A catheter for plaque stabilisation by cryotherapy, the catheter comprising: a first tube with a first end for receiving a flow of a coolant and a second end for supplying the flow of the coolant to a cooling element; a second tube, with a first end and a second end, wherein the second end of the second tube is configured to receive a flow of the coolant from the cooling element such that the second tube provides a flow path of the coolant from the second end of the second tube to the first end of the second tube; an inflatable flexible heat transfer element on the outer surface of the catheter, wherein the cooling element is encompassed by the flexible heat transfer element; a conduit for supplying an inflation fluid for inflating the flexible heat transfer element, wherein, when the flexible heat transfer element is inflated by said inflation fluid, the cooling element is within said inflation fluid and thereby in direct thermal conductivity with the inflation fluid; and a guide wire lumen extending through the flexible heat transfer element along a path that is external to the first tube, the second tube and the cooling element; wherein the cooling element is non-expandable, has a diameter that is larger than a diameter of the second tube, and comprises an elongate tubular wall defining an elongate cooling chamber therein, the cooling chamber has a first end that is in fluid communication with the second end of the second tube and the cooling chamber has a second end that is closed; and wherein the cooling chamber is configured such that when the coolant is supplied to the first end of the first tube as a liquid, at least some of the coolant undergoes a phase change in the cooling chamber and returns through the second tube as a gas. a first tube with a first end for receiving a flow of a coolant and a second end for supplying the flow of the coolant to a cooling element; a second tube, with a first end and a second end, wherein the second end of the second tube is configured to receive a flow of the coolant from the cooling element such that the second tube provides a flow path of the coolant from the second end of the second tube to the first end of the second tube; an inflatable flexible heat transfer element on the outer surface of the catheter, wherein the cooling element is encompassed by the flexible heat transfer element; a conduit for supplying an inflation fluid for inflating the flexible heat transfer element, wherein, when the flexible heat transfer element is inflated by said inflation fluid, the cooling element is within said inflation fluid and thereby in direct thermal conductivity with the inflation fluid; and a guide wire lumen extending through the flexible heat transfer element along a path that is external to the first tube, the second tube and the cooling element; wherein the cooling element is non-expandable, has a diameter that is larger than a diameter of the second tube, and comprises an elongate tubular wall defining an elongate cooling chamber therein, the cooling chamber has a first end that is in fluid communication with the second end of the second tube and the cooling chamber has a second end that is closed; and wherein the cooling chamber is configured such that when the coolant is supplied to the first end of the first tube as a liquid, at least some of the coolant undergoes a phase change in the cooling chamber and returns through the second tube as a gas.