3-DIMENSIONAL TEMPERATURE CONTROL OF PALLADIUM-NICKEL THERMOSEEDS - ACOMPUTER-AIDED AND EXPERIMENTAL EVALUATION

The capability of self-regulating thermoseeds to compensate for nonuni form cooling along their longitudinal axis has been investigated in th is study. For this purpose a quasi three-dimensional computer model ha s been developed. Calculations of the temperature profile in tissue wi th nonuniform heat loss demonstrated a clear improvement in the longit udinal temperature control of PdNi seeds compared to constant power se eds. Further, two strategies for improved control of nonuniform coolin g along the longitudinal axis of ferromagnetic seeds have been investi gated: (1) application of a 'normal' undivided seed; and (2) division of a long seed in smaller segments of which each segment is able to re spond more directly to local variations in the temperature distributio n. Calculations with the quasi three-dimensional model showed that the loose segments do respond more directly to their close proximity. How ever, the equilibrium temperature of a segment in an area with high lo cal blood flow will be relatively low due the limited heat production of PdNi thermoseeds. In the undivided seeds the high thermal conductiv ity of PdNi causes some levelling of the longitudinal temperature grad ient in the seed. In addition, calorimetric experiments have shown tha t the heat production of a segmented seed is less effective because of a demagnetizing field. Also, the absence of PdNi between the segments reduces the heal production of the seed.