Theoretical analysis of specimen cooling rate during impact freezing and liquid-jet freezing of freeze-etch specimens.

We have carried out a theoretical analysis of specimen cooling rate under ideal conditions during impact freezing and liquid-jet freezing. The analysis shows that use of liquid helium instead of liquid nitrogen as cooling medium during impact freezing results in an increase in a specimen cooling rate of no more than 30-40%. We have further shown that when both impact freezing and liquid-jet freezing are conducted at liquid nitrogen temperature, the two methods give approximately the same specimen cooling rate under ideal conditions except for a thin outer layer of the specimen. In this region impact freezing yields the highest cooling rate

Theoretical analysis of the ice crystal size distribution in frozen aqueous specimens.

To estimate theoretically how suited different freezing techniques are for freezing of freeze-etch specimens, it is necessary to know the relationship between specimen cooling rate and the resulting average ice crystal size. Using a somewhat simplified theoretical analysis, we have derived the approximate ice crystal size distribution of nonvitrified frozen aqueous specimens frozen at different cooling rates. The derived size distribution was used to calculate the relationship between relative change in average ice crystal size, (delta l/l), and relative change in specimen cooling rate delta (dT/dt)/(dT/dt). We found this relationship to be (delta l/l) = -k X delta (dT/dt)/(dT/dt) where k = 1.0 when specimen solidification takes place at about -6 degrees C, and k congruent to 1.3 when it takes place at about -40 degrees C

Therapeutic response during pulsed laser treatment of port-wine stains: Dependence on vessel diameter and depth in dermis

Selective photothermolysis with pulsed lasers is presumably the most successful therapy for port-wine stain birthmarks (flammeus nevi). Selectivity is obtained by using an optical wavelength corresponding to high absorption in blood, together with small absorption in tissues. Further on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to prevent thermal damage to perivascular tissues. The optical wavelength and pulse length are therefore dependent on vessel diameter, vessel wall thickness and depth in dermis. The present work demonstrates that in the case of a 0.45 ms long pulse at 585 nm wavelength, vessels of 40-60 Μm require minimum optical fluence. Smaller vessels require higher fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels also require a higher dose because the attenuation of light in blood prevents the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in the range of 6-7 J cm-2 is expected to inflict vessel rupture rather than thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energy, wavelength and pulse length are optimized with respect to vessel diameter and depth in dermis. © 1995 W.B. Saunders Company Ltd

Therapeutic response during pulsed laser treatment of port-wine stains: Dependence on vessel diameter and depth in dermis

Selective photothermolysis with pulsed lasers is presumably the most successful therapy for port-wine stain birthmarks (flammeus nevi). Selectivity is obtained by using an optical wavelength corresponding to high absorption in blood, together with small absorption in tissues. Further on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to prevent thermal damage to perivascular tissues. The optical wavelength and pulse length are therefore dependent on vessel diameter, vessel wall thickness and depth in dermis. The present work demonstrates that in the case of a 0.45 ms long pulse at 585 nm wavelength, vessels of 40-60 Μm require minimum optical fluence. Smaller vessels require higher fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels also require a higher dose because the attenuation of light in blood prevents the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in the range of 6-7 J cm-2 is expected to inflict vessel rupture rather than thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energy, wavelength and pulse length are optimized with respect to vessel diameter and depth in dermis. © 1995 W.B. Saunders Company Ltd