Showing papers in "Iranian Journal of Medical Physics in 2005"

Noise Pollution and Traffic Noise Index on Mashhad Main Streets during the Busiest Hours of Summer

Abstract: Introduction: Among the environmental pollutions, noise is very important for its physiological and psychological effects on human. Traffic noise is one of the most important pollutants and the hospitals are one of the critical places regarding this type of noise. For these reasons, in the summer of 1382, the traffic noise of Mashhad main streets around the hospitals was assessed during the busiest hours. Materials and Methods: The noise indexes such as L Aeq , L Afmax , L 10 , L 50 and L 90 were measured by a Sound-Level-Meter, model Investigator 2260. The traffic load was also determined. On the basis of these results, Noise Pollution Level (NPL) and Traffic Noise Index (TNI) were calculated. The assessment was done during three different periods of the days in twelve stations. Results: Based on the obtained results, the maximum L Aeq was recorded on Bahar Street during the morning hours and on Koohsangi Street during the noon and night periods. Throughout the three periods the maximum NPL and TNI were estimated on Bahar and Nakhrisi Streets, respectively. The correlation between all of the indexes was analyzed and a logarithmic correlation was observed between L Aeq and the traffic load. Discussion and Conclusion: On the basis of the noise standard in free field in Iran, noise pollution is a serious problem in Mashhad.

A monte carlo simulation of photon beam generated by a linear accelerator

Abstract: ntroduction: Monte Carlo simulation is the most accurate method of simulating radiation transport and predicting doses at different points of interest in radiotherapy. A great advantage of the Monte Carlo method compared to the deterministic methods is the ability to deal accurately with any complex geometry. Its disadvantage is the extremely long computing time required to obtain a dose distribution with good statistical accuracy. Materials and Methods: The MCNP-4C Monte Carlo code was used to simulate a 9 MV photon beam from a Neptun 10PC linear accelerator. The accelerator was modeled as a complete unit consisting of a target, exit window, initial collimator, primary collimator, flattening filter, monitor chamber and secondary collimator. The geometrical details and the composition of each component was either obtained from the manufacturer or was directly measured. The simulation of the source was performed in a two step process. Initially, the electron source was defined. Secondly, the bremsstrahlung energy spectra and the fluence distribution at the scoring planes were used to define the photon source. The simulated electron beam energy followed a Gaussian distribution, with FWHM equal to 12% in nominal energy. The used intensity distribution of the electron beam also followed a Gaussian distribution with a FWHM equal to 0.34 cm. To compute the photon beam data a 50 × 50 × 40 cm 3 water phantom located at SSD = 100 cm was simulated. The depth dose and the dose profile curves were calculated for four different field sizes (5×5, 10×10, 20×20 and 30×30 cm 2 ) and compared against the measured values. The low-energy cut-off for the photons and electrons was 10 and 500 KeV, respectively. The measurements were carried out by using a Scanditronix dose scanning system and a 0.12 cm 3 RK ionization chamber. Results: To verify the simulated model, the calculated Monte Carlo dose data were compared against the corresponding measured values. The energy spectra and the angular distribution of the x-ray beam generated by the Neptun 10PC linac was examined. The result showed an efficiency of about 73% for the production of bermsstrahlung photon by the target. The agreement between the calculated and the measured depth dose and the dose profile was generally better than 2% for all the fields. Discussion and Conclusion: The simulation of the Neptun 10PC linac performed in this work is capable of computing the depth dose data and the beam profiles in water phantom for all the predefined fields including 5×5, 10×10, 20×20 and 30×30 cm 2 . Therefore, it can be concluded that MCNP-4C is a suitable tool for the dose calculation in radiotherapy. The simulated linac machine and the resulting data can be used to predict the dose distribution in all complex fields.

New formula for Calculation of Cobalt-60 Percent Depth Dose

Abstract: Introduction: On the basis of percent depth dose (PDD) calculation, the application of dosimetry in radiotherapy has an important role to play in reducing the chance of tumor recurrence. The aim of this study is to introduce a new formula for calculating the centeral axis percent depth doses of Cobalt-60 beam. Materials and Methods: In the present study, based on the British Journal of Radiology (BJR) table, nine new formulas are developed and evaluated for depths of 0.5 - 30 cm and fields of )4 4( × ) 45 45 ( × − cm2. To evaluate the agreement between the formulas and the table, the average of the absolute differences between the values was used and the formula with the least average was selected as the best fitted formula. The Microsoft Excel 2000 and the Datafit 8.0 soft wares were used to perform the calculations. Results: The results of this study indicated that one amongst the nine formulas gave a better agreement with the PDDs listed in the table of BJR. The new formula has two parts in terms of log (A/P). The first part as a linear function with the depth in the range of 0.5 to 5 cm and the other one as a second order polynomial with the depth in the range of 6 to 30 cm. The average of the differences between the tabulated and the calculated data using the formula ( Δ ) is equal to 0.3152. Discussion and Conclusion: Therefore, the calculated percent depth dose data based on this formula has a better ageement with the published data for Cobalt-60 source. This formula could be used to calculate the percent depth dose for the depths and the field sizes not listed in the BJR table.

Evaluation of temperature rise and thermal lesion dimensions in liver laser interstitial thermotherapy

Abstract: Introduction: Laser interstitial thermotherapy (LITT) is an internal ablation therapy method consisting of a percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. In this treatment method, there isn’t any general information about the relationship between increasing the exposure power, coagulation and carbonization areas with the changes in temperature. In this study, according to the power range of LITT the changes in the temperature of liver tissue and the diameter of the necrotic area were measured. Materials and Methods: In vitro LITT was performed on fleshly sheep liver tissue using a bare-tip optical fiber from a Nd:Yag laser. A power setting of 2, 2.4, 3, 3.4, and 4 watt were used for an exposure time of 300 sec. The temperature monitoring was performed during the heating and cooling down by fixing micro thermocouples at 2.5 mm from the fiber tip. The thermal lesions which include necrosis and carbonization areas were compared for each power. Results: The result of the temperature monitoring was expressed as the mean value for each power. The temperature charts show that at 2.5 mm from the fiber tip the max. tissue temperature is increased from 276.20 oC (for a power setting of 2 watt and a 308 sec of exposure time) to 728.2 oC (for a power setting of 3.4 watt and a 365 sec exposure time). At 6 mm from the fiber tip the max. temperature was measured to be 86.4 oC for a power setting of 4 watt and 325 sec exposure time. For each power a non linear regression analysis was performed during the heating and cooling down for the dependent (temperature) and independent (time) parameters. The max. value for the cubic equation is shown to be R = 0.99 during the heating and for the exponential equation to be R = 0.89 during the cooling down. A p value of 0.01 is considered significant. The diameter of the necrotic liver tissue increases from 12.95 mm at 600 joules to 16.15 mm at 1200 joules of energy. When the total applied energy is increased from 600 to 1200 joules, the thermal ablation increased by 25% while there was a 56% increase in the carbonization area. Increasing the carbonization area caused a decreases in the penetrability of the laser beam. Discussion and Conclusion: A useful treatment planning based on a non-linear regression analysis could be prepared for the treatment of hepatocellular carcinoma. In this analysis, the temperature changes in the necrotic area are monitored as a function of power setting in the range of 2-4 watts in LITT.

Presentation of a Non-invasive Method of Estimating Arterial Stiffness by Modeling Blood Flow and Arterial Wall Based on the Determination of Elastic Module of Arterial Wall

Abstract: Introduction: Arterial stiffness is an important predictor of cardiovascular risk. Several indices have been introduced to estimate the arterial stiffness based on the changes in the brachial blood pressure. Since the substitution of the blood pressure changes in the central arteries such as carotid with the blood pressure changes in the brachial results in error in the blood flow, it is of importance to present an elastic parameter based on the mechanical models without any reliance on the brachial blood pressure. Materials and Methods: Initially, a suitable dynamic model is introduced for pulsatile blood flow in the arteries based on Navier-Stokes the equations in fluid mechanics. Then, according to the theory of elasticity, the equations governing arterial wall are described and coupled with the equations of fluid flow. The attained system of equations is completed by the clinical information obtained from the carotid artery Doppler ultrasound images of healthy male subject. Therefore, the Doppler ultrasound images are recorded and saved in computer after which the center-line blood velocity, the arterial wall thickness, the period of a cardiac cycle and the arterial radius are measured by off-line processing. Results: The results from the analytic solution of the completed equations show that the elastic modulus for this healthy subject is 51 kpa which is in close agreement with the result obtained from other researches. Discussion and Conclusion: By applying this method, a non-invasive method of clinically evaluating the arterial stiffness will be possible by the Doppler ultrasound measurement of common carotid artery without any measurement of the local blood pressure.