Iranian Journal of Medical Physics
Year:2005 | Volume:2 | Issue:8|Papers Count:14

Introduction: Image distortion is one of the major problems of magnetic resonance imaging (MRI) for use in 3DMRI, velocity MRI, FMRI and radiotherapy treatment planning (RTTP). It is widely known that the most obvious effect of the in homogenity of the magnetic fields and the nonlinearity of the gradient is the Geometric Distortion of MR tomograms. In this study, the accuracy of MR images was considered by the phantom study and the reproducibility of images was evaluated by repeating the phantom measurements. Materials and Methods: MRI scans of the phantom with grid pattern inside it were performed using head coil in two 1.5 Tesla MRI systems (picker VISTA TM HPQ, USA & GE Signa Echo Speed, USA). For the Geometric Distortion reproducibility evaluation the T1W, T2W and PDW SE protocols were repeated three times. The Geometric Distortion was measured by an edge, detection program in MATLAB.Results: The geometric distortion in the peripheral area of the images tends to be generally larger than it in the central area in all images. The average displacement in Picker MRI was 3 pixels in the y and 1pixel in the x direction and in GE MRI it was 1pixel in both x and y direction for 24 cm FOV (pixel = 0.9mm).Discussion and Conclusions: Since the positional variation was within 1 to 3 pixels for the head coil, it will be possible to use this MR system in 3DMRI, velocity MRI, FMRI and RTTP. To decrease the geometric distortion the use of suitable coil and protocol are expected.

Introduction: Skin sparing is one of the most desirable characteristics of high energy photon beams. However, the photons emerging from the target of linacs are contaminated by secondary electrons as a result of their interactions with air, collimators, flattening filter and any other objects in their path. This phenomenon tends to increase the skin dose received by the patients. A practical and simple way to reduce the contribution of electron contamination is to place a sheet of medium to high Z material just after the secondary collimator. In this study, filters having different thickness and atomic number were applied and their effectiveness on the reduction of skin dose was evaluated.Materials and Methods: The filters of different thickness and atomic number were applied. The percent depth dose values were determined by the direct measurements made in a Scanditronix water phantom using a PTW 31006 Pin Point chamber having a sensitive volume of 0.015 cm3. A Perspex filter holder was made to be installed on the accessory slot. A plastic bag containing helium was also made using thin plastic sheet to study the effect of the helium bag when it replaces the air column between the head of the linac and the phantom. All of the measurements were carried out for the three field sizes of 10´10, 20x20 and 25x25 cm2. The setups were adjusted for SSD = 100cm. The ratio of the surface dose to maximum dose (Ds) was used as the criterion to determine the optimum filter. Results: The dosimetry results obtained in the water phantom indicated that a 0.4 mm thick Pb filter is the most effective one. This filter reduces the Ds for the field sizes of 10x10, 20x20 and 25x25 cm2 by 5.7, 7.9 and 9.6%, respectively. Also the simultaneous use of the optimum filter and He bag is more effective than the filter alone. It reduces the Ds by 6.3, 10.1 and 12.3% for the field sizes of 10x10, 20x20 and 25x25 cm2, respectively.Discussion and Conclusion: Based on the results of this work it is evident that the contribution of contaminant electrons to dose from the air column between the head and the phantom is much smaller than it from the secondary electrons arising from the head of the linac. On the other hand, the electron contamination originating from the air column is almost independent of the field size. But the surface dose arising from the secondary electrons produced by the head of the linac depends on the field size, which is increased by increasing the field size.

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 LAeq, LAfmax, L10, L50 and L90 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 LAeqwas recorded on Bahar Street during the morning hours and on Koohsangi Street during the morning 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 LAeq 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.

Introduction: In 2000, the UNSCEAR reported that CT constitutes 5% of all the medical x-ray examinations and it contributes 34% of the resultant collective dose worldwide. Children are more sensitive to the ionizing radiations than adults. So, routine quality control tests are expected to be carried out periodically on the CT scanners. The aim of this research was to estimate the effective doses received by the children below two years of age from routine CT examinations carried out at an educational imaging center in Tehran. It was also aimed to evaluate the quality control parameters of the mentioned CT scanner at the same time.Materials and Methods: In this study, the Computed Tomography Dose Index (CTDI) values were measured at the central axis of the CT gantry in air and in the standard quality control phantoms of the head and body (as recommended by the FDA) using a pencil ionization chamber and LiF TLD pellets for a single scan. By using the measured CTDI values and the ImPACT software, the effective doses were calculated for every routine CT examination protocol. In this study, the quality control parameters such as noise, CT number calibration, high and low contrast resolution and the flatness of the CT image were also evaluated. These parameters were also measured using standard procedures and test objects. Results: The effective dose estimated in this research ranged from 2.05 to 21.45 and 2.05 to 15.7 mSv for the female and male children, respectively. The measured values of the CTDI in the standard head and body phantoms were 20.6±2.01 and 11.13±1.04mGy/100mAs, respectively. The high and low contrast resolution was estimated to be 0.8mm and 1.0mm, respectively.Conclusion: The estimated values of the effective doses in this research were less than the values reported for the Netherlands, the USA, and Germany and were comparable with the values reported in the UK. The measured CTDI values were 11% more than that of the ImPACT. Although the estimated doses are comparable with the ones from other countries, but the quality control tests indicated that the CT number was not calibrated as well as the lack of uniformity in CT numbers. An acceptable calibration of the CT scanner not only could provide high quality images, but it could also lead to a lesser patient dose hence abiding by the ALARA principle in radiation protection.

Introduction: The radioimmunoscintigraphy (RIS) has found widespread clinical applications in tumor diagnosis. Human epithelial mucin, MUC1, is commonly over expressed in adenocarcinoma including 80% of breast cancers and represents a useful target for RIS. The PR81 is a new murine anti-MUCI monoclonal antibody that was found to react with the membrane extracts of several human breast cancerous tissues and the cell surface of some MUCI positive cell lines. In this study, a direct method which is very simple, rapid and efficient for the labeling of this MAb with 99mTc, particularly suitable for the development of a 'kit', was developed. The quality control of new radiopharmaceutical and immunoscintigraphy studies in BALB/c mice bearing breast tumor xenografts were also performed. Materials and Methods: The Ab reduction was performed with 2-mercaptoethanol (2-ME) at a molar ratio of 2000:1 (2-ME:MAb) and reduced Ab was labeled with 99mTc via methylene diphosphonate (MDP) as a transchelator. The labeling efficiency was determined by ITLC. The amount of radiocolloids was measured by cellulose nitrate electrophoresis. The stability of the labeled product was checked in fresh human serum by gel filtration chromatography (FPLC) over 24 hrs. The integrity of the labeled MAb was checked by the means of SDS-PAGE. Cell-binding assay was used to test the binding ability of 99mTc-PR81 to MCF7 cells. Biodistribution was studied in normal BALB/c mice at 4 and 24 hrs post-injection. The tumor imaging was performed in female BALB/c mice with breast tumor xenografts 24 hrs after the new complex injection.Results: The labeling efficiency was 94.2%±2.3 and radiocolloids were 2.5%±1.7. In vitro stability was 70%±5.7 in fresh human serum over 24 hrs. There was no significant Ab fragmentation due to the labeling procedure. Both the labeled and unlabeled PR8l were able to compete for binding to MCF7 cells. The biodistribution studies in normal BALB/c mice showed that there was no important accumulation in any organ. The immunoscintigraphy studies demonstrated definite localization of the preparation at the site of tumors with high sensitivity. Discussion and Conclusion: The results show that by using the Schwarz method of radiolabeling MAb PR8l, a labeling yield higher than 90% with high stability of the complex in human serum can be obtained. These findings demonstrated that the new radiopharmaceutical can be considered as a promising candidate for imaging of human breast cancer.

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 central 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 (4x4)-(45 x 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 (`D) is equal to 0.3152.Discussion and Conclusion: Therefore, the calculated percent depth dose data based on this formula has a better agreement 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.

Introduction: One of the major causes of image non-uniformity in MRI is due to the existence of non-homogeneity in RF receive and transmit. This can be the most effective source of error in quantitative studies in MRI imaging. Part of this non-homogeneity demonstrates the characteristics of RF coil and part of it is due to the interaction of RF field with the material being imaged. In this study, RF field non-homogeneity of surface and volume coils is measured using an oil phantom. The method employed in this work is based on a routine Spin Echo based sequence as proposed by this group previously.Materials and Methods: For the determination of RF non-uniformity, a method based on Spin Echo sequence (8-180) was used as reported previously by the same author. In this method, several images were obtained from one slice using different flip angles while keeping all other imaging parameters constant. Then, signal intensity at a ROI from all of these images were measured and fitted to the MRI defined mathematical model. Since this mathematical model describes the relation between signal intensity and flip angle in a (8-180) Spin Echo sequence, it is possible to obtain the variation in receive and transmit sensitivity in terms of the variation of signal intensity from the actual expected values. Since surface coils are functioning as only receiver (RF transmission is done by Body coil), first the results of receive coil homogeneity is measured, then characteristic of transmit coil (for the body coil) is evaluated.Results: The coefficient of variation (C.V.) found for T(r) value obtained from images using head coils was in the order of 0.6%. Since the head coil is functioning as both transmitter and receiver, any non-uniformity in either transmit or receive stage can lead to non-homogeneity in RF field. A part from the surface coils, the amount of non-homogeneity due to receive coil was less than that of the transmit coil. In the case of the surface coils the variation in receive homogeneity measurement was high due to inherent sensitivity of the coil in different locations, which was as expected.Discussion and Conclusion: Since the RF interaction of RF field is very low in oil phantom, the variation shown in the T(r) is due to BI non-homogeneity of coil system. Since the head coil is functioning as both transmitter and receiver, any non-uniformity in either transmit or receive stage could lead to non-homogeneity of RF field.

Introduction: The success of hyperthermia depends on the accuracy of the temperature monitoring in the tumor and the surrounding normal tissue. In this study, the temperature changes were determined by computing the speckle displacement in the ultrasound digital images. Speckle tracking algorithm was used to compute the displacement.Materials and Methods: The experiment was performed on Tissue Mimicking (TM) phantoms. The imaging probe used was a commercial type linear array operating at 10 MHz. The speckle displacement along the axial direction was computed using Speckle Tracking method and the temperature changes were obtained from the displacement data. Additionally, the temperature changes during the heating were invasively measured by NTC thermistors. The results of the two methods were compared against each other.Results: There was a good agreement between the Ultrasonic temperature estimation and the measurements made by the thermistors. The maximum error and the standard deviation obtained were 0.53 and 0.2°C, respectively. Discussion and Conclusion: Speckle tracking algorithm is capable of extracting motion information from both the ultrasounic digital images and radiofrequency (RF) echo signals. The temperature changes can be obtained from the displacement data.