Numerical Modelling of E-M Occupational Exposures associated with MRI

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1 Numerical Modelling of E-M Occupational Exposures associated with MRI Jeff Hand and Yan Li Imaging Sciences Dept, Imperial College London, Hammersmith Hospital Campus

2 Physical Agents Directive (EMFs) 2004/40/EC Sets limits for acute occupational exposure to time-varying magnetic fields and to RF, adopted from guidelines previously issued by ICNIRP in Limits are legally enforceable Original limits would prevent some clinical procedures using MRI from being performed MRI community raised concerns, new scientific evidence suggested concerns were realistic EU funded a project to investigate examples of occupational exposure associated with MRI The Directive came into force on 30 April Originally Member States had until 30 April 2008 to implement the Directive. It has now been amended by a new Directive (2008/46/EC), which has reset the implementation date to 30 April 2012.

Limits are legally enforceable Original limits would prevent some clinical procedures using MRI from being performed MRI community raised concerns, new scientific evidence suggested

3 An investigation into occupational exposure An investigation into occupational exposure to electromagnetic fields for personnel working with and around medical magnetic resonance imaging equipment. Capstick M, McRobbie D, Hand J, Christ A, Kühn S, Hansson Mild K, Cabot E, Li Y, Melzer A, Papadaki A, Prüssmann K, Quest R, Rea M, Ryf S, Oberle M, Kuster N. Employment, Social Affairs and Equal Opportunities DG, European Commission 2008 Consortium: Foundation for Research on Information Technology in Society - IT'IS (Zurich) Imperial College London Imperial College Health Care NHS Trust (Radiological Sciences Unit) London Umeå University, Department of Radiation Physics MR Center, University Hospital Zurich MRI/background-dokumentation.htm

Employment, Social Affairs and Equal Opportunities DG, European Commission 2008 Consortium: Foundation for Research on Information Technology in Society - IT'IS (Zurich) Imperial College London

4 An investigation into occupational exposure The European Society of Radiology proposed four centres for observation of procedures and measurement of EM fields. These were selected to reflect a range of clinical and research practice and field strengths. If measured fields exceeded Action Values, then detailed numerical simulations were required to estimate exposure Use of at least 2 numerical methods was required Will discuss examples of simulations carried out using Microwave Studio at Imperial College London The following statements and recommendations do not necessarily reflect the position of the European Commission.

These were selected to reflect a range of clinical and research practice and field strengths.

5 Fields associated with MRI Static magnetic field conventional cylindrical bore scanners: 1.5 T, 3 T, 7 T open scanners: up to ~ 1 T Switched gradient fields 3 sets of orthogonal gradient coils Typically mt/m gradient T/m/s slew rate ~ 1 khz (but spectral content form 100 Hz to 10 khz) RF field (B 1 field) related to the static field through 2 f= o where is the gyromagnetic ratio Static field (T) Frequency (MHz)

25-50 mt/m gradient 100-200 T/m/s slew rate ~ 1 khz (but spectral content form 100 Hz to 10 khz) RF field (B 1

6 Modelling occupational exposure due to open 1 T MR University of Cologne Clip placement on breast prior to surgery located under real time scanning in a Philips 1 T Panorama scanner

7 Gradient Field Simulations: Frequency Scaling The simulation is carried out at a higher frequency (typically 1-5 MHz), but at which a quasi-static solution is still valid, and the tissue conductivity σ is taken to be that at the (low khz) frequency of interest f. The induced E-field E (x,y,z) at the simulation frequency f is scaled to that at the frequency of interest f, E(x,y,z) using and the induced current density J (x,y,z) at frequency f is found from the computed J (x,y,z) at frequency f using In the investigations reported here, the transient fields were simulated at 1 MHz and frequency scaling to 1 khz was applied.

The induced E-field E (x,y,z) at the simulation frequency f is scaled to that at the frequency of interest f, E(x,y,z) using and the induced current density

8 Modelling RF exposure due to open 1 T MR

Simulated data In each figure, the horizontal axis represents left to right

9 Modelling RF exposure due to open 1 T MR B 1 field in central horizontal plane for an isocentric B 1 of 5 μt (peak) and 33 % duty cycle. Measured data. Simulated data In each figure, the horizontal axis represents left to right through the isocentre of the scanner and the vertical axis from the isocentre towards the patient couch.

10 Modelling RF exposure due to open 1 T MR SAR scaled to experimentally measured B 1 fields: H = 0.27 A/m SAR whole body = W/kg SAR 10g = 0.44 W/kg Compliant with EC Directive 2004 Whole body SAR 10g head & trunk SAR 10g limbs 0.4 W/kg 10 W/kg 20 W/kg

27 A/m SAR whole body = 0.053 W/kg SAR 10g = 0.

11 Modelling gradient fields exposure due to open 1 T MR Current density distribution in section in the plane containing the maximum single voxel value (1.2 A/m 2 RMS) in CNS tissue, due to Panorama z-gradient coil. The insert is an enlargement of the distribution within the head. The colour scale is normalised to the maximum in plane value and a gradient of 26 mt/m is assumed. Current density in CNS (1 cm 2 average) x-coil y-coil z-coil ma/m 2 Up to 14x proposed legally enforceable limit of 10 ma/m 2!

The insert is an enlargement of the distribution within the head.

Modelling gradient fields exposure due to open 1 T MR E-field distribution within coronal plane containing the maximum single voxel E- field value (0.

12 Modelling gradient fields exposure due to open 1 T MR E-field distribution within coronal plane containing the maximum single voxel E- field value (0.74 V/m RMS) due to Panorama z-gradient coil. This occurs in the skin of the head. The axes are located at the scanner isocentre and the gradient is 26 mt/m. Compatible with IEEE C Safety guidelines

74 V/m RMS) due to Panorama z-gradient coil. This occurs in the skin of the head.

13 Exposure when standing close to conventional MR scanner Katholieke Universiteit Leuven 3T cylindrical bore scanner Cardiac stress test Technician s position during scanning for cardiac stress test.

14 Induced current density due to x-gradient coil Current density (ma/m 2 ) within the mid-coronal and mid-sagittal sections of the body model for the case of the maximum gradient (40 mt/m) The peak single voxel value for current density was 119 ma/m 2 RMS and occurred in the arm/hand closest to the scanner. In CNS tissues, single voxel values up to 25 ma/m 2 RMS were present, with local enhancement around the thoracic and cervical spinal regions. The maximum spatially averaged (over 1 cm 2 ) current density in CNS was 21 ma/m 2 RMS. PAD limit exceeded when closer than 45 cm form end of bore

15 Summary Microwave Studio used for numerical dosimetry of real examples of occupational exposure to E-M fields during MRI procedures In general, RF exposures are compliant with ELVs proposed in the EU Directive on Physical Agents (E-M fields) In the examples discussed, the ELV for current density in CNS tissue is exceeded for exposure to switched gradient fields Exposures to switched gradient fields were compliant with IEEE C which is based on avoidance of peripheral nerve stimulation EU Directive currently under discussion and revised limits are expected to will take account of recent data and updated guidance.

exceeded for exposure to switched gradient fields Exposures to switched gradient fields were compliant with IEEE C95.

16 Acknowledgement This work was supported by funding from EUROPEAN COMMISSION Employment, Social Affairs and Equal Opportunities DG Statements and recommendations made do not necessarily reflect the position of the European Commission. Colleagues from IT IS (Zurich), Imperial College Healthcare Trust (London), Department of Radiation Physics, Umeå University, and MR Center, University Hospital, Zurich

the position of the European Commission.

COUNCIL RECOMMENDATION of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz) (1999/519/EC)

COUNCIL RECOMMENDATION of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz) (1999/519/EC) THE COUNCIL OF THE EUROPEAN UNION, Having regard to