4 DISCUSSION
Lung densitometry, that is, lung density measurement, is a quantitative
CT measurement method capable of measuring structural abnormalities in
the lung based on the characteristics of lung tissue that variably
attenuate X-rays. Parenchymal abnormalities can typically result in
reduced attenuation in emphysema or cystic lung disease or in increased
attenuation, as in pulmonary fibrosis6. The presence
of pathologic air trapping in the expiratory phase on CT scan has been
defined as a parameter for detecting diseases in small
airways7. In particular, in patients with BO,
radiologic detection of bronchial dilatation infrequently precedes the
clinical diagnosis of disease; air flow limitation in expiratory CT
scans has been suggested as the most appropriate indicator for early
diagnosis compared to PFTs and transbronchial biopsy8,9.
Previous studies performed quantitative CT analysis to detect air
trapping in adult patients with COPD10-12,
asthma13, and lung
transplantation14. Parameters including density
mapping, emphysema or air trapping index, MLD, and 15th percentile
density index were analyzed. To the best of our knowledge, there have
only been a few quantitative densitometry studies in pediatric patients15-17.
Our results suggest the possibility of quantitative pulmonary function
evaluation through CT in pediatric BO patients, as in 19 (23.5%) out of
81 patients, who were unable to complete spirometry measurement. In our
experience, in children aged 4–5, who are younger than the age at which
spirometry can be performed, the cooperation of 3D CT to control
inspiration and expiration was sufficiently possible. LAA from both -850
and -900 HU thresholds showed a statistically significant correlation
with specific airway resistance sRaw in multivariate analysis,
indicating high OR and high AUC values for BO diagnosis. In previous
studies in adult patients, the attenuation threshold range from -856 to
-950 HU has been shown to be the most applicable for identifying
pathologic air trapping 10,18-22.
According to previous studies regarding lung development, the lung
continuously grows in proportion to body size after birth, but the
formation of new alveoli stops around the ages of 2–823,24. During the period of formation of new alveoli,
lung density is relatively constant. However, the lung density is
expected to decrease according to the growth of the lung, following the
increased alveoli airspace volume. Robert et al. showed that the lung
structure of subjects with larger lungs differs from those with smaller
lungs and that larger lungs have a thinner septum and lower parenchymal
density as the air volume in the alveoli increases 25.
Interestingly, our study showed that the densitometry expiratory
threshold of airflow limitation is similar to or slightly lower than
that of adults. This finding is supported by a previous study, where the
overall MLD of a group with an average age of 2 was similar to that of
adults 26. It is also consistent with Christian et
al.’s suggested -925 to -950 HU threshold in a recent study, which
included adolescents with HIV BO patients with mean ages similar to
those in our study 15.
The methods frequently used for quantitative analysis of air trapping
using CT include (a) the difference or the ratio of the
expiratory/inspiratory MLD and (b) the measurement of the percentage of
the lung area showing below the threshold HU during exhalation.
Pathologic air trapping was defined as a voxel difference of less than
80 to 110 HU between expiration and inspiration within segmented lung
parenchyma 14,27,28. In our study, the MLDD cut-off of
BO diagnosis between expiration and inspiration was 109.0, which is
consistent with previous studies. In multivariate linear regression
analysis, the difference (MLDD) or the ratio of the
expiratory/inspiratory MLD (E/I MLD) showed a strong correlation with
FEV1, and the -850 to -950 HU threshold method showed a significant
correlation with sRaw (Table 2). This indicates that each LDI and PFT
parameter can be independently specific to reflect different anatomical
areas that are responsible for pathological lung function.
Previous studies on the E/I ratio of MLD have shown a stronger
correlation with the spirometric airflow limitation parameters than the
density threshold-based method of air trapping 22,29.
E/I MLD showed a strong correlation with FEV1, FEV1/FVC, FEF25-75, and
FRC/TLC ratios as well as markers with respiratory morbidities14,29. In our study, in contrast, expiratory -900 HU
(E900) showed the highest AUC in the ROC analysis for BO diagnosis
compared to other LDIs; this was a single factor that consistently
showed a significant strong correlation with the PFT parameter in
various correlation analyses.
Our study has multiple strengths. This is a study of lung densitometry
in pediatric BO patients, and it extensively investigated the
correlation between various densitometry and pulmonary function
parameters. We suggested an additional role of quantitative CT in
evaluating pulmonary function in difficult pediatric BO patients with
poor PFT cooperation and a low reliability of test results. The main
limitation of our study is that it is a retrospective study using a
small sample size of patients. Since young children have varying lung
volumes and structures with age, the patients included in our study are
likely heterogeneous populations with varying lung densities. Further
study is needed to determine whether a fixed lung density threshold can
be applied to pediatric BO patients of various ages.
In conclusion, quantification LDIs obtained through chest CT scans have
a significant correlation with conventional PFTs. The ratio of the E/I
MLD and the LAA below the threshold -900 HU complements the diagnosis of
pediatric BO patients by providing additional quantitative indications
of progressive expiratory air flow limitation.