Debate surrounding the EU Physical Agents (EMF) Directive (2004/40/EC) has focused on the validity of chosen thresholds and the likely impact that these would have on daily practice. Most controversially, regulators want to impose a 10 mA/m2 limit on the current density induced in the body by a changing magnetic field. This, they believe, will prevent workers from experiencing physiological ill-effects. However, critics have questioned the rationale for what they claim is an overly low threshold.
Numerical modelling studies indicate that imposition of such a limit would severely hamper daily R&D activities. Researchers would be barred from standing near an operating MR scanner, or moving at normal speed when close to a high-field magnet bore. To date, these predictions have been solely based on calculation. Researchers at the Sir Peter Mansfield MR Centre at the UK's University of Nottingham, have now shown how they may be validated experimentally (Phys. Med. Biol. 53 361).
"People have calculated electric fields due to movement in static magnetic field or during imaging with switched magnetic gradients, but nobody has tried to put those numerical calculations together with measurements and show that they are correct," said physicist Paul Glover. "We have demonstrated that we can do this."
Exceeding the limits
Glover and co-author Richard Bowtell, professor of physics at Nottingham, used a dual dipole electric field probe to make in vivo measurements on a human volunteer over a frequency range of 0.1 to 800 Hz. The surface electric field was taken to be the potential difference between the probe's two electrode tips divided by the distance between them. This simple technique is perfectly valid for magnetically induced effects so long as the probe is constructed correctly (see Probing MRI-induced electric fields).
The pair investigated two separate scenarios. A human subject was asked to walk, turn and move their head near a 3T magnet and to lean into the bore. Readings were taken from electrodes on the volunteer's abdomen, head and tongue. Abdominal electric field measurements were also acquired during a standard echo-planar imaging sequence with the subject positioned inside the scanner. In each case, the rate of change of magnetic field (dB/dt) was recorded simultaneously using three orthogonal search coils placed close to the electric field probe.
Using known values of tissue conductivity to calculate current density, Glover and Bowtell found that even moderate movements around the scanner produced peak current densities of around 100 mA/m2 in the volunteer's body: 10 times the maximum limit proposed in the EMF Directive. Yet the volunteer experienced none of the bioeffects linked to switching magnetic fields, such as peripheral nerve stimulation (PNS), vertigo or a metallic taste in the mouth.
The researchers also noted a striking consistency in the geometric multiplier relating dB/dt to the measured electric field. This implies that the more easily-measured magnetic-field changes could be used for dose monitoring purposes. "If we know what the value of the relevant geometric multiplier is, then a simple measurement of dB/dt could provide enough information to alert an MR operator when they were exceeding a recommended limit," Glover explained.
Glover and Bowtell have yet to compare their experimental electric field measurements against numerical calculations for the same subject. This is to be addressed next. "We want to try a range of body sizes and check that the numerical calculations that predict an electric field for certain movements are correct," Glover told medicalphysicsweb.
They also want to use the in vivo measurement technique to gain better understanding of the process of PNS. "Simulations haven't been able to predict the site of nerve stimulation, or what quantity is most important for triggering this effect. Is it the electric field, the gradient of the electric field or the electric field perpendicular to a nerve? By making real measurements, we hope that we can put this on a sound, scientific footing." Bowtell said.