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Quantitation of Emphysema by Computed Tomography Using a “Density Mask” Program: METHODS

We studied 85 patients, 52 males and 33 females, ranging in age from 40 to 78 years (mean age ± SD = 64 ± 8 yr). They were selected from patients tested in the Vancouver General Hospital Pulmonary Function Laboratory during investigation for suspected malignancy. Patients with tumors were included if the tumor was peripheral and less than 7 cm in diameter or central but obstructing one segment or less. The limit for a peripheral lesion of 7 cm in diameter was chosen since the volume of a spherical lesion 7 cm in diameter would be only 180 ml, while obstruction of one segment would affect only about l/20th of the lung volume. Thus, both types of lesions would be expected to have a negligible effect on lung function. We excluded patients with previous lung surgery, evidence of interstitial lung disease, pleural disease, pneumonia or previous history of congestive heart failure or tuberculosis.

Density Mask

All patients had CT of the chest as part of their investigation. The “density mask” was obtained as previously described. Briefly, the CT scans were obtained on a CE 9800 scanner at 10-mm intervals using 10-mm collimation. The CT scans were obtained during breath holding at end inspiration. Hard-copy images were photographed at a 1,500-HU window width and — 690-HU window level. The “density mask” software program of the CE 9800 scanner was used to highlight on each slice voxels with attenuation values less than — 910 HU, which were taken to indicate areas of emphysema as shown previously. The “density mask” program also automatically gave the total area and the area of the highlighted voxels for each slice (Fig 1), as well as the overall density of each slice. As each CT slice is 1-cm thick, the volume of each slice is equal to the area in cm2 X the 1-cm thickness. Therefore, one can obtain the overall CT lung volume from the sum of the volumes of all slices. Similarly, the volume of lung with emphysema can be determined from the sum of the volumes of all highlighted voxels for all slices. This, expressed as a percentage of the overall CT lung volume, gives the overall percentage of emphysema in l>oth lungs. The mean overall lung density was calculated by adding the sum of the products of mean density and volume of each slice, and div iding that sum by the overall lung volume of all slices. levitra plus

Figure 1. Panel

Figure: 1. Panel A: CT scan with right and left lungs outlined to measure lung area. The computer program indicates that the right lung area is 106.6 cnr and the left lung area, 103.7 cm2. Since the CT slice is 1-cm thick, the lung volume at this level = sum of the right and left lung areas x 1 cm = 210.3 cm’. By adding all the CT images, the total lung volume can be calculated. Panel B: The “density mask” program outlines regions with attenuation < —910 11V to indicate emphysema and gives the area in cm2 being outlined. Tin* area of emphysema on the right is 53.8 cm2 and on the left is 20.5 cm-. The volume oflung with emphysema in this slice = sum of the right and left areas of emphysema X 1 cm thickness = 74.3 cm’. By adding all the CT images, the total lung volume with emphysema can be determined.

Pulmonary Function Tests

All patients had spirometry and expiratory flow rates measured on a rolling seal spirometer (PK Morgan equipment, Chatham, Kent, England), according to standard techniques. Single-breath diffusing capacity for CX) was determined using an automated valve and timing device, and a bag in а 1юх system (PK Morgan). A breath-holding time of about 10 s was used and was determined by the Jones and Meade” method. The washout volume was 900 ml and the alveolar sample volume was 900 ml. At least two determi­nations of Dsb that were within 5 percent of each other were determined; the highest value was reported. In 11 patients (seven men and four women), whose hemoglobin level was reduced below 13 g/100 ml for men and 12 g/100 ml for women, Dsb was corrected for hemoglobin according to the method of Cotes and co-workers.

Fifty-seven subjects had lung volumes measured by the closed circuit helium dilution technique. Forty-six subjects had lung volumes measured in a constant-volume body plethysmograph according to the technique of Du Bois and associates, using at least three determinations which were within 5 percent of each other; the mean value was used. In 18 patients, lung volumes were determined by both helium dilution and plethysmography.
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Spirometric values were expressed as percent predicted using the prediction equations of Crapo and co-workers. Expiratory flow rates from the flow volume curve were expressed as percent of predicted according to the equations of Knudson and co-workers. Predicted values for Dsb and the Dsb/VA ratio were based on the prediction equations of Miller and co-workers for nonsmokers. Lung volumes were expressed as percent predicted based on the equations of Crapo and co-workers.

Statistical Analysis

We used linear regression analysis to determine the correlation between the “density mask” results and different parameters of lung function. Analysis of variance was used to evaluate differences in lung function results between groups with different extent of emphysema as quantitated by the CT “density mask” program.

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Tags: Computed Tomography, Emphysema, Quantitation

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