Irene Hernandez-Giron BSc, Presenter: Nothing to Disclose

Jacob Geleijns PhD, Abstract Co-Author: Nothing to Disclose

Alfonso Calzado MD, PhD, Abstract Co-Author: Nothing to Disclose

Marçal Salvadó Artells PhD, Abstract Co-Author: Nothing to Disclose

Raoul M. S. Joemai, Abstract Co-Author: Research grant, Toshiba Corporation

Wouter J. H. Veldkamp PhD, Abstract Co-Author: Nothing to Disclose

Low contrast sensitivity of CT scanners is frequently determined by human observers scoring visibility of low contrast objects within CT phantom images. This method is biased as the scored objects are arranged in known fixed patterns and a wide intra and inter-observer variability may also appear. The purpose of this work is to introduce an automated objective method relating dose and low contrast detectability (LCD).

Images of the low contrast module of the Catphan 500 phantom were acquired using a 320-detector row CT scanner (Aquilion ONE, Toshiba, Japan). The phantom contains three contrast series (contrast 0.3, 0.5 and 1.0%). Each contrast series contains nine circular objects of varying size (diameter 2-15 mm). Software was developed for automated determination of LCD using a non-prewhitening matched filter with eye filter (NPWE) model observer in a two alternative forced choice (2-AFC) experiment. The obtained detectability indexes (d’) were transformed into proportion correct (PC) responses that vary between 0.5 (chance) and 1.0 (100% detectability). Objects were considered visible when PC≥75% in the 2-AFC experiment. Psychometric fits were performed with PC as a function of object diameter and the minimum object diameter that is just visible (α), related to PC=75%, was determined. The influence of dose on LCD was studied for the intermediate 0.5% contrast series and different mAs (scan parameters: 80x0.5mm; 120 kV, 25, 50, 100, 150, 200 mAs) using 75 reconstructed slices (5 mm; FC12 soft reconstruction kernel) taking into account the kernel PSF in the method. The influence of mAs (dose) in object diameter visibility (α) was studied.

The smallest object, 2 mm is only visible for the highest dose considered (200 mAs). For all acquired series except for 25 mAs and 50 mAs, all objects with diameter ≥3mm appeared visible. Increasing values of mAs improved LCD as expected: the minimum object diameter that is just visible, α, decreased with increasing mAs, ranging from 4.8 to 1.8 mm for the lowest and highest doses considered (25 and 200 mAs, respectively).

The proposed method for low contrast detectability assessment allows for objective measurement of dose efficiency for CT.

The NPWE model observer can be applied for objective evaluation of image quality in CT and can help to establish optimized image quality at low patient dose.

Hernandez-Giron, I, Geleijns, J, Calzado, A, Salvadó Artells, M, Joemai, R, Veldkamp, W, Dose Efficiency of a 320-detector Row CT Scanner by Objective Assessment of Low Contrast Detectability (LCD).  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11009910.html