Simulation of RF heating in a human voxel model.

<p>Temperature simulations performed using the <i>in vivo</i> human voxel model “Ella” <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061661#pone.0061661-Christ1" target="_blank">[37]</a> in conjunction with the hybrid applicator. Positioning of the voxel model and eight bow tie dipole antennas (<b>a</b>). Axial and coronal slices through the human brain together with the dielectric medium adjusted to T = 20°C (<b>b–c</b>). Simulated temperature maps for a axial and coronal slice of the human brain (<b>d–e</b>). For this purpose RF heating was conducted over 5 min using an average RF power of 50 W per channel at 298 MHz. For the center of the brain the maximum temperature was 48.6°C upon completion of the RF heating paradigm (<b>d</b>). In comparison the cranium's surface did not exceed a temperature of 43.3°C for the same heating paradigm.</p

Experimental setup of the hybrid applicator used at a magnetic field strength of 7.0 T.

<p>Picture photograph of the eight channel TX/RX hybrid applicator implemented at 7.0T together with annotations that induce the transmission channel number (<b>left</b>). Picture photograph of the experimental setup which uses the hybrid applicator together with a cylindrical phantom at 7.0T (<b>right</b>).</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

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

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

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

<i>In vivo</i> imaging of the human brain and the human heart using the bow tie antennas.

<p>Illustration of the imaging capabilities of the hybrid TX/RX applicator driven by bow tie antennas. High spatial resolution MR images of the human brain (<b>a, b</b>). A gradient echo technique was used with a spatial resolution of: (0.5×0.5×2.0) mm³, FOV = (200×175) mm², TR = 989 ms, TE = 25 ms, reference transmitter voltage U_ref = 170 V, nominal flip angle = 35°, receiver bandwidth = 30 Hz/pixel. Minimum intensity projection derived from susceptibility weighted 3D gradient echo imaging of the human brain (<b>c</b>). Imaging parameters: spatial resolution: (0.5×0.4×1.2) mm³, FOV = (184×184) mm², TR = 25 ms, TE = 14 ms, reference transmitter voltage U_ref = 170 V, nominal flip angle = 24°, 16 slices per slab, receiver bandwidth = 120 Hz/pixel, flow compensation. Short axis view of the human heart (<b>d</b>). Images were acquired using a 2D CINE FLASH technique, FOV = (360×326) mm², TE = 2.7 ms, TR = 5.6 ms, receiver bandwidth = 444 Hz/px, 30 cardiac phases, 8 views per segment, slice thickness 4 mm, spatial resolution: (1.4×1.4×4) mm³, nominal flip angle = 35°, reference transmitter voltage U_ref = 400 V.</p