Compton spectrometry applied to clinical CT axial beams from tubes stopped and in revolution

Abstract

In order to reduce radiation dose in computed tomography (CT) procedures, a deeper knowledge of the characteristics of the incident beam and its interactions with patient tissues are much desirable. The energy spectra of X-ray beams used in CT serve to get parameters that characterize beam quality and equipment performance, as well as transmission and absorption characteristics in patient tissues. However, CT photon fluxes are too intense to be directly measured with most photon counting detectors. This work describes a Compton spectrometer designed at LRDMP, composed by a CdTe detector and suitable collimators and shields. It was designed to obtain the spectrum of intense beams like those used in CT, from the measured spectrum of the beam scattered by a PMMA rod. A MatLab® computer program was developed, including the Waller-Hartree formalism, to correct and reconstruct the spectrum of the beam incident on the scatterer, from the measured one. Influence of shielding and scatterer thickness were carefully investigated. The system was tested in clinical measurements in three CT scanners, ensuring practical alignment with CT lasers and scan projection radiographies. Spectra obtained with stationary and rotatory tube presented HVL values compatible (±6%) with those measured in QC tests with ionization chambers. The methodology also enabled an accurate double-check of the scattering angle and kVp through the energy shift of K-lines and spectra end point, respectively. It was possible to verify clearly the role of different bow tie filters in modifying the beam, as well as to quantify the spectrum of a dual energy CT scanner. Moreover, the total acquisition time was no longer than few minutes for each measured spectrum.