Bernhard Krauss PhD, Abstract Co-Author: Employee, Siemens AG

Bernhard Schmidt PhD, Abstract Co-Author: Employee, Siemens AG

Thomas G. Flohr PhD, Presenter: Employee, Siemens AG

To evaluate the dose efficiency of various spectra and dedicated beam filtrations for phantoms of different attenuation and size in case of non-contrast CT scans.

We used a third generation dual source CT (SOMATOM Force, Siemens AG) which was equipped with a movable pre-patient beam filter to enable data acquisition with standard spectra (70 to 150kV in steps of 10kV), and shaped spectra with 0.6mm additional tin beam filtration (Sn). The standard deviation of the image pixel noise (SD) at constant radiation dose (in terms of CTDIvol) was evaluated for circular water phantoms with diameters of 10-40cm, representing different patient attenuations. For each phantom diameter, the relative image noise normalized to the standard 120kV spectrum was determined. Subjective image quality was assessed with an anthropomorphic phantom.

The relative image noise at constant radiation dose depends strongly on the phantom size and on the beam spectrum. For all phantom diameters, relative image noise was lowest for 100kV plus Sn. At a phantom diameter of 30 cm (equivalent to the mean attenuation of a thorax), image noise at constant radiation dose noise was reduced by 30% for 100kV plus Sn, compared with the standard 120 kV spectrum. For larger diameters, the 150kV plus Sn had the lowest relative image noise. In addition, the use of Sn substantially reduced beam hardening at all kV levels. In particular, subjective image quality at equal radiation dose was significantly better at 100kV plus Sn than at 80kV, which would traditionally be used for low dose CT scanning.

Additional tin filtration of the beam allows for a substantial reduction of image noise and therefore increase in radiation dose efficiency for non-contrast CT examinations. Image noise reduction of up to 30% and better subjective image quality at constant dose are feasible.

In non-contrast CT scans, the use of addition filtration allows for a substantial reduction of patient dose without compromising image noise and subjective image quality. Further on, the additional beam filtration strongly reduces the tube output, thus providing the technical prerequisite for very low dose scans beyond previous limits.

Krauss, B, Schmidt, B, Flohr, T, Assessment of the Impact of Additional Tin Filtration for Spectral Shaping on Image Quality and Dose.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14011477.html