Experimental analysis of sapphire contact probes for Nd-YAG laser angioplasty

Laser angioplasty may offer percutaneous recanalization of occluded vessels where conventional guidewire and balloon techniques fail. Metal laser thermal angioplasty probes may, however, cause excessive thermal damage due to high tip temperatures (greater than 400.C). Therefore, contact probes made from artificial sapphire crystal designed for general laser surgery are currently being evaluated for use in laser angioplasty with continuous wave Nd-YAG energy. The sapphire modifies the laser energy in various ways, and this paper examines the physical characteristics of five types of rounded sapphire probe (SMTR, MTR, MTRL, OS, LT) and how these properties are affected by clinical usage. The laser beam profile emitted by these probes demonstrates a focal spot 1-2 mm in front of the tip. However, the forward transmission of Nd-YAG energy through the sapphires varied (SMTR, 85%; MTR, 83%; MTRL, 75%; OS, 54%; LT, 69%). Probe heating occurs owing to energy absorption within the sapphire. The surface temperature of the sapphires was measured in air by infrared thermography and the hottest region within the probes localized by an isothermographic technique. At energy settings used clinically (20 J, 10 watts for 2 s) the SMTR, MTR, and MTRL probes exhibited higher temperature rises (94-112.C) than the OS and LT probes (30.C), and heating was localized to the front surface of the former probes. Peak sapphire temperatures remained lower than those of metal probes even at higher energies. After clinical use, the MTR probe demonstrated reduced transmission, beam defocusing, and increased heating, due to surface pitting. Thus, recanalization with sapphire probes occurs by a combination of photothermal and contact thermal effects that are localized to the probe tip and may reduce the degree of thermal injury associated with metal probes. Understanding these basic properties is important to the application and development of contact probes for laser recanalization.