Thorax organ dose estimation in computed tomography based on patient CT data using Monte Carlo simulation

Background: this study presents patient specific and organ dose estimation in computed tomography [CT] imaging of thorax directly from patient CT image using Monte Carlo simulation. Patient's CT image is considered as the patient specific phantom and the best representative of patient physical index in order to calculate specific organ dose

Materials and Methods: EGSnrc /BEAMnrc Monte Carlo [MC] System was used for CT scanner simulation and DOSXYZnrc was used in order to produce patient specific phantom and irradiation of photons to phantom in step and shoot mode [axial mode]. In order to calculate patient thorax organ dose, patient CT image of thorax as voxelized phantom was divided to a 64x64x20 matrix and 6.25 x 6.25 x 6.25 mm[3] voxel size and this phantom was imported to DOSXYZnrc code. MC results in unit of Gy/particle were converted to absorbed dose in unit of mGy by a conversion factor [CF]. We calculated patient thorax organ dose in MC simulation from all irradiated slices, in 120 kV and 80 kV photon energies

Results: effective dose was obtained from organ dose and organ weighting factor. Esophagus and spinal cord received the lowest, and bone received the highest dose. In our study, effective dose in CT of thorax was 7.4 mSV and 1.8 mSv in 120 and 80 kV, respectively

Conclusion: the results of this study might be used to provide the actual patient organ dose in CT imaging and calculation of real effective dose based on organ dose

Verifying the accuracy of dose distribution in gamma Knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and MC simulation

Polymer gel dosimetry is still the only dosimetry method for direct measuring of threedimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. In this study the obtained isodose maps with PAGAT polymer gel dosimeter were compared to those calculated with EGSnrs for singleshot irradiations of 8 and 18 mm collimators of Gamma Knife [GK] unit in homogeneous and inhomogeneous phantoms. A custom-built, 16 cm diameter spherical Plexiglas head phantom was. Inside the phantom, there was one cubic cutout for insertion of gel phantoms, and another cutout for inserting the inhomogeneities. The phantoms were scanned with a Siemens clinical 1.5 T MRI scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. The results of measurement and simulation in homogeneous and inhomogeneous phantoms showed that the presence of inhomogeneities in head phantom could cause spatial uncertainty higher than +/- 2 mm and dose uncertainty higher than 7%. the presence of inhomogeneities could cause dose differences which were not in accordance with accuracy in treatment with GK radiosurgery. Moreover, the findings of Monte Carlo calculation revealed that the applied simulation code [EGSnrc] was a proper tool for evaluation of 3D dose distribution in GK unit

diagnostic accuracy of digitized mammography

Digitized mammography has several advantages over screen-film radiography in data storage and retrieval, making it a useful alternative to screen-film mammography in screening programs. The purpose of this study was to determine the diagnostic accuracy of digitized mammography in detecting breast cancer. 185 women [845 Images] were digitized at 600 dpi. All images were reviewed by an expert radiologist. The mammograms were scored on a scale of breast imaging reporting and data system [BIRADS]. The definite diagnosis was made either on the pathologic results of breast biopsy, or upon the follow-up of at least one year. The overall diagnostic accuracy of digitized mammography was calculated by the area under receiver operating characteristic curve. 242 sets of mammograms had no lesions. The total counts of masses, microcalcifications or both in one breast were 39 [11%], 42 [12%], and 25 [7%], respectively. There were 321 [92%] benign and 27 [8%] definite malignant lesions. The diagnostic accuracy of digitized images was 96.34% [95% CI: 94%-98%]. The diagnostic accuracy of digitized mammography is comparably good or even better than the published results. The digitized mammography is a good substitute modality for screen-film mammography in screening programs

FMRI study of human visual cortex in response to spatiotemporal properties of visual stimuli

The brain response to temporal frequencies [TF] has been already reported, but with no study reported for different TF with respect to various spatial frequencies [SF]. fMRI was performed by 1.5T GE-system in 14 volunteers during checkerboard, with TFs of 4, 6, 8 and 10Hz in low and high SFs of 0.5 and 8cpd. Average percentage BOLD signal change demonstrated the amplitude of the fMRI response to different TFs was maximal in 6Hz for high SF of 8cpd, while, it was maximal at TF of 8Hz for low SF of 0.5cpd. The results are useful for vision therapy [such as the treatment of Amblyopia] and visual task selecting in fMRI studies

Spatial frequency dependence of the human visual cortex response on temporal frequency modulation studied by fMRI

The brain response to temporal frequencies [TF] has been already reported. However, there is no study on different TF with respect to various spatial frequencies [SF]. Functional magnetic resonance imaging [fMRI] was done by a 1.5 T General Electric system for 14 volunteers [9 males and 5 females, aged 19'26 years] during square-wave reversal checkerboard visual stimulation with different temporal frequencies of 4, 6, 8 and 10 Hz in 2 states of low SF of 0.4 and high SF of 8 cycles/degree [cpd]. All subjects had normal visual acuity of 20/20 based on Snellen's fraction in each eye with good binocular vision and normal visual field based on confrontation test. The mean luminance of the entire checkerboard was 161.4 cd/m2 and the black and white check contrast was 96%. The activation map was created using the data obtained from the block designed fMRI study. Pixels with a Z score above a threshold of 2.3, at a statistical significance level of 0.05, were considered activated. The average percentage blood oxygenation level dependent [BOLD] signal change for all activated pixels within the occipital lobe, multiplied by the total number of activated pixels within the occipital lobe, was used as an index for the magnitude of the fMRI signal at each state of TF and SF. The magnitude of the fMRI signal in response to different TF's was maximum at 6 Hz for a high SF value of 8 cpd; it was however, maximum at a TF of 8 Hz for a low SF of 0.4 cpd. The results of this study agree with those of animal invasive neurophysiologic studies showing SF and TF selectivity of neurons in visual cortex. These results can be useful for vision therapy and selecting visual tasks in fMRI studies

[Optimization of brain MRA with contrast injection in 1.5 T field]

Selection of suitable parameters for brain MRA requires accurate measures, because the image quality depends on the location of arteries, veins and also the velocity differences of blood, taking into account the low blood flow in small veins and arteries, use of paramagnetic contrast media is recommended. Hence, in present study, we investigated the imaging optimization of brain vessels using contrast media in 1.5 T field. For image optimization blood T1 was estimated after the injection of 0.1mmol/kg of Gd-DTPA and the relative blood signals were measured at T1=300, 600, 900 and 1200ms using TR=20ms and TE=7ms parameters. Ernest angle and relative signal increased as the T1 decreased. MRA was obtained in three groups, each including five volunteer patients using parameters TR=20ms, TE=7ms and flip angle 10, 20 and 30 degrees in two series without and during contrast injection. Signals of carotid, M.C.A and thorcolar herofili and SD in air were measured and it was shown that in 20 degrees flip angle, C/N was maximum. At the last stage, three series of MRA, without, during c.i and 15 minutes after c.i where obtained in 20 volunteer patients using parameters TR=20ms, TE=7ms and flip angle 20 degrees and calculated C/N. After statistical analysis the highest C/N was observed during c.i MRA. Paired t-student test was performed to compare the differences between the C/N ratios. For clinical purposes one vein and two arterles were graded in 5 definite levels. Results indicated an important effect of paramagnetic contrast media on better observing of small arteries and vein. The best quality was taken during c.i, but in some arteries contrast media did not improve the quality of MRA