Fig 1.
In A, the TEM and TTT coil designs with anatomically detailed head model as the load; in B, the relative permittivity maps of one of the head models; In C, the superimposed outlines of all the head models; In D, a constructed TTT coil side.
Table 1.
Anatomically detailed head models.
The 22 tissue volumes (inside each head model), identified by their constitutive properties (conductivity and dielectric constants) (σ, εr) of the different head models H1-5, are shown. The anatomically detailed head models H1-5 were created by morphing the Duke male model (H2) of the Virtual family head models (34). Models are arranged in increasing volume/mass.
Fig 2.
Simulations and network analyzer measurements.
In A), reflection (Sxx) of a representative port of the TTT coil with the 5 different head models (H1-5) using full wave FDTD simulations. The maximum variation (between different head models) in input impedance amongst all the ports using the five different head models (80 cases) was 2.3%; In B), reflection of a representative port of the TEM coil with the 5 different head models using FDTD simulations. The maximum variation (between different head models) in input impedance amongst all the ports using the five different head models (20 cases) is 3.5%. There was no re-tuning or re-matching for any of the ports in both coils; the coils were tuned/matched to H2 and used in the same configuration for the other head models; In C), experimental impedance measurements (Smith Chart) for 2 representative ports for the TTT & TEM coils. Both coils showed consistent (< 5% variation) input impedance among all four volunteers.
Fig 3.
Performance comparison of the TTT and TEM coils with three different RF shimming techniques: Quadrature/pseudo-quadrature excitation, phase-only RF shimming (per coil, one RF shim set is applied to all 5 head models), and amplitude-and-phase RF shimming (per coil, one RF shim set is applied to all 5 head models).
FDTD Calculated stats for the B1+ field and SAR for the five head models described in Fig 1 and Table 2 are shown. B1+ field homogeneity is quantified in terms of max/min, and CV in the region of interest (ROI). The ROI is defined as the whole head above and including the cerebellum and excluding the nasal cavities for all head models. The SAR performance is presented in terms of relationships between peak local SAR, average SAR, and B1+ field. Each line in each subfigure represents the mean value.
Fig 4.
B1+ distribution for quadrature (TEM) and pseudo-quadrature (TTT) excitation.
FDTD simulation data matching stats and conditions described in Table 2 and Fig 3. Axial slices were plotted with steps of 6.35 or 12.7 mm, slice numbers are indicated on top of each slice for instance (30, 26, … 2, -2, -4, …-10 etc.). Slices are plotted every 6.35 mm from (slice -10) through (slice 2) to capture the end of cerebellum in the head model, and every 12.7 mm subsequently to visualize the B1+ field distribution for the five head models.
Fig 5.
B1+ distribution for phase-only RF shimming.
FDTD simulation data matching stats and conditions described in Table 2 and Fig 3. Description is provided in Fig 4 caption.
Fig 6.
B1+ distribution for amplitude-and-phase RF shimming.
FDTD simulation data matching stats and conditions described in Table 2 and Fig 3. Description is provided in Fig 4 caption.
Table 2.
Statistics for TEM and TTT coils for quadrature and pseudo-quadrature excitation, phase-only RF shimming, and phase-and-amplitude RF shimming.
The average head model mass is 4.56kg (with 14% maximum variation among the models), the average brain volume is 1.51 L (with 15% maximum variation), and the average Eccentricity (major/minor axes) is 1.25 (with 37% maximum variation).
Fig 7.
Experimentally obtained B1+ maps in two volunteers using the TEM and TTT coils.
The color scale ranges from 0 to the maximum B1+ for each subject.
Fig 8.
SAR (W/Kg for 10 g) distribution for 2μT mean B1+ in the ROI in all head models H1-5 in the TEM and TTT coils.
Exemplar axial, sagittal, coronal slices of SAR are shown. The distributions are plotted to the same maximum of 12 W/Kg for 10 g. The SAR is plotted for pseudo-quadrature arrangement, phase-only RF shimming (one RF shim set for coil applied on all 5 head models), and amplitude-and-phase RF shimming (one RF shim set for coil applied on all 5 head models). Please see Figs 2–6 and Table 2 for the conditions under which the SAR distributions are plotted.