Biomedical subjects

Guillermo Aguilar

Publications and source records attributed to Guillermo Aguilar.

At least 19 recordsLinked to original sources

PubMed2006

Laser-assisted cryosurgery of prostate: numerical study.

A new methodology for preventing freezing damage beyond pre-specified boundaries during prostate cryosurgery is proposed herein. It consists of emitting controlled laser irradiation from the urethra, across the wall and into the prostate while conventional cryoprobes freeze the unwanted prostate tissue. The purpose of this methodology is to protect the urethral wall better and confine the desired cryoinjured region more accurately than the current cryosurgery approach. We also explore the potential use of light-absorbing dyes to further enhance the laser light absorption and corresponding heat generation to increase the thickness of the protected region. A finite difference heat diffusion model in polar coordinates with temperature-dependent thermophysical properties simulates the prostate freezing while laser irradiation across the urethral wall is emitted. This approach maintains the temperature of the urethral wall and the adjacent tissue above a pre-specified threshold temperature of -45 degrees C, independent of application time. Temperature contours resulting from prostate cryoablation with (a) conventional constant temperature heating; (b) laser irradiation heating; and (c) laser irradiation heating with pre-injected light-absorbing dye layers indicate that the thickness of the protected region increases in this order, and that the latter two methodologies may be more effective in limiting cryoinjury to a predefined region compared to constant temperature heating. An analysis of laser power requirements and sensibility of laser-assisted cryosurgery (LAC) of prostate is also presented. It is shown that tissue temperature may vary as much as +/-20 degrees C with variations of +/-10% in laser power relative to the nominal power required to maintain the tissue at 37 degrees C. This demonstrates the sensitivity to laser power and the need of an accurate laser power control algorithm.

Algorithms↗

PubMed2006

Improvement of port wine stain laser therapy by skin preheating prior to cryogen spray cooling: a numerical simulation.

BACKGROUND AND OBJECTIVES: Although cryogen spray cooling (CSC) in conjunction with laser therapy has become the clinical standard for treatment of port wine stain (PWS) birthmarks, the current approach does not produce complete lesion blanching in the vast majority of patients. The objectives of this study are to: (1) experimentally determine the dynamic CSC heat flux when a skin phantom is preheated, and (2) numerically study the feasibility of using skin preheating prior to CSC to improve PWS laser therapeutic outcome. STUDY DESIGN/MATERIALS AND METHODS: A fast-response thin-foil thermocouple was used to measure the surface temperature and thus heat flux of an epoxy skin phantom during CSC. Using the heat flux as a boundary condition, PWS laser therapy was simulated with finite element heat diffusion and Monte Carlo light distribution models. Epidermal and PWS blood vessel thermal damage were calculated with an Arrhenius-type kinetic model. RESULTS: Experimental results show that the skin phantom surface can be cooled to a similar minimum temperature regardless of the initial temperature. Numerical simulation indicates that upon laser irradiation, the epidermal temperature increase is virtually unaffected by preheating, while higher PWS blood vessel temperatures can be achieved. Based on the damage criterion we assumed, the depth and maximum diameter of PWS vessels that can be destroyed irreversibly with skin preheating are greater than those without. CONCLUSIONS: Skin preheating prior to CSC can maintain epidermal cooling while increasing PWS blood vessel temperature before laser irradiation. Numerical models have been developed to show that patients may benefit from the skin preheating approach, depending on PWS vessel diameter and depth.

Cryotherapy↗

PubMed2006

Thermal responses of ex vivo human skin during multiple cryogen spurts and 1,450 nm laser pulses.

BACKGROUND AND OBJECTIVE: Although cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery, concern has been expressed that CSC may induce cryo-injury. The objective of this study is to measure temperature variations at the epidermal-dermal junction in ex vivo human skin during three clinically relevant multiple cryogen spurt-laser pulse sequences (MCS-LPS). STUDY DESIGN/MATERIALS AND METHODS: The epidermis of ex vivo human skin was separated from the dermis and a thin-foil thermocouple (13 microm thickness) was inserted between the two layers. Thermocouple depth and epidermal thickness were measured using optical coherence tomography (OCT). Skin specimens were preheated to 30 degrees C before the MCS-LPS were initiated. Three MCS-LPS patterns, with total cryogen spray times of 38, 30, and 25 milliseconds respectively, were applied to the specimens in combination with laser fluences of 10 and 14 J/cm(2), while the thermocouple recorded the temperature changes at the epidermal-dermal junction. RESULTS: The thermocouple effectively recorded fast temperature changes during three MCS-LPS patterns. The lowest temperatures measured corresponded to the sequences with longer pre-cooling cryogen spurts. No sub-zero temperatures were measured for any of the MCS-LPS patterns under study. CONCLUSIONS: The three clinically relevant MCS-LPS patterns evaluated in this study do not cause sub-zero temperatures in ex vivo human skin at the epidermal-dermal junction and, therefore, are unlikely to cause significant cryogen induced epidermal injury.

Aerosols↗

PubMed2006

Cutaneous effects of cryogen spray cooling on in vivo human skin.

BACKGROUND: Despite widespread clinical use of cryogen spray cooling (CSC) in conjunction with laser dermatologic surgery, in vivo cutaneous effects have not been systematically evaluated. OBJECTIVE: The authors characterize the in vivo cutaneous effects for Fitzpatrick skin types I through VI after CSC exposures of varying spurt durations and spurt delivery patterns (single vs. multiple spurts). MATERIALS AND METHODS: Twenty-seven normal human subjects were exposed to single cryogen spurts from 10 to 80 milliseconds, and multiple spurt patterns consisting of two 20-millisecond spurts, four 10-millisecond spurts, and eight 5-millisecond spurts. Subjects were evaluated by clinical observation and photography at 1 hour, 1 day, and 1, 4, 8, and 12 weeks after CSC exposure. RESULTS: Acute erythema and urticaria (1-24 hours) were noted in 14 of 27 and 3 of 27 subjects, respectively. Transient hyperpigmentation occurred in 4 of 27 subjects (skin types III-VI) but resolved spontaneously without medical intervention in all subjects by 8 weeks. No permanent skin changes were noted in any subjects. Skin reactions were more common with longer single-spurt durations (50 milliseconds or greater) and multiple spurt patterns. CONCLUSION: Acute erythema, urticaria, and, less commonly, transient hyperpigmentation were observed after CSC exposure. Permanent skin injury was not observed and is unlikely.

Administration, Topical↗

PubMed2005

Effect of ambient humidity on light transmittance through skin phantoms during cryogen spray cooling.

Cryogen spray cooling (CSC) is a technique employed to reduce the risk of epidermal damage during dermatologic laser surgery. However, while CSC protects the epidermis from non-specific thermal damage, it might reduce the effective fluence reaching the target chromophore due to scattering of light by the spray droplets and subsequent water condensation/freezing on the skin surface. The objective of this work was to study the effect of ambient humidity (omega) on light transmittance during CSC. An integrating sphere was employed to measure the dynamics of light transmittance through a deformable agar phantom during CSC. The study included two representative CSC spurt patterns studied using four omega: 57, 40, 20 and 12%. Results show that during CSC, as omega increased, light transmittance decreased. For the highest humidity level (57%) studied, light transmittance reached a minimum of 55% approximately 30 ms after spurt termination. In a controlled environment with omega = 12%, light transmittance reached a minimum of 87% approximately 30 ms after spurt termination. The reduced light transmittance immediately after spurt termination was most likely because of scattering of light caused by condensation of water vapour due to aggressive cooling of ambient air in the wake of the cryogen spurt.

Aerosol Propellants↗

PubMed2005

Evaluation of single versus multiple cryogen spray cooling spurts on in vitro model human skin.

Many commercially available dermatologic lasers utilize cryogen spray cooling for epidermal protection. A previous tissue culture study demonstrated that single cryogen spurts (SCS) of 80 ms or less were unlikely to cause cryo-injury in light-skinned individuals. More recently, multiple cryogen spurts (MCS) have been incorporated into commercial devices, but the effects of MCS have not been evaluated. The aim was to study an in vitro tissue culture model and the epidermal and dermal effects of SCS vs patterns of shorter MCS with the same preset total cryogen delivery time (Deltat(c)) and provide an explanation for noted differences. Four different spurt patterns were evaluated: SCS: one 40-ms cryogen spurt; MCS2: two 20-ms cryogen spurts; MCS4: four 10-ms cryogen spurts; MCS8: eight 5-ms cryogen spurts. Actual Deltat(c) and total cooling time (Deltat(Total)) were measured for each spurt pattern. RAFT tissue culture specimens were exposed to cryogen spurt patterns and biopsies were taken immediately and at days 3 and 7. Actual Deltat(c) was increased while Deltat(Total) remained relatively constant as the preset Deltat(c) of 40 ms was delivered as shorter MCS. Progressively more epidermal damage was noted with exposure to the MCS patterns. No dermal injury was noted with either SCS or MCS. For a constant preset Deltat(c) of 40 ms, delivering cryogen in patterns of shorter MCS increased the actual Deltat(c) and consequently the observed epidermal cryo-injury as compared to an SCS.

Apoptosis↗

PubMed2005

Comparison of diffusion approximation and Monte Carlo based finite element models for simulating thermal responses to laser irradiation in discrete vessels.

Both diffusion approximation (DA) and Monte Carlo (MC) models have been used to simulate light distribution in multilayered human skin with or without discrete blood vessels. However, no detailed comparison of the light distribution, heat generation and induced thermal damage between these two models has been done for discrete vessels. Three models were constructed: (1) MC-based finite element method (FEM) model, referred to as MC-FEM; (2) DA-based FEM with simple scaling factors according to chromophore concentrations (SFCC) in the epidermis and vessels, referred to as DA-FEM-SFCC; and (3) DA-FEM with improved scaling factors (ISF) obtained by equalizing the total light energy depositions that are solved from the DA and MC models in the epidermis and vessels, respectively, referred to as DA-FEM-ISF. The results show that DA-FEM-SFCC underestimates the light energy deposition in the epidermis and vessels when compared to MC-FEM. The difference is nonlinearly dependent on wavelength, dermal blood volume fraction, vessel size and depth, etc. Thus, the temperature and damage profiles are also dramatically different. DA-FEM-ISF achieves much better results in calculating heat generation and induced thermal damage when compared to MC-FEM, and has the advantages of both calculation speed and accuracy. The disadvantage is that a multidimensional ISF table is needed for DA-FEM-ISF to be a practical modelling tool.

Blood Vessels↗

PubMed2005

Radial and temporal variations in surface heat transfer during cryogen spray cooling.

Cryogen spray cooling (CSC) is a heat extraction process that protects the epidermis from thermal damage during dermatologic laser surgery. The objective of the present work is to investigate radial and temporal variations in the heat transferred through the surface of a skin phantom during CSC. A fast-response thermal sensor is used to measure surface temperatures every 1 mm across a 16 mm diameter of the sprayed surface of the phantom. An analytical expression based on Fourier's law and Duhamel's theorem is used to compute surface heat fluxes from temperature measurements. Results show that radial and temporal variations of the boundary conditions have a strong influence on the homogeneity of heat extraction from the skin phantom. However, there is a subregion of uniform cooling whose size is time dependent. It is also observed that the surface heat flux undergoes a marked dynamic variation, with a maximum heat flux occurring at the centre of the sprayed surface early in the spurt followed by a quick decrease. The study shows that radial and temporal variations of boundary conditions must be taken into account and ideally controlled to guarantee uniform protection during CSC of human skin.

Body Temperature↗

PubMed2005

Skin model surface temperatures during single and multiple cryogen spurts used in laser dermatologic surgery.

BACKGROUND: Although cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery, concern has been expressed that CSC may induce cryo-injury. In order to address this concern, it is necessary to evaluate the effects of prolonged exposure of human skin phantoms (HSP) to CSC. OBJECTIVE: To measure the minimum surface temperature (T(min)) and the time at which it occurs (t(Tmin)) as well as determine the time the sprayed HSP surface remains below 0 degrees C (sub-zero time, Deltat(s)) and -26 degrees C (residence time, Deltat(r)) during the application of single (SCS) and multiple (MCS) cryogen spurts. Two initial HSP substrate temperatures were studied, T(i): 23 and 70 degrees C. STUDY DESIGN/MATERIALS AND METHODS: An epoxy-based HSP was constructed to measure T(min), t(Tmin), Deltat(s), and Deltat(r), for 17 spray patterns: 1 SCS with a total cryo-delivery time (Deltat(c)) of 40 milliseconds; 8 MCS patterns with identical Deltat(c), but with a total cooling time (Deltat(total)) varying from 50 to 280 milliseconds; and 8 SCS patterns that matched the Deltat(total) of the MCS patterns. RESULTS: For both T(i), our results show that it is possible to distinguish between two different cooling regimes. For Deltat(total) </= 110 milliseconds, the differences between SCS and MCS patterns with the same Deltat(total) for all variables (T(min), t(Tmin), Deltat(s), Deltat(r)) are negligible. Most importantly, all these variables show a remarkable linear dependence with Deltat(total). In the interval 110 milliseconds < Deltat(total) < 280 milliseconds, T(min) and t(Tmin) are similar for SCS and MCS, while Deltat(s) and Deltat(r) show more pronounced differences between the two spray patterns. In this interval, the values of T(min) and Deltat(s) for MCS remain invariant and similar to the corresponding values for Deltat(total) = 110 milliseconds. CONCLUSIONS: These results suggest that: (1) similar epidermal protection may be attained with SCS and MCS for Deltat(total) </= 110 milliseconds; and (2) for 110 milliseconds < Deltat(total) </= 280 milliseconds, MCS help to maintain Deltat(s) similar to that of SCS at Deltat(total) = 100 milliseconds, which may be beneficial to prevent cryo-injury.

Aerosols↗

PubMed2005

Effects of hypobaric pressure on human skin: feasibility study for port wine stain laser therapy (part I).

BACKGROUND AND OBJECTIVES: Since the development of laser-induced photothermolysis for the therapy of port wine stain (PWS) birthmarks, clinical results have shown that dark purple lesions usually respond well to the first three to five treatments. However, for most PWS, complete blanching is never achieved, and the lesion stabilizes at a red-pink color. The aim of this feasibility study is to demonstrate that with the aid of a local vacuum applied to the lesion site prior to laser exposure, photocoagulation of the smaller PWS blood vessels may be successfully achieved. STUDY DESIGN/MATERIALS AND METHODS: Suction cups were designed to fit onto the hand pieces of commercial laser devices used for PWS laser therapy. One subject with normal skin and another with PWS skin were recruited for this study. Laser pulses of various fluences were applied at atmospheric pressure or shortly after (5-15 seconds) hypobaric pressures (17-51 kPa) were placed as test sites on the forearm of both subjects. The laser-induced purpura at the test sites was documented over the course of 1 week on both subjects and the resulting PWS blanching was optically quantified by visible reflectance spectrometry 7 months after therapy. RESULTS: For the subject with normal skin, the laser fluence needed with hypobaric pressure (51 kPa) to induce similar purpura intensity to that observed with atmospheric pressure was approximately 35% lower. For PWS skin, all suction application times (5-15 seconds) and hypobaric pressures (17-51 kPa) resulted in more intense purpura and the PWS blanching 7 months after treatment was clinically significant for test sites treated with hypobaric pressures ranging from 17 to 34 kPa. CONCLUSIONS: The temporary and controlled dilation of the targeted blood vessels achieved with a local vacuum can significantly reduce the "small-vessel-limitation" in the treatment of PWS without increasing the risk of epidermal damage.

Feasibility Studies↗

PubMed2005

Effects of hypobaric pressure on human skin: implications for cryogen spray cooling (part II).

BACKGROUND AND OBJECTIVES: Clinical results have demonstrated that dark purple port wine stain (PWS) birthmarks respond favorably to laser induced photothermolysis after the first three to five treatments. Nevertheless, complete blanching is rarely achieved and the lesions stabilize at a red-pink color. In a feasibility study (Part I), we showed that local hypobaric pressure on PWS human skin prior to laser irradiation induced significant lesion blanching. The objective of the present study (Part II) is to investigate the effects of hypobaric pressures on the efficiency of cryogen spray cooling (CSC), a technique that assists laser therapy of PWS and other dermatoses. STUDY DESIGN/MATERIALS AND METHODS: Experiments were carried out within a suction cup and vacuum chamber to study the effect of hypobaric pressure on the: (1) interaction of cryogen sprays with human skin; (2) spray atomization; and (3) thermal response of a model skin phantom. A high-speed camera was used to acquire digital images of spray impingement on in vivo human skin and spray cones generated at different hypobaric pressures. Subsequently, liquid cryogen was sprayed onto a skin phantom at atmospheric and 17, 34, 51, and 68 kPa (5, 10, 15, and 20 in Hg) hypobaric pressures. A fast-response temperature sensor measured sub-surface phantom temperature as a function of time. Measurements were used to solve an inverse heat conduction problem to calculate surface temperatures, heat flux, and overall heat extraction at the skin phantom surface. RESULTS: Under hypobaric pressures, cryogen spurts did not produce skin indentation and only minimal frost formation. Sprays also showed shorter jet lengths and better atomization. Lower minimum surface temperatures and higher overall heat extraction from skin phantoms were reached. CONCLUSIONS: The combined effects of hypobaric pressure result in more efficient cryogen evaporation that enhances heat extraction and, therefore, improves the epidermal protection provided by CSC.

Aerosols↗

PubMed2004

Heat-transfer dynamics during cryogen spray cooling of substrate at different initial temperatures.

Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic therapy. However, the dominant mechanisms of heat transfer during the transient cooling process are incompletely understood. The objective of this study is to elucidate the physics of CSC by measuring the effect of initial substrate temperature (T0) on cooling dynamics. Cryogen was delivered by a straight-tube nozzle onto a skin phantom. A fast-response thermocouple was used to record the phantom temperature changes before, during and after the cryogen spray. Surface heat fluxes (q") and heat-transfer coefficients (h) were computed using an inverse heat conduction algorithm. The maximum surface heat flux (q"max) was observed to increase with T0. The surface temperature corresponding to q"max also increased with T0 but the latter has no significant effect on h. It is concluded that heat transfer between the cryogen spray and skin phantom remains in the nucleate boiling region even if T0 is 80 degrees C.

Aerosol Propellants↗

PubMed2004

Influence of angle between the nozzle and skin surface on the heat flux and overall heat extraction during cryogen spray cooling.

High speed video imaging and an inverse heat conduction problem algorithm were used to observe and measure the effect of the angle between the nozzle and surface of a skin phantom on: (a) surface temperature; (b) heat flux q; and (c) overall heat extraction Q during cryogen spray cooling (CSC). A skin phantom containing a fast-response temperature sensor was sprayed with 50 ms cryogen spurts from a commercial nozzle placed 30 mm from the surface. The nozzle was systematically positioned at angles ranging from 5 to 90 degrees (perpendicular) with respect to the phantom surface. It is shown that angles as low as 15 have an insignificant impact on the surface temperature, q and Q. Only exaggerated angles of 5 show up to 10% lower q and 30% lower Q with respect to the maximal values measured when nozzles are aimed perpendicularly. This study proves that the slight angle that many commercial nozzles have does not affect significantly the CSC efficiency.

Algorithms↗

PubMed2004

Evaluation of cryogen spray cooling exposure on in vitro model human skin.

BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is commonly used during dermatologic laser surgery. The epidermal and dermal effects of CSC have not been adequately evaluated. To study the potential for epidermal and dermal injury after CSC using an in vitro model of human skin (RAFT). STUDY DESIGN/MATERIALS AND METHODS: RAFT specimens were exposed to continuous CSC spurt durations of 10, 20, 40, 80, 100, 200, or 500 milliseconds. Biopsies were taken acutely, 3 and 7 days post-CSC exposure. Sections were stained with hematoxylin and eosin for evaluation of possible injury, Ki-67 to determine keratinocyte viability, and Melan-A, to identify and evaluate melanocytes. RESULTS: Minimal, transient epidermal changes were noted in specimens exposed to continuous CSC spurts of 80 milliseconds or less. Keratinocytes and melanocytes remained viable. Continuous CSC spurts of 100, 200, or 500 milliseconds (much longer than recommended for clinical use) resulted in significant epidermal injury acutely, with partial or full thickness epidermal necrosis at 7 days. Only the 500 milllisecond specimen demonstrated dermal change, decreased fibroblast proliferation at 3 days. CONCLUSIONS: Continuous CSC spurts of 80 milliseconds or less induce minimal, if any, epidermal or dermal damage and are unlikely to produce cryo-injury when used during dermatologic laser surgery.

Administration, Topical↗

PubMed2004

Increase of dermal blood volume fraction reduces the threshold for laser-induced purpura: implications for port wine stain laser treatment.

BACKGROUND AND OBJECTIVES: The average success rate in achieving total blanching of port wine stain (PWS) lesions treated with laser-induced selective photothermolysis is below 25%, even after multiple treatments. This is because smaller diameter (5-20 microm) PWS blood vessels are difficult to destroy with selective photothermolysis since the volumetric heat generated by absorption of laser light is insufficient to adequately heat the entire vessel wall. The aim of this study was to investigate a potential technique for more efficient photocoagulation of small diameter PWS blood vessels in PWS that respond poorly to selective photothermolysis. STUDY DESIGN/MATERIALS AND METHODS: The blood volume fraction (BVF) in the upper dermis of the forearm of human volunteers was increased by placing an inflated blood pressure cuff on the upper arm. Applied pressures were in the range of 80-100 mm Hg for up to 5 minutes. The increased BVF was determined by matching reflectance spectra measured with that computed using a diffusion model. The impact of increased BVF on purpura formation induced by a 0.45 milliseconds pulsed dye laser (PDL) at 585 nm wavelength was investigated in normal and in PWS skin. RESULTS: In the presence of a 100 mm Hg pressure cuff, the BVF, as determined from the diffusion model, increased by a factor of 3 in the forearm and by 6 in the hand. Increasing BVF by a factor of 3 corresponds to an increase in blood vessel diameters by a factor of radical 3 approximately 1.7. BVF increased at 1-3 minutes after application of the pressure cuff, remained constant at 3-5 minutes, and returned to baseline values at 3 minutes after removal of the pressure cuff. Approximately 40% less radiant exposure was needed to induce the same amount of purpura after PDL irradiation when the blood pressure cuff was used. Applying an 80 mm Hg pressure cuff reduced the required radiant exposure for purpura formation by 30%. Heating of blood vessels was calculated as a function of vessel diameter and of radiant exposure (at 585 nm and at 0.5 and 1.5 milliseconds pulse duration). CONCLUSIONS: Enlarging the vessel lumen, for example, by obstructing venous return, can significantly reduce the "small-vessel-limitation" in PDL treatment of PWS. Dilation of PWS blood vessels enables a more efficient destruction of smaller vessels without increasing the probability of epidermal damage.

Adult↗

PubMed2004

Effect of skin indentation on heat transfer during cryogen spray cooling.

BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used to pre-cool the epidermis during dermatological laser procedures such as treatment of port wine stain (PWS) birthmarks, hair removal, and non-ablative photorejuvenation. Thus far, heat transfer studies related to CSC optimization have assumed a flat surface but clinical observation suggests that human skin indents due to the force of an impinging cryogen spray. STUDY DESIGN/MATERIALS AND METHODS: High-speed videos of cryogen spray impingement on in vivo human skin were taken and the resulting indentations characterized as a function of both nozzle-to-skin distance and anatomic location. Detectors with pre-formed indentations were constructed and heat flux measurements were performed at two nozzle-to-surface distances. RESULTS: Indentation causes cryogen accumulation that reduces the efficiency of heat transfer when compared to spray impingement on a flat surface. Large indentations, however, encourage convective flow within the cryogen pool that mitigates many of the negative effects of liquid layer thickening and improves the heat flux. CONCLUSIONS: Flat surfaces produce the most efficient heat transfer, but once indentation exists (as it does in all clinically relevant cases), larger indentations produce a higher maximum heat flux. This suggests that higher momentum sprays (which produce larger skin indentations for identical spurts) than those in current clinical use may improve CSC efficiency.

Administration, Topical↗

PubMed2004

Passive mass deposition control of cryogen sprays through the use of wire meshes.

BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used for epidermal protection during laser dermatologic surgery. However, further research is necessary to optimize the use of CSC for many applications, particularly in patients with darker skin types, and in procedures with deep-targeted chromophores. STUDY DESIGN/MATERIALS AND METHODS: With the objective to enhance cryogen spray atomization and introduce a passive mass deposition control, stainless steel wire meshes of various sizes were positioned in the spray path of two nozzles of different diameters (0.7 and 1.4 mm). A temperature sensor placed at a fixed distance below the nozzle tips measured variations in temperature as a function of time. Calculations for heat flux (q), and overall heat extraction (Q) were done through an inverse heat conduction problem (IHCP) algorithm. RESULTS: The use of wire meshes reduced maximum q and thus cooling efficiency for both nozzles. However, for the larger diameter nozzle (1.4 mm), cooling duration was prolonged significantly when compared to the case without mesh, which led to an increase in Q. CONCLUSIONS: Wire meshes, within the range of wire diameters used, are not desirable for use with the smaller diameter nozzle because they reduce cooling efficiency. For the larger diameter nozzle, however, the prolonged cooling duration and increase in Q may be beneficial for laser dermatological procedures directed towards deeper targeted chromophores, such as hair removal.

Administration, Topical↗

PubMed2004

A comparative study of photoacoustic and reflectance methods for determination of epidermal melanin content.

Although epidermal melanin content has been quantified non-invasively using visible reflectance spectroscopy (VRS), there is currently no way to determine melanin distribution in the epidermis. We have developed a photoacoustic probe that uses a Q-switched, frequency-doubled Nd:YAG (neodymium, yttrium, aluminum, garnet) laser operating at 532 nm to generate acoustic pulses in skin in vivo. The probe contained a piezoelectric element that detected photoacoustic waves that were then analyzed for epidermal melanin content using a photoacoustic melanin index (PAMI). Melanin content was compared between results of photoacoustics and VRS. Spectra from human skin were fitted to a model based on diffusion theory that included parameters for epidermal thickness, melanin content, hair color and density, and dermal blood content. Ten human subjects with skin phototypes I-VI were tested using the photoacoustic probe and VRS. A plot of PAMI v. VRS showed a good linear fit with r2=0.85. Photoacoustic and VRS measurements are shown for a human subject with vitiligo, indicating that melanin was almost completely absent. We present preliminary modeling for photoacoustic probe design and analysis necessary for depth profiling of epidermal melanin.

Acoustics↗