The need to optimize CT protocols with respect to radiation dose is widely recognized. This study uses
phantom-based methodology to investigate the affect of changes in exposure and slice thickness on observer
performance for the detection of low contrast opacities with multislice computed tomography to determine dose-
optimized slice thickness and image noise for routine liver imaging. Methods: A phantom containing an opacity with diameter 9.5mm and density 10HU below background was scanned at various exposure and slice thickness settings (range 50-125mAs and 1-3mm). An image set consisting of 120 images containing background-only and 60 images containing the opacity in random locations was created. Following Institutional Review Board approval, nine experienced observers viewed the images and scored opacity visualization using a four-point confidence scale. Noise,
contrast-to-noise ratio (CNR), sensitivity, specificity and area under the curve (AUC) were calculated. Comparisons between exposure and slice thickness settings were performed using ROC, Spearman and Wilcoxon techniques. Results: Significant (p<0.05) reductions in AUC and sensitivity occurred when CNR dropped to 0.71 or below and 0.68 or below,
respectively. There was strong correlation between noise and AUC (r = -0.79, p<0.01), noise and sensitivity (r = -0.92,
p<0.001), CNR and AUC (r = -0.90, p<0.001) and CNR and sensitivity (r = 0.61, p<0.05). Conclusion: Observer
performance for the detection of opacities is strongly related to quantum noise and CNR. Dose optimized lesion
detection was achieved with 5mm slice thickness and CNR of 0.72 and noise of 9.05.
Purpose: The purpose of this study was to compare observer performance for the detection of
randomly-positioned lesions to that of location-known lesions to determine if randomization of
lesion placement is necessary for optimization of hepatic lesion detection with CT. A phantom
containing fixed lesions (diameter 2.4mm, 4.8mm and 9.5mm) was scanned at various exposure and
slice thickness settings. A second image set was created by electronically cutting lesions from the
phantom images and pasting them into background-only images. Nine observers, blinded to lesion
location in the second image set, reviewed all images under standardized viewing conditions.
Visualization of lesions was scored using a four-point scale. Observer scores for the two methods
were correlated for all lesions, and for each lesion size using Spearman's rank correlation coefficient
(r). There was very high correlation between the observer scores for all lesions (r=0.919, p<0.0001)
and for the 9.5mm lesion (r=0.963, p<0.0001). There was moderate correlation for the 4.8mm and
2.4mm lesions (r=0.509, p=0.084, r=0.640, p=0.028). Discussion: When considering all lesions, or
the 9.5mm lesion independently, randomization did not alter observer scores, suggesting random
location of large lesions is unnecessary for dose optimization. For the smaller lesion sizes correlation
between the two methods is less robust. Conclusion: If lesion size is large or unimportant, dose
optimization can be performed using a phantom with fixed lesions. For small lesions, randomized
lesion location may be warranted, thus having implications for phantom design.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.