Nır-ır Laserlerin Karaciğer Üzerindeki Etkilerinin In Vıtro Ortamda Araştırılması Ve Karşılaştırılması

Abstract

Theodore Maiman’ın Hughes Araştırma Laboratuvarı’nda 1960 yılında ilk kez laseri çalıştırmasından bu yana laserler tıbbi, askeri, ticari ve bilimsel amaçlar için araştırılmaktadır. Laserler göz hastalıkları ve cilt hastalıklarının tanı ve sağaltımında yoğun biçimde kullanılmakla birlikte, diğer disiplinler (üroloji, ortopedi) tarafından da araştırılmaktadır. Laser-doku etkileşimleri ışılısıl (photo-thermal), ışıl-kimyasal, ışıl-mekanik ve ışıl-parçalama biçiminde veya bunların bir kombinasyonu olarak gelişebilir. Laserin dokudaki ısıl etkileri ve ışılısıl haraplama, 43°C’den başlayarak koagülasyon (pıhtılaşma), ablasyon (doku buharlaşması/kaldırılması), ısıl yanıklar ve doku erimesi şeklinde gözlenmektedir. Elektromagnetik spektrumun yakın kızılaltı (NIR) bölgesi, morötesi (UV) ve kızılaltı (IR) bölgesine göre dokudaki görece yüksek girginlikten dolayı, ısıl tedavide kullanılmaktadır. 2 µm dalgaboyu civarı da doku kaldırma uygulamaları için uygundur. Laser-doku etkileşiminde uygulanma (ışıma) süresi güç yoğunluğundan daha önemli bir parametredir. Bu çalışmada 1070 nm YLF , 980 nm diyot ve 1940 nm fiber laserlerinin karaciğer dokusu üzerindeki ışılısıl etkileri gözlenmiştir. Bu dalgaboylarıyla, ışılısıl haraplama başlama süresi, ışılısıl yanık başlama süresi, güvenli süre tayini ve ablasyon etkinliği ölçülerek karşılaştırılmıştır. Ölçümler, karaciğer dokusunda, 4 mm x 6 cm kesitler alınarak gerçekleştirilmiştir. Laserler dokuya 1 cm mesafeden ve 1070 nm ve 980 nm için 4-10 W güç aralığında, 1940 nm için 1-3 W güç aralığında uygulanmıştır. Makroskopik olarak ışılısıl haraplama ve ışılısıl yanık çapı, derinliği ölçümleri kalipiyer ve mikroskop altında tekrarlanmıştır. Makroskopik ölçümleri karşılaştırmak için yapılan mikroskopik ölçüm, hemotoksilen&eosin boyası ile yapılmıştır. Sonuç olarak diğer iki dalgaboyuna göre 980 nm dalgaboyu dokuda daha büyük ışılısıl hasar oluşturmaktadır. 1070 nm en “güvenli” dalgaboyudur ve görece daha küçük ışılısıl haraplama hacmi oluşturmaktadır. 1070 nm için ışılısıl haraplama ve yanık derinliği, 1940 nm için de yanık derinliği güç yoğunluğundan bağımsızdır. 1940 nm dalgaboyu, suyun soğurma tayfından dolayı karaciğer dokusunda yüzeysel olarak küçük ve etkin bir doku kesimi için uygundur.

Since its first demonstration by Theodore Maiman in 1960 at Hughes Research Laboratories, lasers have been investigated for medical, scientific, military and commercial applications. Although in ophthalmology and dermatology lasers are used frequently, they are investigated for other medical disciplines, such as, in gynecology, urology, and orthopedics. Unlike conventional light sources, lasers produce monochromatic, coherent, and highly collimated intense beams of light. Coherent light can be focused to a very small spot size for practical applications. Thanks to this characteristics intense and small spot size laser beams can be produced. Laser-tissue interactions include photothermal, photochemical, photomechanical, and photodissociation effects. When laser light strikes the tissue surface , interaction with tissue can be absorption, scattering, reflection and transmission. Main absorbers in biological tissue are water, hemoglobin (Hb) and in skin and retina, melanin. When laser light is absorbed, heat is produced and can cause thermal damage to the tissue. Thermal effects of lasers start when tissue reaches 43 ºC leading to coagulation. Vaporization, ablation carbonization and melting occur for longer exposure and higher power densities. In literature, laser–tissue interactions depend on a combination of laser and tissue parameters. These are 1. laser parameters: wavelength, laser mode, spot size, exposure time, energy and power density (J/cm2 and W/cm2) , 2. tissue parameters: absorption coefficient, scattering coefficient, anisotropy, heat conductivity, heat capacity. In laser-tissue interactions energy density varies between 1 and 1000 J/cm2. In contrast to energy density, power density varies over 15 orders of magnitude. Considering laser parameters only, laser tissue interaction mechanisms depend on exposure time, power density and energy density. Correlation between exposure time and power density shows us that almost same energy density is required for any type of interaction mechanism. Thus, exposure time is considered to be the main parameter determining the nature of laser tissue interactions. Other important parameters responsible for the interaction mechanisms are wavelength and power density. Laser wavelength is considered the second important parameter. Response of biological tissue is largely determined by irradiation wavelength. Last parameter is applied power density (irradiance) that governs the type of interaction and extent of photothermal damage. The NIR region of the electromagnetic spectrum (600 to 1200 nm) was used in therapy because of relatively deeper penetration at these wavelengths when compared to UV or IR. Specific absorbers in this spectral range are water and protein molecules like hemoglobin. Wavelengths around 2 &#956;m are considered to be in the “eye-safe” window making them more suitable for superficial interstitial phototherapy. The goal of laser induced thermal therapy is precise treatment of local lesions while preserving surrounding healthy tissue. In order to achieve this goal, laser beam size and exposure times must be closely controlled or monitored because laser tissue interactions depend on exposure time more than power density. The outcome of photothermal interactions depend on dosimetry and may be limited to coagulation only, but may also extent to ablation and carbonization when higher doses are used. The aim of our study was to compare thermal effects and extent of laser irradiation at three wavelengths (980 nm, 1070 nm, and 1940 nm, with varying penetration depths) on liver tissue. The reasons for selecting 1070 nm, 980 nm and 1940 nm in our study are summarized below: 1. We chose those three laser wavelengths in our experimental study because their photothermal effects on tissue were expected to vary considerably. 2. 1064 nm laser is little absorbed by biological tissue ( µa<< ) and it is used for interstitial LITT due to high penetration depth in tissue. There are 6 nm differences to 1070 nm ytterbium fiber laser (YLF). 3. 1940 nm is more absorbed (µa >>µs ) when compared to 980 nm diode and 1070 nm fiber laser. Wavelengths around 2 &#956;m may create superficial lesions due to local absorption peak of water. Up to now just two wavelengths were studied for this purpose , 1.9 µm and 2.01 µm. 4. 980 nm diode laser is considered more practical and efficient in medical applications due to portability, and compactness. 980 nm is more absorbed by water and hemoglobin (Hb) when compared to 1070 nm. For tissue stained with blood this property can be used for coagulative and ablative treatment. 5. Moreover , liver tissue attracts metastatic tumors. This manuscript consists of six chapters; 1. Introduction 2. Basic laser physics 3. Laser-tissue interaction 4. Materials and methods 5. Results 6. Discussion and conclusions We studied onset of coagulation, carbonization and ablation, as well as ablation efficiency. Our in vitro study was performed on bovine liver tissue. 4 mm axial and 6 cm radial sections were taken from fresh liver specimens from a local abattoir. Irradiation was performed with laser output ranging from 4-10 W (3 - 7,5 W/mm2) for 1070 nm and 980 nm , 1-3 W (0,3 - 0,9 W/mm2) for 1940 nm. CW laser beam was applied using a bare fiber at 1 cm distance from the tissue surface. Thermal lesions were studied under a microscope. Radius and depth of coagulation and carbonization were measured using a caliper. Subsequently, we estimated carbonized and coagulated tissue volumes. In addition the macroscopic measurements, carbonized and coagulated tissue samples were stained with hematoxylin and eosin (H&E) in order to more precisely evaluate the effects of laser. We compared carbonization efficiency (carbonization volume/coagulation volume), safe time interval (from onset of coagulation to carbonization), normalized carbonization radius/carbonization depth, and normalized coagulation radius/coagulation depths at those three wavelengths. The measured parameters were given as mean values with their standard deviations. The level of significance was set to p < 0,01 for 1940 nm and to p<0,007 for 980 nm and 1070 nm. Histological measurements were performed to investigate cellular tissue damage. In coagulation region we observed that due to the relaxation of collagen and elastin, fiber tissue integrity was destroyed, matrix was separated and central and bile duct were damaged. Same as the coagulation region, in carbonization region collagen and elastin fibers were broken. With hepatocytes damaged, tissue shrinkage occurred and intercellular space was increased. Also we saw granulation under the microscope. Our results indicated that at 980 nm tissue damage was more pronounced than at other wavelengths. For ablation purposes 1070 nm has a longer safe time interval and created smaller coagulation volumes. Carbonization depth and coagulation depth were comparable at 1070 nm. Moreover, carbonization depth was independent of applied power density at 1940 nm (at 1W, 2 W, 3 W). For small and precise cutting of liver tissue 1940 nm was the wavelength of choice due to local absorption peak of water. Macroscopic and microscopic results were in agreement.