Iranian Journal of Medical Physics
Volume:6 Issue: 23, 2009

One of the most important parameters in x-ray CT imaging is the noise induced by detected scattered radiation. The detected scattered radiation is completely dependent on the scanner geometry as well as size, shape and material of the scanned object. The magnitude and spatial distribution of the scattered radiation in x-ray CT should be quantified for development of robust scatter correction techniques. Empirical methods based on blocking the primary photons in a small region are not able to extract scatter in all elements of the detector array while the scatter profile is required for a scatter correction procedure. In this study, we measured scatter profiles in 64 slice CT scanners using a new experimental measurement. To measure the scatter profile, a lead block array was inserted under the collimator and the phantom was exposed at the isocenter. The raw data file, which contained detector array readouts, was transferred to a PC and was read using a dedicated GUI running under MatLab 7.5. The scatter profile was extracted by interpolating the shadowed area. The scatter and SPR profiles were measured. Increasing the tube voltage from 80 to 140 kVp resulted in an 80% fall off in SPR for a water phantom (d=210 mm) and 86% for a polypropylene phantom (d = 350 mm). Increasing the air gap to 20.9 cm caused a 30% decrease in SPR. In this study, we presented a novel approach for measurement of scattered radiation distribution and SPR in a CT scanner with 64-slice capability using a lead block array. The method can also be used on other multi-slice CT scanners. The proposed technique can accurately estimate scatter profiles. It is relatively straightforward, easy to use, and can be used for any related measurement.

In recent decades, several Monte Carlo codes have been introduced for research and medical applications. These methods provide both accurate and detailed calculation of particle transport from linear accelerators. The main drawback of Monte Carlo techniques is the extremely long computing time that is required in order to obtain a dose distribution with good statistical accuracy. In this study, the MCNP-4C Monte Carlo code was used to simulate the electron beams generated by a Neptun 10 PC linear accelerator. The depth dose curves and related parameters to depth dose and beam profiles were calculated for 6, 8 and 10 MeV electron beams with different field sizes and these data were compared with the corresponding measured values. The actual dosimetry was performed by employing a Welhofer-Scanditronix dose scanning system, semiconductor detectors and ionization chambers. The result showed good agreement (better than 2%) between calculated and measured depth doses and lateral dose profiles for all energies in different field sizes. Also good agreements were achieved between calculated and measured related electron beam parameters such as E0, Rq, Rp and R50. The simulated model of the linac developed in this study is capable of computing electron beam data in a water phantom for different field sizes and the resulting data can be used to predict the dose distributions in other complex geometries.

Biological effects of magnetic fields have been investigated by scientists for many years and a vast amount of research using different frequencies and intensities have been performed on humans and laboratory animals. The results of these studies, which have mostly been done by applying high frequencies, have shown that electromagnetic fields are effective on structure and function of biological systems. Sixty male Sprague Dawly rats were randomly divided into 6 groups of ten each. The first group was sham exposed and the other groups were exposed to fields with frequencies of 5, 10, 15, 25 and 40 Hz and intensity of 50 μT. To investigate the effects on learning, 20 trials were undertaken for each animal using a shuttle box that was sandwiched between the arms of the field generator, which was adjusted to produce the desired frequency and intensity. To evaluate the reversibility of the possible effects of the fields and also to evaluate memory, the 20 trials were repeated 24 hours later under the same condition but in the absence of the field. In this study, we measured the latency and number of conditioning responses. According to our findings, a significant difference was found between latency and number of conditioning responses of the second day of the 25 Hz group and the sham and other experimental groups. For that group, latency increased and number of conditioning responses decreased on the second day compared to the first day, in contrast to all other groups. Exposure to the field with frequency of 25 Hz caused disruption of memory, however, there was no significant difference between the groups regarding learning.

In patients with cardiac artery disease, a myocardial perfusion scan, which is a non-invasive method, is utilized. This study is conducted to develop an advantageous software applicable to quantitative myocardial SPECT perfusion. Each cross-section of the left ventricle was segmented by applying a fuzzy clustering method. After obtaining the myocardial skeleton of the left ventricle from its short axis cross sections, we made use of fuzzy logic to decide whether the pixel belongs to the myocardial muscle and any perfusion perturbation or not. The reconstructed image was divided into 18 equivolume sectors. The features were extracted in each sector and, finally, were compared with a normal data bank. Abnormal critical conditions in rest and stress studies and coronary artery disease diagnosis were investigated in a set of about 317 images. Measurement and allocation of different myocardial sectors to specific coronary arteries were accomplished by utilizing collected information about the patients (75 men and 62 women), and the validity of the artery obstruction diagnosis has been proven in 40 patients undergoing coronary angiography. Our developed software DCAD (b) has demonstrated a considerably good performance in the diagnosis of coronary artery occlusion and can be a promising method aiding nuclear medicine specialists in their diagnosis.

An efficient method of tomographic imaging in nuclear medicine is positron emission tomography (PET). Compared to SPECT, PET has the advantages of higher levels of sensitivity, spatial resolution and more accurate quantification. However, high noise levels in the image limit its diagnostic utility. Noise removal in nuclear medicine is traditionally based on Fourier decomposition of images. This method is based on frequency components, irrespective of the spatial location of the noise or signal. The wavelet transform presents a solution by providing information on the frequency content while retaining spatial information. This alleviates the shortcoming of the Fourier transform and thus, wavelet transform has been extensively used for noise reduction, edge detection and compression. In this research, we used the SimSET software to simulate PET images of the NCAT phantom. The images were acquired using 250 million counts in a 128×128 matrix. For the reference image, we acquired an image with high counts (6 billion). Then, we reconstructed these images using our own software developed in MATLAB. After image reconstruction, a 250 million counts image (noisy image) and a reference image were normalized and then root-mean-square error (RMSE) was used to compare the images. Next, we wrote and applied de-noising programs. These programs were based on using 54 different wavelets and 4 methods. De-noised images were compared with the reference image using RMSE. Our results indicate that the Stationary Wavelet Transform and Global Thresholding are more efficient at noise reduction compared to the other methods that we investigated. The wavelet transform is a useful method for de-noising of simulated PET images. Noise reduction using this transform and loss of high-frequency information are simultaneous with each other. It seems that we should attend to the mutual agreement between noise reduction and the visual quality of the image.

The selection of suitable beam angles and weights in external-beam radiotherapy is at present generally based upon the experience of the planner. Therefore, automated selection of beam angles and weights in forward-planned radiotherapy will be beneficial. In this work, an efficient method is presented within the MATLAB environment to investigate how to improve the dose distributions by selecting suitable coplanar beam angles and weights for the beam. In the beam angle and weight selection algorithm, the optimal beam angles and weights correspond to the lowest objective function value of the dose distributions of each group of candidate beams. Optimal weights and angles reach a balance between all the objectives. In this work, we used a genetic algorithm and adopted a real-number encoding method to represent both beam weights and angles with an assignable number of repetitions. For the evolution of this algorithm, we used both monophasic and biphasic methods. In monophasic evolution, the chromosome containing the weights and angles is evolved in a single phase. In biphasic evolution, the chromosome is evolved while keeping one parameter (e.g., weight) constant and then, in the second phase, the evolution is continued while keeping the other parameter (e.g., angle) constant. The dose calculation was carried out using “correction-based techniques”. Simple and simulated clinical cases are presented to test the algorithm. They show that the biphasic evolution requires more computation time compared to monophasic (typically 40 and 20 minutes respectively) but results in a better optimization. The results show the efficacy of the algorithm and its fairly acceptable computation time.

Computed tomography (CT) has numerous applications in clinical procedures but its main problem is its high radiation dose to the patients compared to other imaging modalities using x-ray. CT delivers approximately high doses to the nearby tissues due to the scattering effect, fan beam (beam divergence) and limited collimator efficiency. The radiation dose from multi-slice scanners is greater than the single-slice scanners and since multi-slice scanners increasingly employ a wide beam, 100 mm ion chambers currently used in measuring the CTDI100, are not capable of accurately measuring the total dose profile of the slice width. Therefore, the CT dose is underestimated by using them. The purpose of this study is to measure the Computed Tomography Dose Index (CTDI) of a GE multi-slice CT scanner (64-slice) using polymer gel dosimetry based on MRI imaging (MRPD). CTDI is the sum of point doses along the central axis and estimates the average patient dose during CT scanning. For measuring CTDI, after designing and fabricating the phantom and preparing the MAGIC gel, MRI imaging using a 1.5 T Siemens MRI scanner was performed with the imaging parameters of ST = 2 mm, NEX = 1, TE = 20-640 ms and TR = 2000 ms. CTDI was measured with a 100 mm ion chamber (CTDI100) and also the MAGIC gel with MRPD method for 10 mm and 40 mm CT scan nominal widths. Following the measurement of the CTDI100 for 10 mm and 40 mm nominal slice widths of the multi-slice scanner using both ion chamber and MAGIC gel, the results showed that the ion chamber underestimates CTDI100 by 28.71% and 14.03% compared to gel for 10 mm and 40 mm respectively.Discussion and It was concluded from this study that gel dosimeters have the capability to measure CTDI in wide beams of multi-slice CT scanners whereas 100 mm standard ion chamber due to its limited length is not reliable even for a 10 mm beam width. In addition, due to the 3 dimensional nature of gel dosimetry, by using a MAGIC polymer gel, it is possible to obtain a lot of important information from the mentioned profiles such as the actual slice thickness and z-axis geometric efficiency. In addition to the stated parameters, the percentages of the total and partial homogeneities in the slice plane can be obtained only from gel dosimetry. The results of this study show that MAGIC polymer gel dosimetry based on MRI can be used as a supplementary method to using conventional ion chamber dosimetry especially in measurements for slice widths greater than 2 mm.