A method and system of ablating tissue is provided, including positioning an expandable element of a catheter in a blood vessel inflating the expandable element with a volume of refrigerant to substantially occlude the blood vessel measuring the volume of refrigerant used to inflate the expandable element correlating the measured volume to an inflated dimension of the expandable element defining at least one of a target pressure within the expandable element and a target flow rate for refrigerant delivery to the expandable element based at least in part on the inflated dimension regulating refrigerant delivery to the expandable element to attain the at least one defined target pressure within the expandable element or defined target flow rate for fluid delivery to the expandable element and ablating at least a portion of the blood vessel with the expandable element.
