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A type of miniature catheter, capable of conveying therapy directly to brain tumors, can pass through narrow, tortuous vessels inaccessible to conventional catheters without damaging the endothelium.
The toposcopic—or "topo"—catheter, a flexible thin-walled polyurethane tube measuring 1 mm in diameter, can travel through vessels in which other narrow catheters would either buckle because they are too flexible, or get caught in turns because they are too firm. The difference is in the toposcopic catheter's way of advancing.
Instead of being pushed from behind or pulled by the action of blood flow, the topo everts from the distal end of a conventional catheter that has been advanced as far as it can go. After each portion of the topo emerges, it remains motionless where it has been placed, like a miniature tank tread, while the continuing eversion lays down additional portions further on in the vessel (Figure). The tube thus advances
Doherty J. Toposcopic catheter traverses narrow vessels. JAMA. 1984;252(9):1108–1109. doi:10.1001/jama.1984.03350090006004
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