Numerical study of RF exposure and the resulting temperature rise in the foetus during a magnetic resonance procedure.
Numerical simulations of specific absorption rate (SAR) and temperature changes in a 26-week pregnant woman model within typical birdcage body coils as used in 1.5 T and 3 T MRI scanners are described. Spatial distributions of SAR and the resulting spatial and temporal changes in temperature are determined using a finite difference time domain method and a finite difference bio-heat transfer solver that accounts for discrete vessels. Heat transfer from foetus to placenta via the umbilical vein and arteries as well as that across the foetal skin/amniotic fluid/uterine wall boundaries is modelled. Results suggest that for procedures compliant with IEC normal mode conditions (maternal whole-body averaged SAR(MWB) < or = 2 W kg(-1) (continuous or time-averaged over 6 min)), whole foetal SAR, local foetal SAR(10 g) and average foetal temperature are within international safety limits. For continuous RF exposure at SAR(MWB) = 2 W kg(-1) over periods of 7.5 min or longer, a maximum local foetal temperature >38 degrees C may occur. However, assessment of the risk posed by such maximum temperatures predicted in a static model is difficult because of frequent foetal movement. Results also confirm that when SAR(MWB) = 2 W kg(-1), some local SAR(10g) values in the mother's trunk and extremities exceed recommended limits.
Hand
J W
JW
Imaging Sciences Department, Imperial College London, London W12 0NN, UK. j.hand@imperial.ac.uk
Li
Y
Y
Hajnal
J V
JV
D003198:Computer Simulation; D005260:Female; D005333:Fetus / Q000502:physiology / Q000528:radiation effects*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008954:Models, Biological; D009035:Mothers; D009068:Movement; D011247:Pregnancy; D011827:Radiation; D011846:Radio Waves; D013696:Temperature; D013997:Time Factors