikes, whose magnitude will depend on the patient anatomy. Dose uncertainties can be decreased by modeling nuclear interactions directly in bones, without applying the tissue-to-water conversion.

A Dopamine Acrylamide Molecule for Promoting Collagen Biomimetic Mineralization and Regulating Crystal Growth Direction.

The reconstruction of the intra/interfibrillar mineralized collagen microstructure is extremely important in biomaterial science and regeneration medicine. However, certain problems, such as low efficiency and long period of mineralization, are apparent, and the mechanism of interfibrillar mineralization is often neglected in the present literature. Thus, we propose a novel model of biomimetic collagen mineralization that uses molecules with the dual function of cross-linking collagen and regulating collagen mineralization to construct the intrafibrillar and interfibrillar collagen mineralization of the structure of mineralized collagen hard tissues. In the present study completed in vitro, N-2-(3,4-dihydroxyphenyl) acrylamide (DAA) is used to bind and cross-link collagen molecules and further stabilize the self-assembled collagen fibers. The DAA-collagen complex provides more affinity with calcium and phosphate ions, which can reduce the calcium phosphate/collagen interfacial energy to promote hydroxyapatite (HA) nucleation and accelerate the rate of collagen fiber mineralization. Besides inducing intrafibrillar mineralization, the DAA-collagen complex mineralization template can realize interfibrillar mineralization with the c-axis of the HA crystal on the surface of collagen fibers and between fibers that are parallel to the long axis of collagen fibers. The DAA-collagen complex, as a new type of mineralization template, may provide a new collagen mineralization strategy to produce a mineralized scaffold material for tissue engineering or develop bone-like materials.

Effects of Mn-doping on the structure and in vitro degradation of β-tricalcium phosphate

β-tricalcium phosphate (β-TCP) is an ideal biomaterial for the bone repair because of its biocompatibility and biodegradability. In this study, 0 mol%, 5 mol%, 15 mol% and 30%mol bivalent manganese ion (Mn2+) doped β-TCP (Mn-TCP) powders were synthesized by a sol-gel method. The amount of the dopants significantly influences the crystallinity and the parameters related with structure of β-TCP, such as the lattice parameters and crystallite dimensions. The particle size and the particle distribution of doped β-TCP powers were evaluated as well. Meanwhile, the as-synthesized powders were consolidated by sintering at 1000 °C in muffle furnace for 5 h to get Mn-TCP porous material and the degradation experiment was carried out in Simulated Body Fluid (SBF) solution for 28 days. Then, Mn-TCP porous material were characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM). Significantly, there were bone-like apatite materials deposited on the surface of bone-like porous materials. With the increasing doping amount of Mn2+, the newly formed apatite-like materials decreased, while the crystallinity increased significantly. Besides, pH results showed that alkaline environment was more favorable for the formation of sedimentary materials.

Dynamic behaviors and protection mechanisms of sulcata tortoise carapace

This paper presents the compressive behavior of tortoise carapace at high strain rates and its protection mechanisms under impact loading. Both experimental and numerical results are reported. Tortoise is a land-based desert-dwelling animal taxonomically classified in the order of Testudines. The carapace is the dome-shaped upper part of the tortoise shell that protects its body from predator attacks. The carapace structure is composed of four layers formed as a composite structure with a porous core. The outer surface is keratin scutes made of fibrous structural proteins. The remaining layers are bone-like materials which are dorsal cortex, cancellous interior and ventral cortex. The compressive behavior at high rate of deformation is examined using split Hopkinson pressure bar (SHPB) technique. The results shown in the stress-strain plot illustrate a strain-rate hardening effect. The impact test is conducted using a gas gun with 6.35-mm diameter steel bearing balls as projectiles. The responses of carapace sample under a range of impact velocities are investigated to analyze its protection mechanisms. The numerical model of impact test is created to obtain an insight into mechanical behaviors of the carapace structure that cannot be observed in the experiments. The strain rate dependent material model is defined based on the SHPB test results. The distributions of stress and rebound velocity are presented and discussed.

A novel quantitative and reference-free ultrasound analysis to discriminate different concentrations of bone mineral content

Bone fracture is a continuous process, during which bone mineral matrix evolves leading to an increase in hydroxyapatite and calcium carbonate content. Currently, no gold standard methods are available for a quantitative assessment of bone fracture healing. Moreover, the available tools do not provide information on bone composition. Whereby, there is a need for objective and non-invasive methods to monitor the evolution of bone mineral content. In general, ultrasound can guarantee a quantitative characterization of tissues. However, previous studies required measurements on reference samples. In this paper we propose a novel and reference-free parameter, based on the entropy of the phase signal calculated from the backscattered data in combination with amplitude information, to also consider absorption and scattering phenomena. The proposed metric was effective in discriminating different hydroxyapatite (from 10 to 50% w/v) and calcium carbonate (from 2 to 6% w/v) concentrations in bone-mimicking phantoms without the need for reference measurements, paving the way to their translational use for the diagnosis of tissue healing. To the best of our knowledge this is the first time that the phase entropy of the backscattered ultrasound signals is exploited for monitoring changes in the mineral content of bone-like materials.

Quantitative assessment of dentine mineralization and tubule occlusion by NovaMin and stannous fluoride using serial block face scanning electron microscopy

Dentine hypersensitivity (DH) is one of the most common dental conditions affecting most adults during their lifetime. Tubule occlusion is a widely accepted method for treating DH. Current in-vitro techniques such as focused ion beam, scanning electron microscopy (SEM), or hydraulic conductance that are used to determine tubule occlusion do not provide the depth of occlusion, are time-consuming, expensive and the volume of dentine tested is limited. The presented study aimed to assess the ability of serial block-face SEM (SBF-SEM) to section dentine, to quantify the number of occluded tubules including the depth of penetration by NovaMin and stannous fluoride (SnF2 ) and to compare mineral density between the control and treated dentine. Results demonstrated that NovaMin provided a better occlusion with 100% of the tubules blocked at the surface compared to 83% for SnF2 . The grayscale value (230.42) was significantly higher (p ≤ 0.05) after treatment with NovaMin compared to SnF2 (222.06) and the control (196.37), indicating increased mineral density and dentine mineralization. SBF-SEM has the potential to be used for large volume analysis of bone-like materials at high resolution with minimal sample preparation over a short period. It can be significantly useful in the development and research of new biomaterials.

X-ray diffraction and in situ pressurization of dentine apatite reveals nanocrystal modulus stiffening upon carbonate removal

Bone-like materials comprise carbonated-hydroxyapatite nanocrystals (c-Ap) embedding a fibrillar collagen matrix. The mineral particles stiffen the nanocomposite by tight attachment to the protein fibrils creating a high strength and toughness material. The nanometer dimensions of c-Ap crystals make it very challenging to measure their mechanical properties. Mineral in bony tissues such as dentine contains 2~6 wt.% carbonate with possibly different elastic properties as compared with crystalline hydroxyapatite. Here we determine strain in biogenic apatite nanocrystals by directly measuring atomic deformation in pig dentine before and after removing carbonate. Transmission electron microscopy revealed the platy 3D morphology while atom probe tomography revealed carbon inside the calcium rich domains. High-energy X-ray diffraction in combination with in situ hydrostatic pressurization quantified reversible c-Ap deformations. Crystal strains differed between annealed and ashed (decarbonated) samples, following 1 or 10 h heating at 250 °C or 550 °C respectively. Measured bulk moduli (K) and a-/c-lattice deformation ratios (η) were used to generate synthetic Ksyn and ηsyn identifying the most likely elastic constants C33 and C13 for c-Ap. These were then used to calculate the nanoparticle elastic moduli. For ashed samples, we find an average E11=107 GPa and E33 =128 GPa corresponding to ~5% and ~17% stiffening of the a-/c-axes of the nanocrystals as compared with the biogenic nanocrystals in annealed samples. Ashed samples exhibit ~10% lower Poisson's ratios as compared with the 0.25~0.36 range of carbonated apatite. Carbonate in c-Ap may therefore serve for tuning local deformability within bony tissues. Statement of SignificanceCarbonated apatite nanoparticles, typical for bony tissues, stiffen the network of collagen fibrils. However, it is not known if the biogenic apatite mechanical (elastic) properties differ from those of geologic mineral counterparts. Indeed the tiny dimensions and variable carbonate composition may have strong effects on deformation resistance. The present study provides experimental measurements of the elastic constants which we use to estimate Young's moduli and Poisson's ratio values. Comparison between ashed and annealed dentine samples quantifies the properties of both carbonated and decarbonated apatite nanocrystals. The results reveal fundamental attributes of bony mineral and showcase the additive advantages of combining X-ray diffraction with in situ hydrostatic compression, backed by atom probe and transmission electron microscopy tomography.

Mechanically robust and thermally insulating polyarylene ether nitrile with a bone-like structure

Inspired by the excellent mechanical performance of bone materials characterized by compact shell and porous core, this study proposes a lightweight, mechanically robust, and thermal-insulating polyarylene ether nitrile (PEN) with a bone-like structure. The supercritical fluid technique was applied to provide PEN with two types of tunable bone-like structures, which makes it simple to manipulated under supercritical treatment conditions. Depending on different bone-like microstructures, their tensile strength can reach up to 75 MPa with the impact strength range of 50–190 KJ/m2. Notably, the impact strength of PEN foam with a saturation time of 4 h is 1.6% higher than that of pure PEN. Furthermore, lightweight PEN foams capable of thermal insulation and their conductivity can reach 0.06 W/m.K without compromising the actual mechanical performance. These lightweight PENs with a remarkable performance in thermal insulation and mechanical robustness show a massive potential in automobile application for effectiveness in energy conservation and reducing air pollution. Based on this experiment, a new method is proposed to prepare biomimetic bone-like materials.

Investigating mineralization species in cultured bone from human mesenchymal stem cells using synchrotron-based XANES

In this context, we successfully demonstrate the use of X-ray absorption near edge spectroscopy (XANES) to identify calcium species and compare the mineralization of bone-like materials from human mesenchymal stem cells (MSC) in 3 culture conditions in vitro; non-osteogenic medium (non-OM), osteogenic medium (OM) and OM supplemented with bone morphogenetic protein 9 (OM-BMP9). X-ray photoelectron spectroscopy (XPS) was also used to examine the bonding of mineralized species on the surface of MSC-mediated mineralization. The results from Alizarin Red staining exhibited calcium deposits in all 3 culture conditions with different degrees of calcium deposition (OM-BMP9> OM > non-OM). While XANES disclosed that the majority of calcium species deposited in the non-OM culture was calcium sulfate (CaSO4; 27.7%), tricalcium phosphate (TCP; 27.1%) and calcium chloride (CaCl2; 22.2%) with lower levels of calcium carbonate (CaCO3; 10.5%) and calcium hydroxide (Ca(OH)2; 12.5%). Hydroxyapatite (HA) was not detected in the non-OM sample. On the other hand, the majority of calcium species in OM and OM-BMP9 cultures presented in calcium phosphate forms. The amount of calcium species in the OM-BMP9 culture comprised 33.1% HA, 40.5% TCP, 5.7% CaCl2, 15.4% CaSO4, and 5.0% CaCO3 and the OM culture comprised 22% HA, 33.5% TCP, 8.5% CaCl2, 24.3% CaSO4, 5% CaCO3 and 3.9% Ca(OH)2. Amorphous calcium phosphate (APC) was not detected in all the 3 groups. XPS results confirmed the existence of calcium phosphate species on the surface of MSC-mediated biomineralization. The successful use of XANES and XPS to qualitatively and quantitatively determine calcium phosphates in vitro is highly beneficial and provides not only an insight into chemical composition of the mineral deposit in the cultured bone but also presents more biologically-relevant information than those from conventional analytical methods. This is also the first report to demonstrate, by synchrotron-based XANES, the osteogenic role of BMP9 in enhancing HA formation specifically. This will help optimizing culture conditions to produce highly osteogenic bone-like materials containing similar compositions and functions to native human bone.

An overview of the acoustic studies of bone-like porous materials, and the effect of transverse acoustic waves

In this paper, the effects of transverse acoustic waves in characterizing a bone-like, porous medium filled with a viscous fluid are analyzed for the first time. Scattering operators along with stress fields are derived by using the standard Biot-JKD model. A short duration acoustical pulse is applied to one side of a bone-like, porous medium so that both longitudinal and transverse waves travel through the intermediate medium which is filled with a viscous fluid. The reflection and transmission operators along with stresses in the medium are expressed in terms of these waves. The numerical implementation is validated for the longitudinal wave by comparison with the numerical simulation performed by Fellah, Chapelon, Berger, Lauriks, and Depollier (2004). The effects of the transverse waves on the reflection and transmission coefficients as well as the stress field are studied by considering different viscosities and porosities. It is shown that when the fluid viscosity in the medium is relatively high (such as bone marrow), the effect of the transverse wave dominates. However, this effect is negligible when the medium is filled with a relatively low viscous fluid (such as air). Furthermore, it is shown that the role of transverse waves in characterizing bone structures and bone loss is imperative since the acoustical response of such media at specific frequencies can be triggered only by considering the effects of transverse waves.

Transient acoustic wave propagation in bone-like porous materials using the theory of poroelasticity and fractional derivative: a sensitivity analysis

In this paper, the transient acoustic wave propagation in a bone-like porous material saturated with a viscous fluid was investigated using Biot’s theory. Due to the interaction between the viscous fluid and solid skeleton, the damping behavior is proportional to a fractional power of frequency, i.e., the dynamic tortuosity was written in terms of the fractional power of frequency. Furthermore, to describe the viscous interaction of fluid and solid in the time domain, the fractional derivative was used. The fast and slow waves, which are the solutions to Biot’s equations, were described by fractional calculus in the time domain. The reflection and transmission operators were expressed in the Laplace domain and inverted into the time domain using Durbin’s numerical inversion. Once the numerical implementation was validated, the effects of porosity and viscosity on the stress, and reflected and transmitted waves were investigated. The results showed that by increasing the porosity the stress in a bone-like material filled with either air or bone marrow increases. The transmitted pressure decreases by increasing the porosity. The reflected pressure decreases for low viscous fluid when the porosity increases while it increases when the viscosity of the fluid is high. In addition, the results showed the importance of taking into account the fractional derivatives in the transient wave propagation in such porous materials.

A RESONANCE FREQUENCY EXTRACTION METHOD FROM LOW Q-FACTOR MATERIALS BASED ON RESONANT ULTRASOUND SPECTROSCOPY

Resonant ultrasound spectroscopy (RUS) allows identification of the elastic coefficients of solid materials vibrating under an ultrasonic excitation from the measurement of their inherent frequencies. Retrieving the resonant frequencies is therefore a key signal processing step in RUS. However, according to the attenuation characteristics of low $Q$-factor (quality factor) materials, the resonance spectrum obtained by the experiment is flat and the resonance frequencies can not be directly observed from the spectrum. Therefore, in order to retrieve more effective resonance frequencies than traditional approach from the low $Q$-factor materials, a new extraction method of resonance frequencies was proposed to solve the limitation in this paper. The empirical mode decomposition method was used to decompose the frequency response of the specimen into finite Intrinsic Mode Function (IMF) components with special oscillation characteristics. According to the prior information of resonant ultrasound spectroscopy (RUS),the relevant IMF component was selected to retrieve reliable resonance frequencies from the resonance spectrum. The short fiber filled epoxy (a kind of bone-like materials, $Q \approx $25) was adopted as the specimen to calculate the elastic coefficients and engineering moduli compared with the traditional linear prediction method. The experimental results show that the new method has high computational efficiency and is more sensitive to the weak excitation modes of low $Q$-factor materials. The number of effective resonance frequencies (26) are more than traditional linear prediction methods (21), which also satisfied 5 times estimation requirement of elastic constants. In addition, the optimized elastic moduli are closer to the standard values of the short fiber filled epoxy. In conclusion, the EMD-based method can retrieve a sufficient quantity and effective resonance frequencies from the flat spectrum of low $Q$-factor materials, which can not only improve the reliability of the estimation of mechanical parameters, but also extend the application range of resonant ultrasound spectroscopy.

Development of bone seeker–functionalised microspheres as a targeted local antibiotic delivery system for bone infections

ObjectiveBone infections are challenging to treat because of limited capability of systemic antibiotics to accumulate at the bone site. To enhance therapeutic action, systemic treatments are commonly combined with local antibiotic-loaded materials. Nevertheless, available drug carriers have undesirable properties, including inappropriate antibiotic release profiles and nonbiodegradability. To alleviate such limitations, we aim to develop a drug delivery system (DDS) for local administration that can interact strongly with bone mineral, releasing antibiotics at the infected bone site. MethodsBiodegradable polyesters (poly (ε-caprolactone) or poly (D,l-lactic acid)) were selected to fabricate antibiotic-loaded microspheres by oil in water emulsion. Antibiotic release and antimicrobial effects on Staphylococcus aureus were assessed by zone of inhibition measurements. Microsphere bone affinity was increased by functionalising the bisphosphonate drug alendronate to the microsphere surface using carbodiimide chemistry. Effect of bone targeting microspheres on bone homeostasis was tested by looking at the resorption potential of osteoclasts exposed to the developed microspheres. ResultsIn vitro, the antibiotic release profile from the microspheres was shown to be dependent on the polymer used and the microsphere preparation method. Mineral binding assays revealed that microsphere surface modification with alendronate significantly enhanced interaction with bone-like materials. Additionally, alendronate functionalised microspheres did not differentially affect osteoclast mineral resorption in vitro, compared with nonfunctionalised microspheres. ConclusionWe report the development and characterisation of a DDS which can release antibiotics in a sustained manner. Surface-grafted alendronate groups enhanced bone affinity of the microsphere construct, resulting in a bone targeting DDS. The Translational Potential of this ArticleThe DDS presented can be loaded with hydrophobic antibiotics, representing a potential, versatile and biodegradable candidate to locally treat bone infection.

The effect of different CT scanners, scan parameters and scanning setup on Hounsfield units and calibrated bone density: a phantom study

This study investigated the effect of different CT scanners, reconstruction protocols, scan positions, and air gaps on HU and/or calibrated calcium hydroxyapatite concentrations (CaHA). Phantoms containing bone-like materials were scanned on four different CT scanners. The effect of slice thickness, field of view (FOV) and reconstruction kernel on HU was investigated. Using clinical scan and reconstruction protocols and a calibration phantom, HU and CaHA were determined for different positions, and air gaps between phantom and calibration phantom. Changing reconstruction kernel considerably affected the HU (range −31 HU to 64 HU), whereas slice thickness and FOV did not. Inter-scanner differences in HU were <88 HU and decreased to <47 CaHA after calibration. Different positions of the phantom resulted in differences (on average up to −26 HU and −24 CaHA). An air gap of 2.5 cm atop the calibration phantom resulted in errors up to −41 CaHA. If absolute HU and calibrated CaHA are needed, different reconstruction kernels and changes in position within the FOV as well as air gaps should be avoided.