A catheter system comprising: an outer elongated element having an outer elongated element proximal end and an outer elongated element distal end, said outer elongated element having an outer elongated element lumen extending from said outer elongated element proximal end to said outer elongated element distal end and having an outlet port at said outer elongated element distal end, and a proximal occlusion element located at said outer elongated element distal end, said proximal occlusion element proximal to said outlet port, said proximal occlusion element comprising a proximal occlusion element proximal end and a proximal occlusion element distal end; an inner elongated element having an inner elongated element proximal end and an inner elongated element distal end, and a distal occlusion element located at said inner elongated element distal end, said inner elongated element positioned within said outer elongated element lumen wherein said outer elongated element is coaxially arranged with respect to said inner elongated element, said coaxial arrangement providing a delivery lumen between said inner and outer elongated elements (22, 20) for delivery of a delivery substance to a vessel through said delivery lumen and through said outlet port, wherein said inner elongated element distal end is distal to and movable with respect to said outer elongated element distal end, and wherein said inner elongated element further comprises a pressure relief element at said inner elongated element distal end, characterised in that said pressure relief element provides controlled removal of blood from the treatment area to a location outside of the treatment area. an outer elongated element having an outer elongated element proximal end and an outer elongated element distal end, said outer elongated element having an outer elongated element lumen extending from said outer elongated element proximal end to said outer elongated element distal end and having an outlet port at said outer elongated element distal end, and a proximal occlusion element located at said outer elongated element distal end, said proximal occlusion element proximal to said outlet port, said proximal occlusion element comprising a proximal occlusion element proximal end and a proximal occlusion element distal end; an inner elongated element having an inner elongated element proximal end and an inner elongated element distal end, and a distal occlusion element located at said inner elongated element distal end, said inner elongated element positioned within said outer elongated element lumen wherein said outer elongated element is coaxially arranged with respect to said inner elongated element, said coaxial arrangement providing a delivery lumen between said inner and outer elongated elements (22, 20) for delivery of a delivery substance to a vessel through said delivery lumen and through said outlet port, wherein said inner elongated element distal end is distal to and movable with respect to said outer elongated element distal end, and wherein said inner elongated element further comprises a pressure relief element at said inner elongated element distal end, characterised in that said pressure relief element provides controlled removal of blood from the treatment area to a location outside of the treatment area.