Xinhui Duan PhD, Presenter: Nothing to Disclose

Jia Wang PhD, Abstract Co-Author: Nothing to Disclose

Lifeng Yu PhD, Abstract Co-Author: Nothing to Disclose

Cynthia H. McCollough PhD, Abstract Co-Author: Research grant, Siemens AG

To estimate the X-ray spectra of a clinical CT scanner using transmission measurements and an expectation maximization (EM) method.

We developed an EM technique that allows an accurate estimation of x-ray spectrum from transmission measurements. The major advantages of this technique are its inherent positivity, i.e. it will not produce negative values from a positive initial guess, and its robustness to noise. Two types of step-wedge phantoms, one aluminum and one polycarbonate, were used to represent different attenuation levels. Three spectra, including 80kV, 140kV, and 140kV with an additional tin filter, were measured when the scanner (SOMATOM Definition Flash, Siemens) was operated in a static mode with a tube position of 6 o’clock (tube current 300mA, collimation 14.4mm). The step-wedge phantoms were placed on the bottom of the gantry and transmission data through each of 28 unique combinations of step thickness and material were separately measured by the CT detector. The transmission values, obtained from the raw projection data, were downloaded to an external computer. A set of linear equations were used to model the X-ray transmission and were solved with the EM technique. The parameters for the method include the thickness of each step, the mass density and composition of the phantoms, and an initial guess to iteratively solve the equations using EM. The estimated spectra were compared with the spectra simulated by the manufacturer. We validated our estimated spectra by comparing calculated and measured transmission data through 10 depths of water (2.25~20.25cm).

Mean energies of the estimated and simulated spectra were in excellent agreement (< 0.9% difference), although slight differences in shape were observed. The average difference between the calculated and the measured log transmission data through water was 2.01% (standard deviation 16.6%) for all water thicknesses and spectra.

X-ray spectra can be accurately estimated from transmission measurements with an EM approach. The method allows user assessment of X-ray spectra for commercial CT systems.

With improved knowledge of X-ray spectra, the accuracy of dual-energy CT material characterization and Monte Carlo dose estimate can be increased.

Duan, X, Wang, J, Yu, L, McCollough, C, Estimating the X-ray Spectrum of a Clinical CT Scanner from Transmission Measurements.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9006963.html