Induced radiofrequency fields in patients undergoing MR examinations: insights for risk assessment.

Purpose. To characterize and quantify the induced radiofrequency (RF) electric (E)-fields andB1+rmsfields in patients undergoing magnetic resonance (MR) examinations; to provide guidance on aspects of RF heating risks for patients with and without implants; and to discuss some strengths and limitations of safety assessments in current ISO, IEC, and ASTM standards to determine the RF heating risks for patients with and without implants.Methods. InducedE-fields andB1+rmsfields during 1.5 T and 3 T MR examinations were numerically estimated for high-resolution patient models of the Virtual Population exposed to ten two-port birdcage RF coils from head to feet imaging landmarks over the full polarization space, as well as in surrogate ASTM phantoms.Results. Worst-caseB1+rmsexposure greater than 3.5µT (1.5 T) and 2µT (3 T) must be considered for all MR examinations at the Normal Operating Mode limit. Representative inducedE-field and specific absorption rate distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure.B1shimming can cause +6 dB enhancements toE-fields along implant trajectories. The distribution and magnitude of inducedE-fields in the ASTM phantom differ from clinical exposures and are not always conservative for typical implant locations.Conclusions.Field distributions in patient models are condensed, visualized for quick estimation of risks, and compared to those induced in the ASTM phantom. InducedE-fields in patient models can significantly exceed those in the surrogate ASTM phantom in some cases. In the recent 19ε2revision of the ASTM F2182 standard, the major shortcomings of previous versions have been addressed by requiring that the relationship between ASTM test conditions andin vivotangentialE-fields be established, e.g. numerically. With this requirement, the principal methods defined in the ASTM standard for passive implants are reconciled with those of the ISO 10974 standard for active implantable medical devices.

Radiofrequency-induced heating of broken and abandoned implant leads during magnetic resonance examinations.

PURPOSE: The risks of RF-induced heating of active implantable medical device (AIMD) leads during MR examinations must be well understood and realistically assessed. In this study, we evaluate the potential additional risks of broken and abandoned (cut) leads. METHODS: First, we defined a generic AIMD with a metallic implantable pulse generator (IPG) and a 100-cm long lead containing 1 or 2 wires. Next, we numerically estimated the deposited in vitro lead-tip power for an intact lead, as well as with wire breaks placed at 10 cm intervals. We studied the effect of the break size (wire gap width), as well as the presence of an intact wire parallel to the broken wire, and experimentally validated the numeric results for the configurations with maximum deposited in vitro lead-tip power. Finally, we performed a Tier 3 assessment of the deposited in vivo lead-tip power for the intact and broken lead in 4 high resolution virtual population anatomic models for over 54,000 MR examination scenarios. RESULTS: The enhancement of the deposited lead-tip power for the broken leads, compared to the intact lead, reached 30-fold in isoelectric exposure, and 16-fold in realistic clinical exposures. The presence of a nearby intact wire, or even a nearby broken wire, reduced this enhancement factor to <7-fold over the intact lead. CONCLUSION: Broken and abandoned leads can pose increased risk of RF-induced lead-tip heating to patients undergoing MR examinations. The potential enhancement of deposited in vivo lead-tip power depends on location and type of the wire break, lead design, and clinical routing of the lead, and should be carefully considered when performing risk assessment for MR examinations and MR conditional labeling.

Osteogenic differentiation of human mesenchymal stem cells in the absence of osteogenic supplements: A surface-roughness gradient study.

The use of biomaterials to direct osteogenic differentiation of human mesenchymal stem cells (hMSCs) in the absence of osteogenic supplements is thought to be part of the next generation of orthopedic implants. We previously engineered surface-roughness gradients of average roughness (Ra) varying from the sub-micron to the micrometer range (∼0.5-4.7 µm), and mean distance between peaks (RSm) gradually varying from ∼214 µm to 33 µm. Here we have screened the ability of such surface-gradients of polycaprolactone to influence the expression of alkaline phosphatase (ALP), collagen type 1 (COL1) and mineralization by hMSCs cultured in dexamethasone (Dex)-deprived osteogenic induction medium (OIM) and in basal growth medium (BGM). Ra∼1.53 µm/RSm∼79 µm in Dex-deprived OI medium, and Ra∼0.93 µm/RSm∼135 µm in BGM consistently showed higher effectiveness at supporting the expression of the osteogenic markers ALP, COL1 and mineralization, compared to the tissue culture polystyrene (TCP) control in complete OIM. The superior effectiveness of specific surface-roughness revealed that this strategy may be used as a compelling alternative to soluble osteogenic inducers in orthopedic applications featuring the clinically relevant biodegradable polymer polycaprolactone. STATEMENT OF SIGNIFICANCE: Biodegradable polymers, such as polycaprolactone (PCL), are promising materials in the field of tissue engineering and regenerative medicine, which aims at creating viable options to replace permanent orthopedic implants. The material, cells, and growth-stimulating factors are often referred to as the key components of engineered tissues. In this article, we studied the hypothesis of specific surface modification of PCL being capable of inducing mesenchymal stem cell differentiation in bone cells in the absence of cell-differentiating factors. The systematic investigation of the linearly varying surface-roughness gradient showed that an average PCL roughness of 0.93 µm alone can serve as a compelling alternative to soluble osteogenic inducers in orthopedic applications featuring the clinically relevant biodegradable polymer polycaprolactone.

Regulation of human mesenchymal stem cell osteogenesis by specific surface density of fibronectin: a gradient study.

The success of synthetic bone implants requires good interface between the material and the host tissue. To study the biological relevance of fibronectin (FN) density on the osteogenic commitment of human bone marrow mesenchymal stem cells (hBM-MSCs), human FN was adsorbed in a linear density gradient on the surface of PCL. The evolution of the osteogenic markers alkaline phosphatase and collagen 1 alpha 1 was monitored by immunohistochemistry, and the cytoskeletal organization and the cell-derived FN were assessed. The functional analysis of the gradient revealed that the lower FN-density elicited stronger osteogenic expression and higher cytoskeleton spreading, hallmarks of the stem cell commitment to the osteoblastic lineage. The identification of the optimal FN density regime for the osteogenic commitment of hBM-MSCs presents a simple and versatile strategy to significantly enhance the surface properties of polycaprolactone as a paradigm for other synthetic polymers intended for bone-related applications.

Differential regulation of osteogenic differentiation of stem cells on surface roughness gradients.

Tissue engineering using scaffold-cell constructs holds the potential to develop functional strategies to regenerate bone. The interface of orthopedic implants with the host tissues is of great importance for its later performance. Thus, the optimization of the implant surface in a way that could stimulate osteogenic differentiation of mesenchymal stem cells (MSCs) is of significant therapeutic interest. The effect of surface roughness of polycaprolactone (PCL) on the osteogenic differentiation of human bone-marrow MSCs was investigated. We prepared surface roughness gradients of average roughness (Ra) varying from the sub-micron to the micrometer range (∼0.5-4.7 µm), and mean distance between peaks (RSm) gradually varying from ∼214 µm to 33 µm. We analyzed the degree of cytoskeleton spreading, expression of alkaline phosphatase, collagen type 1 and mineralization. The response of cells to roughness divided the gradient into three groups of elicited stem cell behavior: 1) faster osteogenic commitment and strongest osteogenic expression; 2) slower osteogenic commitment but strong osteogenic expression, and 3) similar or inferior osteogenic potential in comparison to the control material. The stem-cell modulation by specific PCL roughness surfaces highlights the potential for creating effective solutions for orthopedic applications featuring a clinically relevant biodegradable material.

Influence of solutes on hydration and lubricity of dextran brushes.

The characteristic lubricity and non-fouling behavior of polymer brushes is critically dependent on the solvation of the polymer chains, as well as the chain-chain interactions. Dextran brushes have shown promise as non-toxic aqueous lubricant films, and are similar in composition to natural lubricating systems, while their comparative simplicity allows for controlled preparation and fine characterization. This project entails measuring the solvation and lubricity of dextran brushes in the presence of additives which modify the inter-chain hydrogen bonding. The thickness and refractive index of the film were measured during adsorption of the brush layer onto a silica substrate and the subsequent immersion in solutions of potassium sulfate and α, α-trehalose. We also studied the lubricity of the system as a function of normal loading using colloidal-probe AFM. Both solutes are shown to have a minimal effect on the hydration of the brush while significantly reducing the brush lubricity, indicating that inter-chain hydrogen bonding supports the load-bearing capacity of polysaccharide brushes.

Load-induced transitions in the lubricity of adsorbed poly(L-lysine)-g-dextran as a function of polysaccharide chain density.

Chain-density gradients of poly(l-lysine)-graft-dextran (PLL-g-dex), a synthetic comblike copolymer with a poly(l-lysine) backbone grafted with dextran side chains, were fabricated on an oxidized silicon substrate. The influence of the changing dextran chain density along the gradient on the local coefficient of friction was investigated via colloidal-probe lateral force microscopy. Both in composition and structure, PLL-g-dex shares many similarities with bottlebrush biomolecules present in natural lubricating systems, while having the advantage of being well-characterized in terms of both architecture and adsorption behavior on negatively charged oxide surfaces. The results indicate that the transition of the dextran chain density from the mushroom into the brush regime coincides with a sharp reduction in friction at low loads. Above a critical load, the friction increases by more than an order of magnitude, likely signaling a pressure-induced change in the brush conformation at the contact area and a corresponding change in the mechanism of sliding. The onset of this higher-friction regime is moved to higher loads as the chain density of the film is increased. While in the low-load (and low-friction) regime, increased chain density leads to lower friction, in the high-load (high-friction) regime, increased chain density was found to lead to higher friction.