Numerical optimization of sequential cryogen spray cooling and laser irradiation for improved therapy of port wine stain

Monitoring of Caffeine Consumption Effect on Skin Blood Properties by Diffuse Reflectance Spectroscopy

Caffeine is the most widely consumed psychoactive substance worldwide, affecting numerous tissues and organs, with notable impacts on the central nervous system, heart, and blood vessels. The effect of caffeine on vascular smooth muscle cells is an initial transient contraction followed by significant vasodilatation. In this study we investigate the use of diffuse reflectance spectroscopy (DRS) for monitoring of vascular changes in human skin induced by caffeine consumption. DRS spectra were recorded on volar sides of the forearms of eight healthy volunteers at time intervals of 0, 30, 60, 120, and 180 min after consumption of caffeine, while one subject served as a negative control. Analytical diffusion approximation solutions for diffuse reflectance from three-layer structures were used to assess skin composition (e.g. dermal blood volume fraction and oxygen saturation) by fitting these solutions to experimental data. The results demonstrate that cutaneous vasodynamics induced by caffeine consumption can be monitored by DRS, while changes in the control subject not consuming caffeine were insignificant.

Measurements of hair temperature avalanche effect with alexandrite and Nd:YAG hair removal lasers

BACKGROUND AND OBJECTIVES

In this study, we investigate the photothermal response of human hair using a pulsed laser source employed in the hair removal treatment. The purpose is to understand the dynamics behind the most common clinical practice to better define the salient features that may contribute to the efficiency of the process.

STUDY DESIGN/MATERIALS AND METHODS

Temperature changes of hair samples (dark brown color) from a human scalp (skin type Fitpatrick II) were measured by a thermal camera following irradiation with single and multiple neodymium: yttrium-aluminum-garnet (Nd:YAG) (1064 nm) and alexandrite (755 nm) laser pulses. Particularly, the hair was treated with an individual laser pulse of a sufficiently high fluence, or with a series of lower fluence laser pulses. We investigated the temperature increase in a broad range of fluence and number of pulses. From the data analysis we extrapolated important parameters such as thermal gain and threshold fluence that can be used for determining optimal parameters for the hair removal procedure. Our experimental investigations and hypothesis were supported by a numerical simulation of the light-matter interaction in a skin-hair model, and by optical transmittance measurements of the irradiated hair.

RESULTS

An enhancement of the temperature response of the irradiated hair, that deviates from the linear behavior, is observed when hair is subjected to an individual laser pulse of a sufficiently high fluence or to a series of lower fluence laser pulses. Here, we defined the nonlinear and rapid temperature built-up as an avalanche effect. We estimated the threshold fluence at which this process takes place to be at 10 and 2.5 J/cm² for 1064 and 755 nm laser wavelengths, respectively. The thermal gain expressed by the degree of the deviation from the linear behavior can be higher than 2 when low laser fluence and multiple laser pulses are applied (n = 50). The comparison of the calculated gain for the two different laser wavelengths and the number of pulses reveals a much higher efficiency when low fluence and multiple pulses are delivered. The avalanche effect manifests when the hair temperature exceeds 45°C. The enhanced temperature increase during the subsequent delivery of laser pulses could be ascribed to the temperature-induced changes in the hair's structural properties. Simulations of the hair temperature under Nd:YAG and alexandrite irradiation indicate that the avalanche phenomenon observed in the hair suspended in air may apply also to the hair located within the skin matrix. Namely, for the same fluence, similar temperature increase was obtained also for the hair located within the skin.

CONCLUSION

The observed "avalanche" effect may contribute to the reported clinical efficacy of laser hair removal and may at least partially explain the observed efficacy of the brushing hair removal procedures where laser fluence is usually low. The repeated irradiation during the brushing procedure may lead to an avalanche-like gradual increase of the hair's thermal response resulting in sufficiently high final hair temperatures as required for effective hair reduction.

Radiofrequency impedance monitoring of the biological tissues under local heating by optical radiation

Development of methods for simultaneous control of state of biological tissues during optical treatment is the important tasks in laser surgery. We introduce a novel approach for the monitoring of the state of biological tissues in the process of its local heating by optical radiation. It is based on measurements of the electrical radiofrequency impedance kinetics of the sample during irradiation. The obtained data are processed using interconnected mathematical modeling of corresponding thermodynamic, optical and electrical problems. Experimental applications of this approach, represented in the paper, reveal its high sensitivity, repeatability and consistency with the model. The introduced method can be used for the selection and optimization of radiation parameters of medical laser sources as an alternative or an addition to histological techniques. Radiofrequency impedance measurement can be used directly in the course of surgical operations for monitoring the treated tissues state, including its temperature and degree of damage.

Wavelength optimization for the laser treatment of port wine stains

Based on the well-known principle of selective photothermolysis, laser has been a promising way for the treatment of port wine stains (PWSs). The laser wavelengths used for PWS's clinical treatment include but are not limited to pulsed dye laser (PDL) in 585-600 nm, long-pulse 755-nm alexandrite, and 1064-nm Nd:YAG lasers. The objective of this study was to investigate the optimal wavelength for PWS's laser treatment. A two-scale mathematic model was constructed to simultaneously quantify macroscale laser energy attenuation in two-layered bulk skin and microscale local energy absorption on target blood vessels within Krogh unit. The effects of morphological parameters, including epidermal melanin content, epidermal thickness, dermal blood content, blood vessel depth, and diameter on laser energy deposition within target blood vessels, were investigated from the visible to near-infrared bands (500-1100 nm). The energy deposition ratio of target blood vessel to epidermal surface was proposed to determine the optimal laser wavelength for PWS with different skin morphological parameters. The bioheat transfer modeling and animal experiment are also conducted to prove our wavelength optimization. The optimal wavelengths for lightly pigmented skin with small and shallow target blood vessels are 580-610 nm in the visible band. This wavelength coincides with commercially used PDL. The optimal wavelength shifts to 940 nm as the epidermal pigmentation increases or the size and blood vessel depth increases. The optimal wavelength changes to 1005 nm as the epidermal pigmentation or the size and burying depth of target blood vessel further increases. Nine hundred forty nanometers can be selected as a general wavelength in PWS treatment to meet the need in most widely morphological structure. Lasers with wavelengths in the 580-610, 940, and 1005 nm regions are effective for treating PWS because of their high optical selectivity in blood over the epidermis.

Vascular damage mechanism and parameter optimization under alexandrite laser irradiation: a theoretical study

The 755-nm Alexandrite Laser has a good clinical effect in treating resistant port wine stain, without causing thermal damage of normal tissue and side effects such as purpura. However, little is known about the mechanism of vascular damage induced by 755-nm laser irradiation, which restricts the optimization of laser parameters. In this work, the thermal damage model and the pressure damage model were used to study the damage mechanism of 755-nm laser irradiation on vessels, and the incident energy density and pulse width required for vascular damage were determined according to the damage mode. Under the irradiation of 755-nm laser, the vascular injury pattern was the co-occurrence of vessel rupture and vessel constriction, and the energy density required for the treatment of vessels with a diameter of 200 μm to reach the damage threshold was the lowest.

In vivo parameter identification in arteries considering multiple levels of smooth muscle activity

In this paper an existing in vivo parameter identification method for arteries is extended to account for smooth muscle activity. Within this method a continuum-mechanical model, whose parameters relate to the mechanical properties of the artery, is fit to clinical data by solving a minimization problem. Including smooth muscle activity in the model increases the number of parameters. This may lead to overparameterization, implying that several parameter combinations solve the minimization problem equally well and it is therefore not possible to determine which set of parameters represents the mechanical properties of the artery best. To prevent overparameterization the model is fit to clinical data measured at different levels of smooth muscle activity. Three conditions are considered for the human abdominal aorta: basal during rest; constricted, induced by lower-body negative pressure; and dilated, induced by physical exercise. By fitting the model to these three arterial conditions simultaneously a unique set of model parameters is identified and the model prediction agrees well with the clinical data.

The influence of morphological distribution of melanin on parameter selection in laser thermotherapy for vascular skin diseases

Port wine stains (PWSs) are congenital vascular malformations that progressively darken and thicken with age. Currently, laser therapy is the most effective way in clinical management of PWS. It is known that skin pigmentation (melanin content) affects the radiant exposure that can be safely applied to treat PWS. However, the effect of melanin distribution in the epidermis on the maximum safe radiant exposure has not been studied previously. In this study, 10 different morphological distributions of melanin were proposed according to the formation and migration characteristics of melanin, and the two-scale heat transfer model was employed to investigate the influence of melanin distribution on the threshold radiant exposure of epidermis and blood vessels. The results show that melanin distributions do have a strong effect on laser parameter selection. When uniform melanin distribution is assumed, the threshold radiant exposure to damage a typical PWS blood vessel (50 μm diameter) is 8.62 J/cm² lower than that to injure epidermis. The optimal pulse duration is 1-5 ms for a typical PWS blood vessel of 50 μm when melanin distribution is taken into consideration. PWS blood vessels covered by non-uniformly distributed melanin are more likely to have poor response to laser treatment.

Temperature Depth Profiles Induced in Human Skin In Vivo Using Pulsed 975 nm Irradiation

BACKGROUND AND OBJECTIVES

The aim of this study was to determine the temperature depth profiles induced in human skin in vivo by using a pulsed 975 nm diode laser (with 5 ms pulse duration) and compare them with those induced by the more common 532 nm (KTP) and 1,064 nm (Nd:YAG) lasers. Quantitative assessment of the energy deposition characteristics in human skin at 975 nm should help design of safe and effective treatment protocols when using such lasers.

STUDY DESIGN/MATERIALS AND METHODS

Temperature depth profiles induced in the human skin by the three lasers were determined using pulsed photothermal radiometry (PPTR). This technique involves time-resolved measurement of mid-infrared emission from the irradiated test site and reconstruction of the laser-induced temperature profiles using an earlier developed optimization algorithm. Measurements were performed on volar sides of the forearms in seven volunteers with healthy skin. At irradiation spot diameters of 3-4 mm, the radiant exposures were 0.24, 0.36, and 5.7 J/cm² for the 975, 532, and 1,064 nm lasers, respectively.

RESULTS

Upon normalization to the same radiant exposure of 1 J/cm ² , the assessed maximum temperature rise in the epidermis averaged 0.8 °C for the 975 nm laser, 7.4 °C for the 532 nm, and 0.6 °C for the 1,064 nm laser. The characteristic subsurface depth to which 50% of the absorbed laser energy was deposited was on average 0.31 mm at 975 nm irradiation, and slightly deeper at 1,064 nm, and 0.15 mm at 532 nm. The experimentally obtained relations were reproduced in a dedicated numerical simulation.

CONCLUSIONS

The assessed energy deposition characteristics show that the pulsed 975 nm diode laser is very suitable for controlled heating of the upper dermis as required, for example, for nonablative skin rejuvenation. The risks of nonselective overheating of the epidermis and subcutis are significantly reduced in comparison with irradiation at 532 and 1,064 nm, respectively. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Numerical Modeling and Clinical Evaluation of Pulsed Dye Laser and Copper Vapor Laser in Skin Vascular Lesions Treatment

Introduction: Different yellow lasers have been successfully used for the treatment of vascular lesions. This study is aimed to ascertain the role and efficiency of copper vapor lasers (CVLs) and pulsed dye lasers (PDLs) for the treatment of vascular lesions using numerical modeling and to compare results with our clinical experience. In this study we aimed to develop criteria for the choice of more efficient laser exposure mode, investigate more relevant modes of laser irradiation to ensure selective photothermolysis of target vessels, and compare the CVL and PDL efficiency in the course of patients with skin vascular lesions (SVL) treatment. Methods: We performed numerical simulation of the processes of heating a vessel with CVL and PDL to temperatures at which its coagulation could occur. Calculated fluencies were compared with clinical results of laser therapy performed on 1242 patients with skin hemangiomas and vascular malformations (SHVM), including 635 patients treated with CVL and 607 patients treated with PDL. PDL and CVL provided excellent results in 40 and ten days after treatment. The treatment was not painful. Patients did not need anesthesia. Postoperative crusts were greater with PDL than with CVL. Results: Results of computer simulation of a selective vessel heating using PDL and CVL radiation are presented. By results obtained, depth of the location and sizes of vessels that could be selectively heated to more than 75°C are determined. Conclusion: Based on calculated and clinical data, the heating mode for dysplastic vessels using a series of CVL micropulses could be regarded to be safer and more efficient than the mode of a PDL short, powerful pulse.

Groupwise Multichannel Image Registration

Multichannel image registration is an important challenge in medical image analysis. Multichannel images result from modalities such as dual-energy CT or multispectral microscopy. Besides, multichannel feature images can be derived from acquired images, for instance, by applying multiscale feature banks to the original images to register. Multichannel registration techniques have been proposed, but most of them are applicable to only two multichannel images at a time. In the present study, we propose to formulate multichannel registration as a groupwise image registration problem. In this way, we derive a method that allows the registration of two or more multichannel images in a fully symmetric manner (i.e., all images play the same role in the registration procedure), and therefore, has transitive consistency by definition. The method that we introduce is applicable to any number of multichannel images, any number of channels per image, and it allows to take into account correlation between any pair of images and not just corresponding channels. In addition, it is fully modular in terms of dissimilarity measure, transformation model, regularisation method, and optimisation strategy. For two multimodal datasets, we computed feature images from the initially acquired images, and applied the proposed registration technique to the newly created sets of multichannel images. MIND descriptors were used as feature images, and we chose total correlation as groupwise dissimilarity measure. Results show that groupwise multichannel image registration is a competitive alternative to the pairwise multichannel scheme, in terms of registration accuracy and insensitivity towards registration reference spaces.

Nd:YAG laser-induced morphology change and photothermal conversion of gold nanorods with potential application in the treatment of port-wine stain

Based on the principle of selective photothermolysis, 1064 nm Nd:YAG laser has great potential for the treatment of deeper and larger PWS. However, the clinical effectiveness is limited because of the weak absorption of blood to Nd:YAG laser. The aim of this study is to obtain the optimal irradiation conditions to effectively destroy vascular lesions with the assistance of PEG-modified gold NRs to enhance blood absorption of Nd:YAG laser. In our study, PEG-modified gold NRs were prepared by the seeded growth method. Gold NRs after exposure to Nd:YAG laser were characterized using absorption spectra and transmission electron microscope images. The tissue-like phantom containing a glass capillary with blood was prepared and exposed to Nd:YAG laser to investigate the laser energy density and pulse number required for blood coagulation before and after the addition of gold NRs in blood. The results show that the millisecond Nd:YAG laser irradiation does not result in the shape change of gold NRs. After injection of gold NRs into the bloodstream (4.60 mg/kg), the absorbance of blood at 1064 nm increased 3.9 times. The threshold energy density for the treatment of PWS decreased by 33% (from 30 to 20 J/cm²). Our findings provide an experimental guide for choosing laser parameters and gold NRs concentration for the treatment of deeper and larger PWS with the assistance of PEG-modified gold NRs in vivo in the future.

Model development and experimental validation for analyzing initial transients of irradiation of tissues during thermal therapy using short pulse lasers

BACKGROUND AND OBJECTIVES

Short pulse lasers with pulse durations in the range of nanoseconds and shorter are effective in the targeted delivery of heat energy for precise tissue heating and ablation. This photothermal therapy is useful where the removal of cancerous tissue sections is required. The objective of this paper is to use finite element modeling to demonstrate the differences in the thermal response of skin tissue to short-pulse and continuous wave laser irradiation in the initial stages of the irradiation. Models have been developed to validate the temperature distribution and heat affected zone during laser irradiation of excised rat skin samples and live anesthetized mouse tissue.

STUDY DESIGN/MATERIALS AND METHODS

Excised rat skin samples and live anesthetized mice were subjected to Nd:YAG pulsed laser (1,064 nm, 500 ns) irradiation of varying powers. A thermal camera was used to measure the rise in surface temperature as a result of the laser irradiation. Histological analyses of the heat affected zone created in the tissue samples due to the temperature rise were performed. The thermal interaction of the laser with the tissue was quantified by measuring the thermal dose delivered by the laser. Finite element geometries of three-dimensional tissue sections for continuum and vascular models were developed using COMSOL Multiphysics. Blood flow was incorporated into the vascular model to mimic the presence of discrete blood vessels and contrasted with the continuum model without blood perfusion.

RESULTS

The temperature rises predicted by the continuum and the vascular models agreed with the temperature rises observed at the surface of the excised rat tissue samples and live anesthetized mice due to laser irradiation respectively. The vascular model developed was able to predict the cooling produced by the blood vessels in the region where the vessels were present. The temperature rise in the continuum model due to pulsed laser irradiation was higher than that due to continuous wave (CW) laser irradiation in the initial stages of the irradiation. The temperature rise due to pulsed and CW laser irradiation converged as the time of irradiation increased. A similar trend was observed when comparing the thermal dose for pulsed and CW laser irradiation in the vascular model.

CONCLUSION

Finite element models (continuum and vascular) were developed that can be used to predict temperature rise and quantify the thermal dose resulting from laser irradiation of excised rat skin samples and live anesthetized mouse tissue. The vascular model incorporating blood perfusion effects predicted temperature rise better in the live animal tissue. The models developed demonstrated that pulsed lasers caused greater temperature rise and delivered a greater thermal dose than CW lasers of equal average power, especially during the initial transients of irradiation. This analysis will be beneficial for thermal therapy applications where maximum delivery of thermal dose over a short period of time is important.

A comparison of microvascular responses to visible and near-infrared lasers

BACKGROUND AND OBJECTIVE

Pulsed dye laser (PDL) is a commonly used treatment for Port Wine Stain birthmarks (PWS). However, deeper components of PWS are often resistant to PDL. Deeper penetrating lasers, including the long pulsed Neodymium:Yttrium Aluminum Garnet (Nd:YAG) laser have been used, but carry greater risk. This study evaluates the distinct blood vessel thermal responses to visible (595 nm) and near infrared (1,064 nm) lasers using animal and numerical models.

STUDY DESIGN/MATERIALS AND METHODS

Blood vessels in the rodent dorsal skin chamber (DSC) were irradiated by a 595 nm PDL and a long-pulsed 1,064 nm Nd:YAG laser. Laser-induced immediate and 1-hour post-structural and functional changes in the vessels were documented. Numerical simulations were conducted using a 1,000 µm depth SD mouse skin fold to simulate experimental conditions.

RESULTS

PDL irradiation produced immediate blood vessel hemorrhage. Modeling indicated this occurs due to preferential heating of the superior parts of large blood vessels. Nd:YAG irradiation resulted in blood vessel constriction; modeling indicated more uniform heating of vessel walls.

CONCLUSION

PDL and Nd:YAG lasers result in distinct tissue responses. This supports different observable clinical treatment end points when using these devices. Vessel constriction associated with the Nd:YAG may be more difficult to observe and is one reason this device may carry greater risk.

Vascular anomalies: classification, imaging characteristics and implications for interventional radiology treatment approaches

The term vascular anomaly represents a broad spectrum of vascular pathology, including proliferating vascular tumours and vascular malformations. While the treatment of most vascular anomalies is multifactorial, interventional radiology procedures, including embolic therapy, sclerotherapy and laser coagulation among others, are playing an increasingly important role in vascular anomaly management. This review discusses the diagnosis and treatment of common vascular malformations, with emphasis on the technique, efficacy and complications of different interventional radiology procedures.

A two-temperature model for selective photothermolysis laser treatment of port wine stains

Selective photothermolysis is the basic principle for laser treatment of vascular malformations such as port wine stain birthmarks (PWS). During cutaneous laser surgery, blood inside blood vessels is heated due to selective absorption of laser energy, while the surrounding normal tissue is spared. As a result, the blood and the surrounding tissue experience a local thermodynamic non-equilibrium condition. Traditionally, the PWS laser treatment process was simulated by a discrete-blood-vessel model that simplifies blood vessels into parallel cylinders buried in a multi-layer skin model. In this paper, PWS skin is treated as a porous medium made of tissue matrix and blood in the dermis. A two-temperature model is constructed following the local thermal non-equilibrium theory of porous media. Both transient and steady heat conduction problems are solved in a unit cell for the interfacial heat transfer between blood vessels and the surrounding tissue to close the present two-temperature model. The present two-temperature model is validated by good agreement with those from the discrete-blood-vessel model. The characteristics of the present two-temperature model are further illustrated through a comparison with the previously-used homogenous model, in which a local thermodynamic equilibrium assumption between the blood and the surrounding tissue is employed.

Energy deposition profile in human skin upon irradiation with a 1,342 nm Nd:YAP laser

BACKGROUND AND OBJECTIVES

Nd:YAP laser emitting at 1,342 nm appears promising for nonablative skin rejuvenation treatment, based on favorable absorption properties of water and melanin in this part of the spectrum. A quantitative determination of energy deposition characteristics of Nd:YAP in normal human skin should enable design of a safe and effective treatment protocol for future human studies.

STUDY DESIGN

Energy deposition profile of a prototype Nd:YAP laser was determined using pulsed photothermal radiometry. This technique involves time-resolved measurement of mid-infrared emission from a sample after pulsed laser irradiation. The laser-induced temperature depth profile is reconstructed from the radiometric transients using a custom optimization algorithm, developed and tested earlier in our group. Measurements were performed on the extremities of four healthy volunteers at low radiant exposure (2.8 J/cm(2) ). For the purpose of comparison, energy deposition characteristics of commercial Nd:YAG and KTP lasers (at 1,064 and 532 nm, respectively), were also determined at the same test sites.

RESULTS

On average, the Nd:YAP laser deposits 50% of the absorbed energy within the top 0.36 mm of skin and 90% within 0.86 mm, which is significantly shallower than the Nd:YAG laser. The ratio between the dermal versus epidermal heating is more favorable and shows a smaller inter- and intra-patient variance as compared to both Nd:YAG and KTP laser.

CONCLUSIONS

Energy deposition characteristics of the 1,342 nm Nd:YAP laser are very suitable for controlled heating of the upper dermis, as required for nonablative skin rejuvenation. The risks of overheating the epidermis or subcutis should be significantly reduced in comparison with the 1,064 nm Nd:YAG laser.

Photocoagulation of dermal blood vessels with multiple laser pulses in an in vivo microvascular model

BACKGROUND/OBJECTIVES

Current laser therapy of port wine stain (PWS) birthmarks with a single laser pulse (SLP) does not produce complete lesion removal in the majority of patients. To improve PWS therapeutic efficacy, we evaluated the performance of an approach based on multiple laser pulses (MLP) to enhance blood vessel photocoagulation.

STUDY DESIGN

The hamster dorsal window chamber model was used. Radiant exposure (RE), pulse repetition rate (f(r)), total number of pulses (n(p)), and length of vessel irradiated were varied. Blood vessels in the window were irradiated with either SLP with RE of 4-7 J/cm(2) or MLP with RE per pulse of 1.4-5.0 J/cm(2), f(r) of 0.5-26.0 Hz, and n(p) of 2-5. The laser wavelength was 532 nm and pulse duration was 1 ms. Either a 2 mm vessel segment or entire vessel branch was irradiated. Digital photographs and laser speckle images of the window were recorded before and at specific time points after laser irradiation to monitor laser-induced blood vessel structural and functional changes, respectively.

RESULTS

We found that: (1) for a SLP approach, the RE required to induce blood vessel photocoagulation was 7 J/cm(2) as compared to only 2 J/cm(2) per pulse for the MLP approach; (2) for MLP, two pulses at a repetition rate of 5 Hz and a RE of 3 J/cm(2) can induce photocoagulation of more than 80% of irradiated blood vessel; and (3) irradiation of a longer segment of blood vessel resulted in lower reperfusion rate.

CONCLUSIONS

The MLP approach can induce blood vessel photocoagulation at much lower RE per pulse as compared to SLP. The 5 Hz f(r) and the need for two pulses are achievable with modern laser technology, which makes the MLP approach practical in the clinical management of PWS birthmarks.

Three-dimensional Monte Carlo model of pulsed-laser treatment of cutaneous vascular lesions

We present a three-dimensional Monte Carlo model of optical transport in skin with a novel approach to treatment of side boundaries of the volume of interest. This represents an effective way to overcome the inherent limitations of "escape" and "mirror" boundary conditions and enables high-resolution modeling of skin inclusions with complex geometries and arbitrary irradiation patterns. The optical model correctly reproduces measured values of diffuse reflectance for normal skin. When coupled with a sophisticated model of thermal transport and tissue coagulation kinetics, it also reproduces realistic values of radiant exposure thresholds for epidermal injury and for photocoagulation of port wine stain blood vessels in various skin phototypes, with or without application of cryogen spray cooling.