RESUMO
Thermal dose models are metrics that quantify the thermal effect on tissues based on the temperature and the time of exposure. These models are used to predict and control the outcome of hyperthermia (up to 45°C) treatments, and of thermal coagulation treatments at higher temperatures (>45°C). The validity and accuracy of the commonly used models (CEM43) are questionable when heating above the hyperthermia temperature range occurs, leading to an over-estimation of the accumulation of thermal damage. A new CEM43 dose model based on an Arrhenius-type, Vogel-Tammann-Fulcher, equation using published data, is introduced in this work. The new dose values for the same damage threshold that was produced at different in-vivo skin experiments were in the same order of magnitude, while the current dose values varied by two orders of magnitude. In addition, the dose values obtained using the new model for the same damage threshold in 6 lesions in ex-vivo liver experiments were more consistent than the current model dose values. The contribution of this work is to provide new modeling approaches to inform more robust thermal dosimetry for improved thermal therapy modeling, monitoring, and control.
Assuntos
Hipertermia Induzida , Eletrocoagulação , Temperatura Alta , Fígado , TemperaturaRESUMO
Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phantoms, the results demonstrate an unexpected rise in optical power transmitted approximately I cm away from the source during laser exposure at low power (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulation and possibly charring. Similar trends were observed in liver tissue, with a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagulation. At 1.5 W irradiation an additional, later decrease was also seen which was most likely due to tissue charring. Independent spectrophotometric studies in Naphthol Green dye indicate the rise in fluence observed in the heated albumen phantoms may have been primarily due to light exposure causing photobleaching of the absorbing chromophore. and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating changes in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and control of laser thermal therapy via interstitial fluence sensors.
Assuntos
Temperatura Alta , Fotocoagulação a Laser/métodos , Lasers , Luz , Animais , Bovinos , Corantes/farmacologia , Fígado/efeitos da radiação , Modelos Teóricos , Óptica e Fotônica , Imagens de Fantasmas , Radiometria , Espectrofotometria , Temperatura , Tempo , Fatores de TempoRESUMO
Multipoint optical fluence measurements can potentially be used to detect coagulation-induced changes in optical propagation during interstitial laser thermal therapy. Estimating the dimensions of coagulation using on-line optical monitoring, which is applicable to treatments where the tip of the source fibre is not precharred, may be limited by the accuracy of the placement of optical sensors with respect to source fibres. A strategy has been developed to determine accurately the position of a four-sensor linear array, prior to treatment, using optical fluence data obtained from the sensors for low-power (< or = 0.5 W) irradiation. A minimum of four sensors in an array was required in order to develop a mathematical formulation for position determination that did not require tissue optical properties or laser power as input. Optical propagation was based on diffusion theory for homogeneous tissues in spherical geometry. Low input laser power is needed to ensure that there are no thermally induced changes in tissue optical properties not accounted for in the mathematical description. Experimental evaluation was performed in a tissue-equivalent liquid phantom using 0.5 W of 805 nm optical energy and a translatable isotropic optical sensor. For sensor locations with 2 mm spacing, placement accuracy of 0.67 mm was achieved. The accuracy improved to 0.13 mm as the sensor spacing increased to 5 mm.
Assuntos
Temperatura Alta , Fotocoagulação a Laser/instrumentação , Fotocoagulação a Laser/métodos , Lasers , Modelos Teóricos , Imagens de FantasmasRESUMO
BACKGROUND AND OBJECTIVE: Interstitial Laser Photocoagulation (ILP) is a minimally invasive cancer treatment technique, whereby optical energy from implanted optical fibers is used to therapeutically heat small, solid tumors. In this work, the potential of ILP without tissue charring is investigated. STUDY DESIGN/MATERIALS AND METHODS: Optical diffusion and bio-heat transfer equations were used to develop dynamic models of interstitial laser heating in liver in vivo. Modifications in the optical properties due to tissue coagulation (T > or = 60 degrees C) were incorporated into the physical description. In addition, the effect of three different blood perfusion patterns on temperature distributions was explored. Model-predicted temperatures were used as an index for thermal damage based on an accumulated temperature injury (Arrhenius) model. Thermal damage dimensions were determined with tissue temperatures constrained to remain below 100 degrees C, so as to minimize the potential for tissue charring and smoke production. RESULTS: The model predicts that increases in scattering due to coagulation and choice of perfusion pattern affect substantially thermal damage dimensions. The results indicate that, for single fiber ILP at 2.55 W for 600 s, the maximum achievable thermal damage diameter in liver, without charring, is 9.6 mm. In addition, ILP performed with high-low power ramping may have an advantage over constant power treatments, in that, larger volumes of thermal damage can be realized earlier in an irradiation. CONCLUSIONS: For ILP performed with a single spherical emitting fiber, optimal irradiation parameters exist such that thermal lesions in liver up to approximately 10 mm in diameter can be induced while the maximum tissue temperature remains below 100 degrees C, avoiding tissue charring.
Assuntos
Fotocoagulação a Laser , Fígado/fisiopatologia , Procedimentos Cirúrgicos Minimamente Invasivos/métodos , Modelos Teóricos , Temperatura Corporal , Humanos , Fígado/irrigação sanguínea , Fígado/cirurgia , Circulação HepáticaRESUMO
Interstitial laser heating of tissues is influenced by blood flow in the treatment region. Temperature gradients around large blood vessels may result in local underheating of tissues. A three-dimensional, time-dependent finite difference model of interstitial laser heating around large vessels is presented. A thermal conduction model was developed using a transport theory approximation for the energy distribution from an optical line source. Calculated transient temperature profiles and temperature reductions around 0.144 and 0.400 cm diam vessels show qualitative agreement with those measured in a series of tissue phantom studies. Experiments and calculations for a large vessel located approximately 1.0 cm from the optical source indicate that temperature reductions are less than 1 degree C at distances greater than approximately 1.0 cm from the vessel surface. The model also indicates that significant reductions in the extent of a thermal coagulation boundary can occur if a large vessel is situated inside the normal coagulation zone.
Assuntos
Hipertermia Induzida/instrumentação , Vasos Sanguíneos , Temperatura Alta , Modelos Estruturais , Modelos TeóricosRESUMO
A theoretical basis for interstitial laser photocoagulation (ILP) practiced with point-emitting fiber tips has been established by solving the bioheat transfer equation, using basic Green's function methods, for steady and instantaneous point sources of both optical energy and direct heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during ILP. These are defined here as optothermal heat capacities and an optothermal diffusion length, all of which characterize how a thermal diffusion temperature profile is flattened and reduced when optical diffusion is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of previous empirical observations that point optical sources heat tissues less than point heat sources of the same power. A comparison of normalized theoretical temperature transients with published measurements suggests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6 +/- 1.7.
Assuntos
Hipertermia Induzida/métodos , Fotocoagulação a Laser/métodos , Fenômenos Biofísicos , Biofísica , Difusão , Temperatura Alta , Humanos , Circulação Hepática/fisiologia , Modelos Teóricos , Neoplasias/fisiopatologia , Neoplasias/terapia , TermodinâmicaRESUMO
Interstitial laser photocoagulation (ILP) was performed in vitro in lean bovine and chicken muscle by delivering 1.6 W of continuous-wave Nd:YAG laser energy (1064 nm) from a 400-microns core optical fiber for 300s. The resulting thermal coagulation lesion was consistently larger when the delivered energy was deposited into a small steel sphere than when it was delivered freely into the tissue. Mathematical modelling confirms this result. This preliminary study suggests that a point heat source produces a larger volume of thermal coagulation than a point optical source (1064 nm) delivering the same power.
Assuntos
Tecnologia de Fibra Óptica , Terapia a Laser/instrumentação , Fotocoagulação/métodos , Modelos Biológicos , Músculos/cirurgia , Animais , Bovinos , Galinhas , Temperatura Alta , Técnicas In Vitro , Fibras ÓpticasRESUMO
The present study was conducted to determine whether or not a statistically significant relationship exists between patients' anxiety levels and dropout rates in an inpatient alcoholism treatment program. The anxiety test scores of two groups (20 veterans completing a 4-week rehabilitation program and 20 veterans terminating treatment against staff advice during the initial 2 weeks of treatment) were compared. No significant differences were measured in a between-group comparison. The need for future research to be conducted with the inclusion of a control group is illustrated in regard to the findings of this study.