Xinhui Duan PhD, Abstract Co-Author: Nothing to Disclose

Joshua Grimes PhD, Presenter: Nothing to Disclose

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

Shuai Leng PhD, Abstract Co-Author: Nothing to Disclose

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

 To measure the effect of focal spot blooming on CT spatial resolution, and to evaluate the ability of quadrapole dynamic focusing to counteract focal spot blooming as x-ray tube current and potential are changed.

High-contrast spatial resolution was evaluated 1) in-plane by scanning a wire phantom (tantalum wire, 0.125 mm diameter, suspended in air) to measure modulation transfer function (MTF) and 2) along the z-axis by scanning a foil phantom (50 micron thick, 1 mm diameter gold foil) to measures slice sensitivity profile (SSP). Phantoms were scanned at 70-150 kV on a Siemens Force scanner with dynamic focal spot control, and 70-140 kV on a Siemens Definition Flash scanner, with tube current ranging from 100 mA to the maximum available on each system. Images were reconstructed using 0.6 mm image thickness and 50 mm field-of-view with smooth (Force: Br36 and Flash: B30) and sharp (Force: Br64 and Flash: B70) kernels.

The variation in spatial resolution in the axial plane was much smaller on Force scanner than the Flash scanner as tube current and voltage changed. Comparing the spatial frequencies at the 50% values of the MTF curves, the difference between the minimum and maximum values for all kV-mA combinations was 3.0% for the smooth kernel and 4.7% for the sharp kernel on the Force scanner. These values were 10.5% and 21.9%, respectively, for the Flash scanner. The full-width-at-half-maximum (FWHM) of the SSP increased on both systems, but increased more slowly as tube current increased on the Force scanner (0.096 mm and 0.11 mm per 1000 mA for the smooth and the sharp kernels, respectively) than the Flash scanner (0.19 mm and 0.17 mm per 1000 mA).

The x-ray tube equipped with dynamic focal spot control on a recently-introduced scanner greatly reduced blooming effects, keeping the in-plane spatial resolution constant over a large range of tube currents and voltages.

Technical measures to limit focal spot blooming are important, especially at low kV and high mA, which is relevant in children and smaller adults, and for dual-energy scanning.

Duan, X, Grimes, J, Yu, L, Leng, S, McCollough, C, Use of Dynamic Focal Spot Control to Reduce Focal Spot Blooming in CT, and Its Impact on High-contrast Spatial Resolution.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14003448.html