MR-safe segmented Nitinol-Core guidewire design
A nitinol guidewire with typical exchange-length geometry (0.035” outer diameter and 260 cm length) was constructed using the segmented design (Fig. 1). Each segment is 10 cm long to prevent standing wave formation and avert RF-induced heating at 1.5 T [6]. Each segment consists of a 0.014” nitinol rod coated with a thin film of parylene for electrical insulation, and jacketed with thermoplastic polymer (Pebax, Zeus Inc, Orangeburg, SC) to impart a consistent profile between the rods and the connectors. The insulated rod segments are inserted and secured into the connector tubes using medical grade UV-cured adhesive (Dymax Corporation, Torrington, CT) to create the non-conducting metallic core (Fig. 2). The connectors are 5 mm long, laser-cut nitinol tubes. Insulation faults, which would risk RF-induced heating, are prevented by insulating both ends of each connector tube. The rod-connector subassembly is surrounded by polymer (Vectran) fiber-braided polyimide tubing (0.026” inner diameter, 0.032” outer diameter) to augment pushability, torque response, kink resistance, and dielectric properties.
The distal nitinol rod segment tapers from 0.014” to 0.005” over 5 cm. A 3 cm long coil (MP35N alloy, Heraeus, MN) is mounted onto the distal tip to enhance flexion and recoil (Fig. 2). This tip is jacketed with non-braided thermoplastic polymer for softness and flexibility. Pre-shaped distal tip configurations are available including J- and angled.
Iron oxide powder (Sigma Aldrich, St. Louis, MO) is blended with a UV-cure adhesive (Dymax Corporation) and applied onto the guidewire for passive visualization, at the distal and proximal end of the distal tip coil, and every 10 cm thereafter. The markers underlie a final layer of thermoplastic jacketing to avoid blood contact and to bring the outer diameter of the guidewire to 0.035”.
Mechanical tests
We tested the NHLBI passive guidewire alongside a high performance nitinol based commercial (non-segmented) comparator (Glidewire Standard GR3509, Terumo, Tokyo). Both are depicted in Fig. 3. Rod-connector subassemblies and the coiled distal tip subassemblies (n = 5 each) were tested for tensile breakage force against a stationary jaw. The industry standard minimum tensile strength for an 0.035” guidewire is 5N [15].
Flexibility of the tapered, coiled distal tip was measured as the force required to deflect 45° and 60° at 5 mm, 10 mm, and 20 mm from the tip using an Intravascular Device Testing Equipment (IDTE 2000, Machine Solutions Inc., Arizona) according to US Food and Drug Administration (FDA) guidance [16] .
Torque response and pushability tests were performed in a custom vascular model of a left heart cardiac catheterization trajectory from femoral artery across the aortic arch and aortic valve into the left ventricle. The number of rotations applied at the proximal end was plotted against the number of rotations transmitted to the distal end to evaluate torque transmission inside the vascular model. Force required to advance 65 cm through this trajectory at a constant speed was measured to assess guidewire pushability [16].
In Vitro RF-induced heating tests
Heating tests were performed in a 1.5T MR system (Aera, Siemens, Erlangen, Germany) using a balanced steady-state free precession (bSSFP) pulse sequence under typical real-time CMR operating conditions (TR/TE, 2.9/1.4 ms; flip angle, 45°, bandwidth, 1000 Hz/pixel; matrix, 192 × 108; FOV, 300 × 300 mm; GRAPPA Factor 2). The tests were performed under high-flip-angle (75°) conditions to induce a high Specific Absorption Rate (SAR).
In vitro RF-induced heating tests used an ASTM 2182 phantom [17]. Temperature was measured using a fiberoptic temperature probe (OTG-M170, Opsens Inc., Canada) with a thermal resolution of 0.1 °C and an accuracy of 0.3 °C. The probe was fed through a polyimide channel (0.009” ID, 0.011” OD) affixed alongside the guidewire using heat shrink tubing (Advanced Polymers, Salem, NH). The guidewire tip was positioned at the designated hot-spot of the phantom (6 cm depth; 11 cm off-center). The temperature probe channel extended 1cm beyond the guidewire tip.
Temperature was recorded continuously for 30 s before initiating CMR scanning, for 30 s after initiating CMR scanning, for 60 s after probe withdrawal to the guidewire tip, and then while the probe was withdrawn.
In Vivo RF-induced heating tests
Animal experiments were approved by the NHLBI Animal Use and Care Committee and performed according to contemporary NIH standards, in swine under general anesthesia after percutaneous femoral artery and vein access. Four animals underwent RF-induced heating tests (weight = 24–63 kg).
In vivo heating data were acquired through a fiber optic probe positioned at the distal tip of the guidewire to monitor RF-induced temperature rise in each animal. Probe, rectal core body temperature, and their instantaneous difference were recorded continuously. Upon femoral access through an introducer sheath (Pinnacle Terumo, 5 F), the guidewire was advanced to the aortic arch while acquiring 60-s stationary temperature data at various insertion lengths inside the body. Upon reaching the arch, a temperature-probe pull-back was performed by retracting the probe while holding the guidewire stationary to evaluate heating along the length of the guidewire.
Guidewire conspicuity under CMR
In vitro images were obtained in a phantom prepared according to ASTM F2119-07 “Standard Test Method for Evaluation of MR Artifacts from Passive Implants” [18] to assess guidewire conspicuity under CMR. The two markers tested used iron oxide powder consisting of 97 % or 99.99 % purity (trace metal-basis, product numbers: 637106, 518158, Sigma Aldrich). Conspicuity of the two markers was evaluated on GRE images (TR/TE, 612/10 ms; thickness, 5 mm; FOV, 300 × 300 mm; matrix, 128 × 128). Contrast-to-noise ratio (CNR) between the markers and phantom was calculated according to the difference method [19], and the size of The marker susceptibility artifacts was evaluated according to ASTM standard F2119-07 [18].
In vivo images were acquired using an interactive, real-time bSSFP sequence (TR/TE, 2.88/1.44 ms; thickness, 6 mm; FOV, 350 × 350 mm; matrix, 192 × 144) typically used to for CMR catheterization at our institution. This sequence was chosen over GRE by the operators because it is faster and provides higher overall Signal-to-Noise Ratio, even though the size of the susceptibility artifacts may be larger with unbalanced gradient echo techniques [20].
In Vivo heart catheterization
Left heart catheterization was performed on seven swine, including the four used for heating experiments. The NHLBI guidewire was introduced through the femoral artery and navigated without a support catheter from the femoral artery around the aortic arch and across the aortic valve into the left ventricle to assess guidewire pushability, steerability, and torquability. The guidewires were re-sterilized and re-used after catheterization cases to evaluate durability.