Introduction: This study investigates basic dosimetric properties of unflattened 6 MV photon beam shaped by multileaf Collimator and compares them with those of flattened beams. Materials and Methods: Monte Carlo simulation model using BEAM code was developed for a 6MV photon beam based on Varian Clinic 600 unique performance linac operated with and without a flattening filter in beam line. Dosimetric features including lateral profiles, central axis depth dose, photon and electron spectra were calculated for flattened and unflattened cases, separately. Results: An increase in absolute depth dose with a factor of more than 2. 4 was observed for unflattened beam which was dependent on depth. PDDs values were found to be lower for unflattened beam for all field sizes. Significant decrease in calculated mlc leakage was observed when the flattening filter was removed from the beam line. The total scatter factor, SCP was found to show less variation with field sizes for unflattened beam indicating a decrease in head scatter. The beam profiles for unflattened case are found to have lower relative dose value in comparison with flattened beam near the field edge, and it falls off faster with distance. Conclusion: Our study showed that increase in the dose rate and lower peripheral dose could be considered as realistic advantages for unflattened 6MV photon beams.

Introduction: SPECT is a diagnostic imaging technique the main disadvantage of which is the existence of Poisson noise. So far, different methods have been used by scientists to improve SPECT images. The Wavelet Transform is a new method for de-noising which is widely used for noise reduction and quality enhancement of images. The purpose of this paper is evaluation of noise reduction in SPECT images by wavelet.Material and Methods: To calculate and simulate noise in images, it is common in nuclear medicine to use Monte Carlo techniques. The SIMIND software was used to simulate SPECT images in this research. The simulated and real images formed using the current typical (hexagonal) Collimators were de-noised by different types of wavelets.Results: The best type of wavelet was selected for SPECT images. The results demonstrated that the best type of wavelet in the simulated and real images increased Signal to Noise Ratio (SNR) by 33% and 45% respectively. Also, Coefficient of Variation (CV) decreased by 77% and 71% respectively, while Contrast of Recovery (CR) was reduced by only 4% and 9% respectively.Conclusion: Comparing the results for real SPECT images in this paper with previously acquired results in real PET images, it can be concluded that the images of both nuclear medicine systems using Wavelet Transform differ in SNR and CR by only 5% and 7% respectively, and in CV by about 20%. Therefore, wavelet transform is applicable for nuclear medicine image de-noising.

Introduction: Geometric changes in the multileaf Collimator (MLC) led to dosimetric considerations in intensity-modulated radiation therapy (IMRT) due to the number and size of the pixels in the intensity map, which are determined by the MLC leaf width. In this study, we evaluated the dosimetric effects of different MLC widths on physical dose distributions for IMRT plans.Materials and Methods: Forty-two IMRT plans based on different MLC devices were generated and analyzed to study the effect of MLC width on plan quality.Results: Improvements in IMRT plan quality using 0.4 cm leaf width in comparison with 1 cm leaf width were evaluated. The 0.4 cm leaf-based plans resulted in significantly higher Dmean, D98%, D95%, D5%, and V95 (58.86 Gy, 95.11%, 96.57%, 104%, and 97.92%, respectively) compared to the 1 cm leaf plans (58.66 Gy, 92.56%, 94.56%, 104.14%, and 95.72%, respectively). Conformation number (CN) for planning target volume in 0.4 cm leaf plans was significantly higher than the 1 cm leaf plans (0.74 vs.0.67; P<0.05). In addition, the 0.4 cm leaf plans significantly improved dose homogeneity compared to the 1 cm leaf plans (1.08 vs.1.10; P<0.05). We found that 0.4 cm leaf width significantly decreased the integral dose to normal tissue compared to the 1 cm leaf width (from 56.09 to 49.46 Gy.Kg P<0.05).conclusion: No significant clinical differences were observed between the two plans for a serially functioning tissue, while the differences in mean doses were statistically significant for parallel functioning normal tissues.

Introduction: The Collimator design and optimization are essential in small animal molecular imaging for preclinical studies. In this study, a mathematical model was derived and used to optimize the slit Collimator for small animal imaging applications.  Materials and Methods: The geometric efficiency was formulated as a source-to-detector distance for a certain amount of the Collimator resolution (). The first-order derivative of the derived formula gives the optimized parameters. The detector performance was modeled in terms of intrinsic resolution . Furthermore, the edge penetration effect was considered using the validated model. Results: Optimum source-to-detector distance  was found as . For an ideal detector, optimal, geometric efficiency  and slit aperture width  were found as ,  and , respectively. Where   and  are the source-to-Collimator distance and detector length, respectively. For the fixed resolution of 1.0 mm, the sensitivity for different source-to-Collimator distances of 50.0, 100.0, and 150.0 mm was calculated as, , and , respectively. In addition, for a sub-millimeter resolution of 0.5 mm at 15.0, 30.0, and 50.0 mm, the geometric efficiency was calculated as, , , and . For a typical source-to-Collimator distance (15.0 mm), the optimal geometric efficiencies are, , , , and   for the resolutions of 0.25, 0.50, 1.0, 1.5, and 2.0 mm, respectively. Conclusion: Based on the analytic model predictions, the performance characteristics of the slit Collimator in terms of geometric efficiency and resolution were extracted. The importance of the proposed model lies both in its speed and ease of application.

Introduction: 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.Material and Methods: 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.Result: The Collimator system weighs approximately 37kg excluding the leaves. The projected width of each leaf on the isocenter of a Varian linear accelerator is 10mm. 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 1.4mm.Discussion and Conclusion: 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

Purpose: The main goal of this study was to determine the optimal Collimator in the absence of medium energy Collimators along with the impact of Attenuation Correction (AC) and different iterative reconstruction protocols on the quantitative evaluation of Gallium-67 (67Ga) SPECT/CT imaging. Materials and Methods: A GE Discovery 670 dual-head SPECT/CT scanner and a NEMA phantom filled with 67Ga solution were used to scan the patients. The projections were acquired with both Low Energy High Resolution (LEHR) and High Energy General Purpose (HEGP) Collimators, and CT images were acquired to evaluate the effect of attenuation correction. SPECT data were reconstructed using the ordered subset expectation maximization (OSEM) method with various combinations of iterations and subsets. The performance was quantified, and a clinical study validated the phantom study. Results: Acquired images by the HEGP Collimator yielded higher Contrast Recovery (CR) and Contrast to Noise Ratio (CNR) in images with AC than those without non-AC (41.6% and 74.2%, respectively). The CNR in all spheres after AC was increased by 80.4% (82.1%) for the HEGP Collimator against the LEHR Collimator. Also, an increase in iterations × subsets from 16 to 48 led to the Coefficient of Variation (COV) increasing by 17.2%, 16.67%, 15.50%, 14.4%, 14.2%, and 14.1% for 10 mm to 37 mm sphere diameter, respectively. Conclusion: CT-based AC and HEGP Collimators can yield improved 67Ga SPECT quantification compared to Non-AC and LEHR Collimators. The choice of the optimal Collimator with the reconstruction protocol led to changes in the image quality and quantitative accuracy, emphasizing the need to carefully select the appropriate combination of data acquisition factors.

Introduction: Converging Collimator has special application in nuclear medicine. Converging Collimator are used primarily with cameras having large-area detectors to permit full utilization of the available detector area for imaging of small organs such as thyroid. Because of inherent magnification of converging Collimator, it can provide higher image quality of small objects. Accurate spatial resolution and efficiency Measurement of this Collimator, therefore, can have significant role in design and development of these Collimators. The aim of this research is accurate modeling of converging Collimator for surveying of spatial resolution and efficiency as a function of axial distance from Collimator face by Monte Carlo code.Methods: A simplified model of converging Collimator based on an actual model was simulated by MCNP Monte Carlo code. The spatial resolution was measured in 2cm, 5cm, 8cm and 10 cm from Collimator face. The efficiency was also measured in 1cm, 2cm, 3cm, 4cm and 5cm from Collimator face.Results: From the results, the spatial resolution of the system (MTF=0.1) increased from 2.15 cycle/cm at axial distance of 2 cm to 1.05 cycle/cm at axial distance of 10 cm. (1.75 and 1.4 cycle/cm at axial distances 5 and 8 cm, respectively). The efficiency of the system at mentioned axial distances increased from 8.9 E-5 to 7.97 E-4. In addition, the results of this simulation have acceptable accordance with prior analytic results in this field that can validate simulated system.Conclusion: From results of simulation one can deduce that spatial resolution of this Collimator will decrease with increase in axial distance from Collimator face. Because of magnification effect, rate of spatial resolution reduction in axial distance is lesser than parallel hole Collimator. Thus, this feature of converging Collimator provides better image quality in nuclear images at expense of decreased field size. The efficiency of these Collimators has a direct proportion with axial distance from Collimator face and improved with increasing distance. Thus, a design of a Collimator that provides both high spatial resolution and acceptable efficiency has noticeable importance.

Background: The present study aims to estimate the effect of Collimator, phantom and Multi-Leaf Collimator (MLC) scatters on dose calculation in different breast cancer cases using acrylic, brass build-up caps and acrylic mini-phantom in the measurements. Materials and Methods: Collimator scatter factors (Sc), phantom scatter factors (Sp) and MLC transmission factors for different field sizes ranging from 1 × 1 cm2 to 40 × 40 cm2 for energies 6 MV and 15 MV were measured using acrylic mini-phantom (PTW and local mini-phantom), acrylic build up cap and brass build up cap where the farmer ionization chamber was used as detector in this study and semiflex detector was used but only with small field sizes from 1 × 1 cm2 to 4 × 4 cm2 and scatter effect on the dose calculation in different breast cancer cases was evaluated. Results: The results in this study show that there is no significant difference between MLC transmission factors using acrylic mini-phantom and brass build-up cap with energy 6 MV where the transmission factor value is 0. 007 and 0. 0071 with acrylic mini-phantom and brass build-up cap, respectively. Also it is clear that brass build-up cap gives the highest Collimator scatter factors results where Collimator scatter factors start at value 0. 963 at field size 4 × 4 cm2 then increase gradually to end at point value 1. 049 at field size 40 × 40 cm2. In breast cancer cases, there is sharp increase in organ at risk doses with brass build-up cap. Conclusion: From this study it is evident that almost there is large variation between the acrylic build-up cap, acrylic mini-phantom and brass build-up cap where brass build-up achieve higher results in most measurements.

Introduction: Dose distribution can be affected by diverse parameters, such as beam orientations, and Collimator angles. These parameters should respect and maintain the international recommended levels during the realization of the quality assurance protocols of linear accelerators. This study aimed at evaluating the dosimetric effects on treatment quality considering the mechanical error fluctuations in the recommended range. Material and Methods: This study included ten patients with head and neck cancer. All of them were treated using three-dimensional conformal radiotherapy with the simple 3-field classic technique. Initially, an optimized treatment plan was computed for each patient. Afterward, similar calculations were executed by varying isocenter position, gantry and Collimator angles. Eventually, dosimetric evaluations based on dose-volume histograms were studied and analyzed by Wilcoxon signed rank test for each plan. Results: The analysis of the dose-volume histograms of tumor volumes and organs at risk, as well as the dosimetry calculation, revealed that the small errors of 0. 5° in gantry and Collimator angles have minimal effects on dose distribution. However, the variation in isocenter coordinating up to 1 mm may influence the patients’ treatment quality, particularly in the spinal cord and the brainstem, in which Wilcoxon's test showed significant effects in all plans. Conclusion: According to the results, the quality of the treatment plans is almost insensitive to the errors of the gantry and the Collimator angles of the order 0. 5° though it is relatively sensitive to isocenter errors (1 mm). These should be reduced in order to avoid overdose when applying the conventional 3-field technique.