Application of OSL strips in CT dosimetry according to the AAPM methodology

Computed tomography (CT) images contribute to high-quality medical diagnosis, but radiation dose can be quite high, requiring accurate assessment. CT dose index (CTDI) was developed for dosimetric purposes, but for scanners operated exclusively in axial mode. Nowadays, CTDI underestimate patient dose in helical CT exams. AAPM report TG111 (2010) suggested a new metric in which the patient's radiation dose is obtained from dose profiles constructed from several measurements made with a small ionization chamber. It is also possible to obtain dose profiles using properly calibrated OSL (optically stimulated luminescence) strips. The main objective of the present work is to contribute to optimizing CT dosimetry, comparing dose profiles obtained with OSL strips with measurements obtained by other authors. In this work, a “pencil” ionization chamber and 20 cm x 0.3 cm OSL strips were X-ray-irradiated, in air and in the holes of two cylindrical CT phantoms, using 100, 120, 140 kV peak voltages, both in lab and in a clinical CT scanner. Irradiated strips were read using an OSL reader built in the GDRFM. OSL profiles were calibrated against ionization chamber. From them, CTDIw and CTDIvol values were determined, differing approximately 3.9% from those of the CT scanner. From the profiles, also the planar equilibrium dose Deq,p (TG111) was evaluated in some CT protocols; Deq,p exceeded the CTDI values from the CT scanner in every case. E.g.: The percentage difference between Deq,p and CTDIvol for the head phantom ranged between 33-25%. Thus, in some cases, it could be advantageous to use calibrated OSL dosimeters instead of ionization chambers to obtain the profiles, saving time, because it is possible to obtain five OSL profiles from a single phantom irradiation

Measurement of the quantity practical peak voltage in the radiology practice

OBJETIVO: O objetivo deste trabalho foi estudar a grandeza practical peak voltage (PPV), determinada a partir da forma de onda de tensão aplicada a tubos radiológicos, e compará-la com algumas definições de kVp para diferentes tipos de geradores: monofásico (onda completa, clínico), trifásico (seis pulsos, clínico) e potencial constante (industrial). MATERIAIS E MÉTODOS: O trabalho envolveu a comparação do PPV medido invasivamente (utilizando um divisor de tensão) com a resposta de dois medidores comerciais não invasivos, além dos valores de outras grandezas usadas para medição da tensão de pico aplicada ao tubo de raios X, e a análise da variação do PPV com a ondulação percentual da tensão (ripple). RESULTADOS: Verificou-se que a diferença entre o PPV e as definições mais comuns de tensão de pico aumenta com o ripple. Os valores de PPV variaram em até 3% e 5%, respectivamente, na comparação entre medições invasivas e não invasivas feitas com os equipamentos trifásico e monofásico. CONCLUSÃO: Os resultados demonstraram que a principal grandeza de influência que afeta o PPV é o ripple da tensão. Adicionalmente, valores de PPV obtidos com medidores não invasivos devem ser avaliados considerando que eles dependem da taxa de aquisição e da forma de onda adquirida pelo instrumento.OBJECTIVE: The present study was aimed at evaluating the practical peak voltage (PPV) determined from the voltage waveform applied to x-ray tubes and comparing it with some kVp definitions for different types of x-ray equipment: single-phase (full-wave) and three-phase (six-pulse) clinical x-ray generators, and an industrial constant potential apparatus. MATERIALS AND METHODS: The study involved the comparison between invasively measured PPV (with voltage dividers) and values obtained with two commercial noninvasive meters, besides values of other quantities utilized for measuring the x-ray tube peak voltage. The PPV variation with the voltage ripple was also analyzed in the present study. RESULTS: The authors observed that the difference between PPV and the most common peak voltage definitions increases with the ripple. PPV values varied up to 3% and 5%, respectively, in the comparison between invasive and non-invasive measurements with single-phase and three-phase devices. CONCLUSION: The results demonstrated that voltage ripple is the main quantity influencing the invasive or non-invasive PPV determination. Additionally, non-invasively measured PPV values should be evaluated taking into consideration their dependence on the data sample rate and waveform obtained by the device.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

X-ray spectroscopy applied to radiation shielding calculation in mammography

The protective shielding design of a mammography facility requires the knowledge of the scattered radiation by the patient and image receptor components. The shape and intensity of secondary x-ray beams depend on the kVp applied to the x-ray tube, target/filter combination, primary x-ray field size, and scattering angle. Currently, shielding calculations for mammography facilities are performed based on scatter fraction data for Mo/Mo target/filter, even though modern mammography equipment is designed with different anode/filter combinations. In this work we present scatter fraction data evaluated based on the x-ray spectra produced by a Mo/Mo, Mo/Rh and W/Rh target/filter, for 25, 30 and 35 kV tube voltages and scattering angles between 30 and 165 degrees. Three mammography phantoms were irradiated and the scattered radiation was measured with a CdZnTe detector. The primary x-ray spectra were computed with a semiempirical model based on the air kerma and HVL measured with an ionization chamber. The results point out that the scatter fraction values are higher for W/Rh than for Mo/Mo and Mo/Rh, although the primary and scattered air kerma are lower for W/Rh than for Mo/Mo and Mo/Rh target/filter combinations. The scatter fractions computed in this work were applied in a shielding design calculation in order to evaluate shielding requirements for each of these target/filter combinations. Besides, shielding requirements have been evaluated converting the scattered air kerma from mGy/week to mSv/week adopting initially a conversion coefficient from air kerma to effective dose as 1 Sv/Gy and then a mean conversion coefficient specific for the x-ray beam considered. Results show that the thickest barrier should be provided for Mo/Mo target/filter combination. They also point out that the use of the conversion coefficient from air kerma to effective dose as 1 Sv/Gy is conservatively high in the mammography energy range and overestimate the barrier thickness. (c) 2008 American Association of Physicists in Medicine

Utilização de métodos computacionais no ensino: a experiência de Geiger e Marsden do espalhamento de partículas alfa

O presente trabalho descreve o programa GEIGER 1.0, que ilustra a experiência realizada por H. Geiger e E. Marsden em 1909 para a análise da distribuição angular do espalhamento de partículas a por folhas metálicas finas. Através de tal experiência foi possível idealizar um modelo para os átomos que incluía a idéia de um núcleo com determinadas características. O software, descrito com detalhes, baseia-se na teoria de Rutherford e permite, entre outras coisas, comparar as suas previsões com os resultados experimentais obtidos por Geiger e Marsden