Preclinical Evaluation of a Novel Fiber Compound MR Guidewire In Vivo:

Current State of MRI-Guided Endovascular Arterial Interventions: A Systematic Review of Preclinical and Clinical Studies

BACKGROUND

MRI guidance of arterial endovascular interventions could be beneficial as it does not require radiation exposure, allows intrinsic blood-tissue contrast, and enables three-dimensional and functional imaging, however, clinical applications are still limited.

PURPOSE

To review the current state of MRI-guided arterial endovascular interventions and to identify the most commonly reported challenges.

STUDY TYPE

Systematic review.

POPULATION

Pubmed, Embase, Web of Science, and The Cochrane Library were systematically searched to find relevant articles. The search strategy combined synonyms for vascular pathology, endovascular therapy, and real-time MRI guidance.

FIELD STRENGTH/SEQUENCE

No field strength or sequence restrictions were applied.

ASSESSMENT

Two reviewers independently identified and reviewed the original articles and extracted relevant data.

STATISTICAL TESTS

Results of the included original articles are reported.

RESULTS

A total of 24,809 studies were identified for screening. Eighty-eight studies were assessed for eligibility, after which data were extracted from 43 articles (6 phantom, 33 animal, and 4 human studies). Reported technical success rates for animal and human studies ranged between 42% to 100%, and the average complication rate was 5.8% (animal studies) and 8.8% (human studies). Main identified challenges were related to spatial and temporal resolution as well as safety, design, and scarcity of current MRI-compatible endovascular devices.

DATA CONCLUSION

MRI guidance of endovascular arterial interventions seems feasible, however, included articles included mostly small single-center case series. Several hurdles remain to be overcome before larger trials can be undertaken. Main areas of research should focus on adequate imaging protocols with integrated tracking of dedicated endovascular devices.

Thermoplastic Pultrusion: A Review

Pultrusion is one of the most efficient methods of producing polymer composite structures with a constant cross-section. Pultruded profiles are widely used in bridge construction, transportation industry, energy sector, and civil and architectural engineering. However, in spite of the many advantages thermoplastic composites have over the thermoset ones, the thermoplastic pultrusion market demonstrates significantly lower production volumes as compared to those of the thermoset one. Examining the thermoplastic pultrusion processes, raw materials, mechanical properties of thermoplastic composites, process simulation techniques, patents, and applications of thermoplastic pultrusion, this overview aims to analyze the existing gap between thermoset and thermoplastic pultrusions in order to promote the development of the latter one. Therefore, observing thermoplastic pultrusion from a new perspective, we intend to identify current shortcomings and issues, and to propose future research and application directions.

Differences in clotting parameters between species for preclinical large animal studies of cardiovascular devices

Several species of domestic animals are used in preclinical studies evaluating the safety and feasibility of medical devices; however, the relevance of animal models to human health is often not clear. The purpose of this study was to compare the clotting parameters of animal models to determine which animals most adequately mimic human clotting parameters. The clotting parameters of the different species were assessed in whole blood by in vitro thromboelastography using the clotting activators, such as tissue factor (extrinsic clotting screening test, EXTEM^®) and partial thromboplastin phospholipid (intrinsic clotting screening test, IINTEM^®). The measurements were performed using normal blood samples from humans (n = 13), calves (n = 18), goats (n = 56) and pigs (n = 8). Extrinsic clotting time (CT) and the intrinsic CT were significantly prolonged in calves compared to humans (249.9 ± 91.3 and 376.4 ± 124.4 s vs. 63.5 ± 11.8 and 192.5 ± 29.0 s, respectively, p < 0.01). The maximum clot firmness (MCF) in domestic animals (EXTEM^®: 77-87 mm, IINTEM^®: 66-78 mm) was significantly higher than that of humans (EXTEM^®: 59.1 ± 6.0 mm, IINTEM^®: 58.8 ± 1.5 mm, p < 0.01), and calves and goats exhibited longer time to MCF (MCF-t) than did humans and pigs (p < 0.01). Our results show that there are relevant differences in the four species' extrinsic and intrinsic clotting parameters. These cross-comparisons indicate that it is necessary to clarify characteristics of clotting properties in preclinical animal studies.

Radial MRI with variable echo times: reducing the orientation dependency of susceptibility artifacts of an MR-safe guidewire

OBJECTIVES

Guidewires are indispensable tools for intravascular MR-guided interventions. Recently, an MR-safe guidewire made from a glass-fiber/epoxy compound material with embedded iron particles was developed. The size of the induced susceptibility artifact, and thus the guidewire's visibility, depends on its orientation against B ₀. We present a radial acquisition scheme with variable echo times that aims to reduce the artifact's orientation dependency.

MATERIALS AND METHODS

The radial acquisition scheme uses sine-squared modulated echo times depending on the physical direction of the spoke to balance the susceptibility artifact of the guidewire. The acquisition scheme was studied in simulations based on dipole fields and in phantom experiments for different orientations of the guidewire against B ₀. The simulated and measured artifact widths were quantitatively compared.

RESULTS

Compared to acquisitions with non-variable echo times, the proposed acquisition scheme shows a reduced angular variability. For the two main orientations (i.e., parallel and perpendicular to B ₀), the ratio of the artifact widths was reduced from about 2.2 (perpendicular vs. parallel) to about 1.2 with the variable echo time approach.

CONCLUSION

The reduction of the orientation dependency of the guidewire's artifact via sine-squared varying echo times could be verified in simulations and measurements. The more balanced artifact allows for a better overall visibility of the guidewire.

Glass-Fiber-based MR-safe Guidewire for MR Imaging-guided Endovascular Interventions: In Vitro and Preclinical in Vivo Feasibility Study

Purpose To evaluate glass-fiber-based guidewires that are safe for magnetic resonance (MR) imaging-guided endovascular interventions by using a phantom and an in vivo swine model. Materials and Methods MR imaging-safe guidewires were made from micropultruded glass and/or aramid fibers and epoxy resin with diameters of 0.89 mm (0.035 inch) for standard and stiff guidewires and 0.36 mm (0.014 inch) for micro guidewires. MR imaging visibility and mechanical properties were assessed in a pulsatile flow model. After approval was obtained from the institutional animal care and use committee, MR imaging guidewires were evaluated for standard endovascular procedures in nine pigs. Real-time steady-state free-precession sequences were used for MR imaging-guided catheterization, balloon dilation, and stent implantation into aorto-iliac/visceral arteries and the vena cava (temporal resolution, five images per second; and spatial resolution, 150-mm field of view, and 128 × 128 matrix) with a 1.5-T clinical imager. Visualization with the guidewires was rated on a four-point scale, handling was rated on a three-point scale, and catheterization times for different vessel regions were determined by two interventional radiologists. Afterward, handling ratings and catheterization times were obtained for standard nitinol guidewires during x-ray-based fluoroscopy. Cannulation times, signal intensity in each vessel region, and visualization and handling ratings were measured for the MR imaging guidewires. Bland-Altman analysis was performed for inter- and intraobserver variability of cannulation time. Spearman rank correlation was used to compare handling of MR imaging guidewires and standard nitinol guidewires. Results MR imaging guidewires were characterized by good to excellent visibility, with a continuous artifact of 2 mm in diameter and 4 × 8-mm ball-shaped tip marker. Stiffness, flexibility, and guidance reflected comparable times for all in vitro and in vivo procedures with both the MR imaging and standard nitinol guidewires. Standard and micro MR imaging guidewires were most suitable for the iliac crossover maneuver. Phantom visceral artery cannulation was easier with standard and micro MR imaging guidewires. The stiff MR imaging guidewire provided the best support for cannulation of the swine aorta and vena cava. All interventional procedures were performed successfully without complications. Conclusion Preliminary results showed that the use of glass-fiber-based guidewires for evaluation of MR imaging-guided endovascular interventions is technically feasible and safe in a swine model, and potentially, in humans. ^© RSNA, 2017 Online supplemental material is available for this article.

Multimodal Imaging of Nanocomposite Microspheres for Transcatheter Intra-Arterial Drug Delivery to Liver Tumors

A modern multi-functional drug carrier is critically needed to improve the efficacy of image-guided catheter-directed approaches for the treatment of hepatic malignancies. For this purpose, a nanocomposite microsphere platform was developed for selective intra-arterial transcatheter drug delivery to liver tumors. In our study, continuous microfluidic methods were used to fabricate drug-loaded multimodal MRI/CT visible microspheres that included both gold nanorods and magnetic clusters. The resulting hydrophilic, deformable, and non-aggregated microspheres were mono-disperse and roughly 25 um in size. Sustained drug release and strong MRI T₂ and CT contrast effects were achieved with the embedded magnetic nano-clusters and radiopaque gold nanorods. The microspheres were successfully infused through catheters selectively placed within the hepatic artery in rodent models and subsequent distribution in the targeted liver tissues and hepatic tumors confirmed with MRI and CT imaging. These multimodal nanocomposite drug carriers should be ideal for selective intra-arterial catheter-directed administration to liver tumors while permitting MRI/CT visualization for patient-specific confirmation of tumor-targeted delivery.

Segmented nitinol guidewires with stiffness-matched connectors for cardiovascular magnetic resonance catheterization: preserved mechanical performance and freedom from heating

BACKGROUND

Conventional guidewires are not suitable for use during cardiovascular magnetic resonance (CMR) catheterization. They employ metallic shafts for mechanical performance, but which are conductors subject to radiofrequency (RF) induced heating. To date, non-metallic CMR guidewire designs have provided inadequate mechanical support, trackability, and torquability. We propose a metallic guidewire for CMR that is by design intrinsically safe and that retains mechanical performance of commercial guidewires.

METHODS

The NHLBI passive guidewire is a 0.035" CMR-safe, segmented-core nitinol device constructed using short nitinol rod segments. The electrical length of each segment is less than one-quarter wavelength at 1.5 Tesla, which eliminates standing wave formation, and which therefore eliminates RF heating along the shaft. Each of the electrically insulated segments is connected with nitinol tubes for stiffness matching to assure uniform flexion. Iron oxide markers on the distal shaft impart conspicuity. Mechanical integrity was tested according to International Organization for Standardization (ISO) standards. CMR RF heating safety was tested in vitro in a phantom according to American Society for Testing and Materials (ASTM) F-2182 standard, and in vivo in seven swine. Results were compared with a high-performance commercial nitinol guidewire.

RESULTS

The NHLBI passive guidewire exhibited similar mechanical behavior to the commercial comparator. RF heating was reduced from 13 °C in the commercial guidewire to 1.2 °C in the NHLBI passive guidewire in vitro, using a flip angle of 75°. The maximum temperature increase was 1.1 ± 0.3 °C in vivo, using a flip angle of 45°. The guidewire was conspicuous during left heart catheterization in swine.

CONCLUSIONS

We describe a simple and intrinsically safe design of a metallic guidewire for CMR cardiovascular catheterization. The guidewire exhibits negligible heating at high flip angles in conformance with regulatory guidelines, yet mechanically resembles a high-performance commercial guidewire. Iron oxide markers along the length of the guidewire impart passive visibility during real-time CMR. Clinical translation is imminent.

Advances in diagnostic radiology

Recent advances in diagnostic radiology are discussed on the basis of current publications in Investigative Radiology. Publications in the journal during 2009 and 2010 are reviewed, evaluating developments by modality and anatomic region. Technological advances continue to play a major role in the evolution and clinical practice of diagnostic radiology, and as such constitute a major publication focus. In the past 2 years, this includes advances in both magnetic resonance and computed tomography (in particular, the advent of dual energy computed tomography). An additional major focus of publications concerns contrast media, and in particular continuing research involving nephrogenic systemic fibrosis, its etiology, and differentiation of the gadolinium chelates on the basis of in vivo stability.