Chapter 1: MR interactions

The following interactions of a medical device can be identified for the MR environment:

1. Magnetically induced displacement force (static, dynamic)
2. Magnetically induced torque (static, dynamic)
3. Radio frequency (RF) induced heating and
4. Gradient induced heating
5. RF and Gradient induced voltages (stimulation, activation)
6. Gradient induced vibration
7. Malfunction of the device within the MR environment (dependent on individual demands)
8. Malfunction of the MR system = image quality issues (SNR, B₀-homogeneity, etc.)
9. Artifacts

Chapter 2: MR standard test procedures

Today's MR implant labeling is based on standardized testing of above mentioned interactions to be categorized under

"MR safety" point of view. ASTM standard test methods cover basic interactions of

• magnetically induced displacement force (ASTM F2052)
• magnetically induced displacement torque (ASTM F2213)
• RF-induced heating (ASTM F2182).

Further direct safety-related MR interactions are necessary to be tested. Currently these are getting introduced into standardized test procedures. The ISO draft TS 10974 is mentioning these interactions  for active implantable medical devices (AIMD) regarding

• RF and gradient interactions as well as covering
• the electromagnetic interference/compatibility (EMI/EMC) related to the malfunction of the medical device.

Interferences to be tested and categorized under "MR compatibility" are such as

• image artifacts (ASTM F2119) due to the implant's magnetic susceptibility of materials or RF-coulpling
• Signal-to-noise ratio (SNR),
• B₀-homogeneity,
• eddy currents,
• RF noise caused in the imaging volume.

These to be covered by individual procedures, whereas some can be considered from IEC 62464-1 for essential image characteristics to be tested for the MR system.

Chapter 3: MR labeling

The content of the MR labeling is based on the interactions mentioned and include according to ASTM F2305 and in FDA Guidance "Safety and Compatibility of Passive Implants" the following as extended MR labeling example; separated here into the different sections an MR implant labeling should consider; marked with bold font headlines:

Non-clinical testing has demonstrated the “DEVICE” is MR conditional. It can be scanned safely under the following conditions:

Force and torque:

By testing of magnetically induced displacement force and torque the devices showed a

magnetically induced force of 3% (equal to 2° deflection) of the limit and a
magnetically induced torque of <1% of the limit;
static magnetic field strength of 3 Tesla with a
spatial gradient magnetic field of 8.4 Tesla/meter and a
spatial gradient field product of 18.0 Tesla²/meter.

According to these test results, entering the MR magnet can be considered safe directly after implantation without safety discussion only for

static magnetic field strengths of 3 Tesla and less with a
spatial gradient magnetic field of <100 Tesla/meter and a
spatial gradient magnetic field product <100 Tesla²/meter
(values extrapolated and cut-off; extrapolation exceeds 100 by far).

Non-clinical testing has not been performed to rule out the possibility of implant migration at static gradient magnetic fields stronger than mentioned above.

RF heating:

In non-clinical testing with a 3 Tesla "MR SYSTEM NAME and MODEL", the “DEVICE” produced a maximum temperature rise of 4.5°C in a static phantom with background temperature increase of 3.4°C at a whole body averaged specific absorption rate (WBA-SAR) software displayed of “3.8” W/kg (3.4 W/kg in a phantom calorimetric test) for 15 min of continous MR scanning with transmit/receive body coil. The local body SAR shall be limited to 5.0 W/kg for using the MR body coil. (theoretical estimated WBA-SAR <1.5 W/kg)
Note that the WBA-SAR is inappropriate to scale exact local temperature increases. Local SAR can deviate and result in much higher values than the WBA-SAR software displayed. Measurement inaccuracies and additional safety margins should be taken into account. Before each individual MR procedure, it might be necessary to discuss the situation with regard to the patient benefit consulting medical experts and MR physicists.

Gradient interactions (to be considered for induced voltages/heating/vibrations (standardization work in progress)

MR image artifacts can affect the implant's surrounding by a distorted "DEVICE" length of +20% in a standard spin echo sequence and +50% in a standard gradient echo sequence. The image artifact of the “DEVICE” diameter can be +300% in a standard spin echo sequence and +540% in a standard gradient echo sequence.

More often a general sentence can be found if small artifacts only have been identified in testing:

MR image quality may be/is compromised if the area of interest is in the same area or relatively close to the position of the device. Therefore, it may be necessary to optimize MR imaging parameters for the presence of this implant.

Chapter 4: Interpreting the MR labeling

Today's Routine and available standardized MR labeling information is not the standard case even though standards are existing. Old and obsolete MR labeling information still exists for several medical devices. Therefore the MR user should be aware of risks and pitfalls if MR scanning a patient with implanted or externally attached medical device. Listed in this Eposter, there are often more parameters to be considered than following a general advice of "Safe at 3 Tesla". Also please note if there is a contraindication for MR scanning patients with devices expressed in the instructions for use of the MR system manufacturer. Important information that is needed for interpretation:

• Exact and secured data about the implant (name, model, no., etc.)
• Written original MR labeling information from instructions for use according to the example in Chapter 3 (from implant manufacturer or cooperating reliable MR safety online database: e.g. MagResource)
• Compatibility datasheet of the MR system acc. to IEC 60601-2-33 (=> system manual)
• Exact and secured data about the RF transmit/receive coils of the MR system

The MR labeling should have a section about magnetic force and torque. Important parameters to be controlled with the compatibility data sheet of the MR system are the static magnetic field with its spatial gradient magnetic field in Tesla/meter and the spatial gradient magnetic field product in Tesla²/meter. Note that static magnetic fields of less  (e.g. 1 T) than that displayed in the labeling (e.g. 3 T) exist, but can have a higher spatial gradient, which is not covered by the static magnetic field only.

The RF heating section of the MR labeling should provide sufficient data (MR system, field strength, temperatue increases, scan duration, local and average SAR (software displayed, calorimetric, background), location, configuration, orientation, MR coil, pulse sequence, etc.) from MR testing about the sensitivity of heating of a device under so called "worst-case" considerations in a specific test phantom.

This information provides an experimental example. Given data for safe MR scanning is difficult in transferring to a specific patient, especially if a high sensitivity for RF heating is displayed in the MR labeling.

The specific absorption rate (SAR) and related B1/ RF mode selections on the MR system are today the only parameter to limitate  RF power and tissue heating of the patient by decreasing these parameter adjustments in the MR sequence.

However: Note that the WBA-SAR is inappropriate to scale exact local temperature increases at implanted devices. Only under specific assumptions made in the MR labeling MR scanning might be feasible.

Before each individual MR procedure, it might be necessary to discuss the situation with regard to the patient benefit consulting medical experts and MR physicists.

Future test methods under development will provide more reliable data covering less uncertainty for the patient population.

Labeling-included data for interaction with switched gradient magnetic field should be considered (software, hardware (device location, orientation).

Information from MR labeling about MR image artifacts should be showing dimensional data for informing the MR user about the extent of the artifacts created by the device.

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Appendix: MR interactions

Following the above summary, below the most important parameters are listed for the different MR interactions (the list does not claim completeness)

1. Magnetic induced displacement force (static, dynamic).

for ferro- and paramagnetic materials (all metals) depending on

• the static magnetic field B (saturation)
• the static field gradient of the fringe field
• the magnetic saturation of the device material

2. Magnetic induced torque (static, dynamic)

torque dependent on

• the static magnetic field B (saturation)
• the device dimensions
• the magnetic saturation of the material.

(1. and 2.) Dynamic forces and torques are possible for electrically conductive devices and structures which are translated in the magnetic field. This interaction is depending on

• the speed of the movement
• the magnitude of the spatial gradient magnetic field
• the effective area of induction
• the conductivity of the device material. 

3a. Radio frequency (RF) induced heating (also 4a. RF induced voltages (stimulation, activation))

Radio frequency (RF) induced heating is a complex and multi-parameter dependent issue. RF pulses are in the area of MHz and apply the main amount of heating energy. Parameters are:

• electric conductivity and permittivity of the device (impedance of connected device parts if electronic)
• geometric dimension of the device and configuration
• surrounding tissue conductivity and permittivity
• geometric arrangement relative to
• the patient body relative to
• the specific used MR coil (electromagnetic field characteristic)
• center frequency of the specific MR system

3b. Gradient-induced heating (also 4b. Gradient-induced voltages (stimulation, activation), 5 Gradient-induced vibration)

due to switched gradient magnetic fields dependent on

• gradient amplitude (x,y,z)
• effective stimulation time of gradient pulse
• (both above = gradient slew rate)
• device position within the gradient coil
• device orientation within the gradient coil
• effective area of induction
• the conductivity of the device material

      vibration

• static magnetic field (providing counterpart)

6. Malfunction of the device within the MR environment (dependent on individual demands) and

7. Malfunction of the MR system = image quality issues (SNR, B0-homogeneity, remaining proton signals of plastics, etc.)

Interferences to or by a medical device are individually based on interactions described above. Static and dynamic magnetic fields as well as electromagnetic field and the acoustic noise pressure level can inhibit or distort the safe operation to a certain determinable amount. Device configurations and orientations in the MR environment are always considerable parameter of the matrix.