Occupational exposure measurements of static and pulsed gradient magnetic fields in the vicinity of MRI scanners

Abstract

Recent advances in magnetic resonance imaging (MRI) have increased occupational exposure to magnetic fields. In this study, we examined the assessment of occupational exposure to gradient magnetic fields and time-varying magnetic fields generated by motion in non-homogeneous static magnetic fields of MRI scanners. These magnetic field components can be measured simultaneously with an induction coil setup that detects the time rate of change of magnetic flux density (dB/dt). The setup developed was used to measure the field components around two MRI units (1 T open and 3 T conventional). The measured values can be compared with dB/dt reference levels derived from magnetic flux density reference levels given by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The measured motion-induced dB/dt values were above the dB/dt reference levels for both MRI units. The measured values for the gradient fields (echo planar imaging (EPI) and fast field echo (FFE) sequences) also exceeded the dB/dt reference levels in positions where the medical staff may have access during interventional procedures. The highest motion-induced dB/dt values were 0.7 T s(-1) for the 1 T scanner and 3 T s(-1) for the 3 T scanner when only the static field was present. Even higher values (6.5 T s(-1)) were measured for simultaneous exposure to motion-induced and gradient fields in the vicinity of the 3 T scanner.

MR safety: simultaneous B 0, dΦ/dt, and dB/dt measurements on MR-workers up to 7 T

“…Only one study exists in which the movement of the volunteers' heads was measured in the vicinity of 1 and 3 T MR systems [15]. It has been reported that head movements in the stray field of 3 and 7 T MR magnets (e.g.…”

Left-Ventricular Mechanical Activation and Aortic-Arch Orientation Recovered from Magneto-Hydrodynamic Voltages Observed in 12-Lead ECGs Obtained Inside MRIs: A Feasibility Study

“…Due to variations in the direction of the magnetic field outside of the MRI bore, (Siemens) manufacturer specified magnetic field information was obtained to measure the subject rotations relative to changes in the magnetic field direction(36). VMHD vectors were extracted from recorded 12-lead ECGs using techniques described in the Methods section and then rotated to the MRI reference plane (Fig.…”

“…A second limitation lies in the assumption that the signal contributions of the true ECG recordings taken inside and outside the MRI bore are equal, allowing for extraction of a pure MHD signal, which will not always be the case. The magnetic field outside the MRI scanner can, experience a variation of up to 7.3%, subject to the specific MRI scanner type utilized (36), so the results reported in Figure 3 may have large associated errors.…”

Magnetic resonance safety

“…The fast switching gradient fields are applied for spatial encoding of the MRI signal and can cause peripheral nerve stimulation and implant heating. They are also responsible for the noise in the MRI scanner room, which can reach levels of 100 dB or more and potentially lead to hearing damages [74,119–121,118,122–126,30,31,127,128,16,90,129,37,130–133,57,58,134,135,69]. …”