Figure 1.
Basic design of the virtual antenna configuration used for electromagnetic field simulations.
Basic design of the proposed bow tie dipole antenna building block used in numerical EMF simulations (a). Eight bow tie dipole antennas placed radially around a cylindrical phantom (b). Transversal view of the virtual phantom setup together with the bow tie dipole antennas (c).
Table 1.
Synopsis of the excitation frequencies and antenna dimensions used for electromagnetic field simulations.
Figure 2.
Experimental version of the bowtie antenna used in the hybrid applicator.
Basic design and dimensions of the bow tie dipole building block used for MR imaging, MR thermometry and RF heating at 7.0 T (a). Picture photographs taken from the front, back and side of the bow tie antenna building block (b). Picture photograph of the cable trap design using semi rigid cable. Schematic diagram of the matching and tuning network connected to the antenna (d).
Figure 3.
Experimental setup of the hybrid applicator used at a magnetic field strength of 7.0 T.
Picture photograph of the eight channel TX/RX hybrid applicator implemented at 7.0T together with annotations that induce the transmission channel number (left). Picture photograph of the experimental setup which uses the hybrid applicator together with a cylindrical phantom at 7.0T (right).
Figure 4.
Synopsis of SAR simulations for frequencies ranging from 64 MHz (1.5 T) to 600 MHz (14.0 T).
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 (a). Point SAR distribution of the central axial slice of the cylindrical phantom (b). Point SAR distribution of the mid-coronal slice through the cylindrical phantom (c). A decrease in the size of the SAR hotspot was found for the axial and coronal view when moving to higher field strengths.
Table 2.
Synopsis of the specific absorption rate distribution derived from electromagnetic field simulations.
Figure 5.
RF performance of the experimental hybrid applicator.
Noise correlation matrix obtained for the decoupling of the 8 elements included in the proposed 8 channel TX/RX applicator (left). Simulated B1+-map in [µT/√kW] derived from a single element; channel 5 in this case (middle). For this purpose a transversal slice through the center of the phantom was used. For comparison the measured B1+-map is shown [µT/√kW] for the same slice and bow tie antenna element (right).
Figure 6.
Transmission fields (B1+) of the hybrid applicator at 7.0 T in the human brain.
In vivo brain B1+ maps obtained from Bloch Siegert mapping of the eight independent channels of the applicator (left). 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 (right). The B1+map shows rather uniform B1+distribution.
Figure 7.
In vivo imaging of the human brain and the human heart using the bow tie antennas.
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 (a, 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 Uref = 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 (c). Imaging parameters: spatial resolution: (0.5×0.4×1.2) mm3, FOV = (184×184) mm2, TR = 25 ms, TE = 14 ms, reference transmitter voltage Uref = 170 V, nominal flip angle = 24°, 16 slices per slab, receiver bandwidth = 120 Hz/pixel, flow compensation. Short axis view of the human heart (d). 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 Uref = 400 V.
Figure 8.
Targeted RF heating in a phantom: simulation and experiment.
Axial and coronal views of specific absorption rate (left) and temperature (middle) 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 (right). 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.
Figure 9.
2D steering of targeted RF heating in a phantom: simulation and experiment.
Axial and coronal views of specific absorption rate (left) and temperature (middle) distribution derived from EMF and temperature simulations using the 8 channel applicator, a cylindrical phantom and a 1H excitation frequency of 298 MHz. For comparison, a temperature map derived from MRTh acquisitions at 7T (298 MHz) using the TX/RX applicator is shown (right). For the experimental setup a heating period of 120 s was used. A set of phase shifts (Ch1:0°, Ch2:45°, Ch3:180°, Ch4:225°, Ch5:0°, Ch6:225°, Ch7:135°, Ch8:45°) between the bow tie antennas was used to steer the SAR and temperature hotspot towards the surface of the phantom.
Figure 10.
Simulation of RF heating in a human voxel model.
Temperature simulations performed using the in vivo human voxel model “Ella” [37] in conjunction with the hybrid applicator. Positioning of the voxel model and eight bow tie dipole antennas (a). Axial and coronal slices through the human brain together with the dielectric medium adjusted to T = 20°C (b–c). Simulated temperature maps for a axial and coronal slice of the human brain (d–e). 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 (d). In comparison the cranium's surface did not exceed a temperature of 43.3°C for the same heating paradigm.