The Nd:YAG laser in neurosurgery

Part 1. Laboratory investigations: dose-related biological response of neural tissue

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✓ The biological response of normal cat brain to Nd:YAG laser light was studied both in vitro and in vivo to evaluate the potential safety of this laser for coagulation in brain tissue. Transmission studies revealed a blood:brain absorption ratio of 100:1 indicating the selective absorption of Nd:YAG light by hemoglobin and enabling Nd:YAG light to selectively heat blood vessels compared to brain tissue. In vivo temperature recordings and pathological evaluation demonstrated a remarkable ability of the brain to dissipate the thermal energy produced by Nd:YAG light with only a small amount of structural damage. Powers of 10 W applied for 8 seconds using a 1.2-mm focused probe resulted in a penetration depth in normal brain of only 2 mm. Thermal recordings also revealed that blood is heated to 90% of its maximum temperature within 3 seconds, while the brain temperature increases linearly as the duration of the laser pulse is increased. In addition, the localized heating of brain tissue was cooled rapidly within seconds following cessation of the laser pulse. These findings indicate that by using short, intermittent pulses of light focused upon blood vessels, damage to the surrounding tissue can be minimized, and the Nd:YAG laser can be used safely as an adjunctive measure for hemostasis in many neurosurgical procedures.

Article Information

Address reprint requests to: Thoralf M. Sundt, Jr., M.D., Department of Neurologic Surgery, Mayo Clinic, Rochester, Minnesota 55901.

© AANS, except where prohibited by US copyright law.

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Figures

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    Beam profile for Nd:YAG light delivered by the 50-mm focusing probe used for the in vivo and pathological experiments at power outputs of 5, 10, and 15 W. Continuous line: 5 W; dotted line: 10 W; and broken line: 15 W.

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    A: Transmission curves for Nd:YAG light through a 1-cm thick section of cat brain. B: Transmission of Nd:YAG light through 1 cm of heparinized arterial cat blood. C: Transmitted versus incident energy for Nd:YAG light through a 1-cm thick section of cat brain. A linear relationship is evident except at high incident energies where vaporization occurred. Each point is the mean and standard deviation of five independent measurements.

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    Temperature recordings following 10 W of Nd:YAG light applied for 1 to 8 seconds to the cat cortex in vivo at 5 mm (left) and 10 mm (right) from the cortical surface. ΔT represents the rise in temperature above baseline. Point A represents the maximum temperature rise of the blood and Point B represents the maximum temperature rise of the brain. Indicators at the top left represent the duration of the laser pulse.

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    Temperature changes at depths of 5 mm and 10 mm following 20 W of Nd:YAG light applied for 8 seconds to the cortical surface. ΔT represents the rise in temperature above baseline.

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    Maximum temperature rise for brain (left) and blood (right) at a depth of 5 mm following Nd:YAG light applied to the cortical surface at various powers and durations. ΔT is the rise in temperature above baseline. Each point is the mean and standard deviation of five independent measurements.

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    Photomicrographs showing laser lesions following 10 W of Nd:YAG light applied for 8 seconds to the cortical surface. A: Low-power view of the lesion. PAS, × 1.8. B: Interstitial and perivascular edema of the cortex. Note the delicate fibrin accumulation in the vessel lumen at the lower right and the friability of the tissue. LFB-PAS, × 116. C: Extensive interstitial edema of the white matter. H & E, × 116.

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    Gross (upper) and corresponding LBF-PAS photomicrographic sections (lower) of chronic Nd:YAG laser lesions demonstrating the sharp demarcation and corresponding size of the lesions for 5 W (left), 10 W (center), and 40 W (right) applied for 8 seconds to the cortical surface. The magnification for all the gross photographs is indicated by the rule in A, which is calibrated in millimeters. All photomicrographs are magnified × 1.7 the gross photographs above.

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    Chronic lesion demonstrating cavitation and extensive PAS-positive macrophage reaction resembling an organizing infarct following 10 W of Nd:YAG light applied for 8 seconds to the cortical surface. LFB-PAS, × 110.

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    White matter immediately underlying a chronic lesion following 10 W of Nd:YAG light applied for 8 seconds to the cortical surface. Left: Bodian stain demonstrating decreased axonal density reflecting Wallerian degeneration. × 100. Center: LFB-PAS stain showing loss of myelin and extensive gemistocytic gliosis. × 100. Right: Holzer stain demonstrating dense fibrillary gliosis. × 100.

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