Design of a water coupling bolus with improved flow distribution for multi-element superficial hyperthermia applicators

Abstract

A water bolus used in superficial hyperthermia couples the electromagnetic (EM) or acoustic energy into the target tissue and cools the tissue surface to minimise thermal hotspots and patient discomfort during treatment. Parametric analyses of the fluid pressure inside the bolus computed using 3D fluid dynamics simulations are used in this study to determine a bolus design with improved flow and surface temperature distributions for large area superficial heat applicators. The simulation results are used in the design and fabrication of a 19 x 32 cm prototype bolus with dual input-dual output (DIDO) flow channels. Sequential thermal images of the bolus surface temperature recorded for a step change in the circulating water temperature are used to assess steady state flow and surface temperature distributions across the bolus. Modelling and measurement data indicate substantial improvement in bolus flow and surface temperature distributions when changing from the previous single input-single output (SISO) to DIDO configuration. Temperature variation across the bolus at steady state was measured to be less than 0.8 degrees C for the DIDO bolus compared to 1.5 degrees C for the SISO water bolus. The new DIDO bolus configuration maintains a nearly uniform flow distribution and low variation in surface temperature over a large area typically treated in superficial hyperthermia.

Figure 1

Schematic illustration of the bolus with single inlet and single outlet (SISO) flow channels. The 3×6 DCC aperture antenna array with coaxial connectors is overlaid to indicate the 19×32 cm bolus area.

Figure 2

Numerical model of a SISO bolus illustrating the peripheral inlet and outlet flow channels and central “active area” of the bolus available for coupling a hyperthermia applicator to tissue surface.

Figure 3

Steady state velocity field in the central plane of the SISO bolus model (a) of Figure 1, calculated for 2 L/min flow rate.

Figure 4

Normalized pressure above $(P_n^1)$ and below $(P_n^2)$ the 19×32 cm bolus active area computed for the SISO bolus model of Figure 1.

Figure 5

Normalized pressure above $(P_n^1)$ and below $(P_n^2)$ the 19×32 cm bolus active area computed for the SISO bolus model (b) of Table 1.

Figure 6

Normalized pressure above $(P_n^1)$ and below $(P_n^2)$ the 19×32 cm bolus active area computed in the mid plane of the DIDO bolus model (c) of Table 1.

Figure 7

Steady state velocity field in the mid plane of the DIDO bolus with D_s=3 cm, a₀=5 mm, d₀=5 mm, D_r=1.27 cm and logarithmically spaced holes in the outlet channel.

Figure 8

Steady state velocity field inside the active area of the DIDO bolus of Figure 7. Velocity profiles (a)–(c) were calculated for the cross sectional cuts in the bolus mid plane parallel to the center of each row of the 3×6 DCC array for 2 and 4 L/min flow rates as shown in (d).

Figure 9

Prototype DIDO water coupling bolus with dual opposing inlet and outlet flow channels and logarithmically spaced orifices in the return tubing fabricated based on the fluid dynamics simulation outcome; design values: D_s=3 cm, a₀=5 mm, d₀=5 mm, D_r=1.27 cm.

Figure 10

Propagation of isothermal contour on the bolus surface for a step change in the circulating water temperature captured for the (a) SISO and (b) prototype DIDO water boluses at steady state fluid flow and 2 L/min flow rate.