Regional variance of visually lossless threshold in compressed chest CT images: lung versus mediastinum and chest wall.

OBJECTIVE To estimate the visually lossless threshold (VLT) for the Joint Photographic Experts Group (JPEG) 2000 compression of chest CT images and to demonstrate the variance of the VLT between the lung and mediastinum/chest wall. SUBJECTS AND METHODS Eighty images were compressed reversibly (as negative control) and irreversibly to 5:1, 10:1, 15:1 and 20:1. Five radiologists determined if the compressed images were distinguishable from their originals in the lung and mediastinum/chest wall. Exact tests for paired proportions were used to compare the readers' responses between the reversible and irreversible compressions and between the lung and mediastinum/chest wall. RESULTS At reversible, 5:1, 10:1, 15:1, and 20:1 compressions, 0%, 0%, 3-49% (p<.004, for three readers), 69-99% (p<.001, for all readers), and 100% of the 80 image pairs were distinguishable in the lung, respectively; and 0%, 0%, 74-100% (p<.001, for all readers), 100%, and 100% were distinguishable in the mediastinum/chest wall, respectively. The image pairs were less frequently distinguishable in the lung than in the mediastinum/chest wall at 10:1 (p<.001, for all readers) and 15:1 (p<.001, for two readers). In 321 image comparisons, the image pairs were indistinguishable in the lung but distinguishable in the mediastinum/chest wall, whereas there was no instance of the opposite. CONCLUSION For JPEG2000 compression of chest CT images, the VLT is between 5:1 and 10:1. The lung is more tolerant to the compression than the mediastinum/chest wall.

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