HUGUVAC MRI SAFE EVAL MRIdt

The VetORSolutions “MRI Safe HUG-U-VAC” positioning system (“System”) (produced by VetORSolutions, “Client”) is potentially MR-Safe (i.e., poses no potential hazard when used within the MR scanner environment). The purpose of this report is to review this system and provide an assessment of its potential for safe use within the MR scanner environment. The system is depicted in

Figure 1.

Figure 1: Photograph of the “MRI Safe HUG-U-VAC” positioning system.

 

The system composition details as provided by Client are as follows:

• urethane (external shell);
• polystyrene beads (internal fill material);
• plastic, nylon, brass (valve). 

The Client indicated that there are no metallic components (i.e., no components that would be either electrically conductive or magnetic) within the system other than a single brass screw (within the valve structure) of 19 mm length and 2.4 mm diameter. MRIdt did not conduct any evaluation of the composition of the system. 

Client provided MRIdt with a sample of the System for inspection (Figure 1). The sample was introduced into a 3 T MR environment, and no detectable forces or torques on the system were observed (Figure 2). This is consistent with the composition as indicated by Client. The assessment in the 3 T environment was qualitative and did not meet the specific requirements of any ASTM or other test standard. No measurements or simulations of radiofrequency (RF) induced heating (at either 64 or 128 MHz) were included as part of this assessment.

Figure 2: Photograph of the System within the bore of a 3 T MR scanner. The System did not demonstrate any detectable magnetic force or torque within this environment.

 

Physical Parameters of Primary Importance:

• The interactions that a device will have with the MRI environment are primarily based on two physical parameters:
          o the magnetic susceptibility of the material will determine the potential for the device to interact with the static magnetic field of the MR system. In particular, the magnetically induced force, torque, and image artifacts are primarily influenced by the
  magnetic susceptibility of the material comprising the device [1].
          o The electrical conductivity of the material will primarily determine the interactions the device can have with time-varying magnetic fields (either gradient or radiofrequency)
  in the MR system. Devices with very low conductivity (i.e., high resistivity) will generally
  have little to no interaction with the time-varying magnetic fields [2]. Based on the recommendations of [5], materials with electrical conductivities of less than 2 S/m may
  be defined as electrically non conductive.
  
• The relative permittivity of the material can additionally influence the interaction a device would have with the radiofrequency electric and magnetic fields; however, in the absence of significantly high electrical conductivity, the permittivity has negligible effect [2].

Assessment of Critical Physical Parameters for Materials Under Consideration: 

•  Positioner:
     o Composition:
           Urethane and polystyrene beads
     o Magnetic susceptibility: the exact magnetic susceptibility of these materials are not specified by the manufacturer or within known material databases (e.g.,
  www.matweb.com). Because the materials are stable plastic polymers, there are not
  expected to be unpaired electrons within the polymer molecules and therefore the
  materials would be expected to be diamagnetic. Magnetic susceptibilities for most
  polymer materials are diamagnetic and within 1 ppm of water [3,4]. For reference, the susceptibility of average human tissue (equivalent to water [1]) is also approximately - 10 ppm [1]. There is no data to indicate that unfilled urethanes or polystyrenes have a magnetic susceptibility significantly different from any other chemically stable plastic polymer material.
     o Electrical conductivity: the electrical resistivity of these materials is not specified by the manufacturer or within known material databases (e.g., www.matweb.com). In general, unless specific additives or processes are included to increase the electrical conductivity of a urethane or polystyrene, the electrical conductivity of these materials (like most plastic polymers) is very low (i.e., the electrical conductivity may be assumed to be less than 2 S/m), and there is therefore no expectation that the sample would interact with the time-varying electromagnetic fields within the MRI system [5]. 

 

•  Valve (non-metallic components):
  o Composition:
       Plastic, nylon
  o Magnetic susceptibility: the exact magnetic susceptibilities of these specific materials are not specified by the manufacturer or within known material databases (e.g., www.matweb.com). Because the materials are all stable plastic polymers, there are not expected to be unpaired electrons within the polymer molecules and therefore the materials would be expected to be diamagnetic. Magnetic susceptibilities for most plastic polymer materials are diamagnetic within 1 ppm of water [3,4]. For reference, the susceptibility of average human tissue (equivalent to water [1]) is also approximately -10 ppm [1]. There is no data to indicate that any plastic materials used in the dressing components have a magnetic susceptibility significantly different from any other plastic polymer material.
  o Electrical conductivity: the electrical resistivity of these materials is not specified by the manufacturer or within known material databases (e.g., www.matweb.com). In
  general, unless specific additives or processes are included to increase the electrical
  conductivity of a plastic polymer, the electrical conductivities of these materials are
  very low (i.e., the electrical conductivity may be assumed to be less than 2 S/m), and
  there is therefore no expectation that the dressing components would interact with
  the time-varying electromagnetic fields within the MRI system [5].

 

• Valve (brass component):
  o Composition:
       Brass set-screw (the valve control arm contains a small (less than 20 mm length)
  brass screw).  

o Magnetic susceptibility: Brass is generally an alloy of copper (susceptibility of approximately -10 ppm [1]) and zinc (susceptibility of approximately -17 ppm [1])). The specific alloy of brass used for the screw was not known at the time of assessment. The magnetic susceptibility of brass is generally similar to that of its constituent components (slightly diamagnetic, susceptibility typically between -20 and -10 ppm). For reference, the susceptibility of average human tissue (equivalent to water [1]) is also approximately -10 ppm [1]. Brass does not normally demonstrate any significant interaction with the static magnetic fields of an MRI system. This is consistent with the observation that the sample did not demonstrate any detectable interaction with a 3 T scanner (Figure 2).

o Electrical conductivity: the electrical resistivity of brass is very high (essentially equivalent to other excellent conductors like copper). As a result, the brass screw does have the potential to interact with the time-varying electromagnetic fields of the MRI system (both gradient and radiofrequency fields) [5]. However, because of the very small size of the screw (less than 20 mm in length), no significant interactions with the radiofrequency electromagnetic fields of the MR scanner (at either 64 or 128 MHz) are expected. No physical testing or simulation of RF-induced heating was conducted as part of this assessment.

Based on the analysis and considerations described above, the VetORSolutions “MRI Safe HUG-UVAC” positioning system is expected to exhibit no significant or unsafe interactions with the MRI system environment. Per the terminology in ASTM F2503-23, these devices would be considered MR Safe.

Associated labelling for the device would follow the recommendations of Section VIII, Part A of [5]: “The VetORSolutions “MRI Safe HUG-U-VAC” positioning system is MR Safe.”

References:

[1] J.F. Schenck. “The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds”. Med Phys 23(6):815-850 (1996).

[2] J.A. Nyenhuis, et al. “MRI and Implanted Medical Devices: Basic Interactions With an Emphasis on Heating”. IEEE Trans Dev Mat Reliability 5(3):467-480 (2005). 

[3] P.T. Keyser, et al. “Magnetic Susceptibility of some materials used for apparatus construction (at 295 K)”. Rev Scientific Instruments 60:2711-2714 (1989). 

[4] M.C. Wapler, et al. “Magnetic properties of materials for MR engineering, micro-MR and beyond”. Journal of Magnetic Resonance 242:233-242 (2014).

[5] FDA Guidance Document, “Testing and Labeling Medical Devices for Safety in the Magnetic Resonance (MR) Environment, Guidance for Industry and Food and Drug Administration Staff”. 10 October 2023.

Attachments:

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Report Approvals.

Original signatures maintained on-file.

Revision History.

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