[Experimental evaluation of the occupational exposure to static magnetic fields on a 3 T magnetic resonance scanner].

The recent postponement until 31 October 2013 of the deadline for transposition of the EU Directive 2004/40/EC, concerning the minimum health requirementsfor the exposure of workers to the risks arising from electromagnetic fields between 0 and 300 GHz, keeps on suspending the Italian law which was aimed to implement the EU regulations on the occupational exposure to electromagnetic fields, including those generated by Magnetic Resonance Imaging (MRI) units. Waiting for the revision of the exposure limits proposed by the EU Directive taking into account results from new studies and evolution of knowledge, the time-weighted values of static magnetic field proposed by the Italian Ministry of Health (D.M 02/08/91) still survive as limits for worker's exposure. The comparison between the proposed thresholds and the time required to position patients allows to calculate how long the MRI staff can stay at different values of static magnetic field, i.e. the maximum workload of each worker. In order to evaluate more accurately how many time the members of MRI staff are near the magnet bore and the real value of worker's exposure to the static magnetic field during the handling of patients, a teslameter Metrolab THM1176-PDA was used. Personal exposure measurements on the radiologists and the radiographers who worked on a 3 T GE Healthcare Discovery 750 MR were carried out during the positioning of self-sufficient and collaborative patients. The sensor was worn at the chest level on the side that was nearest to the magnet bore. Results show wide variations occurring between individual working procedures concerning the handling of patients, especially during the initial position phase. The mean values of the time spent by radiographers inside the magnet room (B > 0.5 mT) to place the patient and to take him outside at the end of the exam were respectively 220 and 127 seconds. The mean value of the time spent by radiologists was 162 seconds when they had to insert a peripheral vein access (arm) and inject contrast medium. The time fraction spent in magnetic flux density above 200 mT was near 31% for radiographers and about 7% for radiologists. The maximum of the static magnetic field recorded was 1550 mT for radiographers and 409 mT for radiologists. The measuring system has proven to be useful in evaluating the compliance with time weighted exposure limit stated by Italian law and also to find the maximum magnetic flux density to which the staff is actually exposed. This is the quantity of significance in evaluating workers' exposure following international guidelines.