British Journal of Radiology (2007) 80, 483-487
© 2007 British Institute of Radiology
doi: 10.1259/bjr/69843752
EU Directive 2004/40: field measurements of a 1.5 T clinical MR scanner
S F Riches, MPhys, MSc1,
D J Collins, BA, MinstP1,
J W Scuffham, PhD, MSci3 and
M O Leach, PhD, FinstP, FMedSci2
Cancer Research UK Clinical Magnetic Resonance Group, 1 Royal Marsden NHS Foundation Trust and 2 Institute of Cancer Research, Sutton, Surrey SM2 5PT, 3 Royal Surrey County Hospital, Egerton Road, Guildford GU2 7XX, UK
Correspondence: Martin Leach, Clinical Magnetic Resonance Research Group, Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT, UK. E-mail: martin.leach{at}icr.ac.uk
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Abstract
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The European Union (EU) Physical Agents (EMF) Directive [1] must be incorporated into UK law in 2008. The directive, which applies to employees working in MRI, sets legal exposure limits for two of the three types of EMF exposure employed in MRI; time-varying gradient fields and radiofrequency (RF) fields. Limits on the static field are currently not included but may be added at a later date. Conservative action values have been set for all three types of exposure including the static field. The absolute exposure limits will exclude staff from the scanner bore and adjacent areas during scanning, impacting on many clinical activities such as anaesthetic monitoring during sedated scans, paediatric scanning and interventional MRI. When the legislation comes into force, NHS Trusts, scanner companies and academic institutions will be required to show compliance with the law. We present results of initial measurements performed on a 1.5 T clinical MRI scanner. For the static field, the proposed action value is exceeded at 40 cm from the scanner bore and would be exceeded when positioning a patient for scanning. For the RF field, the action values were only exceeded within the bore at distances of 40 cm from the scanner ends during a very RF intensive sequence; MRI employees are unlikely to be in the bore during an acquisition. For the time-varying gradient fields the action values were exceeded 52 cm out from the mouth of the bore during two clinical sequences, and estimated current densities show the exposure limit to be exceeded at 40 cm for frequencies above 333 Hz. Limiting employees to distances greater than these from the scanner during acquisition will have a severe impact on the future use and development of MRI.
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Introduction
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The Physical Agents Directive [1] was adopted by the European Union (EU) in 2004 and must become domestic law by 30 April 2008. It limits occupational exposure to electromagnetic fields (EMF) and covers workers in all sectors including medicine and MRI. It currently includes exposure to the time-varying magnetic field generated by the gradients (gradient fields, 100–1000 Hz) and the radiofrequency fields (RF, 10–100 MHz), which are both generated during image acquisition. In the current version of the Directive, the static magnetic field legal exposure limit of 2 T has been removed; however, it may still be introduced in a future version. The Directive defines absolute legal exposure limits above which even brief exposure will be illegal. For gradient fields, the exposure limits are frequency dependent and are based on the potential current induced in the body. For RF fields, the potential heating is limited by setting the specific absorption rate (SAR) limit. In addition, conservative action values are set in quantities that are easier to measure, such that compliance with the action values ensures conformity with the legal exposure limits. These are set for all three types of EMF exposure including the static field, resulting in the illogical situation in which the action value for the static field ensures compliance with a non-existent legal limit. Table 1
shows the exposure limits and action values for typical MRI frequencies.
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Table 1. Proposed exposure limit and action values for occupational exposure to electromagnetic fields at typical MRI frequencies
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Under the Directive, the employer must assess the levels of occupational EMF exposure and, as areas where the action value is exceeded allow possible overexposure, they must be indicated and working procedures altered to restricted access. Alternatively, sufficient risk assessments, calculation and modelling must be undertaken to show that the legal limits are not exceeded. Under the current version of the Directive, the biggest threat to MR practice is the limit on gradient field exposure, which will outlaw common practices, such as anaesthetists checking on sedated patients during scans and clinical staff accompanying nervous or distressed patients, and will halt the development of interventional MR procedures [2] that are replacing X-ray techniques with their associated staff and patient doses. Staff movement through a spatially varying static field will also result in exposure to low-frequency time-varying fields [3], which may breach the gradient limits. Even without the introduction of static field exposure limits, simple positioning of patients on the couch of a 1.5 T scanner might be in breach of the action values that are proposed.
The strength of EMF generated during scans is not only dependent on the field strength and scanner used but also on the sequence used for image acquisition. Currently, no full guidance has been given on the assessment or measurement of exposure to EMF, with standards from the European Committee for Electrotechnical Standardisation (Cenelec) awaited. Until publication, the Directive requires employers to use other "scientifically based" standards or guidelines. We mapped the static field around a 1.5 T Philips Intera Scanner (Version 11, Best, Netherlands) and measured the gradient and RF fields during clinical sequences to assess whether the EU Directive would have an impact on the clinical use of MRI.
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Methods
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Static field
The 0.2 T field strength action value line was mapped around the scanner with a THM7025 Hall probe (Narda Safety Test Solutions, Pfullingen, Germany). The distance of the action value from the scanner housing was found at five positions along the front and back faces of the scanner and at five positions on either side. The field strength was then measured at 10 cm intervals along the bore axis out along the scanner couch, measured from the patient landmark point of the scanner. The instrument is not traceable to a primary standard but was calibrated by plotting the measured 0.5 mT field line on the architect's building plans. The maximum discrepancy in the contour line placements was 2 cm within the magnet room. The probe was also found to agree with another Hall probe to within the instruments' errors.
Time-varying gradient fields
The gradient fields were tested using an exposure level tester (ELT) 400 and a 100 cm2 magnetic field probe (Narda Safety Test Solutions), both with valid calibration and traceable to national standards. The meter has a sampling frequency of 1.04 MHz, integrated results output of 4 Hz and operates from 1 Hz to 400 kHz. It was operated in both induced "field strength mode" (FS mode) or in "ExposureSTD". The FS mode displayed the root-mean square (RMS) of the three orthogonal measured field strengths. The ExposureSTD mode displayed the induced field as a percentage of the Directive action values by filtering the signal across the frequency range. It uses an attenuation curve simulated by first-order filters to account for the frequency response, as determined in International Commission on Non-Ionising Radiation Protection (ICNIRP) guidance on measuring compliance to ICNIRP guidelines [4].
The probe was positioned along the axis of the bore, measured from the patient landmark point as shown in Figure 1. The probe head was tilted to align the three internal orthogonal coils contained within the probe with the spatial axes of the magnet. A 40 cm diameter cylindrical phantom with a bottle phantom on either side was used to load the scanner. The integral body coil was used to transmit and receive the RF. The sequences used for the time-varying gradient field tests were a balanced fast-field echo (FFE) scan (TR/TE = 4/1.93 ms) with a 400 mm field of view (FOV) and an echoplanar diffusion-weighted image (DWI) sequence (TR/TE = 2500/69 ms) with 400 mm FOV. The sequences were chosen as frequently used sequences using high gradient strengths. Two sets of readings were taken at each position; once measuring the magnetic field strength in Tesla and once as a percentage of the Directive action values. For the FFE scan, the readings were taken at 0, 20, 40, 60, 80 and 100 cm from the patient landmark point moving out from the scanner. Because of time restraints, the DWI measurements were taken only at 0 cm (for maximum readings) and at 40 cm (around the observed action value for the FFE sequence). A second set of measurements was taken at 0 cm for the FFE scan with the FOV reduced to 200 mm to increase the gradient rate.
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Figure 1. Exposure level meter positioned on the scanner couch to assess time-varying magnetic fields due to gradients during clinical scans at varying distances from the patient landmark point along the bore axis.
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The measurements acquired in FS mode were converted to an induced current density using the ICNIRP model [4]:
where J is the current density, r is the radius of the current loop, f is the frequency of the field, 
is the conductivity of tissue and B is the magnetic flux density. A radius of 0.64 m (body) and 7 cm (head) was assumed for a typical current loop and = 0.2 S m–1 [5]. Initially, we assumed a principal frequency of 500 Hz and then varied the frequency from 0 to 5 kHz to show the effect of the frequency on the exposure level as a fraction of the frequency-based limits.
RF field
The RF field was measured using an EMR-300 Broadband RF survey meter with a Type 18.0 electric field probe (Narda Safety Test Solutions) covering 100 kHz to 3 GHz. Four measurements of the power density during a clinical sequence were performed at 10 cm intervals along the axis of the magnet bore into the scanner, measured from the patient landmark point. A 40 cm diameter cylindrical phantom with a bottle phantom on either side was used to load the scanner, and the integral body coil was used to transmit and receive the RF. The acquisition was a single-shot magnetic resonance cholangiopancreatography (MRCP) sequence (TR/TE = 8000/800 ms, flip angle = 90°) used for breath-hold pancreatic imaging, a frequently used clinical scan requiring a high amount of RF power. The scanner reported a patient SAR for the sequence of 1.19 W kg–1 for a 70 kg input weight.
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Results
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Static field
Figure 2 shows the 0.2 T static field action value mapped around the scanner. This line has been interpolated from the measurements, which were found to be symmetrical. Along the bore of the scanner, the action value was exceeded at distances of less than 42 cm out from the patient landmark point, reducing to 3 cm at the sides of the scanner in line with the isocentre.
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Figure 2. Contour of static field action value(0.2 T) interpolated from measurements taken around the scanner shown over a map of the scanner room and 0.5 mT controlled area line. Example distances (in centimetres) of the action value are shown in grey.
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Figure 3 shows the magnetic field strength decreasing as a function of distance out from the patient landmark point along the bore axis. At the landmark point, the static field is four and half times the action value.
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Figure 3. The static magnetic field as a function of distance along the axis of the scanner bore in both magnetic field strength(Tesla) and as a percentage of the Directive action values.
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Previous preliminary measurements using the same equipment on a long bore 1.5 T Vision scanner (Siemens, Erlangen, Germany) found the field at the face of the scanner above and below the couch to be 0.23 T. This discrepancy in distance of the action value from the patient landmark point results from different shielding methods and highlights the requirement for the assessment of all scanners.
Time-varying gradient fields
Electrical interference due to the exposure level tester during acquisition was seen in the acquired images, but the field measurements were stable. Figure 4 shows the gradient field as a function of distance from the patient landmark point during the FFE sequence, measured both as the RMS magnetic flux density and as a percentage of the Directive action values. The magnetic field strength measurements have been scaled to show the 100% action value as 50 µT (calculated for a 500 Hz body exposure). The action value (and body exposure limit at 500 Hz) is exceeded at a distance of 52 cm from the patient landmark point. The head exposure limit at 500 Hz is exceeded at 10 cm from the landmark point. Halving the FOV from 400 to 200 mm was found to double the exposure at 0 cm and 40 cm.
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Figure 4. The gradient field generated by a clinical balanced fast-field echo (FFE) scan at varying positions along the bore axis in both root-mean square (RMS) magnetic field strength (Tesla) and as a percentage of the Directive action values. The action value is shown as a percentage and as a flux density for exposure to a frequency of 500 Hz.
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Figure 5 shows the calculated current density as a percentage of the legal body exposure limit with varying frequency for the FFE sequence at different distances from the patient landmark point. It can be seen that the percentage increases for higher frequency components up to 1 kHz as the exposure limit remains static at 10 mA m–2. Above 1 kHz, the exposure limit decreases with f/100, and the calculated exposure remains constant as a function of the legal limit. The legal limit for body exposures is exceeded for all frequencies above 37 Hz at 0 cm, 104 Hz at 20 cm and 333 Hz at 40 cm from the landmark point.
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Figure 5. Calculated current density as a percentage of the legal exposure limits as a function of frequency(f) for three positions of the balanced fast-field echo (FFE) sequence.
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Figure 6 shows the exposure during the DWI sequence at 40 cm from the patient landmark point throughout the scan. The action values are shown at 100% for the ExposureSTD mode and at 50 µT in FS mode (calculated for a body exposure at 500 Hz). The magnetic field strength is higher for the DWI sequence than for the FFE sequence and exceeds the action values at both positions. Further analysis of the sequence is needed to explain the rise at the end of the sequence. At 0 cm, the magnetic flux density was above 1500 µT, and the ExposureSTD measurements exceeded the range of the instrument.
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Figure 6. The variation in the gradient field flux density observed during the diffusion-weighted imaging (DWI) sequence at 40 cm from the patient landmark point.
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RF fields
Figure 7 shows the RF during the MRCP sequence increasing as a function of distance into the bore. The action value is not reached for the integral body coil until a distance of 40 cm into the bore during the sequence.
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Figure 7. The radiofrequency(RF) power density observed as a function of distance into the bore from the patient landmark point.
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Discussion
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The Directive has clear implications for clinical scanning. If the static field limit were to be introduced, employers would have to undertake action to reduce employee exposure closer than 42 cm along the bore axis at any time. As the exposure limit has been removed from this version of the Directive, it is unclear what action the employer must take, as this work shows that patient positioning for all clinical MRI will require employees to be exposed to fields in excess of the action value that remains in the Directive.
The gradient field limits will severely affect current clinical MR practice. Using clinical sequences, it is possible to generate fields greatly in excess of the action values. The ICNIRP calculations of the exposure show that the legal limits can be exceeded for body exposure along the bore axis 52 cm out from the patient landmark point.
The conversion of the measurable magnetic field strength into legal current density values will be required whenever the action values are exceeded to ensure that the legal limits are not breached. In the absence of harmonized standards on assessment, measurement and calculation from Cenelec, the ICNIRP model is insufficient for broadband signals, making simple calculations impossible using these instruments.
For this scanner and these sequences and a nominal frequency of 500 Hz, the requirement to restrict access to areas in excess of the action values will restrict access closer than 40 cm along the magnet bore axis during acquisition, severely affecting interventional work, patient monitoring and patient support by staff during scanning.
Further work is needed using frequency analysis of the measured field compared with reported exposures; this is especially important as the equipment was forced out of range by routine scanning producing fields too far in excess of the Directive action values. Future measurements at a range of angles to the bore axis are also required to assess the whole-body exposure of an employee standing next to the scanner couch. Compliance with the gradient field exposure limits due to movement in the spatially varying static field has not been considered here and needs to be assessed. Full computation modelling of the current density induced in patients inside the scanner [6, 7] needs to be extended to staff standing within time-varying gradient fields close to the magnet bore.
The RF limits will have less impact as the RF fields are contained well within the bore of the scanner, and even an extremely high RF power sequence did not produce signals exceeding action values until about an arm's length into the scanner. However, higher fields and open magnets may produce different results.
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Conclusions
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These measurements have shown that the static field action value will be exceeded during clinical practice when positioning patients on the scanner. While this action value remains in the Directive, further guidance is required to indicate what employers must do to ensure compliance with the Directive in this situation. Time-varying magnetic field measurements show that clinical sequences exceed both the action values and the calculated exposure limits at distances that will severely affect safe clinical practice and will halt interventional MRI completely. The RF limits have been shown to have little impact on clinical practice for this scanner, but may be very different on open magnets where the RF field is less contained.
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Acknowledgments
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Measurement guidance and equipment were supplied by Link Microtek Ltd, with thanks to Jeff Hinsley. This work has been supported by the Cancer Research UK (C1060/A5117).
Received for publication June 15, 2006.
Revision received September 22, 2006.
Accepted for publication October 24, 2006.
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References
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- Directive 2004/40/EC of the European Parliament and of the Council of 29 April 2004 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields). Official Journal of the European Union L159 (30 April 2004) and L184 (24 May 2004)
- Keevil SF, Gedroyc W, Gowland P, Hill D, Leach MO, Ludman CN, et al. Electromagnetic field exposure limitation and the future of MRI. Br J Radiol 2005;78:973–5.[Free Full Text]
- Liu F, Crozier S. Numerical evaluation of the fields induced by body motion in or near high-field MRI scanners. Prog Biophys Mol Biol 2005;87:267–78.[CrossRef][Medline]
- International Commission on Non-Ionising Radiation Protection. Guidance on determining compliance of exposure to pulsed and complex non-sinusoidal waveforms below 100 kHz with ICNIRP guidelines. Health Phys 2003;84:383–7.[CrossRef][Medline]
- International Commission on Non-Ionising Radiation Protection. Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Phys 1998;74:494–522.[Medline]
- Gandhi O, Chen X. Specific absorption rates and induced current densities for an anatomy-based model of the human for exposure to time-varying magnetic fields of MRI. Magn Reson Med 1999;41:816–23.[CrossRef][Medline]
- Liu F, Crozier S. A distributed equivalent magnetic current based FDTD method for the calculation of E-fields induced by gradient coils. J Magn Reson 2004;169:323[CrossRef][Medline]
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Abstract
Introduction
