Journal of Medical and Biological Engineering, 22(1): 25-32
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Dynamic Cooling for Laser Photocoagulation : In Vivo & Ex Vivo Studies Cheng-Jen Chang* Yih-Fong Tzeng1 Fu-Chan Wei Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Taipei, Taiwan, 105 ROC 1 Department of Mechanical Engineering, Chang Gung University, Taoyuan, Taiwan, 333 ROC Received 30 August 2001; Accepted 22 February 2002
Abstract When a cryogen spurt is applied to the skin surface for an appropriately short period of time (on the order of milliseconds), the cooling remains localized in the epidermis, while leaving the temperature of the deeper vessels of hemangiomas unchanged. The purpose of our study is to examine the effectiveness of dynamic cooling in protecting superficial tissue structures during continuous Nd:YAG laser illumination of in vivo and ex vivo models for hemangiomas. The bovine liver and highly vascularized chicken combs were selected as the models for hemangiomas. The Nd:YAG laser illumination ranged from 20 to 60 W. A feedback system utilizing infrared radiometry monitored the surface temperature and controlled delivery time of the cryogen spurt. When the surface temperature during laser illumination reached 36-45℃, a 30-100 m/sec cryogen spurt was delivered. Animals were observed 1 hour to 14 days following each experiment. Gross and histological analyses were performed. Nd:YAG laser illumination resulted in deep (up to 1.0 ± 0.2 mm) tissue photocoagulation, while dynamic cooling preserved the overlying epidermis and papillary dermis. In conclusion, dynamic cooling is effective in protecting the epidermis and papillary dermis, while achieving deep tissue photocoagulation during Nd:YAG laser illumination. This procedure is effective for the treatment of hemangioma in the humans. Keywords: Dynamic cooling, Hemangiomas, Chicken combs, Nd:YAG laser.
Introduction Hemangiomas are benign vascular tumors that occur in up to 10% of children during the first year of life.[1] They differ from vascular malformations, such as port wine stains, in that they are not conglomerates of dilated vessels, but consist of plump, proliferating endothelial cells that may infiltrate the entire dermis, and extend several millimeters in depth.[2] Due to psychological and social factors, as well as functional impairments such as difficulty in eating, visual and breathing obstructions, early treatment is indicated.[3,4] Apfelberg et al [5] and Hobby [6] first reported use of the argon laser ( λ =488 and 514 nm) for treatment of hemangiomas in early infancy. However, due to relatively shallow penetration of the argon laser into the tumor, therapeutic effect is restricted to superficial lesions. For thick hemangiomas, the Nd:YAG laser has been shown effective due to deep penetration of 1064 nm light.[7-9] A particularly problematic complication that can occur when using lasers is thermally induced damage to the epidermis and papillary * Corresponding author: Cheng-Jen Chang Tel: +886-2-27135211 ext.3502; Fax: +886-2-25140600 E-mail:
[email protected]
dermis.[10] Cooling of skin, using ice or chilled water in conjunction with laser illumination, has been used to prevent epidermal thermal injury.[11-13] However, computed temperature distributions following sustained cooling (e.g., 15-60 s ) by 0oC ice at the skin surface show that in addition to cooling the epidermis, temperature of blood vessels is also reduced.[14] Thermal energy removed to protect the epidermis from injury will be offset by additional laser energy required to heat the blood vessels to a sufficiently high temperature for destruction. When a cryogen is sprayed on skin surface, the epidermis can be cooled selectively.[15-17] For an appropriately short cryogen spurt duration (on the order of tens of milliseconds), the spatial distribution of cooling remains localized in the epidermis, while leaving the temperature of deeper vessels unchanged. In this paper, we present: (1) a theory to predict temperature distributions, and thicknesses of protected and photocoagulated tissue in response to repetitive cryogen spurts during continuous Nd:YAG laser illumination; (2) experimental results of a study performed in vivo and a study of ex vivo on utilizing highly vasculized bovine liver tissue
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and the chicken comb animal model that demonstrate the feasibility of inducing deep tissue photocoagulation while protecting the epidermis from thermal injury: and (3) clinical application on the use of dynamic cooling in conjunction with Nd:YAG laser illumination for treatment of hemangiomas.
Methods and Materials The highly vasculized bovine liver tissue were utilized for repetitive application with cryogen spurt during continuous Nd:YAG laser illumination for ex vivo study. The highly vascular chicken comb was used as a in vivo model for hemangiomas since its histoanatomy is analogous to that found in selected vascular birthmarks, and has been extensively studied.[18] Five adult female Leghorn chickens were anesthetized by intravenously injecting 0.3 ml of ketamine and xylazine in a 9:1 volumetric ratio 10-15 minutes prior to beginning experiment. The experimental set-up for laser illumination and dynamic cooling is show in Figure 1. Laser light was delivered through a 600 µm core-diameter silica multimode optical fiber, and directly incident onto the comb. Diameter of the laser illuminated site, d, was maintained at 7 mm in all experiments. Incident laser power, p, ranged from 5 to 90 W; illumination time, tι, from 10 to135 sec; and the radiant exposure, Eo, from 130 to 3,500 J⋅cm-2. Depending on the size of the comb, three to eleven sites were illuminated; combs with smaller surface areas allowed fewer illumination sites.
Chlorodifluoromethane (Aldrich Chemical Company, Milwaukee, WI) (boiling point (BP) = -40 ℃ ), a hydrochlorofluorocarbon (HCFC-22) was used as test cryogen on the combs. Due to their relatively minimal ozone depletion potential, HCFCs are considered suitable alternatives to chlorofluorocarbons (CFCs).[19,20] Cryogen was sprayed onto the comb through an electronically controlled standard automobile fuel injection valve positioned 4 cm from the surface at 30o angle from the tissue. Cryogen spurt duration (τ) was set by a programmable digital delay generator (DG535, Stanford Research Systems, Sunnyvale, CA), and ranged between 30 and 100 ms. The cooled site on the comb surface was concentric with the laser illuminated site, and about 10 mm in diameter. No indications of cryogen induced thermal injury were observed outside the laser illuminated site. Radiometric measurement of surface temperature at the laser illuminated site was used to trigger the delivery of cryogen spurts. When the radiometric surface temperature reached a pre-specified trigger value, Ttrig, (ranging between 36 and 41℃), a cryogen spurt was delivered onto the comb. In this way, repetitive pulsed dynamic cooling during continuous laser illuminated was accomplished through a feedback system. Infrared emission from the comb was detected using a 1 mm2 liquid N2 cooled HgCdTc detector (MDD-10EO-SI, Cincinnati Electronics, Mason, OH), optically filtered at the cold stop by a 10.6-14 µm bandpass filter. Because the infrared absorption coefficient of water in this range is approximately 60 mm-1, [21] we expect that contributions to the infrared signal originate predominantly from superficial depths (1/60 mm-1 ≈ 0.017 mm).
Figure 1. Schematic of experimental set-up for dynamic cooling in conjunction with Nd:YAG laser illumination. Measurements of surface temperature were made by infrared radiometry.
Dynamic Cooling with Selective Photocoagulation
The HgCdTe detector was placed at the focal plane of a 25 mm diameter f/I Ge lens configured for unit magnification. For improved signal to noise ratio, pupil was stopped to 5 mm diameter, and the infrared signal was amplitude modulated by switching the detector on and off by a highly stable synthesized function generator (Model DS345, Stanford Research Systems) at a rate of approximately 25 kHz. Modulated signal was synchronously detected by a lock-in amplifier (Model SR850, Stanford Research Systems). The output signal of the lock-in amplifier was used to define the trigger level for the digital delay generator. The infrared detection system was calibrated by measuring the lock-in amplifier output voltage as a function of the surface temperature of an aluminum block coated with highly emissive (ε = 0.97) black paint (TC303 black, GIE Corp., Provo, UT) and heated by a resistive element from 23℃ to 75℃. Surface temperature of the aluminum block was measured using a precision thermistor (8681, Keithley Instruments, Cleveland, OH) attached to the block; the measure output voltage varied linearly with temperature.[22] Temperature distributions within tissue can be computerized by superposition of thermal response to laser illumination and DCS. T(z,t) =ΔTL + ΔTDCS + Ttrig
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(a)
(1) (b)
Where z (m) is the distance into the tissue; t (s) is time; ΔTL is the temperature increase due to laser illumination; Δ TDCS is the induced temperature decrease due to dynamic cooling spray (DCS); Ttrig is a pre-specified surface temperature above which a cryogen spurt is delivered. Following each experiment the protection of the superficial tissue of bovine liver and photocoagulations of deeper structures were examined. In the in vivo study chicken comb animal model, illumination at cooled sites were examined grossly for surface protection due to dynamic cooling, while the opposite sides of the combs were examined for blanching due to laser induced photocoagulation. Chickens were observed at various times (1h-14days) following the experiments.
Results Temperature measurement in response to laser illumination (P=20W, tι=12s, d=7 mm) without dynamic cooling showed a linear increase to 70℃ (Figure 2A). The observed linear increase indicates that both thermal diffusion and heat loss at tissue-air interface were negligible during illumination. Under these conditions, surface temperature is directly related to the radiant exposure, tissue thermal properties, and absorption coefficient[22]. Once the laser was turned off, surface temperature decreased montonically. Rapid surface temperature reductions to approximately 5℃ were observed in response to 50 ms chlordifluoromethane spurts sprayed onto the comb during laser illumination
Figure 2. (a) Radiometric surface temperature of the chicken comb in response to Nd:YAG laser illumination without dynamic cooling. (b) Radiometric surface temperature of the chicken comb in response to Nd:YAG laser illumination and 50ms repetitive cholrodifluoromethane spurt.
(P=35W, tι=20 s, d=7 mm, Ttrig=42℃) (Figure 2B). When the infrared radiometry feedback system remained on after the laser was turned off, cryogen spurts (with decreasing frequency) were released in response to heat diffusing from within the comb to the surface (Figure 2B). In the ex vivo study, with dynamic cooling of repetitive application of a 50 ms cryogen spurt during continuous Nd:YAG laser (40W) illumination was shown to result in protection of the superficial tissue (≈ 400 µm in thickness) from the thermal injury while achieving photocoagulation of deeper structures (Figure 3). Histological sections obtained were shown the thickness of protection and the tissue damage (Figure 4). In the in vivo study, without dynamic cooling, the illuminated comb surface was always blanched in response to laser illumination at Eo as low as 520 j⋅cm-2 ( p=20W, tι=10s, d=7mm). With dynamic cooling, however, protection of superficial tissue structures from thermal injury was achieved even when illuminating the comb surface at higher values of Eo ranging from 910 (P=35W, tι=10s, d=7mm) to 2,600 J⋅cm-2 (P=50W, tl=20s, d=7mm). The thickest comb site that was blanched on the opposite surface (P=40W, tl=20s, d=7mm; Eo=2.080J⋅cm ), and protected on the illuminated surface due to dynamic cooling was 1.0±0.2 mm. Superficial tissue structures were not protected by dynamic cooling when Eo exceeded 2,340 J⋅cm-2 (P=60W, tι=15s, d=7mm).
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(a)
Figure 3. Photograph of a bovine liver illuminated with the Nd:YAG laser of 40W and repetitively cooled with 50 ms chlorodifluoromethane spurts demonstrated protection of the superficial tissue with photocoagulation of deeper structures.
(b) Figure 5. (a) Photographs of a chicken comb, the sites 3 and 4, illuminated with the Nd:YAG laser and repetitively cooled with 50 and 80ms chlorodifluoromethane spurts. Illuminated surfaces were not blanched due to dynamic cooling. (b) On the opposite side of the comb, sites 3 and 4 were blanched.
Figure 4. Histologic sections of bovine livers demonstrated epidermis intact (arrow head) on the illuminated and cooled surface with damage of deeper tissue (arrow) (magnification: 73X).
Dynamic Cooling with Selective Photocoagulation
(a)
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(b)
Figure 6. (a) A three-month-old baby boy with a rapidly proliferative hemangioma with obstruction on his upper lip. (b) At age of two years, considerable shrinkage of the hemangioma with significantly good result was observed.
Laser illumination parameters were: P=35W, tl=20s, d=7mm on sites 1 and 2; P=35W, tl=30s, d=7mm on site 3. Sites 1and 2 were cooled by repetitive application of 50 ms chlorodifluoromethane spurts. On site 3, the spurt duration was 80 ms. Tissue thickness was 4.0, 9.1, and 7.0 mm for sites 1-3, respectively. Surface of the comb was not blanched since with dynamic cooling temperature was maintained below the necessary threshold required for thermal damage (Figure 5A). However, the surface opposite to illumination at site 1 was blanched (Figure 5B) indicating a sufficient laser induced temperature increase was obtained at this location to cause thermal damage. The opposite surface at sites 2 and 3 were not blanched due to greater tissue thickness at these locations. Although in this study, up to approximate 1.0±0.2 mm of tissue was photocoagulated, successful treatment of hemangiomas might be achieved by inducing smaller coagulation depths to initiate the “involution” process.[10] In our clinical evaluation, a three-month-old baby boy with a proliferative hemangioma on his upper lip was treated under general anesthesia using Nd:YAG laser illuminated with dynamic cooling (Figure 6A). The lesion was treated by Nd:YAG laser with P ranging from 30 to 45W, and tι=15 or 20 s (Eo=1,170 to 2,340 j⋅cm-2 ). Diameter of illuminated
sites was 7mm. Chlorodifluoromethane spurts (τ=50ms) were released during the illumination at a non-uniform frequency ranging between 0.9 and 2.0 Hz. After the procedure, considerable shrinkage of the hemangioma with significantly good result was observed at the age of two years (Figure 6B).
Discussion The clinical objective in the laser treatment of patients with cutaneous hemangiomas is to maximize thermal damage to targeted blood vessels, while minimizing nonspecific injury to the overlying epidermis. A potential technique to achieve this objective is to selectively cool the most superficial layers of the skin. The role of the any cooling modality in conjunction with laser treatment of cutaneous hemangiomas is governed by two crucial constraints: the cooling must produce a rapid, large reduction in the skin surface temperature, and it must provide a spatially selective epidermal temperature reduction while at the same time leaving the temperature of the deeper blood vessels unchanged.
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Light emitted from the Nd:YAG laser (λ =1064nm) penetrates deeply (5-7mm) into tissue, and has been used to photocoagulate thick hemangiomas.[4,7-9,23] For this reason, we decided to treat hemangiomas with Nd:YAG laser if complication or rapid growth of the hemangioma made therapeutic intervension necessary. However, for the cutaneous hemangiomas, laser induced thermal damage to the epidermis remains a major concern.[10] In patients with cutaneous hemangiomas undergoing laser treatment, optimal clinical results should be obtained when the ratio of heat generated in the blood vessels is high compared to that in the epidermis. The ratio of heat generated in the blood vessels (QBr) to that in the epidermis (QE) is a measure of the relative heating of the blood vessels with respect to the epidermis. The best clinical results in patients with hemangioma undergoing laser therapy are obtained when this ratio (QB/Q E) is high ( ≧1). Unfortunately, for “dark skin” patients (skin type III-V), it is not possible to treat the hemangioma with the higher therapeutic light dosage due to epidermal damage. For example, when the skin surface temperature exceeds 70°C immediately after laser exposure (
