,.s,Ji,. lu,. Ghosting of Pulmonary Nodules with Respiratory Motion: Comparison of Helical and Conventional CT Using an In Vitro Pediatric Model Gary 0. Luker1 Kyongtae 1. Bae Marilyn J. Siegel Steven Don James A. Brink Ge Wang Thomas E. Herman OBJECTIVE. The study was designed to compare helical CT with varying pitch and reconstruction intervals and conventional CT for revealing pulmonary nodules in a model that simulates respiratory motion in children. MATERIALS AND METHODS. CT scans were obtained in an experimental model with one nodule (3 or 10 mm) in each scan. One-second scans were obtained at rates of 10. 20. and 30 respirations per minute using conventional CT with 4-mni collimation and table incrementation and helical CT with 4-mm collimation and either 4-mm/sec (pitch. I:I)or 8-mm/sec (pitch. 2: 1 ) table speed. Reconstructions were at I -. 2-. and 4-mni intervals for scans obtained using 4-mm/sec table speed and at I - and 4-mm intervals for scans obtained using 8-mm/sec table speed. Images were independently reviewed by three radiologists who estimated the number of nodules on each image. RESULTS. Ghosting (depiction of more than one nodule in a study) was seen in 79(4 80%. and 75% of helical CT scans obtained with a I : I pitch using I -. 2-. and 4-mm recoil- struction intervals, respectively. By comparison. ghosting was seen in only 54’4 and 5S’4 of helical CT scans with a 2: 1 pitch using I -mm reconstruction intervals and 4-mm recon- struction intervals, respectively, and in Sfl% of conventional CT scans (p < .0(X)l . A sin- gle nodule was detected on all other scans. and at least one nodule was seen on all scans. CONCLUSION. Ghosting of nodules is common in this model. Ghosting was seen less often on conventional scans and helical scans with 2: I pitch than it was on helical scans with I:I pitch. Nonetheless. ghosting was seen on more than 50C% of all scans with each technique. Received March 14, 1996; accepted after revision May 7, 1996. 1All authors: Mallinckrodt Institute of Radiology, Wash- ington University School of Medicine, 510 S. Kingshigh- way Blvd., St Louis, MO 631 10. Address correspondence toM. J.Siegel. AJR 1996;167:1 189-1193 0361-803X196/1675-1 189 © American Roentgen Ray Society P revious studies of chest CT in adults have shown better detec- tion of pulmonary nodules with helical CT than with conventional CT 11. 2J. In addition to increasing detection of nodules. helical CT has led to identification of more small nodules (0.5 cm) I I 1. The improved detection of pulmonary nodules with helical CT results from scanning dur- ing a single breath-hold, eliminating the misnegistration between contiguous slices that can occur with conventional CT. Detection of nodules and diagnostic confi- dence are also increased by overlapping reconstructions from the helical data set 131- Unlike adults. many children cannot cooperate with commands to suspend respi- ration, so chest CT examinations on chil- dren approximately 6 years old or less are typically performed during normal quiet respi- ration. As a result, slice misregistration arti- facts can occur. even with helical CT examinations. Techniques have been described fr performing chest CT with respiratory gat- ing. but these methods are not routinely avail- able for clinical use 14. 51. Even ultrafast CT scans are unable to completely eliminate respi- ratory motion 161. Although bronchiectasis and duplication of fissures have been described as artifacts resulting from respira- tory motion, the effects of respiratory motion on detection of pulmonary nodules have not been investigated to our knowledge 17. l. The purposes of this study are to compare how well helical and conventional CT reveal pul- monary nodules in a model designed to simu- AJR:167, November 1996 1189