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

摘要

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 90°-scattered beams. A MatLab~R 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 radiographics 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.