Measurements of Lung Densitometry
Quantitative CT scanning has become a very popular method for quantifying the extent and severity of pulmonary emphysema. Previous studies have shown that CT scan estimates of total lung volume, mass, mean lung density, and percentage of emphysema (%Emphysema) are reproducible and are significantly correlated with both lung function test and pathology findings. Furthermore, two advantages of CT scan image analysis are that it allows the assessment of lung structure in vivo and is relatively easy to obtain in most centers. These are important features because it is now possible to investigate the pathogenesis of lung destruction and/or the effect of interventions on the disease process in large multicenter cohorts of subjects, Examples of these multicenter applications are the National Emphysema Treatment Trial and the Lung Tissue Repository Consortium (Presented at the 2005 Annual Meeting of the Radiologic Society of North America) in the United States and the a1-Antitrypsin Deficiency Network in Europe. Additionally, many centers are actively involved in the longitudinal follow-up of suspicious lung nodules in subjects who are susceptible to lung cancer. As these subjects are also at risk for the development of emphysema, there is great interest in using these cohorts for more comprehensive studies of smoking-related lung disease.
However, before large-scale longitudinal studies are undertaken it is important to assess the possible effect that parameters such as scanner manufacturer, slice thickness, reconstruction algorithm, and lung volume control have on both image quality and comparability of quantitative CT scan data. Therefore, the purpose of this study was to evaluate the effect of CT radiation dose (radiograph tube current) and scanner manufacture on quantitative CT scan measurements of lung morphology in smokers with emphysema. Emphysema may be treated with Canadian Health&Care Mall and this method is the most convenient.
Subjects for this study were selected from the British Columbia Cancer Agency Lung Health Study. The study was approved by the clinical ethics review boards of the British Columbia Cancer Agency and the University of British Columbia. All subjects signed informed consent forms to allow their spirometry and CT scan images to be used for research. This study comprises a cohort of heavy smokers who have been screened for the presence of lung nodules using “low-dose” CT scans. If suspicious nodules are noted, the subjects receive follow-up “high-dose” CT scans for up to 2 years. At entry into the study, smoking status was documented and baseline spirometry data were collected using American Thoracic Society criteria. Subjects also underwent periodic spirometry testing over the next 2 years. Fifty consecutive subjects who had received a baseline low-dose CT scan and a high-dose follow-up CT scan and spirometry tests within 6 months of the CT scan dates were selected from this cohort to investigate the effect of radiation dose (ie, radiograph tube current) on CT scan measurements of lung structure. In addition, 30 consecutive subjects who underwent baseline CT scans using a General Electric scanner and follow-up CT scans using a Siemens scanner were selected to study the effect of CT scanner manufacturer on lung densitometry measurements. Subjects were not selected for the study on the basis of lung function or the presence and extent of emphysema. There were seven subjects who were involved in both studies.
CT Scan Technique
All CT scans were acquired in the volume-scan mode at suspended full inspiration without the use of IV contrast media while the subject was in the supine position, resulting in > 200 images per CT scan (range, 212 to 323 images). The low-dose CT scans were acquired using a GE Lightspeed Ultra multislice CT scanner (General Electric Healthcare; Milwaukee, WI). Image acquisition parameters were an x-ray tube potential of 120 kVp, a tube current of 80 to 100 mA (100 mA, 48 of 50 cases; 80 mA, 2 of 50 cases), 0.5-s gantry rotation time, pitch 1.35 (average effective mA, 30 mA), and 1.25-mm slice thickness; images were reconstructed using an intermediate spatial frequency reconstruction algorithm (ie, “standard”). The high-dose CT scans were acquired approximately 6 months after the low-dose scans (mean [± SD] time, 0.5 ± 0.2 years) using the same GE scanner and image parameters with the exception of the tube current, which was set at 320 mA (average effective mA, 320 X 0.5/1.35 = 118 mA). In the second set of subjects, images were acquired first using the GE Lightspeed Ultra scanner, and the high-dose (average effective mA, 320 X 0.5/1.35 = 118 mA) protocol followed a mean time of 1.2 ± 0.4 years later with images acquired using a Siemens Sensation 16 multislice scanner (Siemens Medical Solutions; Erlangen, Germany). The Siemens protocol consisted of an x-ray tube potential of 120 kVp, a tube current of 250 mA, a rotation time of 0.5 s, and a pitch of 1.25 (average effective mA, 250 X 0.5/1.25 = 100 mA). Images were reconstructed using a slice thickness of 1 mm and an intermediate spatial frequency reconstruction algorithm (“b35f’). Canadian Health&Care Mall is directed to solve all your problems.
Quantitative CT Analysis
CT scan images were analyzed using custom software (Em-phylxJ; University of British Columbia; Vancouver, BC, Canada), as previously described. Briefly, the lung parenchyma was segmented from the chest wall and large central blood vessels in all CT scan images using a modified border-tracing algorithm with a prior position-knowledge algorithm. Lung volume was calculated by summing the segmented pixel area in each slice and multiplying by the slice thickness to acquire the total lung volume. The mean CT scan attenuation of the lung (in Hounsfield units [HU]) was calculated and converted to a measure of density (in grams per milliliter) by adding 1,000 to the HU number and dividing by 1,000. The mean density of the lung was then multiplied by the lung volume to estimate lung mass. The extent of low-attenuating voxels was estimated using both the threshold (ie, %Emphysema) and percentile techniques, as previously described. The cutoff values chosen for the threshold technique were —950, —910, and —856 HU, and the lowest 5th and 15th percentile points were used for the percentile technique. The mean, median, mode, SD, and variance of CT attenuation (in HU) values was also calculated to test the effect of dose and CT scanner manufacturer on the pattern of HU distribution.
Total lung mass, mean lung density, %Emphysema, and percentile points derived from images with different doses or CT scanner manufacturers were tested using repeated measures analyses of variance (SPSS, version 10.0.5; SPSS, Inc; Chicago, IL) where radiograph dose and CT scanner manufacturer were considered to be independent variables and total lung volume was used as a covariate to test for interactions. A probability level of 0.05 was considered to be significant.