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. <EOS>