World Congress Thoracic Imaging June 18-21, 2017, Hynes Convention Center, Boston, Massachusetts June 18-21, 2017, Hynes Convention Center, Boston, Massachusetts

Sponsoring Societies:

Fleischner Society Society of Thoracic Radiology European Society of Thoracic Imaging Japanese Society of Thoracic Radiology Korean Society of Thoracic Radiology
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Externally Placed "Pocket Phantom" for in vivo Pulmonary Nodule Visualization Assurance on Low Dose CT, a Phantom and Clinical Study
H. H. Guo1, J. Rosenberg2, A. N. Leung2, D. Fleishmann2, J. Wang2;
1Radiology, Stanford University Medical Center, Stanford, CA, 2Stanford University Medical Center, Stanford, CA.

Purpose: A fundamental problem in low dose thoracic CT (LD-CT) and CT dose reduction is that low contrast lesions such as ground-glass pulmonary nodules (GGNs) may no longer be adequately visualized. This can be aggravated by variability in patient body sizes and scanning protocols. We propose a positive control approach in which a pocket phantom containing “ground truth” GGNs of known sizes and density is placed on the patient’s chest and scanned with the patient, thus serving as an indicator of GGN visualization on each and every LD-CT. The feasibility of this positive control strategy for in vivo GGN lesion visibility was evaluated.
Materials and Methods: Two identical pocket phantoms containing -700 HU foam GGNs amidst lung density background were created and placed inside and outside of an anthropomorphic chest phantom (Kyoto Kagaku). Multiple CT scans were conducted at dose ranges from 0.41 to 8.15mGy CTDIvol. Contrast-to-noise ratios (CNR) of the internal and external GGNs were calculated, and four radiologists blinded to nodule location rated the visual conspicuity of the nodules on a 5-point scale. After IRB approval, pocket phantoms were scanned with patients undergoing LD-CT.
Results: CNR of nodules increased as a function of increasing CT dose (p<0.001). The concordance correlation of CNR between internal and external GGNs was high at 0.93 (95% CI; 0.80-0.98). Overall agreement of nodule visual conspicuity between readers was very high (alpha = 0.93). Ratings were significantly higher for higher CT dose (p<0.001) and were not significantly different between internal and external GGNs (Kendall’s tau=0.90; symmetry test p=0.25). Pocket phantom was demonstrated not to increase clinical CT radiation dose. More than 20 patients have been scanned to date with pocket phantoms, with visualization of reference and in vivo GGNs on LD-CT.
Conclusions: Contrast-to-noise ratio and visual conspicuity of externally placed simulated GGNs strongly correlated with those of internal GGNs, thus validating reference pocket phantom’s ability to serve as an indicator of in vivo low contrast nodule visualization. This positive control strategy can further benefit efforts in CT dose reduction and lung cancer screening LD-CT.

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