A collapsible treatment implement guard for a device for treating a target area of a vessel wall of a vessel within a human or animal body, the device comprising: an expandable portion for radially expanding the device from a contracted configuration allowing travel within the vessel to the target area to an expanded configuration allowing treatment of the target area; and at least one treatment implement extending radially outwardly from the expandable portion, wherein in the device's contracted configuration the implement is at least partially shielded by the guard, and in its expanded configuration the guard collapses to expose the implement for contact with the target area of the vessel wall; the guard being arranged to expel fluid from the device under the collapsing action of the guard. A protective sheath for fitting to a device for treating a target area of a vessel wall of a vessel within a human or animal body, a device for treating a target area of a vessel wall of a vessel within a human or animal body, and a method of treating one or more target areas of a vessel wall within a human or animal body are also disclosed.