Xiang Li, Presenter: Research grant, General Electric Company

Ehsan Samei PhD, Abstract Co-Author: Advisory Board, Ion Beam Applications, SA Consultant, Siemens AG

Donald P. Frush MD, Abstract Co-Author: Research Consultant, General Electric Company Research Consultant, Siemens AG

To assess the impact of dose reduction on the detection of small lung nodules in pediatric CT, we recently conducted a receiver operating characteristic (ROC) observer study, in which diagnostic accuracy (area under the ROC curve, Az) was measured as a function of nodule detectability (product of nodule contrast and displayed diameter to noise ratio, CDNRdisplay). For a 64-slice CT, we have also studied noise, radiation dose, and cancer risk as functions of patient size and scan parameters. This work aimed to combine these findings to derive the tradeoff between accuracy and dose/risk.

Az as a function of effective dose (ED) and effective risk (ER) was studied for nodule characteristics of interest in pediatric CT (3-5 mm diameter and 200-500 HU contrast) and for a set of clinical chest scan parameters on a 64-slice CT (120 kVp, 1.375 pitch, 5-mm slice). For representative pediatric body sizes (average chest diameter of 10-23 cm), ED and ER were calculated for a range of tube current-time products (mAs) using the relationship between ED/ER, chest diameter, and scan parameters determined in a prior Monte Carlo study (abstract # 9004642). At each mAs, noise was estimated using the relationship between noise, chest diameter, and scan parameters determined in a prior phantom study, from which CDNRdisplay was calculated and used to determine Az using the relationship between the two assessed in a prior observer study (abstract # 9005637).

For a given patient size, Az  increased with increasing ED/ER, but reached a plateau at a threshold ED/ER value. For the average nodule diameter (4 mm) and contrast (350 HU) in this study, ED required to achieve an Az of 0.90 was 0.3, 0.6, 0.9, and 1.3 mSv for male patients with average chest diameters of 10, 14, 18, and 22 cm, respectively. The corresponding ER would be 1.0, 1.3, 1.7, and 2.0 in 10000 exposed male patients, respectively.

For given lung nodule characteristics, accuracy increases with dose/risk, but reaches a plateau at a threshold dose/risk value. The threshold increases with patient size. As the goal of dose reduction is to reduce risk, risk is more useful than dose for the purposes of protocol design and optimization.

The relationship between accuracy and dose/risk can guide CT protocol design to achieve the desired accuracy at the minimum dose/risk. Risk is more useful than dose in the design of CT protocols.

Li, X, Samei, E, Frush, D, The Tradeoff between Nodule Detection Accuracy and Radiation Dose or Cancer Risk in Pediatric CT.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9007162.html