CT densitometry of the lungs: scanner performance

Effect of threshold on the correlation between airflow obstruction and low attenuation volume in smokers assessed by inspiratory and expiratory MDCT

BACKGROUND

The estimation of emphysematous changes is very sensitive to computed tomography (CT) threshold level. In clinical practice, the predetermined threshold is usually set at -950 Hounsfield units (HU) for the detection of low attenuation volume (LAV). However, threshold levels that are tightly connected to pulmonary function abnormalities have not been determined.

PURPOSE

To determine the threshold level for calculating an LAV that closely reflects airflow limitation in patients with chronic obstructive pulmonary disease (COPD).

MATERIAL AND METHODS

Seventy-six consecutive non-COPD smokers and COPD patients underwent paired inspiratory and expiratory multidetector CT (MDCT). LAV% was segmented every 10 HU between -1000 and -750 HU to examine the correlation between LAV% and indexes of obstructive impairment.

RESULTS

LAV% gradually increased as the threshold level increased on both inspiratory and expiratory images. LAV% on inspiratory images was higher than that on expiratory images at all threshold levels between -1000 and -750 HU. The threshold level that correlated with obstructive impairment differed between the two images: -930 HU on inspiratory and -870 or -880 HU on expiratory images.

CONCLUSION

LAV% dramatically changed according to the threshold level on both inspiratory and expiratory images, indicating that LAV% is dependent on the attenuation threshold level in patients with COPD. The threshold linking LAV% to airflow limitation was higher on expiratory than on inspiratory images.

Quantitative computed tomography-derived clusters: redefining airway remodeling in asthmatic patients

BACKGROUND

Asthma heterogeneity is multidimensional and requires additional tools to unravel its complexity. Computed tomography (CT)-assessed proximal airway remodeling and air trapping in asthmatic patients might provide new insights into underlying disease mechanisms.

OBJECTIVES

The aim of this study was to explore novel, quantitative, CT-determined asthma phenotypes.

METHODS

Sixty-five asthmatic patients and 30 healthy subjects underwent detailed clinical, physiologic characterization and quantitative CT analysis. Factor and cluster analysis techniques were used to determine 3 novel, quantitative, CT-based asthma phenotypes.

RESULTS

Patients with severe and mild-to-moderate asthma demonstrated smaller mean right upper lobe apical segmental bronchus (RB1) lumen volume (LV) in comparison with healthy control subjects (272.3 mm(3) [SD, 112.6 mm(3)], 259.0 mm(3) [SD, 53.3 mm(3)], 366.4 mm(3) [SD, 195.3 mm(3)], respectively; P = .007) but no difference in RB1 wall volume (WV). Air trapping measured based on mean lung density expiratory/inspiratory ratio was greater in patients with severe and mild-to-moderate asthma compared with that seen in healthy control subjects (0.861 [SD, 0.05)], 0.866 [SD, 0.07], and 0.830 [SD, 0.06], respectively; P = .04). The fractal dimension of the segmented airway tree was less in asthmatic patients compared with that seen in control subjects (P = .007). Three novel, quantitative, CT-based asthma clusters were identified, all of which demonstrated air trapping. Cluster 1 demonstrates increased RB1 WV and RB1 LV but decreased RB1 percentage WV. On the contrary, cluster 3 subjects have the smallest RB1 WV and LV values but the highest RB1 percentage WV values. There is a lack of proximal airway remodeling in cluster 2 subjects.

CONCLUSIONS

Quantitative CT analysis provides a new perspective in asthma phenotyping, which might prove useful in patient selection for novel therapies.

Equating quantitative emphysema measurements on different CT image reconstructions

PURPOSE

To mathematically model the relationship between CT measurements of emphysema obtained from images reconstructed using different section thicknesses and kernels and to evaluate the accuracy of the models for converting measurements to those of a reference reconstruction.

METHODS

CT raw data from the lung cancer screening examinations of 138 heavy smokers were reconstructed at 15 different combinations of section thickness and kernel. An emphysema index was quantified as the percentage of the lung with attenuation below -950 HU (EI950). Linear, quadratic, and power functions were used to model the relationship between EI950 values obtained with a reference 1 mm, medium smooth kernel reconstruction and values from each of the other 14 reconstructions. Preferred models were selected using the corrected Akaike information criterion (AICc), coefficients of determination (R2), and residuals (conversion errors), and cross-validated by a jackknife approach using the leave-one-out method.

RESULTS

The preferred models were power functions, with model R2 values ranging from 0.949 to 0.998. The errors in converting EI950 measurements from other reconstructions to the 1 mm, medium smooth kernel reconstruction in leave-one-out testing were less than 3.0 index percentage points for all reconstructions, and less than 1.0 index percentage point for five reconstructions. Conversion errors were related in part to image noise, emphysema distribution, and attenuation histogram parameters. Conversion inaccuracy related to increased kernel sharpness tended to be reduced by increased section thickness.

CONCLUSIONS

Image reconstruction-related differences in quantitative emphysema measurements were successfully modeled using power functions.

Influence of CT reconstruction settings on extremely low attenuation values for specific gas volume calculation in severe emphysema

RATIONALE AND OBJECTIVES

Emphysema is characterized by lung tissue destruction and trapped gas. On computed tomographic (CT) images, this may be expressed by widespread areas with high specific gas volume (SV(g)). SV(g) is highly sensitive to very low attenuation values, which frequently occur in the CT images of patients with severe emphysema. The purpose of the present work was to study if and how different reconstruction settings and different scanners significantly influence SV(g) distribution, particularly in the very low attenuation range.

MATERIALS AND METHODS

Two sets of CT images taken from two different CT scanners at two different lung volumes in 10 healthy volunteers and 18 subjects with severe emphysema were analyzed. Images were reconstructed using two different settings of reconstruction parameters: (1) thin slice thickness and sharp filter and (2) thick slice thickness and smooth filter. For each set of images, average values of SV(g) and their variation (ΔSV(g)) from total lung capacity to residual volume were calculated in the whole lung.

RESULTS

Very low attenuation values are always present in CT images when reconstructed with thin slice thickness and a sharp filter and in very large numbers in patients with severe emphysema. SV(g) values were in general significantly higher in patients with emphysema than in healthy subjects, at both total lung capacity and residual volume (P < .001), and were significantly influenced by the reconstruction filter (P < .001) and CT scanner (P < .001). ΔSV(g) was lower in patients with emphysema than in healthy subjects (P < .001) and was significantly affected by the reconstruction setting but not by the CT scanner.

CONCLUSIONS

The disproportionate effect of low-attenuation pixels on SV(g) likely causes overestimation of the severity of emphysema and trapped gas. This can be significantly reduced, however, by using thick slices and a smooth filter for image reconstruction. ΔSV(g) is generally robust for quantifying the functional impairment of the lung in severe emphysema.

Repeated low-dose computed tomography in current and former smokers for quantification of emphysema

OBJECTIVE

To quantify different emphysema evolution in current and former smokers.

METHODS

We retrospectively analyzed low-dose computed tomography scans from a lung cancer screening study of 59 current and 75 former smokers. The quantitative emphysema analysis was performed using a home-built software (YACTA version 0.9), yielding the parameters lung volume, emphysema volume (EV), emphysema index (EI), mean lung density, and 15th percentile.

RESULTS

The baseline EV and EI were significantly different (median EVformer =422 mL vs EVcurrent =249 mL, P = 0.0003; and median EIformer =7.6 % vs EIcurrent =4.1 %, P = 0.0001, respectively). On the annual repeat scan, the median EI and EV for former smokers had decreased significantly (ΔEIformer = -0.257%, P = 0.004; and ΔEVcurrent = -0.203 mL, P = 0.020), whereas there was no emphysema change in current smokers.

CONCLUSIONS

We were able to demonstrate different emphysema evolution in current versus former smokers; emphysema parameters decreased in the former smokers and remained stable in current smokers.

Emphysema quantification in inflation-fixed lungs using low-dose computed tomography and 3He magnetic resonance imaging

OBJECTIVE

To evaluate the use of inflation-fixed lung tissue for emphysema quantification with computed tomography (CT) and He magnetic resonance (MR) diffusion imaging.

METHODS

Fourteen subjects representing a range of chronic obstructive pulmonary disease severity who underwent complete or lobar lung resection were studied. Computed tomographic measurements of lung attenuation and MR measurements of the hyperpolarized 3He apparent diffusion coefficient (ADC) in resected specimens fixed in inflation with heated formalin vapor were compared with measurements obtained before fixation.

RESULTS

The mean (SD) CT emphysema indices were 56% (17%) before and 58% (19%) after fixation (P = 0.77; R = 0.76). Index differences correlated with differences in lung volume (R = 0.47). The mean (SD) 3He ADCs were 0.40 (0.15) cm/s before and 0.39 (0.14) cm/s after fixation (P = 0.03, R = 0.98). The CT emphysema index and the 3He ADC were correlated before (R = 0.89) and after fixation (R = 0.79).

CONCLUSIONS

Concordance of CT and 3He MR imaging measurements in unfixed and inflation-fixed lungs supports the use of inflation-fixed lungs for quantitative imaging studies in emphysema.

Acurácia da mensuração do enfisema pulmonar na tomografia computadorizada: pontos importantes

Para garantir a confiabilidade dos dados de quantificação computadorizada do enfisema pulmonar (densitovolumetria pulmonar) na tomografia computadorizada, alguns aspectos técnicos devem ser considerados. A alteração das densidades na tomografia computadorizada com as mudanças no nível de inspiração e expiração do pulmão, com a espessura de corte da tomografia computadorizada, com o algoritmo de reconstrução e com o tipo de tomógrafo dificulta as comparações tomográficas nos estudos de acompanhamento do enfisema pulmonar. No entanto, a densitovolumetria pulmonar substituiu a avaliação visual e compete com as provas de função pulmonar como método para medir o enfisema pulmonar. Esta revisão discute as variáveis técnicas que alteram a aferição do enfisema na tomografia computadorizada e sua influência nas medições de enfisema.

Update on radiology of emphysema and therapeutic implications

Emerging treatments require appropriate CT targeting of a selected lobe or lobes and target airways to obtain a successful response. CT scan is used in pretreatment planning to select patients and plan treatment strategy and posttreatment to confirm correct deployment of devices and assess treatment response. Increasingly treatments are being developed to treat patients who have emphysema who require accurate quantitation of extent and distribution of the process. Functional assessment can be made by inference of detailed anatomic correlates and by direct measurement of regional function using dynamic scan protocols. This article summarizes the current role of imaging in the assessment of patients who have emphysema.

Computed tomography measurement of lung volume in preoperative assessment for living donor lung transplantation: volume calculation using 3D surface rendering in the determination of size compatibility

The objective of this study was to describe the use of CT volume quantification assessment of candidates for LLDLT. Six pediatric candidates for LDLLT and their donors were investigated with helical chest CT, as part of the preoperative assessment. The CT images were analyzed as per routine and additional post-processing with CT volume quantification (CT densitovolumetry) was performed to assess volume matching between the lower lobes of the donors and respective lungs of the receptors. CT images were segmented by density and region of interest, using post-processing software. Size matching was also assessed using the FVC formula. Compatible volumes were found in three cases. The other three cases were considered incompatible. All three recipients with compatible sizes survived the procedure and are alive and well. One patient with incompatible size was submitted to the procedure and died because of complications attributed to the incompatible volumes. One patient with incompatible size has subsequently grown and new measurements are to be taken to check the current volumes. Different donors are being sought for the remaining patient whose lung volumes were considered too big for the prospective transplant donor lobes. Under FVC formula criteria, all cases were considered compatible. CT volume quantification is an easy to perform, non-invasive technique that uses CT images for the preassessment of candidates for LDLLT, to compare the volume of the lower lobes from the donors with volume of each lung in the prospective recipients. Size matching based on CT densitovolumetry and FVC may differ.

A low-cost density reference phantom for computed tomography

The authors characterized a commercially available foam composed of polyurethane and polyisocyanurate which is marketed for modeling parts in the aircraft, automotive, and related industries. The authors found that the foam may be suitable for use as a density reference standard in the range below -400 Hounsfield units. This range is coincident with the density of lung tissue. The foam may be helpful in making the diagnosis of lung disease more systematic.

Airway wall thickening and emphysema show independent familial aggregation in chronic obstructive pulmonary disease

RATIONALE

It is unclear whether airway wall thickening and emphysema make independent contributions to airflow limitation in chronic obstructive pulmonary disease (COPD) and whether these phenotypes cluster within families.

OBJECTIVES

To determine whether airway wall thickening and emphysema (1) make independent contributions to the severity of COPD and (2) show independent aggregation in families of individuals with COPD.

METHODS

Index cases with COPD and their smoking siblings underwent spirometry and were offered high-resolution computed tomography scans of the thorax to assess the severity of airway wall thickening and emphysema.

MEASUREMENTS AND MAIN RESULTS

A total of 3,096 individuals were recruited to the study, of whom 1,159 (519 probands and 640 siblings) had technically adequate high-resolution computed tomography scans without significant non-COPD-related thoracic disease. Airway wall thickness correlated with pack-years smoked (P < or = 0.001) and symptoms of chronic bronchitis (P < 0.001). FEV(1) (expressed as % predicted) was independently associated with airway wall thickness at a lumen perimeter of 10 mm (P = 0.0001) and 20 mm (P = 0.0013) and emphysema at -950 Hounsfield units (P < 0.0001). There was independent familial aggregation of both the emphysema (adjusted odds ratio, 2.1; 95% confidence interval, 1.1-4.0; P < or = 0.02) and airway disease phenotypes (P < 0.0001) of COPD.

CONCLUSIONS

Airway wall thickening and emphysema make independent contributions to airflow obstruction in COPD. These phenotypes show independent aggregation within families of individuals with COPD, suggesting that different genetic factors influence these disease processes.

Quantification of emphysema: a composite physiologic index derived from CT estimation of disease extent

The combination of functional indices best reflecting the extent of emphysema is not known. High-resolution computed tomography (HRCT) studies of 106 patients with emphysema [men=71; median age=61 (range=26-86 years)] were reviewed and the extent of emphysema was quantified: (a) visually (emphysema(vis)) and (b) by automated estimation (emphysema(auto)). Functional-morphologic relationships were compared for the two scoring systems, and a composite physiologic index (CPI) (providing the best fit of functional indices against emphysema extent) was derived. Emphysema(vis) and emphysema(auto) were strongly correlated (r=0.90; p<0.0005), but the extent of emphysema(vis) was consistently greater (p<0.00005). Emphysema(vis) correlated most strongly with indices of gas transfer [percent predicted single-breath carbon monoxide diffusing capacity (Dl(co)) and alveolar volume (K(co)); r=-0.70, both p<0.0005]. The combination of physiologic indices most representative of emphysema extent on CT (using visual or automated methods) consisted of K(co) and forced expiratory volume in 1 s (FEV(1)) levels. The equation explanatory power was higher for visual scoring [emphysema(vis)=96.8-(0.67 x % predicted K(co))-(0.41 x % predicted FEV(1)); equation r(2)=0.57] than automated estimation (equation r(2)=0.48). Weighted combinations of K(co) and FEV(1) levels provide a CPI best reflecting morphologic emphysema extent. CPI has the potential to refine the stratification of patients in epidemiological and therapeutic studies.

Variability in densitometric assessment of pulmonary emphysema with computed tomography

OBJECTIVES

The objectives of this study were to investigate whether computed tomography (CT) densitometry can be applied consistently in different centers; and to evaluate the reproducibility of densitometric quantification of emphysema by assessment of different sources of variation, ie, intersite, interscan and inter- and intraobserver variability, in comparison with intersubject variability.

MATERIALS AND METHODS

In 5 different hospitals, 119 patients with emphysema were scanned using standardized protocols. In each site, an observer performed a quantitative densitometric analysis (including blood recalibration) on the corresponding patient group (n=23-25) and one observer analyzed the entire group of 119 patients. After several months, the latter observer analyzed all data for a second time. Subsequently, different sources of variation were assessed by variance component analysis with and without volume correction of the data.

RESULTS

Inter- and intraobserver variability marginally contributes to the total variability (<0.001%). The interscan variability was 0.02% of the total variation after application of volume correction. The intersite variability was 48% as a result of one deviating CT scanner. Air recalibration normalized deviating air densities in CT scanners. Within sites, the intersubject variability ranged between 93% and 99% based on the analysis of 2 subsequent CT scans of the patients.

CONCLUSIONS

Almost all variability in the density measurement of emphysema originates from differences between scanners and from differences in severity of emphysema between patients. Lung densitometry with multislice CT scanners is a highly reproducible measurement, especially if corrected for lung volume, because this reduces interscan variability.

Emphysema: effect of reconstruction algorithm on CT imaging measures

In the current study, the effects of reconstruction algorithms on quantitative measures derived from computed tomographic (CT) lung images were assessed in patients with emphysema. CT image data sets were reconstructed with a standard algorithm and alternative algorithm(s) for 42 subjects. Algorithms were grouped as overenhancing, sharp, standard, or smooth. Density mask and volume measurements from the alternative algorithm data sets were compared with standard algorithm data sets. The overenhancing category yielded an average shift of 9.4% (ie, a shift in average score from 35.5% to 44.9%); the sharp category, a shift of 2.4%; and the smooth category, a shift of -1.0%. Differences in total lung volume measurements were less than 1%. In conclusion, the CT reconstruction algorithm may strongly affect density mask results, especially for certain reconstruction algorithms.

Influence of calibration on densitometric studies of emphysema progression using computed tomography

The fundamental importance of calibration for any measuring device is indisputable, but computed tomography (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 are 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 three successive annual scans acquired with a fully calibrated scanner was intimately associated with changes in CT air densitometry, sampled from patient images. Images were therefore reanalyzed, using a correction technique validated in phantom studies that adjusted for changes in measured air density, and the reliability of the voxel index as a measure of emphysema progression was improved. Comparison of adjusted voxel index thresholds indicated the optimum threshold was -950 Hounsfield units. Internal air calibration is therefore critical in longitudinal and multicenter lung densitometry studies of emphysema and incorporation of a correction factor is essential for quantitative image analysis.

Structure and function of small airways in smokers: relationship between air trapping at CT and airway inflammation

PURPOSE

To use quantitative computed tomography (CT) to compare lung attenuation with both inflammatory infiltration and in vitro reactivity of peripheral airways in smokers scheduled to undergo lung resection for localized pulmonary lesions.

MATERIALS AND METHODS

Attenuation was measured in nine ex-smokers, 13 current smokers, and eight nonsmoking control subjects by using CT with respiratory gating and a contour-tracing algorithm. After lung resection in smokers, peripheral bronchi were dissected and studied in terms of both inflammation (by using immunohistochemistry to examine glycolmethacrylate-embedded specimens) and mechanical activity (by using an isolated organ bath system). Comparisons between groups were made by using analysis of variance and subsequent unpaired t tests. Correlations were evaluated by using the Pearson coefficient and stepwise multiple regression analysis.

RESULTS

The difference between inspiratory and expiratory attenuation was significantly higher in control subjects (-128 HU +/- 11 [SD]) than in ex-smokers (-77 HU +/- 10; P =.004) or current smokers (-67 HU +/- 11; P =.001). Cells infiltrating the smooth muscle increased with the decrease in expiratory attenuation (r = -0.46; P =.03) and the increase in inspiratory versus expiratory attenuation (r = 0.66; P =.001). Mast cell and neutrophil infiltration of smooth muscle was the most important factor in this relationship. Cellular infiltration of the smooth muscle increased with the decrease of in vitro relaxation response to salbutamol.

CONCLUSION

In smokers, air trapping is correlated with inflammatory infiltration of the smooth muscle layer of small airways.

Bronchiolitis obliterans syndrome in lung transplant recipients: use of spirometrically gated CT

PURPOSE

To assess the potential use of spirometrically gated lung computed tomographic (CT) findings in the diagnosis of bronchiolitis obliterans syndrome after lung transplantation.

MATERIALS AND METHODS

Forty-nine lung transplant recipients were examined at least 8 months after surgery with spirometrically gated thin-section CT of the lung. In addition to visual signs of small-airway disease at CT, mean lung attenuation and the SD were numerically determined and compared with the results of lung function testing at the time of the CT examination and 1 year later by using factorial analysis of variance.

RESULTS

Mean lung attenuation was significantly lower in patients who developed bronchiolitis obliterans syndrome within 1 year after the CT study (-837 HU +/- 3) than in patients with persistent normal lung function (-812 HU +/- 3, P <.001). With an optimal threshold, sensitivity was 69%, specificity was 71%, and accuracy was 84%. Visual analysis did not significantly contribute to the prognostic power of CT.

CONCLUSION

Spirometrically gated CT measurements of lung attenuation can be used to predict the onset of bronchiolitis obliterans syndrome after lung transplantation.

Ability of calibration phantom to reduce the interscan variability in electron beam computed tomography

OBJECTIVE

To test the hypothesis that a calibration phantom would improve interpatient and interscan variability in coronary artery calcium (CAC) studies.

METHODS

We scanned 144 patients twice with or without the calibration phantom and then scanned 93 patients with a single calcific lesion twice and, finally, scanned a cork heart with calcific foci.

RESULTS

There were no linear correlations in computed tomography Hounsfield unit (CT HU) and CT HU interscan variation between blood pool and phantom plugs at any slice level in patient groups (p > 0.05). The CT HU interscan variation in phantom plugs (2.11 HU) was less than that of the blood pool (3.47 HU; p < 0.05) and CAC lesion (20.39; p < 0.001). Comparing images with and without a calibration phantom, there was a significant decrease in CT HU as well as an increase in noise and peak values in patient studies and the cork phantom study.

CONCLUSION

The CT HU attenuation variations of the interpatient and interscan blood pool, calibration phantom plug, and cork coronary arteries were not parallel. Therefore, the ability to adjust the CT HU variation of calcific lesions by a calibration phantom is problematic and may worsen the problem.

Diagnostic yield of computed tomography and densitometric measurements of the lung in thoracoscopically-defined idiopathic spontaneous pneumothorax

In the present study, the diagnostic yield of high resolution computed tomography (HRCT) is evaluated in patients with thoracoscopically-verified idiopathic spontaneous pneumothorax (SP). Visual assessment as well as densitometry of lung parenchyma was performed. In eight of the 20 prospectively-evaluated SP patients, emphysema-like (EL) changes such as blebs and bullae could be detected. The SP patients with EL changes were significantly older and were more heavy smokers. Spirometrically-controlled CT lung densitometry showed no differences between the patient group with or without these EL changes. Comparing the densitometric measurements of the patient group with a healthy control group no significant differences in densitometry between both groups were found. In conclusion, this study confirms that HRCT is a reliable method of detecting blebs and bullae in patients with spontaneous pneumothorax. Furthermore CT lung densitometry revealed no parenchymal abnormalities or signs of air trapping in patients with spontaneous pneumothorax.

Pulmonary function and regional distribution of emphysema as determined by high-resolution computed tomography

In patients with pulmonary emphysema, emphysematous changes are not uniform and vary from minimum alveolar destruction to advanced bullous formation, depending on the lobe or site in the lungs. However, we have little knowledge on whether or how this nonuniformity or localization affects pulmonary function in PE patients. Therefore, we measured the computed tomography (CT) density of divided sites in lungs with high-resolution CT images from 25 PE patients (FEV1.0%, mean +/- SD 36 +/- 9%, %DLCO 48 +/- 16%, all men, 68 +/- 4 years) and compared them to various parameters of pulmonary function. The mean CT density of whole lungs correlated with 12 pulmonary function parameters including FEV1.0 and diffusion capacity. When both lung fields were divided into peripheral, intermediate and central portions, the CT density of the central portion correlated with all pulmonary function parameters with which CT density of whole lungs correlated. In contrast, the CT density of the peripheral portion significantly correlated with only 7 parameters with smaller correlation coefficient values than those of the central portion. When divided into upper, middle and lower portions, the CT densities of upper, middle and lower portions correlated with 6, 8 and 10 of the 12 pulmonary function parameters which correlated with the density of whole lungs, respectively. The delta value of CT densities between the upper and lower portions or between the lateral and medial portions correlated with obstructive impairment (FEV1.0 and FEV1.0%). These findings suggest that (1) central rather than peripheral emphysematous changes affect pulmonary function, and (2) uniformity of emphysematous change correlates with the severity of airway obstruction in PE patients.

Gradient correction and classification of CT lung images for the automated quantification of mosaic attenuation pattern

PURPOSE

The detection of density differences, or "mosaic attenuation pattern," on CT images may be difficult when the regional inhomogeneity of the density of the lung parenchyma is subtle. The purpose of this work was to develop a fully automated method for the reproducible quantification of the underattenuated areas of the lung parenchyma. This technique may be useful in increasing the precision of investigation of structure/function relationships.

METHOD

Anatomical segmentation was achieved by a structure-filtering operator based on mathematical morphology. To compensate for the density gradient visible on lung CT scans, a model-based iterative deconvolution filter and an adaptive clustering algorithm were developed. Validation was performed with CT images from a lung phantom, 15 patients with constrictive obliterative bronchiolitis, and 8 normal subjects.

RESULTS

The accuracy of the estimate of the density gradient on phantom studies was 93.3%. The automated quantification of the areas of decreased attenuation on scans of constrictive obliterative bronchiolitis was within 8.2% from the average scoring of two experienced observers.

CONCLUSION

The proposed technique is fully automated and can accurately correct for density gradient and classify areas of decreased attenuation on lung CT images.

Sources of error in lung densitometry with CT

RATIONALE AND OBJECTIVES

To determine and analyze the most important error sources in lung CT densitometry in vivo.

METHODS

The authors examined the influences of CT acquisition errors, physiologic changes, and image segmentation errors on lung densitometry. Among others, spatial dependency and long-term reproducibility of the density measurements of blood and air were examined over a period of 4 years in a group of 28 patients with pulmonary emphysema. These results were related to the measured lung densities in this group.

RESULTS

The density measurement of blood and air is strongly dependent on the position in the thorax. Despite full-scanner calibrations, x-ray tube replacement can induce a significant increase in measured blood density.

CONCLUSIONS

A change in a lung density parameter over time can actually be the result of tube replacement or changing blood density. A simple postprocessing technique can correct for these changes.

Mechanism of mosaic attenuation of the lungs on computed tomography in induced bronchospasm

The purpose of this study was to investigate whether hypoxic pulmonary vasoconstriction is the major determinant of the computed tomography (CT) pattern of mosaic attenuation in asthmatic patients with induced bronchoconstriction. Thin-section CT was performed at suspended full inspiration immediately and 30 min after methacholine bronchoprovocation in 22 asthmatic subjects, who were randomly assigned to breathe room air (group A, n = 8), oxygen via nasal prongs at 5 l/min (group B, n = 8), and oxygen via face mask at 12 l/min (group C, n = 6). CT changes were quantified in terms of global lung density and density in hypodense and hyperdense areas. Lung parenchymal density increases were greatest in group C and greater in group B than in group A, globally (P = 0.03) and in hypodense regions (P = 0.01). On bivariate analysis, the only change in cross-sectional area was related to change in global density. In hypodense regions, density change was related both to reduction in cross-sectional area (P < 0.0005) and to oxygen administration (P = 0.01). After correction for changes in global lung density, only oxygen was independently related to density increase in hypodense areas (P = 0.02). In induced bronchoconstriction, the CT appearance of mosaic attenuation can be largely ascribed to hypoxic vasoconstriction rather than to changes in lung inflation.

CT lung densitometry: dependence of CT number histograms on sample volume and consequences for scan protocol comparability

PURPOSE

Our goals were to determine the dependence of CT number histograms of the lung on section thickness and reconstruction filter and to evaluate the consequences for scan protocol conformity required for universally comparable densitometry of the lungs.

METHOD

The effects of section thickness and reconstruction filter were parameterized with the CT's sample volume [V approximately (section thickness x in-plane resolution2)]. In a study of 31 patients, we determined as a function of V the following CT number histogram parameters: percentiles P(10) and P(90), pixel indexes PI(-905) and PI(-950), and standard deviation.

RESULTS

Patient histogram parameters depended strongly on sample volume. Large differences were found between protocols using 1 and 10 mm sections. For small variations in somewhat larger sample volumes (> 8 mm3), discrepancies were much smaller.

CONCLUSION

To obtain comparable histogram parameters, nearly identical sample volumes (> or = 8 mm3) should be used. When this condition is satisfied, available data suggest that universally comparable densitometry is feasible.