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Papers by Parisa Aklg

In order to construct a library of Iranian pediatric voxel phantoms for radiological protection a... more In order to construct a library of Iranian pediatric voxel phantoms for radiological protection and dosimetry applications, an Iranian eight-year-old phantom was constructed from a series of CT images. Organ and effective dose conversion coefficients to this phantom were calculated for head, chest, abdominopelvis and chest–abdomen– pelvis scans at tube voltages of 80, 100 and 120 kVp. To validate the results, the organ and effective dose conversion coefficients obtained were compared with those of the University of Florida eight-year-old voxel female phantom as a function of examination type and anatomical scan area. For a detailed study, depth distributions of organs together with the thickness of surrounding tissues located in the beam path, which are shielding the internal organs, were determined for these two voxel phantoms. The relation between the anatomical differences and the level of delivered dose was investigated and the discrepancies among the results justified.

In order to establish an organ and effective dose database for Iranian children undergoing comput... more In order to establish an organ and effective dose database for Iranian children undergoing computed tomography (CT) examinations, in the first step, two Iranian 11-year-old phantoms were constructed from image series obtained from CT and magnetic resonance imaging (MRI). Organ and effective doses for these phantoms were calculated for head, chest, abdomen–pelvis and chest–abdomen–pelvis (CAP) scans at tube voltages of 80, 100 and 120 kVp, and then they were compared with those of the University of Florida (UF) 11-year-old male
phantom. Depth distributions of the organs and the mass of the surrounding tissues located in the beam path, which shield the internal organs, were determined for all phantoms. From the results, it was determined that the main organs of the UF phantom receive smaller doses than the two Iranian phantoms, except for the urinary bladder of the Iranian girl phantom. In addition, the relationship between the anatomical differences and the size of the dose delivered was also investigated and the discrepancies between the results were examined and justified.

This paper reports on the methodology applied to select suitable tissue equivalent materials of a... more This paper reports on the methodology applied to select suitable tissue equivalent materials of an 8-year phantom for use in computed tomography (CT) examinations. To find the appropriate tissue substitutes, first physical properties (physical density, electronic density, effective atomic number, mass attenuation coefficient and CT number) of different materials were studied. Results showed that, the physical properties of water and polyurethane (as soft tissue), B-100 and polyvinyl chloride (PVC) (as bone) and polyurethane foam (as lung) agree more with those of original tissues. Then in the next step, the absorbed doses in the location of 25 thermoluminescent dosimeters (TLDs) as well as dose distribution in one slice of phantom were calculated for original and these proposed materials by Monte Carlo simulation at different tube voltages. The comparisons suggested that at tube voltages of 80 and 100 kVp using B-100 as bone, water as soft tissue and polyurethane foam as lung is suitable for dosimetric study in pediatric CT examinations. In addition, it was concluded that by considering just the mass attenuation coefficient of different materials, the appropriate tissue equivalent substitutes in each desired X-ray energy range could be found.

The global increase in the number of computed tomography (CT) examinations have enhanced concerns... more The global increase in the number of computed tomography (CT) examinations have enhanced concerns
regarding stochastic radiation risks to patients, especially for children. Considering that cancer risk is cumulative over
a lifetime and each CT examination contributes to the lifetime exposure, there is a need for a better understanding
of radiation-induced cancer incidence and mortality, and better dose estimates. Accordingly, some authors estimated
organ and effective dose in reference phantoms, but still there is a critical need to expand these data to larger groups of
non-reference children. As an initial step to address this issue, in this study organ and effective doses were calculated
in common CT procedures in non-reference pediatric phantoms and were compared with those of reference phantoms
with the similar ages. Thirteen pediatric phantoms, BABY CHILD, five voxel-based UF pediatric phantoms (B-series)
and six phantoms developed at The Foundation for Research on Information Technologies in Society (IT’IS) were
implemented into MCNP. According to the results, there were no consistent differences between the doses of organs
exposed indirectly and effective doses of these three phantom types, but it was observed that for organs located in
the scan region, there was a relation between absorbed doses and pediatric age group, as expected. Generally, using
the results of this study one can estimate the absorbed doses more accurately. But it should be noted that these low
expansion data are not comprehensive enough for finding a reasonable relationship between phantom size and effective
dose except in chest-abdomen-pelvis (CAP) imaging.

In computed tomography (CT), some superficial organs which have increased sensitivity to radiatio... more In computed tomography (CT), some superficial organs which have increased sensitivity to radiation, receive doses that
are significant enough to be matter of concern. Therefore, in this study, the effects of using shields on the amount of dose
reduction and image quality was investigated for pediatric imaging. Absorbed doses of breasts, eyes, thyroid and testes
of a series of pediatric phantoms without and with different thickness of bismuth and lead were calculated by Monte Carlo
simulation. Appropriate thicknesses of shields were chosen based on their weights, X‑ray spectrum, and the amount of dose reduction. In addition, the effect of lead shield on image quality of a simple phantom was assessed quantitatively using region of interest (ROI) measurements. Considering the maximum reduction in absorbed doses and X‑ray spectrum, using a lead shield with a maximum thickness of 0.4 mm would be appropriate for testes and thyroid and two other organs (which are exposed directly) should be protected with thinner shields. Moreover, the image quality assessment showed that lead was associated with significant increases in both noise and CT attenuation values, especially in the anterior of the phantom. Overall, the results suggested that shielding is a useful optimization tool in CT.

CT scanning technology is a valuable tool to diagnose many diseases; however, the level of the ra... more CT scanning technology is a valuable tool to diagnose many diseases; however, the level of the radiation dose is a source of concern, especially for children. CT scan systems and dose measurement methods have evolved over the years; but reported pediatric effective doses (EDs) have sometimes exceeded the annual dose limit recommended by the ICRP (1 mSv per year for persons under 18 years) (ICRP, 2007a). Efforts have been made to reduce organ doses and EDs by adjusting the scan parameters. This paper describes the determinants of the ED, and the dose reduction techniques in pediatric imaging from the early age of CT examinations until now. The first epidemiological results regarding the associated risk of cancer are also briefly presented.

The term 'thermal flux' implies a Maxwellian distribution of velocity and energy corresponding to... more The term 'thermal flux' implies a Maxwellian distribution of velocity and energy corresponding to the most probable velocity of 2200 m s −1 at 293.4 K. In order to measure the thermal neutron flux density, the foil activation method was used. Thermal neutron flux determination in paraffin phantom by counting the emitted rays of indium foils with two different detectors (Geiger-Muller counter and NaI(Tl)) was the aim of this project. The relative differences of the outcome of the experiments were between 2.5% and 5%. The final results were compared with MCNP4C outputs and the best agreement was generated using NaI(Tl) by a minimum discrepancy of about 0.6% for the foil placed 8.5 cm from the neutron source.

Conference Presentations by Parisa Aklg

Physical anthropomorphic phantoms have an important role in radiation dosimetry studies and quali... more Physical anthropomorphic phantoms have an important role in radiation dosimetry studies and quality assurance of medical imaging. Therefore, the phantoms' equivalent materials should have the same physical properties as human's tissues. Hitherto, different tissue equivalent materials loaded in different physical phantoms for internal and external dosimetry purposes. Although wide range of equivalent materials has been introduced, the purpose of our study was finding the best tissue equivalent materials in xray radiations from CT examinations. The principal characteristic which should be considered for selecting the best equivalent material in photon studies is density and mass attenuation coefficient. Achieving this goal, in the first phase, the gamma's mass attenuation coefficients for different materials such as K2HPO4, B-100, A-150, Urethane, Polyurethane, SL5510, Tissue-equivalent gas (methane based or propane based), the composition recommended by White and even water was calculated. The results showed that using Polyurethane as soft tissue or White compound provides better agreements. Then, absorbed doses in TLD which were set in 8 organs were calculated by MCNPX simulation for ORNL hermaphrodite phantom for three different tube voltages (80, 100 and 120 kVp). This step was repeated by replacing the original tissue of ORNL phantom by best equivlaent materials determined from the previous phase. Finally, it was observed that differences between amounts of dose in TLDs placed in original tissues and equivalent tissues of White compound was less than that of Polyurethane.

The 1 st MEFOMP International Conference of Medical Physics parameters of motility, morphology, a... more The 1 st MEFOMP International Conference of Medical Physics parameters of motility, morphology, and viability.Semen samples were collected from 12 fertile men, and each sample was then divided into two aliquots. The experimental samples were placed in the ELF, while the control one was left intact. The applied fields were pulsed with distance of 6 m/ sec and effective intensity of 1mT and different frequencies of 10, 25 and 45 Hz at different time intervals. The constant field intensity was 1mT in all experiments.

CT is a major source of medical radiation and its availability is responsible for increasing dose... more CT is a major source of medical radiation and its availability is responsible for increasing dose from CT practice. The breast tissue is one of the the structures that has an increased sensitivity to radiation and its received dose is significant enough to be a matter of concern. Therefore, the purpose of our study was to measure breasts dose during chest CT with and without bismuth and lead shielding. In this study, the amount of dose reduction achievable by shielding the breast tissues were calculated for UF revised ORNL family of six anthropomorphic phantoms using MCNPX. Siemens DRH was simulated and the Monte Carlo model included parameters that specify its source geometry and x-ray energy spectrum. Considering the fact that these shields would use for children as well as adult, their weights should be low enough to not feel uncomfortable. Therefore, the effects of bismuth and lead shields up to 0.5 mm thickness were investigated in this research. We selected the size of shields that would cover the anterior surface of the phantom, but not significantly exceed the width of the anterior surface. The accuracy of simulation was investigated by comparing the effective doses (without considering any shields) by published results. An exponential relationship was found between breasts absorbed dose and the thickness of shielding. The breasts dose in mGy/mAs for newborn at 80 kVp was 3.02×10 -2 which was reduced to 1.17×10 -2 and 5.29×10 -3 for 0.1 and 0.5 mm of bismuth, respectively. It was observed that for tube voltage of 80 and 100 kVp using a shield with a thickness of 0.4 mm and for tube voltage of 120 and 140 kVp using a shield with a thickness of 0.5 mm would be appropriate. In addition, further reduction in radiation dose can be achieved using a lead shield instead of bismuth one with the same thickness.