A catheter capable of irradiating blood vessel walls to inhibit restenosis after angioplasty. Catheters are capable of simultaneous irradiation and angioplasty. Preferred catheters include a helical perfusion balloon having closely spaced or contacting adjacent windings when inflated. A helical balloon with strand windings having little space therebetween decreases the amount of blood between the balloon exterior surface and vessel walls. A removable, sheathed, elongate radiation source is used with some embodiments. A fixed, flexible sheath partitioning the catheter lumen into sterile and non-sterile portions is included in another embodiment. Internal rather than external perfusion and use of gas to inflate the balloon decrease the absorption of beta radiation by blood, and liquid inflation fluid, respectively.