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Effect of CT Image Compression on Computer-assisted Lung Nodule Volume Measurement¹

¹ From the Thoracic Imaging Section, Department of Radiology, New York University Medical Center, 560 First Ave, New York, NY 10016. Received June 18, 2004; revision requested August 26; revision received October 19; accepted November 26. J.P.K. supported by National Institutes of Health grant K23 CA096604. Address correspondence to J.P.K. (e-mail: jane.ko{at}nyumc.org).

PURPOSE: To evaluate the effect of two-dimensional wavelet-based computed tomographic (CT) image compression according to the Joint Photographic Experts Group (JPEG) 2000 standard on computer-assisted assessment of nodule volume.

MATERIALS AND METHODS: This HIPAA-compliant study was approved by the research board at the authors' institution; patients' informed consent was not required. Fifty-one nodules in 23 patients (seven men, 16 women; mean age, 59 years; age range, 39–75 years) were selected on low-dose CT scans that were compressed to levels of 10:1, 20:1, 30:1, and 40:1 by using a two-dimensional JPEG 2000 wavelet-based image compression method. Nodules were classified according to size (5 mm or >5 mm in diameter), location (central, peripheral, or abutting pleura or fissures), and attenuation (solid, calcified, or subsolid). Regions of interest were placed on the original images and transposed onto compressed images. Nodule volumes on original (noncompressed) and compressed images were measured by using a computer-assisted method. A mixed-model analysis of variance was conducted for statistical evaluation.

RESULTS: Nodule volumes averaged 388.1 mm³ (range, 34–3474 mm³). There were three calcified, 33 solid noncalcified, and 15 subsolid nodules (13 with ground-glass attenuation). Average volume decreased with increasing compression level, to 383 mm³ (10:1), 370 mm³ (20:1), 360 mm³ (30:1), and 354 mm³ (40:1). No significant difference was identified between measurements obtained on original images and those compressed to a level of 10:1. Significant differences were noted, however, between original images and those compressed to a level of 20:1 or greater (P < .05). Compression level significantly interacted with nodule size, location, and attenuation (P < .001). The effect of compression was greater for nodules with ground-glass attenuation than for those with higher attenuation values. The difference in mean volumes between original images and those compressed to a level of 20:1 was 34.9 mm³ for nodules with ground-glass attenuation, compared with 8.3 mm³ for higher-attenuation nodules, a 4.2-fold difference.

CONCLUSION: Nodule volumes measured on images compressed to a level of 20:1 differed significantly from those measured on noncompressed images, especially for nodules with ground-glass attenuation. This difference could affect the assessment of nodule change in size as measured with computer-assisted methods.

© RSNA, 2005

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