Magnetic resonance imaging can cause focal heating in a nonuniform phantom

Abstract

To test if the radiofrequency fields of a magnetic resonance imager could cause focal heating, two cylindrical phantoms were made from a mixture of agar and saline. The first phantom was uniform; the second was nonuniform in that a narrow bridge of agar was produced. Both phantoms were exposed to high levels of radiofrequency power (140 W) at 63 MHz and the temperature rises were measured. In the uniform phantom, the temperature increased as the radius increased. In the bridge phantom, the narrow bridge heated three times greater than at the opposite uniform periphery, and over five times the average of the uniform phantom. This experiment demonstrates that the radiofrequency fields of magnetic resonance imagers can cause focal heating if the exposed object is nonuniform. Since nonuniformity is present in the human body, as the radiofrequency power of magnetic resonance imaging techniques increases, focal heating in patients is a concern.

Optimized quadrature surface coil designs

“…The local hot spots of maximum power deposition all occur close to the conductors and where they overlap, and may be ameliorated by modifying the spacing and/or insulation. Note that, in practice, tissue heterogeneity may cause local peak SAR values to differ from those deduced for the present homogeneous models [24,25].…”

MR Imaging and Cardiac Pacemakers: In Vitro Evaluation and in Vivo Studies in 51 Patients at 0.5 T

“…MR imaging-induced heating is a possible hazard in patients with biologic implants made of metallic materials (25)(26)(27). It is generally accepted that the thermal effects resulting from gradient switching during MR imaging are negligible (28,29).…”

RF Safety Evaluation of a Breast Tissue Expander Device for MRI: Numerical Simulation and Experiment

“…When metallic implants are present in the body, the currents induced in the implants can generate a significant increase in local SAR, and related temperature in the surrounding tissue [10]–[14]. In addition, the large volume of embedded dielectric material also has the potential to alter surrounding electric fields [15]. …”