An apparatus includes a feedback loop for controlling an extent of a gap between a wall of a balloon mounted on a catheter and a wall of a lumen into which the catheter is inserted. The apparatus includes a radiation detector mounted within the balloon for generating a feedback signal having information indicative of whether the extent of the gap is greater than or less than a desired value. The feedback loop receives the feedback signal and controls a size of the balloon to cause the extent of the gap to approach the desired value.
