IRANIAN JOURNAL OF MEDICAL PHYSICS
Year:2008 | Volume:5 | Issue:1 (18-19)|Papers Count:8

Introduction: Nowadays the medical, therapeutic and pharmacological application of magnetic fields (MF) and its biological effect has raised question about the safety of MF. This study aimed at scrutinizing the effect of static MF on the resistance of S. aureus to antibiotic.Materials and Methods: This prospective, case–control study was conducted to evaluate the effect of low intensity (0.5 mT) static MF on the growth rate and the antibiotic resistance of S. aureus sensitive to gentamicin. The studied bacterium is a nosocomial type and the growth rate was calculated by colony counting to understand the effect of MF on it. In the next stage, the rate of bacterial growth along with the different concentration of antibiotic was studied and the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) were determined.Results: It is concluded that the 0.5 mT MF didn't affect the growth rate of S. aureus after 24 and 48 hours. The 0.5 mT MF induced a 50 percent decline of MIC and MBC of gentamicin after a 48 hour incubation (MIC = 4 mg /cc, MBC = 8 mg /cc in the case group vs. MIC = 8 mg /cc, MBC = 16 mg /cc in the control group).Conclusion: Low–intensity MF didn't affect the bacterial growth rate. However, the bactericidal effects of gentamicin were greater in the presence of MF. It is possible to apply the static MF for enhancing the effect of antibiotic on S. aureus.

Introduction: The use of low energy isotopes such as 103Pd in brachytherapy for the treatment of cancers such as prostate, eye, head, neck, breast and cervix is increasing. In this regard, different models of Pd-103 seeds have been designed and manufactured at the Agricultural, Medical and Industrial Research School (AMIRS) of Atomic Energy Organization of Iran. In this research, the dosimetric parameters of the second model of Pd-103 seed manufactured at AMIRS have been calculated and measured.Materials and Methods: The dosimetric parameters of the second Pd-103 seed manufactured at AMIRS were determined according to TG-43U1 protocol using Monte Carlo calculations (MCNP4C computer code) and measurements performed using TLD-GR200A dosimeters in a Perspex phantom. The parameters include dose rate constant, geometry function, radial dose function, anisotropy function, anisotropy factor and anisotropy constant.Results: It was found that by using MCNP4C code the calculated dose rate constantin water and Perspex was0.706±0.001 and 0.501±0.001 cGyh-1U-1, respectively. Using the calculated geometry function, the radial dose function and the anisotropy function were determined by experimental and theoretical methods in water and Perspex phantom. Also, the calculated value of anisotropy constant in water was equal to 0.88.Discussion and Conclusion: Adiscrepancy of less than 10% between the calculated and the measured values indicates a reasonable agreement between the simulation and the measurement method. Also, the dosimetric parameters of this seed have been compared to the dosimetric parameters of the first Pd-103 seed manufactured at AMIRS and some other seeds. The obtained results indicate that the seeds manufactured at AMIRS have acceptable dosimetric parameters suitable for brachytherapy applications.

Introduction: The advent of dual-modality PET/CT scanners has revolutionized clinical oncology by improving lesion localization and facilitating treatment planning for radiotherapy. In addition, the use of CT images for CT-based attenuation correction (CTAC) decreases the overall scanning time and creates a noise-free attenuation map (mmap). CTAC methods include scaling, segmentation, hybrid scaling/segmentation, bilinear and dual energy methods. All CTAC methods require the transformation of CT Hounsfield units (HU) to linear attenuation coefficients (LAC) at 511 keV. The aim of this study is to compare the results of implementing different methods of energy mapping in PET/CT scanners.Materials and Methods: This study was conducted in 2 phases, the first phase in a phantom and the second one on patient data. To perform the first phase, a cylindrical phantom with different concentrations of K2HPO4 inserts was CT scanned and energy mapping methods were implemented on it. For performing the second phase, different energy mapping methods were implemented on several clinical studies and compared to the transmission (TX) image derived using Ga-68 radionuclide source acquired on the GE Discovery LS PET/CT scanner.Results: An ROI analysis was performed on different positions of the resultant µmaps and the average mvalue of each ROI was compared to the reference value. The results of the µmaps obtained for 511 keV compared to the theoretical values showed that in the phantom for low concentrations of K2HPO4 all these methods produce 511 keV attenuation maps with small relative difference compared to gold standard. The relative difference for scaling, segmentation, hybrid, bilinear and dual energy methods was 4.92, 3.21, 4.43, 2.24 and 2.29%, respectively. Although for high concentration of K2HPO4 the three methods; hybrid scaling/segmentation, bilinear and dual energy produced the lowest relative difference of 10.91, 10.88 and 5%, respectively. For patients it was found that for soft tissues all the mentioned energy mapping methods produce acceptable attenuation map at 511 keV. The relative difference of scaling, segmentation, hybrid, and bilinear methods compared to TX method was 6.95, 4.51, 7, and 6.45% respectively. For bony tissues, the quantitative analysis showed that scaling and segmentation method produce high relative difference of 26 and 23.2%, respectively and the relative difference of hybrid and bilinear in comparison to TX method was 10.7 and 20%, respectively.Discussion and Conclusion: Based on the result obtained from these two studies it can be concluded that for soft tissues all energy mapping methods yield acceptable results while for bony tissues all the mentioned methods except the scaling and segmentation yield acceptable results.

Introduction: Nowadays, considerable developments in the field of radiotherapy have been achieved. They include the advances made in the equipments and treatment planning techniques which require highly complex calculations. Such achievements have made it possible to treat cancer patients not only with higher radiation dose but also with higher precision and consequently increasing the chance of curing the cancer. However, the conventional techniques requiring physical wedge are still being used but with a lesser frequency. One of the wedge parameters needed to be measured is the wedge angle. It is the angle that the horizontal line creates with the tilted isodose curve at a specific depth and for a certain field size.In this study, the variation of wedge angle for different field sizes was evaluated using dosimetric and mathematical method.Material and Methods: For the wedge fields with a dimension of 6×6 to 20×20 cm2, the wedge angle for two photon energies of 6 and 18 MV was measured by the dosimetric method. For these measurements, the conventional wedges having the nominal wedge angle of 15, 30, 45 & 60 were used. The theoretical method suggested by Saw et al. is also used to indirectly calculate the slope of isodose curve by the dose profile and percent depth dose data. The dose profile, percentage depth dose and isodose curves were drawn for all the field sizes and the tilt of isodose curve at 10 cm depth, according to international definition, is considered as the wedge angle. The data were obtained using the theoretical equation of wedge angle and it was compared to the dosimetric data.Results: The result obtained in this work shows that the wedge angle increases with the field size. For a 6×6 cm2 field size, the calculated wedge angle has the highest difference in comparison to the nominal wedge angle. The difference is equal to 14.7 degree for a 45o wedge and a 6 MV photon. The highest difference for a 45o wedge angle, a field size of 10×10 cm2 and a 6 MV photon is 9.2 degree. Comparing the calculated and measured wedge angles shows a maximum difference of 4 degree for 6 and 18 MV photon beams.Discussion and Conclusion: The wedge angle varies with field size. In order to get a better dose distribution in the conventional radiotherapy, it is necessary to use the appropriate wedge angle which generates the desired slope for the isodose line and for the specific field size.

Introduction: The use of mobile communication systems have dramatically increased over the past decade. Although many studies have been performed to determine the effect of radio frequency (RF) but less attention has been paid to the possible biological impact of exposure to extremely low frequency (ELF) components.The objective of this study is two folds. One is to design the equipments needed for the measurement of the ELF fields of two types of GSM900 mobile phone. Secondly, use a protocol suitable for an accurate assessment of the ELF fields.Materials and Methods: First a home-made search coil was provided and calibrated precisely under several experiments. Using Fast Fourier Transform, the power spectrum density of the induced voltage in the search coil was analyzed and the amplitudes of 217 Hz and its harmonics were extracted and then the distribution of magnetic field in the back side of mobile phones was determined.Results: The values of B-field on the back side of the two kinds of GSM mobile phone were different. They were between 50 to 160 mT in Nokia 3310 and 14 to 30 mT in Nokia 8310. Considering the difference between the amplitudes of frequency components at 217 Hz and its harmonics in the two kinds of mobile phone, a range of magnetic flux density at different times in a five day period was measured.Discussion and Conclusion: These findings emphasize the need for considering the distribution of low frequency magnetic field from mobile phone when biological effects of magnetic fields are studied. To determine the intensity windowing effect, one must consider the physical characteristics of the fundamental frequency component wave (217 Hz) and its harmonics produced by the mobile phone similar to the one generated under a real situation.

Introduction: Content Based Image Retrieval (CBIR) is a method of image searching and retrieval in a database. In medical applications, CBIR is a tool used by physicians to compare the previous and current medical images associated with patients pathological conditions. As the volume of pictorial information stored in medical image databases is in progress, efficient image indexing and retrieval is increasingly becoming a necessity.Materials and Methods: This paper presents a new content based radiographic image retrieval approach based on histogram of pattern orientations, namely pattern orientation histogram (POH). POH represents the spatial distribution of five different pattern orientations: vertical, horizontal, diagonal down/left, diagonal down/right and non-orientation. In this method, a given image is first divided into image-blocks and the frequency of each type of pattern is determined in each image-block. Then, local pattern histograms for each of these image-blocks are computed.Results: The method was compared to two well known texture-based image retrieval methods: Tamura and Edge Histogram Descriptors (EHD) in MPEG-7 standard. Experimental results based on 10000 IRMA radiography image dataset, demonstrate that POH provides better precision and recall rates compared to Tamura and EHD. For some images, the recall and precision rates obtained by POH are, respectively, 48% and 18% better than the best of the two above mentioned methods.Discussion and Conclusion: Since we exploit the absolute location of the pattern in the image as well as its global composition, the proposed matching method can retrieve semantically similar medical images.

Introduction: 90Sr/90Y source has been used for the intravascular brachytherapy to prevent coronary restenosis in the patients who have undergone angioplasty. The aim of this research is to determine the dose distribution of 90Sr/90Y source in a water phantom.Materials and Methods: In the present work, MCNP code has been applied to calculate the dose distribution around a 3 cm length of 90Sr/90Y source in a 30×30×30 cm3 water phantom. Also, the exact geometry of the source has been used in this simulation. Tally *F8:e which is suitable for beta ray dosimetry has been evaluated with less than %5 relative error in a sphere having 0.2 mm radius.Results: The isodose curve for 10, 20, 40, and 90% depth dose (PDD) were derived based on the calculated dose curves along the parallel and perpendicular axis to the source.Discussion and Conclusion: The results obtained in this work are in a good agreement with the experimental result published by Buckley et al. and the International Atomic Energy Agency (IAEA) report in a water phantom. Therefore, the result of this research can be used in the intravascular brachytherapy.

Introduction: The Monte Carlo simulation techniques are the most accurate methods to predict the dose distribution in radiotherapy. The main drawback of these methods is their very long computing time for a reasonable accuracy although the time of calculations has been reduced by the use of parallel processing techniques.Materials and methods: The MCNP-4C Monte Carlo code was used to simulate the 9 MV photon beam generated by a Neptun 10PC linear accelerator. To verify the model, the calculated dose data (depth dose, beam profile, output factors) were compared to the corresponding measured values for open fields and were found to be within the acceptable limits. The model has been applied to the simulation of asymmetric fields. The depth dose on central beam axis and collimator axis was calculated for three field sizes; 6×6, 10×10 and 15×15 cm2 with an offset of 1.5, 3.5 and 6 cm, respectively. The lateral dose profiles at two depths have also been calculated for the irradiation fields. All of the computed data were compared to the practically measured values. The measurements were carried out using a Scanditronix dose scanning system and a 0.12 cm3 RK ionization chamber.Results: The comparison of the calculated depth dose on the central beam axis and collimator axis versus water phantom measurements in asymmetric fields showed a very good agreement (better than 2%). The result of Monte Carlo calculation of the dose profiles at 2 and 10 cm depth in water for various field sizes were found to be comparable to the corresponding measured data.Discussion and Conclusion: The obtained results demonstrate the accuracy of the geometry simulation and the photon source model for Neptun 10pc accelerator. This model can be used to predict the dose distribution in the other complex fields