Susan Lalondrelle, Abstract Co-Author: Nothing to Disclose

Syed Afzer Aslam Sohaib MBBS, Abstract Co-Author: Nothing to Disclose

Isabel Ana Castellano PhD, Presenter: Nothing to Disclose

Dorothy Mears, Abstract Co-Author: Speaker, Schering-Plough Corporation

Robert Huddart, Abstract Co-Author: Advisory Board, Ferring Group Travel support, sanofi-aventis Group

Vincent Khoo MD, Abstract Co-Author: Nothing to Disclose

To investigate the effect of reconstruction slice thickness on image quality at CT virtual cystoscopy (VC).

10 delayed contrast enhanced CT examinations (GE Lightspeed 16; 120kV; 16x0.625mm collimation; rotation 0.8sec; pitch 0.938; auto mA, noise index 12) were performed in patients with bladder tumours and reconstructed at different slice thicknesses (0.6mm-5mm) and intervals. For each reconstruction optimal VC images were obtained by setting threshold values manually. For each VC reconstruction, image quality was assessed by scoring ridging, holes, floaters and dimpling artefacts, tumour definition and an overall score. The scores were ranked 1(best) to 7 (worst). The CT number and standard deviation (SD) were recorded for bladder contents and bladder wall  at three levels through the bladder. The mean SD was used as a measure of noise, and the contrast to noise ratio between the bladder contents and the bladder wall (CNRwall) was calculated using the formula CNR= (CTcontents- CTwall)/√((SDcontents2)+(SDwall2)). The mean CNR across the three levels was used for analysis. Each image quality measure was compared to noise and CNR measurements.

Mean CNRwall correlated well with the threshold setting for the VC images (r=0.884, p<0.001). For a given CNRwall, the optimal threshold can be determined by (CNR-0.811)/ 0.024. Ridging artefacts are seen with increasing slice thickness (r=0.73). Dimpling artefacts increased with thinner slice reconstruction and correlated with increased noise (r=0.58).This often resulted in poorer tumour definition on the 0.6mm reconstructed VC images. Tumour definition was best on intermediate (2.5mm and 1.25mm) slice thickness. The presence of holes and floaters did not correlate with any of the variables assessed. The relationship between overall score and slice thickness showed a minimum for VC images reconstructed with a 1.25mm slice thickness, probably due to the competing effects of spatial resolution and CNR. These images most commonly resulted in the best virtual pictures.  

Ever thinner slice thickness reconstruction does not provide for better image quality at virtual cystoscopy due to the presence of increased noise. A slice thickness reconstruction of 1.25mm most often generates the best VC images.

A slice thickness of 1.25mm is recommended for CT VC reconstructions. Optimal threshold can be determined from the CNR.

Lalondrelle, S, Sohaib, S, Castellano, I, Mears, D, Huddart, R, Khoo, V, CT Virtual Cystoscopy: Effect of Reconstruction Slice Thickness on Image Quality.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6012245.html