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Stroke is the third leading cause of death (150,000/year) and the leading cause of disability in the United States. Approximately 700,000 strokes occur annually in the U.S., accounting for costs of over $26 billion/year for treatment and rehabilitation. Ischemic strokes account for approximately 80% of strokes; hemorrhage makes up the remainder. Current treatment modalities include mechanical intervention or pharmacologic thrombolytic (drug) therapy to disrupt or dissolve thrombus. Laser thrombolysis describes a process in which short laser pulses are used to disrupt or ablate the occlusive thrombus.

Thrombolytic therapy may be effective at times but thrombolytics are not indicated for all stroke victims, are not effective on all thrombus, and have associated risks, some of which may have severe consequences, particularly hemorrhage. Successful development of a new treatment modality could have significant benefits to the outcomes of stroke patients, ultimately improving mortality rates and decreasing morbidity, thereby decreasing the cost of rehabilitation and improving the quality of life in stroke patients.

Laser ablation of thrombus in a fluid environment proceeds through three stages, (1) absorption of the laser light and heating, (2) vaporization and bubble formation, (3) bubble expansion and collapse, and (4) material removal (see image at left). Many more studies of ablation in air have been performed than in an aqueous environment. The ablation studies that have been done in a fluid may be classified into two categories depending on where the laser light gets absorbed.

If the laser light is absorbed by the fluid above the clot (typically due to water absorption) then the thrombus removal relies on the laser induced vapor bubble to collapse and disrupt the clot. Our group has shown that ablation efficiency is nearly independent of absorption or radiant exposure for 1μs laser pulses. This means that ten 10mJ pulses remove the same amount of clot as a single 100mJ pulse. It also means that any wavelength strongly absorbed by thrombus is suitable for laser thrombolysis. We also reported that ablation commenced when the temperature of the gel target reached 100$^\circ$C. Finally, we have shown that laser pulse durations ranging from 10ns to 10ms can be used to ablate gelatin with equal effectiveness. Using lasers with a 1-10ms pulse duration has the practical advantage of allowing smaller laser fibers to be used and therefore making the interventional catheters more flexible.

Future work will be aimed at finding laser parameters that are safer and more effective than those currently being used. This will inform future catheter design and may lead to an important treatment for occlusive stroke.

1998

R. P. Godwin, E. J. Chapyak, S. A. Prahl, H. Shangguan, "Laser Mass-Ablation Efficiency Measurements Indicate Bubble-Driven Dynamics Dominate Laser Thrombolysis," SPIE Proceedings of Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems VIII, 3245, 4-11 (1998).

H. Shangguan, S. A. Prahl, S. L. Jacques, L. W. Casperson, K. W. Gregory, "Pressure Effects on Soft Tissues Monitored by Changes in Tissue Optical Properties," SPIE Proceedings of Laser-Tissue Interaction IX, 3254, 366-371 (1998).

H. Shangguan, K. W. Gregory, L. W. Casperson, S. A. Prahl, "Enhanced Laser Thrombolysis with Photomechanical Drug Delivery: an In Vitro Study," Lasers Surg. Med., 23, 151-160 (1998).

H. Shangguan, L. W. Casperson, D. L. Paisley, S. A. Prahl, "Photographic Studies of Laser-Induced Bubble Formation in Absorbing Liquids and on Submerged Targets: Implications for Drug Delivery with Microsecond Laser Pulses," Optical Engineering, 37, 2217-2226 (1998).

H. Shangguan, L. W. Casperson, "Estimation of scattered light on the surface of unclad optical fiber tips: a new approach," Optics Communications, 152, 307-312 (1998).

1997

E. J. Chapyak, R. P. Godwin, S. A. Prahl, H. Shangguan, "Comparison of Numerical Simulations and Laboratory Studies of Laser Thrombolysis," SPIE Proceedings of Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems VII, 2970, 28-34 (1997).

S. L. Jacques, A. D. Barofsky, H. Shangguan, S. A. Prahl, K. W. Gregory, "Laser Welding of Biomaterials Stained with Indocyanine Green to Tissues," SPIE Proceedings of Laser-Tissue Interaction VIII, 2975, 54-61 (1997).

U. S. Sathyam, A. Shearin, S. A. Prahl, "Basic Ablation Phenomena During Laser Thrombolysis," SPIE Proceedings of Diagnostic and Therapeutic Cardiovascular Interventions VII, 2970, 19-27 (1997).

U. S. Sathyam, S. A. Prahl, "Limitations in Measurement of Subsurface Temperatures Using Pulsed Photothermal Radiometry," J. Biomed. Opt., 2, 251-261 (1997).

H. Shangguan, L. W. Casperson, K. W. Gregory, S. A. Prahl, "Penetration of Fluorescent Particles in Gelatin During Laser Thrombolysis," SPIE Proceedings of Diagnostic and Therapeutic Cardiovascular Interventions VII, 2970, 10-18 (1997).

H. Shangguan, L. W. Casperson, S. A. Prahl, "Contact versus Non-contact Ablation Efficacy of Thrombus in an Aqueous Environment," Lasers Surg. Med., S9, 10 (1997 abstract only).

H. Shangguan, L. W. Casperson, A. Shearin, D. L. Paisley, S. A. Prahl, "Effects of Material Properties on Laser-Induced Bubble Formation in Absorbing Liquids and on Submerged Targets," Proceedings of the 22nd International Congress on High-Speed Photography and Photonics, 2869, 783-791 (1997).

H. Shangguan, L. W. Casperson, S. A. Prahl, "Pressure Impulses During Microsecond Laser Ablation," Appl. Opt., 36, 9034-9041 (1997).

J. A. Viator, U. S. Sathyam, A. Shearin, S. A. Prahl, "Ablation Efficiency Measurements of Soft Materials with a Small Optical Fiber," Lasers Surg. Med., S9, 4 (1997 abstract only).

1996

S. A. Prahl, H. Shangguan, M. Girsky, K. Gregory, "Localized Drug Delivery in Thrombus and Gelatin Using Microsecond Laser Pulses," Photobiol. Photochem., 63S, 39 (1996 abstract only).

U. S. Sathyam, A. Shearin, S. A. Prahl, "Visualization of Microsecond Laser Ablation of Porcine Clot and Gelatin Under a Clear Liquid," SPIE Proceedings of Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems VI, 2671, 28-35 (1996).

U. S. Sathyam, A. Shearin, E. A. Chasteney, S. A. Prahl, "Threshold and Ablation Efficiency Studies of Microsecond Ablation of Gelatin Under Water," Lasers Surg. Med., 19, 397-406 (1996).

U. S. Sathyam, "Laser Thrombolysis: Basic Ablation Studies", Oregon Graduate Institute of Science and Technology, (1996).

H. Shangguan, L. W. Casperson, A. Shearin, S. A. Prahl, "Investigation of Cavitation Bubble Dynamics Using Particle Image Velocimetry: Implications for Photoacoustic Drug Delivery," SPIE Proceedings of Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems VI, 2671, 104-115 (1996).

H. Shangguan, L. W. Casperson, A. Shearin, K. W. Gregory, S. A. Prahl, "Drug Delivery with Microsecond Laser Pulses into Gelatin," Appl. Opt., 35, 3347-3357 (1996).

H. Shangguan, L. W. Casperson, S. A. Prahl, "Microsecond Laser Ablation of Thrombus and Gelatin under Clear Liquids: Contact vs Non-contact," IEEE J. Selected Topics Quantum Electron., 2, 818-825 (1996).

H. Shangguan, "Local Drug Delivery with Microsecond Laser Pulses: In vitro Studies", Portland State University, (1996).

1995

U. S. Sathyam, A. Shearin, S. A. Prahl, "The Effect of Spotsize, Pulse Energy, and Repetition Rate on Microsecond Ablation of Gelatin Under Water," SPIE Proceedings of Laser-Tissue Interaction VI, 2391, 336-344 (1995).

U. S. Sathyam, A. Shearin, S. A. Prahl, "The Effect of Absorption Coefficient on Microsecond Ablation Thresholds Under Water," Lasers Surg. Med., S7, 5 (1995 abstract only).

U. S. Sathyam, A. Shearin, S. A. Prahl, "Effect of Bubble Dynamics on Ablation Efficiency During Microsecond Laser Ablation of Gelatin Under Water," Proceedings of the Oregon Academy of Science, 31, 52 (1995 abstract only).

U. S. Sathyam, S. A. Prahl, "Pulsed Photothermal Radiometry of Buried Light-Absorbing Layers," Bull. Am. Physical Society, (1995 abstract only).

H. Shangguan, L. W. Casperson, A. Shearin, K. W. Gregory, S. A. Prahl, "Photoacoustic Drug Delivery: The Effect of Laser Parameters on Spatial Distribution of Delivered Drug," SPIE Proceedings of Laser-Tissue Interaction VI, 2391, 394-402 (1995).

H. Shangguan, A. Shearin, S. A. Prahl, "Visualization of Photoacoustic Drug Delivery Dynamics," Lasers Surg. Med., S7, 4-5 (1995 abstract only).

H. Shangguan, L. W. Casperson, S. A. Prahl, "The Effect of Absorption Coefficient and Radiant Exposure on theThreshold of Cavitation Bubble Formation in Light Absorbing Liquids," Proceedings of the Oregon Academy of Science, 31, 51 (1995).

H. Shangguan, L. W. Casperson, L. A. Buckley, S. A. Prahl, "Quantitative Analysis of Psoralen in Blood Serum with Laser-Induced Fluorescence," Bull. Am. Physical Society, (1995 abstract only).

1994

U. S. Sathyam, A. Shearin, E. A. Chasteney, S. A. Prahl, "The Effect of Absorption on 2 μs Ablation Efficiency Under Water," Lasers Surg. Med., S6, 5 (1994 abstract only).

U. S. Sathyam, A. Shearin, S. A. Prahl, "Bubble Dynamics During 2 μs Pulsed-Dye Laser Ablation Under Water," Lasers Surg. Med., S6, 5-6 (1994 abstract only).

H. Shangguan, L. Buckley, A. Shearin, K. W. Gregory, "Integrated Laser Catheter System for Photodynamic Therapy," Lasers Surg. Med., S6, 13 (1994 abstract only).

1993

H. Shangguan, T. W. Haw, K. W. Gregory, L. W. Casperson, "Novel cylindrical illuminator tip for ultraviolet light delivery," SPIE Proceedings of Diagnostics and Therapeutic Cardiovascular Interventions III, 1878, 167-182 (1993).

H. Shangguan, "Novel Cylindrical Illuminator Tip for Ultraviolet Light Delivery: Design and Fabrication", Portland State University, (1993).