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Published online before print June 6, 2008, 10.1148/radiol.2482071025
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(Radiology 2008;248:492-503.)
© RSNA, 2008

Experimental Studies

Effects of Spatial Resolution and Tube Current on Computer-aided Detection of Polyps on CT Colonographic Images: Phantom Study1

Se Hyung Kim, MD, Jeong Min Lee, MD, Cheong-Il Shin, MD, Hyo Chul Kim, MD, Joon-Goo Lee, PhD, Jong Hyo Kim, PhD, Jin Young Choi, MD, Hyo Won Eun, MD, Joon Koo Han, MD, Jae Young Lee, MD, and Byung Ihn Choi, MD

1 From the Department of Radiology (S.H.K., J.M.L., C.I.S., H.C.K., J.H.K., J.K.H., J.Y.L., B.I.C.) and Institute of Radiation Medicine (S.H.K., J.M.L., J.H.K., J.K.H., B.I.C.), Seoul National University Hospital, and Interdisciplinary Program in Radiation Applied Life Science (J.G.L., J.H.K.), Seoul National University College of Medicine, 28 Yongon-dong, Chongno-gu, Seoul 110-744, Korea; Department of Radiology, Yonsei University Hospital, Seoul, Korea (J.Y.C.); and Department of Radiology, Ewha Womans University Hospital, Seoul, Korea (H.W.E.). Received June 13, 2007; revision requested August 23; revision received December 17; final version accepted February 26, 2008. Supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare (no. 0412-MI00-0401-0007) and the Seoul R&BD Program (10888), Republic of Korea. Address correspondence to J.M.L. (e-mail: leejm{at}radcom.snu.ac.kr).

Purpose: To prospectively evaluate the effects of z-axis spatial resolution and tube current on the sensitivity of a commercially available computed tomographic (CT) colonography computer-aided diagnosis (CAD) system for polyp detection by using pig colon phantoms.

Materials and Methods: Ninety-six polyps were created and analyzed in 14 pig colon phantoms. CT colonography was performed by using a 16-detector CT scanner at 0.75-mm collimation; 10, 50, 100, and 160 mAs; and a pitch of 1.5. At each milliampere-second setting, the CT images were reconstructed with a section thickness (ST) of 1.5 mm and a reconstruction increment (RI) of 1.3 mm. To evaluate the effect of z-axis spatial resolution, CT images were also reconstructed at 100 mAs with various SI and RI combinations (respectively: 1.0 and 0.7 mm, 3.0 and 2.0 mm, 3.0 and 3.0 mm, 5.0 and 5.0 mm). The phantom data were then analyzed by using a CAD program. CAD performance with different CT parameters was calculated and compared in terms of per-polyp sensitivity and number of false-positive (FP) findings per data set.

Results: At a constant tube current of 100 mAs, the polyp detection rate was significantly higher in data sets obtained with SI and RI combinations of 1.0 and 0.7 mm, respectively (81% [78/96]), and 1.5 and 1.3 mm, respectively (75% [72/96]), than in those obtained with the three thicker ST-RI settings (27% [26/96] to 64% [61/96]) (P < .01). A similar trend was observed, regardless of polyp size or morphology. However, the number of FP findings at the 1.0 mm and 0.7 mm setting (8.9 per phantom) was also significantly greater than that at the thicker ST-RI settings (4.0–6.1 per phantom) (P < .05). At a constant z-axis spatial resolution (1.5-mm ST, 1.3-mm RI), CAD polyp detection rate and number of FP findings per phantom remained nearly constant—close to 78% (75/96) and 6.1, respectively—at various tube current settings.

Conclusion: CAD performance in polyp detection at CT colonography is highly dependent on z-axis spatial resolution. However, tube current is not an influencing factor in CAD performance at a given z-axis spatial resolution.

Supplemental material: http://radiology.rsnajnls.org/cgi/content/full/2482071025/DC1

© RSNA, 2008