A catheter for delivering radiant energy, such as a laser beam, is used in a technique to controllably apply the radiant energy in a patient's body, such as in a blood vessel. The radiant energy is applied in a manner which erodes biological material and may be used to drill through vascular obstructions. The catheter emtis the radiant energy from its distal end in a pattern which defines a relatively small working region in which the energy density level is sufficiently high to remove the biological material. The energy disribution is substantially uniform across the beam. Distally beyond the working region, the energy density of the beam decays sharply so that biological material beyond the working region is not removed. Biological material is removed in very limited layer-like regions and in a manner in which a distally propagated segment of the beam will not have sufficient energy density to puncture the blood bessel wall. The catheter is a of elongate flexible tubular construction havign a fiber optic wave guide extending through the lumen of the catheter. The distal end of the optical fiber terminates within the catheter and is secured in alignment with an optical system at the distal end of the catheter. The optical system seals the optical fiber from contact with biological material and shapes the beam so that it is emitted from the optical system in manner which defines the working region. The catheter includes openings adjacent the distal end which communicated with the lumen and enable delivery of fluid to the blood vessel or withdrawal of material from the blood vessel.