Figure 2.

(a) Photograph of an actively-tracking interstitial brachytherapy catheter, composed of an active metallic stylet, enclosed by a plastic cylindrical cover. The stylet fits into the conventional cover since it has the same external dimensions as a conventional stylet. The white box at the proximal end contains SMB adaptors to connect the three micro-coaxial cables to the receiver box. (b) Enlargement (red dash box in the upper panel) of the distal portion of the active stylet. Three flexible printed circuit (FPC) tracking coils (yellow arrows) were mounted onto the three slots on the surface. (c) Design pattern used in construct of FPC coil. Each coil was built on a double-layered flexible printed circuit sheet, consisting of four rectangular conductive loops. (d) Circuit diagram for each receiver channel in the electrical isolation box (dashed line), which is placed between each micro-coil on the stylet and a single channel in the 8-channel MRI receiver box. The two 0.1 µF capacitors limit leakage currents to less than 10 µA and the pin diode provides the active decoupling signal expected by the receiver.