Endovascular near critical fluid based cryoablation catheter and related methods

An endovascular near critical fluid based cryoablation catheter for creating an elongated lengthwise-continuous lesion in tissue can comprise an elongated shaft, a flexible distal tissue treatment section, and a distal tip. A plurality of flexible tubes can extend through the distal treatment section to transport a near critical fluid to and from the distal tip. The distal treatment section can also include a flexible fluid-sealed cover or barrier layer surrounding the delivery tubes. The cover and tubes can collectively define a space which is filled with a fluidic thermally conductive media. The thermally conductive media, fluid delivery tubes, and the cover can be arranged such that a flow of the near critical fluid through the tube bundle transfers heat between a target tissue and the distal treatment section of the catheter thereby creating the elongated lengthwise-continuous lesion in the tissue.