Iranian Journal of Medical Physics
Volume:4 Issue: 16, 2007

Well type ionization chamber is a measuring device which is used to determine the activity of brachytherapy sources. The chamber has a cylindrical volume in which a cylindrical tube is mounted in the middle of the chamber. For the measurements, the brachytherapy sources are transferred to the middle of the tube. For designing the well type chamber, the measurement principals of well type chambers were considered and MCNP-4C code as a calculation tool was used. The designed chamber was simulated and the response of the chamber was evaluated. In this investigation, the chamber operational parameters such as operating voltage, leakage current, reproducibility, reference measuring point, recombination and polarization factors as well as response stability for 137Cs, 57Co and 241Am sources were studied. The chamber leakage currents at the operating voltage in comparison to the chamber response for the measurement of the above mentioned sources were negligible. The responses of the fabricated chamber for these sources are reproducible and its reference measurement position for these sources was obtained at 6 cm from the bottom of the chamber. The recombination factor for the well type chamber was negligible and the polarization factor is close to 1. Therefore, these two factors were not considered in the measurements. The reproducibility of the measurements in different intervals shows the stability of the chamber response for each source. Also the results of the chamber current measurement in term of source strength were compared to the response of the simulated chamber for different source positions and energy ranges of the used sources. Discussion and The results show that the measurement of the reference positions for each source in the simulated and fabricated chamber is quite in a good agreement. Regarding the reliable operational properties of the fabricated chamber, this chamber can be used to determine the activities of brachytherapy sources in which the energy ranges are greater than 50 KeV.

Multileaf collimator (MLC) is state of the art among the radiation field shaping systems for conformal radiotherapy techniques. Some advantages of this system are as follow: reduction of treatment time, higher precision in conforming the radiation field to the projection of the target volume and lowering the chance of errors made by technologists. The purpose of this research was to design and construct the mechanical section of a prototype MLC. The MLC consists of a base plate, 52 leaves, 2 leaf carriages, 2 nuts and 52 triangular thread screws. The leaves are arranged in two opposing banks each containing 26 leaves. The carriages are used to extend the movement of the leaves. The components of the MLC are fabricated by machining processes. The MLC is designed to be mounted on the conventional linear accelerators as an add-on accessory. The leaves were made of aluminum. The dimensions of the leaves are 5 x 200 x 50 mm. The collimator system weighs approximately 37kg excluding the leaves. The projected width of each leaf on the isocenter of a Varian linear accelerator is 10 mm. The movement of the leaves is accomplished by using the triangular thread screws. The screws required to have a low axial backlash which in turn results in lower precision positioning of the leaves. The positioning accuracy of each leaf is approximately 1.4 mm. Discussion and Mashhad MLC system, in it’s present form and design weighs 37 kg. but if Aluminum blades are replaced by tungsten ones then the over all weights of MLC may exceed the weight limit a linac may hold. Therefore futher modifications are required to reduce the MLC total weight. It is desirable to use ball screws and roller cages instead of triangular thread screws to achive a higher positioning pereision of the blades.

Because of the importance of vertebral compressive fracture (VCF) role in increasing the patients’ death rate and reducing their quality of life, many studies have been conducted for a noninvasive prediction of vertebral compressive strength based on bone mineral density (BMD) determination and recently finite element analysis. In this study, QCT-voxel based nonlinear finite element method is used for predicting vertebral compressive strength. Four thoracolumbar vertebrae were excised from 3 cadavers with an average age of 42 years. They were then put in a water phantom and were scanned using the QCT. Using a computer program prepared in MATLAB, detailed voxel based geometry and mechanical characteristics of the vertebra were extracted from the CT images. The three dimensional finite element models of the samples were created using ANSYS computer program. The compressive strength of each vertebra body was calculated based on a linearly elastic-linearly plastic model and large deformation analysis in ANSYS and was compared to the value measured experimentally for that sample. Based on the obtained results the QCT-voxel based nonlinear finite element method (FEM) can predict vertebral compressive strength more effectively and accurately than the common QCT-voxel based linear FEM. The difference between the predicted strength values using this method and the measured ones was less than 1 kN for all the samples.Discussion and It seems that the QCT-voxel based nonlinear FEM used in this study can predict more effectively and accurately the vertebral strengths based on every vertebrae specification by considering their detailed geometric and densitometric characteristics.

Electroporation is a practical technique used to transport the molecules across the cell membrane. The utilization of fluorescent molecules is the method widely used to evaluate the electropermeability of cell membrane as a result of pulse application.It is also possible to use mathematical methods to predict the changes caused in cell electropermeability as a result of the changes made in the pulse parameters. So the technique can be managed in such a way that the maximum permeability and minimum cell death can be achieved when desirable. In this study, MCF-7 cell line of human breast adenocarcinoma tumor was used. Propidium Iodide (PI) and Fluorometry technique was used to monitor the cell electropermeability and the cell survival. The cells were incubated in PI and electrical pulses were applied affecting the cell permeability. As a result the cell membrane is disrupted releasing the fluorescent molecules affiliated with PI. The fluorescence signal emitted by the dead cells was determined by fluorometry and the obtained result was used to generate a curve in terms of the signal and the percent cell survival. The curve was used to resolve the fluorescence signal emitted by the permeated live cells. The incubation of the cells in PI and its uptake by the cells affects their electric pulses. The permeability and cell survival for six different doses of electrical pulses was assessed immediately and also 72 hours after the application of the pulses. The maximum fluorescence emission and excitation wavelengths of PI solution attained at 540 and 600 nanometers, respectively. The highest permeability signal and the least cell survival percentage were recorded for electrical pulses of 1500 volts at 25-microsecond duration and 800 volts at 500-microsecond duration, respectively. The cell death rate, immediately after getting the pulses was assessed lower than the one after 72 hours.Discussion and It is predicted that the increased strength or duration of the pulse will enhance the cell electropermeability provided the cell does not die and the cell membrane is not interrupted. A mathematical equation was obtained in terms of the level of electropermeability signals, cell survival percentage and pulse parameters. Based on the equation the permeability is correlated to the pulse duration by the second power and pulse strength by the first power. It seems that there are some processes that not only induce immediate cell death but also inhibit the injured cells to recover or proliferate. These processes are activated as a result of the damage caused by the application of increased strength and duration of the electrical pulses.

It has been reported that cardiovascular effects are among the possible health effects of being exposed to extremely low frequency electromagnetic fields (ELF EMSs). This study has been conducted to evaluate the long term effects of exposure to magnetic and electric fields on electrocardiogram signal changes. Nine power transmission posts having different voltages were chosen. The study was conducted in three stages. First, the intensity of root mean square (rms) of magnetic and electrical fields was assessed in different spots where the operators were spending more time. Then based on each operator’s work habits the time–weighted mean of intensity of rms in magnetic and electrical fields over a 12 hour period was estimated and used as an index for each operator. In the next step, 102 individuals were chosen among the operators according to their job and medical history. Each subject had an eletrocardiogram taken while being relaxed. The cardiograms were used to extract 5 parameters including number of heart bit, wave time of P, complex QRS, interval time of PR and QTC.Using poly regression analysis method, the synchronized effects of independent variables such as age, job history, time–weighted mean of magnetic and electrical field on the extracted parameters of electrocardiogram signal was analyzed. The obtained results show that the synchronized effects of the variables can cause changes on PR and QTC interval of electrocardiogram signal. The most changes on PR and QTC interval are caused by the exposure to magnetic and electrical fields.Discussion and Studying the effects of electromagnetic field on electrocardiogram signal is representative of effects of these fields on repolarization of cardiac cells. On the other hand, the heart rhythm and the number of heart bit are under the control of automatic system of central nervous system. Therefore, it is important to analyze electrocardiogram signal as well as heart rate variability signal in order to evaluate the field effects on the function of heart automatic system of central nervous system.

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS), affecting mostly young people at a mean age of 30 years. Magnetic resonance imaging (MRI) is one of the most specific and sensitive methods in diagnosing and detecting the evolution of multiple sclerosis disease. But it does not have the ability to differentiate between distinct histopathological heterogeneities that occur in MS lesions and brain tissue.Quantitative magnetization transfer imaging (qMTI) is a relatively new MRI technique which can be used to examine the pathological processes of the brain parenchyma which occur in MS patients.This quantitative MRI technique can provide more complete information about the extent and nature of the brain tissue destruction in multiple sclerosis, which cannot be detected by conventional MRI. In this study, twelve patients with relapsing-remitting MS and twelve healthy control subjects underwent conventional MR imaging including: T2-FSE, T1-SE and FLAIR sequences as well as quantitative magnetization transfer imaging. All the focal lesions were identified on T2-weighted images and were classified according to their signal hypointensity on T1-weighted scans. The white matter and MS lesions were segmented using a semi-automated system. MT ratio (MTR) histogram analysis was performed for the brain white matter and the average MTR value was calculated for the classified MS lesions. A significant reduction was found in MTR value of the normal appearing white matter (NAWM) in patients with relapsing-remitting MS, suggesting that MS is a more diffuse disease, affecting the whole brain tissue. A wide range changes in MTR values can be observed in MS lesions. MTR reduction is correlated with the degree of lesion hypointensity on T1-weighted scans. The lower MTR values of lesions that appear progressively more hypointense on T1-weigted images reflect varying degrees of demyelination and breakdown in the macromolecular structure of MS lesions. There was also a significant difference in MTR histogram parameters between the patients and control groups, accompanied by a total shift of NAWM-MTR histogram to the left. Discussion and Based on theobtained result itcan be stated that MS has a diffused nature and the white matter abnormalities occurring outside T2-visible lesions affect the whole brain tissue. Quantitative magnetization transfer imaging can be used as a reliable method to assess both the overall disease burden and the intrinsic nature of the individual lesions in MS patients.

Osteoporosis is a bone disease in which there is a reduction in the amount of bone mineral content leading to an increase in the risk of bone fractures. The affected individuals not only have to go through lots of pain and suffering but this disease also results in high economic costs to the society due to a large number of fractures. A timely and accurate diagnosis of this disease makes it possible to start a treatment and thus preventing bone fractures as a result of osteoporosis. Radiographic methods are particularly well suited for in vivo determination of bone mineral density (BMD) due to the relatively high x-ray absorption properties of bone mineral compared to other tissues. Monte Carlo simulation has been conducted to explore the possibilities of triple photon energy absorptiometry (TPA) in the measurement of bone mineral content. The purpose of this technique is to correctly measure the bone mineral density in the presence of fatty and soft tissues. The same simulations have been done for a dual photon energy absorptiometry (DPA) system and an extended DPA system. Using DPA with three components improves the accuracy of the obtained result while the simulation results show that TPA system is not accurate enough to be considered as an adequate method for the measurement of bone mineral density. The reason for the improvement in the accuracy is the consideration of fatty tissue in TPA method while having attenuation coefficient as a function of energy makes TPA an inadequate method. Using TPA method is not a perfect solution to overcome the problem of non uniformity in the distribution of fatty tissue.

Neural stimulation is a promising new technology for the treatment of medically-intractable seizures. Transcranial Magnetic Stimulation (TMS) is a simple, non-invasive, low cost technique which is widely used in neurophysiology. In a few studies that have been performed at different frequencies the therapeutic effects of this technique were not obvious. Repetition of low frequencies TMS (≤1 Hz) has inhibitory and quenching effects on neuron activities and it decreases the excitability of the cortex. In epilepsy, as a disease of the central nervous system, the excitability of the cortex increases. It seems that such a technique at its optimum frequencies has therapeutic potential in epilepsy. In this study, the effect of 0.1, 0.5, 1 and 2 Hz repetitive Transcranial Magnetic Stimulation (rTMS) at an intensity of 80% Resting Motor Threshold (RMT) and at 1 Hz with an intensity of 90 and 100% RMT on Amygdala-kindling model in rats in optimum spatial coordinate was investigated. The procedure includes delivering a daily 5 minute rTMS stimulus using a butterfly figure coil having 25 mm in diameter and the coordinate which induces the maximum electrical field in Amygdala area. The stimulus was given within five minutes after the electrical kindling stimulus was delivered. The number of days to reach stage 1, 2, 3, 4 and 5 of kindling model After Discharge Duration (ADD) in each day and the percentage of cumulative ADD between the treated and the control group was statistically compared. Two groups were used to investigate the effect of the intensity of the magnetic field. A frequency of 1 Hz rTMS at an intensity of 90% and 100% RMT was delivered with the same physical parameters as before. Using a frequency of 1 Hz rTMS at an intensity of 80% and RMT of 90% showed a significant inhibitory effect on the spread of seizure into other areas of brain in comparison to the control group. Increasing the intensity of the magnetic field from 80 to 90% RMT increased the inhibitory effect of this technique. However, the quenching effect was significantly decreased when the intensity of the magnetic field was increased to 100% RMT. Discussion and It seems that the frequency of rTMS and Motor Threshold of subject are important factors in causing physiological effects. This study showed that rTMS technique at some frequencies and intensities may have therapeutic effect on epilepsy.