Effects of X-Ray Tube-Voltage Ripple-Rates on Spectra and Exposure Attenuation Curves

Abstract

In x-ray diagnosis, the photon fluence rates on the recording system vary in a range of six to seven order of magnitudes, even if the tube voltage is fixed at 70 kV. The reasons are mainly due to the wide range of tube currents from less than 1 mA (fluoroscopy) to 1 A (radiography) and the wide range of attenuation rates in objects. These reasons result in the difficulty of measuring diagnostic x-ray spectra using semiconductor detector systems. We have solved the difficulties by the two following methods: (1) Low-capacity high-voltage cables were developed, in order to reduce photon fluence rates by decreasing tube currents and keeping radiographic tube-voltage waveforms, (2) Collimators having various diameters from 0.13 to 7mm (the effective diameter of a Ge detector) were prepared.
We carried out the measurement of photon spectra for various tube-voltage ripple-rates (which correspond to various type of generators and tube currents) at 70 kV using objects of various thicknesses made of aluminium and acrylic resin. Photon spectra were transformed to energy spectra and exposure spectra, and then exposure attenuation curves were obtained. The effects of tube-voltage ripple-rates on the spectra and the attenuation curves were quantitatively clarified.