Effects of waterbolus size, shape and configuration on the SAR distribution pattern of the Lucite cone applicator

Figures & data

Figure 1. (a) Schematic representation of the LCA, showing the footprint of the antenna, the orientation of the feeding pin and the lucite and brass sides of the horn aperture. (b) Configuration of the model: Lucite cone applicator, waterbolus and planar muscle-equivalent phantom.

Table I.  Dielectric parameters used in the FDTD simulations.

Material	εr (−)	σ (S m^−1)
Lucite	2.59	3 × 10^−3
PVC	2.2	4 × 10^−3
De-ionized water	76	1 × 10^−3
Muscle phantom	57	1.2

Figure 2. Effective field size as a function of square waterbolus area and waterbolus height. Results from measurements (black markers) and FDTD model (white markers). The dotted line indicates the calculated EFS in the case where no waterbolus is used.

Figure 3. Effective field size as a function of waterbolus aspect ratio, for a bolus area of (a) 250 cm² and (b) 500 cm². Waterbolus height varies from 5 to 40 mm. The grey rectangles depict the extent of the waterboli relative to the LCA footprint (black icon).

Figure 4. Effective field size as a function of distance to the waterbolus edge: effect of waterbolus shift in (a) x-direction (perpendicular to the E-field; bolus shifts towards Lucite window) and (b) y-direction (parallel to the E-field; bolus shifts towards the brass wall of the horn).

Figure 5. Calculated normalized SAR distributions at 1 cm depth in muscle-equivalent phantom illustrating the effect of waterbolus shifts of 0, 25 and 50 mm in two directions. Waterbolus dimensions: 20 × 20 × 4 cm³. The waterbolus area is depicted in grey; the square indicates the LCA footprint; the contours indicate the 25% (outer solid line), 50% (bold line), 75% (inner solid line) and 95% (dotted line) SAR/SAR_max contours.

Table II.  Measured and predicted effective field size values (cm²) for several waterbolus heights and areas.

Waterbolus area (cm^2)
10 × 10			15 × 15		20 × 20		30 × 30
Waterbolus height (cm)	Meas.	Model	Meas.	Model	Meas.	Model	Meas.	Model
0.8	63.3 ± 4.2 (n = 3)	64.9*	80.7 ± 3.6 (n = 4)	79.2*	77.9 ± 4.1 (n = 4)	79.5*	75.6 ± 3.9 (n = 3)	78.1*
2.0	48.4 ± 2.5 (n = 4)	39.8	76.4 ± 5.0 (n = 4)	77.3	75.8 ± 6.3 (n = 5)	74.0	65.0 ± 0.7 (n = 2)	64.9
3.6	27.4 ± 0.7 (n = 4)	24.1*	72.1 ± 9.2 (n = 4)	76.5*	67.4 ± 5.8 (n = 4)	75.2*	53.9 ± 0.9 (n = 4)	54.1*

Model values marked * have been linearly interpolated from the complete set of model values to match the waterbolus height. Measurement values: mean ± SD (cm²).

Figure 6. Measured (thin lines) and predicted (bold lines) 50% SAR/SAR_max contours for waterbolus shifts of 0, 25 and 50 mm in both x- and y-direction. The grey lines indicate the LCA centre and footprint. Waterbolus dimensions: 20 × 20 × 4 cm³.

Table III.  Measured and predicted EFS values (cm²) for waterbolus shifts in x- and y-direction. Waterbolus dimensions: 20 × 20 × 4 cm³. Measurement values: mean ± SD (cm²).

Waterbolus shift in x-direction (cm)
Waterbolus shift in	0.0		2.5		5.0
y-direction (cm)	Meas.	Model	Meas.	Model	Meas.	Model
0.0	75.7 ± 2.3 (n = 3)	75.5	59.8 ± 1.8 (n = 3)	47.8	41.7 ± 2.6 (n = 3)	32.1
2.5	75.3 ± 3.2 (n = 3)	76.9	–	51.3	–	33.2
5.0	77.2 ± 1.0 (n = 3)	68.3	–	20.2	38.7 ± 4.6 (n = 3)	31.0

Figure 7. Illustration of SAR focusing when a small waterbolus is applied. Figures on the left (a, c, e) relate to a 10 × 10 × 4 cm³ waterbolus, on the right (b, d, f) to a 20 × 20 × 4 cm³ waterbolus. (a–d) depict Poynting vectors in the principal antenna planes: (a, b) YZ half-plane, parallel to, and (c, d) XZ half-plane, perpendicular to the E-field. L = lucite sidewall; B = brass sidewall. (e, f) the corresponding normalized SAR distributions at 1 cm depth: LCA footprint (grey) and 25% (thin), 50% (bold), 75% (thin) and 95% (dotted) contours.

Figure 8. Example of the effect of the waterbolus on the SAR distribution in a non-flat shape. (a) Configuration: a small (10 × 10 cm², dotted) and a larger (20 × 20 cm²) waterbolus are applied on a sphere (staircased). (b, c) Normalized SAR on a linear scale in the y = 0 cross-section (perpendicular to the E-field). The smaller waterbolus (b) restricts the SAR distribution as compared to the larger bolus (c).