Magnetic resonance (MR) safety and compatibility of a novel iron bioresorbable scaffold

Abstract

Fully bioresorbable scaffolds have been designed to overcome the limitations of traditional drug-eluting stents (DESs), which permanently cage the native vessel wall and pose possible complications. The ultrathin-strut designed sirolimus-eluting iron bioresorbable coronary scaffold system (IBS) shows comparable mechanical properties to traditional DESs and exhibits an adaptive degradation profile during target vessel healing, which makes it a promising candidate in all-comers patient population. For implanted medical devices, magnetic resonance (MR) imaging properties, including MR safety and compatibility, should be evaluated before its clinical use, especially for devices with intrinsic ferromagnetism. In this study, MR safety and compatibility of the IBS scaffold were evaluated based on a series of well-designed in-vitro, ex-vivo and in-vivo experiments, considering possible risks, including scaffold movement, over-heating, image artifact, and possible vessel injury, under typical MR condition. Traditional ASTM standards for MR safety and compatibility evaluation of intravascular devices were referred, but not only limited to that. The unique time-relevant MR properties of bioresorbable scaffolds were also discussed. Possible forces imposed on the scaffold during MR scanning and MR image artifacts gradually decreased along with scaffold degradation/absorption. Rigorous experiments designed based on a scientifically based rationale revealed that the IBS scaffold is MR conditional, though not MR compatible before complete absorption. The methodology used in the present study can give insight into the MR evaluation of magnetic scaffolds (bioresorbable) or stents (permanent).

Keywords: Artifact; Iron bioresorbable scaffold; MR compatibility; MR safety; Magnetic resonance imaging (MRI).

Graphical abstract

Fig. 1

General and cross-sectional illustrations of the IBS scaffold.

Fig. 2

(a) Schematic diagram of the test fixture mounted on the MR scanner during magnetically induced displacement force test, (b) torsion spring apparatus used for magnetically induced torque test, (c) an apparatus for testing of RF induced heating near the IBS scaffold, and temperature probes distribution in the measurement of RF induced heating.

Fig. 3

(a) Measured displacement forces of various IBS scaffolds and corresponding linear fitting result, (b) magnetically induced torque of IBS-40038 at various directions in the MR scanner under 3 T, (c) the maximum magnetically induced torques and corresponding equivalent forces of all the IBS specifications for coronary arteries.

Fig. 4

Typical MR images showing how the image artifact width was measured in the directions of transverse and sagittal planes (the scaffold was placed perpendicular to the main magnetic field).

Fig. 5

Ex-vivo evaluation of the scaffold-artery system under simulated force-bearing conditions, schematic test setups and corresponding artery inspection: (a) a shear stress was applied on the scaffold-artery system to see whether any relative displacement or artery injury would happen, (b) a normal stress was applied on the scaffold-artery system to see whether any puncture would happen on the dissected vessel.

Fig. 6

Scaffold movement caused by MR scanning immediately after IBS implantation at different scaffold/artery diameter ratios (position of the IBS scaffold could be identified by the distal gold marker under DSA).

Fig. 7

In-vivo and explant (embedded in agar) MR images showing MR artifacts at different implantation periods and corresponding explant micro-CT analysis.

Fig. 8

Optical coherence tomography (OCT) images of one prevailing DES Xience (Abbott), one drug-eluting bioresorbable magnesium-based scaffold Magmaris (Biotronik), and one drug-eluting bioresorbable polymeric scaffold Absorb (Abbott) compared to an IBS scaffold (Biotyx) immediately after implantation [[38], [39], [40]].

Fig. 9

MR image artifacts comparison of different scaffolds/stents (same specifications, 30015) composed of different backbone materials, including one 316L SS bare stent, one IBS scaffold, one CoCr DES (Xience), and one fully corroded IBS scaffold (nitrided iron backbone weight loss ~100%), pattern of the 316L SS bare stent was the same as the tested IBS scaffold.

Fig. 10

The correspondence between explant MR artifact width and implant (backbone) weight loss.