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Published ahead of print on July 21, 2004, doi:10.1164/rccm.200403-326OC

Am. J. Respir. Crit. Care Med., Volume 170, Number 8, October 2004, 883-890

A more recent version of this article appeared on October 15, 2004
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Submitted on April 1, 2004
Accepted on July 21, 2004

Influence of Calibration on Densitometric Studies of Emphysema Progression Using Computed Tomography

David G Parr1, Berend C Stoel2, Jan Stolk3, Peter G Nightingale4, and Robert A Stockley1*

1 Lung Investigation Unit, Queen Elizabeth Hospital, Birmingham, West Midlands, United Kingdom, 2 Division of Image Processing, Leiden University Medical Center, Leiden, The Netherlands, 3 Department of Pulmonology, Leiden University Medical Center, Leiden, The Netherlands, 4 Wellcome Trust Clinical Research Facility, Queen Elizabeth Hospital, Birmingham, West Midlands, United Kingdom

* To whom correspondence should be addressed. E-mail: r.a.stockley{at}bham.ac.uk.

The fundamental importance of calibration in any measuring device is indisputable, but CT calibration in longitudinal lung densitometry studies is largely unexplored. Although the validity of CT as a measure of emphysema has been confirmed in cross-sectional studies there is limited data on long-term reproducibility and this is critically important for validating its use as an outcome measure in therapeutic trials. A general understanding of the strengths and pitfalls of CT densitometry is critical for physicians reviewing the published literature using this methodology. In our study of 57 patients with alpha 1-antitrypsin deficiency (phenotype PiZ), progression of voxel index determined from 3 successive annual scans acquired on a fully calibrated scanner was intimately associated with changes in CT air densitometry, sampled from patient images. Images were therefore re-analyzed using a correction technique validated in phantom studies that adjusted for changes in measured air density and improved the reliability of the voxel index as a measure of emphysema progression. Comparison of adjusted voxel index thresholds indicated the optimum threshold was - 950HU. Internal air calibration is therefore critical in longitudinal and multi-center lung densitometry studies of emphysema and incorporation of a correction factor is essential for quantitative image analysis.
Key words: emphysema, alpha 1-antitrypsin deficiency, lung densitometry




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