Endovascular cryotreatment catheter

An elongated catheter device with a distal balloon assembly is adapted for endovascular insertion. Coolant injected through the device may, in different embodiments, directly cool tissue contacting the balloon, or may cool a separate internal chamber. Plural balloons may be provided, wherein a secondary outer balloon surrounds a primary inner balloon, the primary balloon being filled with coolant and acting as the cooling chamber, the secondary balloon being coupled to a vacuum return lumen to serve as a robust leak containment device and thermal insulator around the cooling chamber. One or more sensors may be disposed between the balloons or the vacuum return lumen, to detect leaks and control the flow of fluid through the device. Examples of sensors include pressure and temperature sensors, optical sensors, magnetic flow switches and flow meters.