Heating applicator based on reentrant cavity with optimized local heating characteristics

Figures & data

Figure 1. Heating applicator based on reentrant cavity (right) and RF unit (left).

Figure 2. Cross section of applicator and electromagnetic field distribution of the lowest (TM010) mode in reentrant cavity.

Figure 3. Outline of electric field distributions in (a) reentrant cavity and (b) RF capacitive heating system.

Figure 4. Control and noise factors for optimal miniaturized applicator and phantom.

Table I.  Taguchi's orthogonal L8 matrix (Example).

	Inner array control factors				Outer array noise factors
Run number	C1	C2	C3	C4	N1	N2
1	1	1	1	1	R1,1	R1,2
2	1	1	1	2	R2,1	R2,2
3	1	2	2	1	R3,1	R3,2
4	1	2	2	2	R4,1	R4,2
5	2	1	2	1	R5,1	R5,2
6	2	1	2	2	R6,1	R6,2
7	2	2	1	1	R7,1	R7,2
8	2	2	1	2	R8,1	R8,2

Table II.  Levels of control factors and degenerated noise factors.

	Level (mm)				Level (mm)
Control factor	1	2	3	Noise factor	Diameter	Height
Applicator height (L)	1000	950	–	Large phantom (N1)	160	160
Outer diameter (R1)	400	350	300
Reentrant diameter (R2)	100	50	30	Small phantom (N2)	115	109
Reentrant gap size (G)	400	350	300

Table III.  Taguchi's orthogonal matrix array (L18 orthogonal array).

	Control factors [mm]				Noise factors
Run number (k)	Applicator height (L)	Outer diameter (R1)	Reentrant diameter (R2)	Reentrant gap size (G)	Large phantom (N1)	Small phantom (N2)
1	1000	400	100	400	R1,1	R1,2
2	1000	400	50	350	R2,1	R2,2
3	1000	400	30	300	R3,1	R3,2
4	1000	350	100	400	R4,1	R4,2
5	1000	350	50	350	R5,1	R5,2
6	1000	350	30	300	R6,1	R6,2
7	1000	300	100	350	R7,1	R7,2
8	1000	300	50	300	R8,1	R8,2
9	1000	300	30	400	R9,1	R9,2
10	950	400	100	300	R10,1	R10,2
11	950	400	50	400	R11,1	R11,2
12	950	400	30	350	R12,1	R12,2
13	950	350	100	350	R13,1	R13,2
14	950	350	50	300	R14,1	R14,2
15	950	350	30	400	R15,1	R15,2
16	950	300	100	300	R16,1	R16,2
17	950	300	50	400	R17,1	R17,2
18	950	300	30	350	R18,1	R18,2

Table IV.  Electric parameters used in 3D-FEM simulation.

	Relative permittivity	Conductivity (S/m)
Cavity wall	–	6.1e + 7
Air	1.0	0.00
Phantom	63	0.47

Figure 5. Definition of SAR distribution along the radical and long-axis directions in the phantom (a) used for the evaluation of heating characteristics.

Table V.  Thermal parameters used in calculation of temperature distribution.

	Density (kg/m^3)	Thermal conductivity (W/(m · K))	Specific heat (kJ/(kg · K))	Initial temperature (K)
Phantom^19	980.0	0.55	4.2	311

Figure 6. Robust optimization based on the variation of the FWHM (SNR) value of SAR distribution.

Figure 7. Optimization based on the minimization of the mean FWHM value of SAR distribution.

Figure 8. Electric field distributions in (a) optimal reentrant cavity and (b) phantom.

Figure 9. Normalized SAR distributions inside small and large phantoms.

Figure 10. Temperature distributions inside (a) small and (b) large phantoms, which correspond to the lower part of the phantom indicated by the red line in (c), after heating for 20 min with RF power adjusted for a SAR value of 20 W/kg at the center of the phantom.

Figure 11. Electric file distributions inside an oblate sphere phantom in (a) optimized applicator and (b) our previous applicator, and (c) those normalized SAR distributions.

Figure 12. Normalized SAR distributions optimized using FWHM for both r- and Z-directions and that only for the r-direction.