Finite volume analysis of temperature effects induced by active MRI implants with cylindrical symmetry: 1. Properly working devices

Table 3 Geometrical data and power losses for example resonators

Resonator	No		1	2	3
Radius of ring resonator	R_imp	[mm]	18	5.55	1.5
Length of ring resonator	L_imp	[mm]	50	9.9	14
Dimension of ring wire axial	x_wire	[mm]	0.25	0.15	0.10
Dimension of ring wire radial	r_wire	[mm]	0.25	0.15	0.10
Number of rings (turns)	n_r		2	6	12
Volume of implant	V_imp	[cm³]	50	1	0.1
Quality factor	Q		5	12.5	80
Total power loss of resonator	P_loss	[mW]	1000	50	32
SAR inside resonator (Eq. (3b))	SAR_internal	[W/kg]	1.8	1.7	5.1
Power loss of resonator due to SAR	P_SAR	[mW]	0.09	0.0016	0.0005

Power losses P_SAR and P_loss are calculated for the MR sequence of Table 2. SAR_internal is evaluated with Eq. (3b). P_SAR can be calculated using SAR_internal and the mass inside the inductor (m_imp = ρ_tissue V_imp), whereas P_loss is calculated with the appropriate constants according to Eq. (7). P_loss for resonator no 2 was verified experimentally with temperature increase measurements after placing the resonator inside a phantom with known heat capacity.

ISSN: 1475-925X