With the advent of clinical 7T MRI, it is necessary to explore the safety of medical implants at higher field strengths and spatial gradients to understand the risk potential in these MR environments. To date, only one model of stent has been tested for magnetically induced displacement force [1], and other work has been published on the effects of RF heating [2]. Currently, there are minimal data regarding the magnetically induced displacement force of coronary artery stents, in 7T MRI.

A range of bare metal (BMS) and drug-eluting (DES) coronary stents were tested for safety in an actively shielded Siemens Magnetom 7 Tesla investigational whole body MR scanner. The maximum gradient field was 11.7 T.m^-1and the maximum gradient product was 58.6 T2.×m^-1. Magnetically induced displacement force was determined according to the ASTM method F2052-14 [3]. The most commonly implanted stents at our large, tertiary interventional cardiology service were selected for testing as follows: Biotronik Prokinetic Energy, Boston Scientific Promus Element, Medtronic Resolute and Resolute Integrity, Abbott MultiLink8 and Xience Prime, Terumo Kaname and Tsunami Gold. Stents were composed of L605 cobalt chromium alloy (L605 CoCr), 316-L stainless steel (316-L SS) or platinum chrome alloys (PtCr); table.

Maximum Displacement Force: Stents were suspended by polypropylene monofilament so that the centre of mass of the stents was within 20 mm of magnet bore centreline plane (y=0). Multiple stents of each model were tested simultaneously such that the mass of the suture was less than 1% of the mass of the stents. Testing apparatus was moved in the z-direction until the location of the maximum deflection angle was located, and measured using a protractor with a digital image capture. The displacement force, Fm, was calculated using the following equation:

$$ Fm = m.g\tan\alpha$$

where m is the total mass of the stents, g is the acceleration due to gravity and α is the deflection angle.

Deflections less than 45˚ indicated that the magnetic attractive displacement force was less than the force due to gravity, and the object would therefore considered safe in that MR environment.

The maximum deflection angle for the L605 CoCr alloy stents (Kaname, Multi-Link8, ProKinetic Energy and Resolute Integrity) were 29 and 32° [figure/table 1]. Thus, magnetic deflection was smaller than the force due to gravity for the L605 CoCr alloy stents, and suggests that these stents are conditionally safe at 7T.

However, the maximum deflection angle for the 316L stainless steel Tsunami Gold stents was 59° and for the Promus Element, made of PtCr alloy, the maximum deflection was 62°. Therefore, 316L stainless steel and PtCr stents exhibited significantly greater deflection than force due to gravity. According to the ASTM standard, these stents may cause unsafe magnetically induced displacement forces at 7T and can not be considered conditionally safe.

According to these data, the L605 CoCr stents appear to have safe deflection properties at 7T. However 316L-SS and PtCr stents exhibit increased magnetically induced displacement forces, and may be not be considered conditionally safe at 7T. Further work involving torque and heating are required to assess coronary stent safety at 7 T.