Interaction between the RF Field of MRI Apparatus and Pacemakers

Abstract

Magnetic resonance imaging (MRI) is a tomography technique that measures the radio frequency (RF) field produced by the magnetic moments of hydrogen nuclei during their precession following the application of RF pulses, superimposed to a static magnetic field. MRI is a widely accepted tool for the diagnosis of a variety of diseases. Nowadays, however, MRI is contraindicated for patients implanted with pacemakers (PM) due to possible adverse effects (ICNIRP, 2004). The most important of such effects seems to be the heating of the heart tissue around the catheter tip resulting from the high currents induced on the catheter by the RF field used in MRI technique (Nyenhuis, 2005). In order to avoid thermal hazards resulting from MRI investigations, international agencies have issued guidelines reporting recommended limits. The International Commission on NonIonizing Radiation Protection (ICNIRP, 2004) considers that for whole-body exposures to MRI apparatus, no adverse health effects are expected if the increase in body core temperature does not exceed 1 °C. With regard to localized heating, ICNIRP assumes that adverse effects are avoided with a reasonable certainty if temperature remains lower than 38 °C in localized regions of the head, lower than 39 °C in the trunk, and less than 40 °C in the limbs. Accordingly, in ICNIRP (2004) limitations have been reported on the maximum power, expressed in terms of specific absorption rate (SAR), namely power per unit mass of tissue (W/kg), that can be dissipated inside the patient body during MRI investigations. More in detail, such limitations refer to whole body SAR (SARWB) and local SAR as averaged over 10 g of tissue (SAR10). In particular, in normal conditions, the SARWB should not exceed 2 W/kg, while the SAR10 is limited to 20 W/kg in the extremities and 10 W/kg in the head and trunk. The above reported SAR limitations should ensure safe MRI investigations for normal patients, but might not guarantee adequate protection for PM holders, mainly because of the previously mentioned catheter tip heating (Bernardi et al., 2009). To clarify this issue, the amount of heating in PM holders during MRI sessions has been investigated in several in vivo, in vitro, and numerical studies and reported temperature elevations spread from not significant values up to tens of degrees. Hence, in this Chapter, the studies available in the literature and concerning SAR distributions and temperature increments in PM holders, exposed to the field generated by MRI apparatus, will be described in order to evidence the main factors influencing the obtained SAR values and temperature increments. After a preliminary description of MRI apparatus in terms of field sources and distributions, the main in vitro (on phantoms), and numerical literature results will be presented. In