Transmission fields (B₁⁺) of the hybrid applicator at 7.0 T in the human brain.

<p><i>In vivo</i> brain B₁⁺ maps obtained from Bloch Siegert mapping of the eight independent channels of the applicator (<b>left</b>). For B₁⁺ mapping an axial slice through the subject's brain was used. The colour scale is in units of 16 µT/√kW. B₁⁺map of the volunteers brain after B₁⁺ shimming (<b>right</b>). The B₁⁺map shows rather uniform B₁⁺distribution.</p

Basic design of the virtual antenna configuration used for electromagnetic field simulations.

<p>Basic design of the proposed bow tie dipole antenna building block used in numerical EMF simulations (<b>a</b>). Eight bow tie dipole antennas placed radially around a cylindrical phantom (<b>b</b>). Transversal view of the virtual phantom setup together with the bow tie dipole antennas (<b>c</b>).</p

MR localizer showing transversal (A) and coronal (B) views of the agarose phantom together with the position of the copper tube (green dotted line).

<p>The yellow dotted line indicates the positioning of the copper tube which corresponds to the positioning used in the numerical simulations (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049963#pone-0049963-g002" target="_blank">Figs. 2A</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049963#pone-0049963-g003" target="_blank">3</a>). Strong MR artifacts are also visible. The same position was used for the stents.</p

Experimental version of the bowtie antenna used in the hybrid applicator.

<p>Basic design and dimensions of the bow tie dipole building block used for MR imaging, MR thermometry and RF heating at 7.0 T (<b>a</b>). Picture photographs taken from the front, back and side of the bow tie antenna building block (<b>b</b>). Picture photograph of the cable trap design using semi rigid cable. Schematic diagram of the matching and tuning network connected to the antenna (<b>d</b>).</p

Synopsis of the specific absorption rate distribution derived from electromagnetic field simulations.

<p>Specific absorption rate (SAR) hotspot diameter in the axial plane for iso-SAR 90%, iso-SAR 75%, iso-SAR 50% and iso-SAR 25% contour lines obtained from EMF simulations using discrete MR frequencies ranging from 1.5 T (64 MHz) to 14.0 T (600 MHz). (O) indicates that the whole object is included in the given iso-SAR contour. (−) indicates that no such iso-SAR value was found in the given ROI.</p

Synopsis of the excitation frequencies and antenna dimensions used for electromagnetic field simulations.

<p>Dimensions of the bow tie antennas used for numerical EMF simulations. Magnetic field strengths ranging from 1.5 T (64 MHz) to 14.0 T (600 MHz) were applied. This approach was used to investigate specific absorption rate (SAR) distribution as a function of the excitation frequency.</p

Synopsis of SAR simulations for frequencies ranging from 64 MHz (1.5 T) to 600 MHz (14.0 T).

<p>Point SAR [W/kg] distributions derived from numerical EMF simulations of an 8 channel bow tie antenna applicator using discrete MR frequencies ranging from 64 MHz (1.5 T) to 600 MHz (14.0 T). Point SAR profile along a middle line through the central axial slice of the cylindrical phantom (<b>a</b>). Point SAR distribution of the central axial slice of the cylindrical phantom (<b>b</b>). Point SAR distribution of the mid-coronal slice through the cylindrical phantom (<b>c</b>). A decrease in the size of the SAR hotspot was found for the axial and coronal view when moving to higher field strengths.</p

Time course of the temperature difference during an RF heating experiment using an agarose phantom retrofitted with the stent and the control phantom.

<p>Delta temperature maps were deduced after 10 minutes (<b>A</b>), 40 minutes (<b>B</b>) and 60 minutes (<b>C</b>) of RF heating. A local temperature increase around the tips of the stents of ΔT = (22) K was observed for all time points.</p

Synopsis of the RF heating induced temperature difference near the stent tips after 10, 40 and 60 minutes of RF heating in the phantom equipped with the 40 mm stent.

<p>The temperature difference between the phantom with the stent and the control phantom (3^rd column) remains constant throughout the course of the experiment, even when the absolute temperature increase in the target slice (2^nd column) reaches ΔT = 24 K after 60 minutes of RF heating.</p

Targeted RF heating in a phantom: simulation and experiment.

<p>Axial and coronal views of specific absorption rate (<b>left</b>) and temperature (<b>middle</b>) distribution derived from EMF and temperature simulations using an 8 channel applicator together with a cylindrical phantom and a ¹H excitation frequency of 298 MHz. For comparison, a temperature map derived from MR thermometry of the same slice at 7T (298 MHz) using the TX/RX applicator is shown (<b>right</b>). For the experimental setup a heating period of 3 min was used. SAR and temperature hotspots were induced in the center of the phantom by using no phase shift between the bow tie antennas. P1–P4 indicate the location of the fiber optic temperature probes.</p