Bernhard Schmidt PhD, Presenter: Employee, Siemens AG

Osama I. Saba, Abstract Co-Author: Employee, Siemens AG

Rosenmarie Banckwitz, Abstract Co-Author: Employee, Siemens AG

Heiko Wolf, Abstract Co-Author: Employee, Siemens AG

Thomas G. Flohr PhD, Abstract Co-Author: Employee, Siemens AG

Previously published studies have already shown that in case of patients with small cross-sections, the use of lower tube voltages allows acquisition of data with a similar image quality in terms of contrast to noise ratio, but with a lower dose. In clinical CT systems, lowest tube voltage is typically limited to 80kV. In our study we assessed the impact of a tube voltage of 70kV on image quality and dose.

Measurements were performed on a prototype of a Definition AS+ (Siemens, Germany), which was modified to allow scanning at a tube voltage of 70kV. To assess image quality, measurements with different phantom sizes simulating pediatric patients (8 to 16 cm diameter, PMMA) were performed. Noise was determined with ROI measurements (STD) in homogeneous areas in the center and the periphery of the phantoms. To assess contrast a tube with diluted iodinated contrast agent was placed in the center/periphery of the phantoms to allow contrast measurements. Dose was determined by CTDIvol measurements on CTDI phantoms with a diameter of 8 and 16 cm. In addition, patient dose for a CT of the abdomen (9 cm scan) was estimated by using a 5-year old anthropomorphic phantom (Alderson) equipped with 130 TLDs. As a reference, all measurement performed at 70kV were repeated with ‘standard’ voltages from 80-120kV with tube currents adapted to have the same CTDIvol as for 70kV.

In case of the 8 cm phantom, for the same CTDIvol, noise does not change significantly for lower voltages. However iodine contrast changes from 427 HU at 120 kV to 998 HU at 80 kV and 1125 HU at 70 kV. This means an increase of iodine contrast by 13% at the same dose level for 70 kV compared to 80 kV. In case of an abdominal scan, the difference of the effective dose values (ICRP103) estimated from the TLD measurements between 70 and 80kV correlated strongly with difference in CTDIvol at the respective voltage (difference<10%).

For small patients a further reduction of tube voltage from 80kV to 70 kV allows acquisition of images either with a 13% increase in iodine contrast for the same dose, or a dose reduction of 28% for the same image quality (iodine contrast to noise). The reduction of effective dose correlates strongly with the change of the measured CTDIvol values.

The use of even lower tube voltages would allow a further reduction of dose to pediatric patients that have to undergo a CT examination.

Schmidt, B, Saba, O, Banckwitz, R, Wolf, H, Flohr, T, Assessment of a 70kV Acquisition Mode for Pediatric CT Scanning.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9013492.html