Ricardo Andrade Terini

A medição da grandeza practical peak voltage na prática radiológica

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.

A portable Compton spectrometer for clinical X-ray beams in the energy range 20-150 keV.

Primary beam spectra were obtained for an X-ray industrial equipment (40-150 kV), and for a clinical mammography apparatus (25-35 kV) from beams scattered at angles close to 90°, measured with a CdTe Compton spectrometer. Actual scattering angles were determined from the Compton energy shift of characteristic X-rays or spectra end-point energy. Evaluated contribution of coherent scattering amounts to more than 15% of fluence in mammographic beams. This technique can be used in clinical environments.

CT Spectrometry with a Portable Compton Spectrometer with Stationary and Moving Tube

The knowledge of energy spectra of CT X-ray beams is essential to completely characterize beam quality and equipment performance. However, CT photon fluxes are too high to be directly measured with most of photon counting detectors. This work describes a Compton spectrometer designed at LRDMP, based on a CdTe detector with proper Al–Pb-Al collimators and shields, to obtain spectra of CT beams, from the measured spectra of 90o-scattered beams. A MatLab® computer program, including the Waller-Hartree formalism, was developed, to correct measured data, and then reconstruct the spectrum of the beam incident on the scatterer. Tests at LRDMP with direct and scattered standard CT beams showed, after data processing and normalization, similarity between correspondent spectra of reconstructed and directly measured beams. Shielding and scatterer thickness influence were carefully investigated. The system was tested in clinical measurements in a GE690 CT scanner, using CT lasers and scout radiographies for alignment. HVL values obtained from the reconstructed spectra, with the stationary tube, agree within 3% with those measured in QC tests. We also double-checked, with good accuracy, the actual scattering angle and the kVp values, through the energy shift of K lines and spectra end point, respectively. Although several exposures might be necessary to acquire each spectrum with good statistics, the total acquisition time was no longer than two minutes for each one. Furthermore, measurements with rotating tube were made, showing that accumulated spectra shape are like those obtained with the stationary tube.

Characterization of OSL dosimeters for use in dose assessment in Computed Tomography procedures

This study describes the characterization of an Al2O3:C OSLD (Landauers Luxel™ tape) for dose evaluation in Computed Tomography. The irradiations were conducted using both a constant potential X-ray equipment and a 64-slice clinical CT scanner, and the readouts were performed using a Risø TL/OSL reader. The following aspects were studied: batch homogeneity, energy response, linearity of dose response, reproducibility, reusability, and effect of uncertainties with the normalization of OSL signals per their response to beta radiation. A group of 330 dosimeters from the 452 irradiated with the same dose presented OSL signals within the interval of 4.7% from the average. The dosimeters presented energy-dependent response in good agreement with results found in the literature. The air kerma response of the OSL signal showed a linear trend for both the constant potential X-ray device and the clinical CT scanner, with differences in their slopes of approximately 10%. Reproducibility, reusability, and effect of beta normalization were analyzed by separating 72 dosimeters in 3 groups. The results obtained in this study together with those of previous works indicate that this type of dosimeter is adequate for dose evaluation in CT clinical applications.

Espectrometria Compton aplicada à medição de doses em mamografia para combinações W/Mo e Mo/Mo

A dose glandular media (DGM) nao pode ser medida diretamente em equipamentos mamograficos. Metodos baseados na espectrometria Compton sao alternativas para avaliar distribuicoes de dose em um simulador de mamas, tornando possivel a estimativa da DGM. Neste trabalho, um detector de CdTe foi usado para as medicoes espectrometricas de feixes de raios X espalhados a aproximadamente 90o , por efeito Compton, por um cilindro de PMMA, inserido entre as placas de um simulador de mama. A reconstrucao do espectro dos feixes primarios, a partir dos medidos, foi feita utilizando a teoria de Klein-Nishina e o formalismo de Compton, seguida pela determinacao do kerma no ar incidente na superficie do simulador, da dose absorvida e, finalmente, da DGM. Os espectros dos feixes de raios X incidentes e as distribuicoes de dose em profundidade em um simulador de BR-12 foram determinados para a faixa de mamografia (28 a 35kV). Os resultados mostraram bom acordo com dados da literatura, obtidos com TLD.