Purpose/objectives
Distinguishing pneumonia and atelectasis may be difficult since both may appear as a consolidated lung, and the clinical symptoms are not specific enough to differentiate them.
It is well known that atelectasis enhances more than pneumonia, but at our knowledge only one publication tried to quantitatively distinguish these two entities.
This study aims to determine if a threshold attenuation value could be used to distinguish pneumonia from atelectasis.
Methods and materials
Computed tomography angiogram scans that did not show pulmonary embolism, were retrospectively analyzed for inclusion and exclusion criteria, between January 1, 2020 and December 31, 2020.
Patients were included in the study if the lungs contained pulmonary non-aereated lung.
CTPA examinations were assessed for each nonaereated lung with drawn of oval regions of interest (ROIs), and the mean was considered. The diagnosis of either pneumonia or atelectasis was based on the presence of the following clinical criteria: fever, respiratory symptoms, leukocytosis/elevation of procalcitonin, antibiotic treatment for pneumonia, or a diagnostic of pneumonia at discharge.
Receiver operating characteristic (ROC) analysis was performed to determine the area under the ROC curve, sensitivity, and specificity of using the attenuation to distinguish pneumonia from atelectasis. A threshold HU value that resulted in the best combined sensitivity and specificity was determined using the Youden index.
Results
The study included 140 nonaerated lungs, with 66 atelectasis and 74 consolidations/pneumonia. The mean enhancement was 66 HU in pneumonia and 109 in atelectasis. For CTPAs using our standard protocol, our results suggest that pneumonia can be confidently diagnosed with a threshold attenuation value of 91 HU. Accuracy, measured as area under the ROC curve, was 0.97 (CI, 0.89–0.99).
In addition, an attenuation threshold was chosen by maximizing the Youden index and a threshold of 84 HU has diagnosed pneumonia with 95% sensitivity and 92% specificity.
Conclusion
This study confirms that consolidation typically has lower attenuation values compared to atelectasis. Our results suggest that a threshold of HU value can be used to distinguish pneumonia from atelectasis.
Further studies are necessary to determine what HU thresholds can be used in different CT protocols.

**Purpose/objectives**
Distinguishing pneumonia and atelectasis may be difficult since both may appear as a consolidated lung, and the clinical symptoms are not specific enough to differentiate them.
It is well known that atelectasis enhances more than pneumonia, but at our knowledge only one publication tried to quantitatively distinguish these two entities.
This study aims to determine if a threshold attenuation value could be used to distinguish pneumonia from atelectasis.
**Methods and materials**
Computed tomography angiogram scans that did not show pulmonary embolism, were retrospectively analyzed for inclusion and exclusion criteria, between January 1, 2020 and December 31, 2020.
Patients were included in the study if the lungs contained pulmonary non-aereated lung.
CTPA examinations were assessed for each nonaereated lung with drawn of oval regions of interest (ROIs), and the mean was considered. The diagnosis of either pneumonia or atelectasis was based on the presence of the following clinical criteria: fever, respiratory symptoms, leukocytosis/elevation of procalcitonin, antibiotic treatment for pneumonia, or a diagnostic of pneumonia at discharge.
Receiver operating characteristic (ROC) analysis was performed to determine the area under the ROC curve, sensitivity, and specificity of using the attenuation to distinguish pneumonia from atelectasis. A threshold HU value that resulted in the best combined sensitivity and specificity was determined using the Youden index.
**Results**
The study included 140 nonaerated lungs, with 66 atelectasis and 74 consolidations/pneumonia. The mean enhancement was 66 HU in pneumonia and 109 in atelectasis. For CTPAs using our standard protocol, our results suggest that pneumonia can be confidently diagnosed with a threshold attenuation value of 91 HU. Accuracy, measured as area under the ROC curve, was 0.97 (CI, 0.89–0.99).
In addition, an attenuation threshold was chosen by maximizing the Youden index and a threshold of 84 HU has diagnosed pneumonia with 95% sensitivity and 92% specificity.
**Conclusion**
This study confirms that consolidation typically has lower attenuation values compared to atelectasis. Our results suggest that a threshold of HU value can be used to distinguish pneumonia from atelectasis.
Further studies are necessary to determine what HU thresholds can be used in different CT protocols.

How to distinguish Pneumonia from Atelectasis? A quantitative approach

The Winter Course 2021 covers a variety of topics from basics in interstitial lung disease, infections including Covid-19, malignancies, lung cancer screening, and artificial intelligence. The combination of topics is aimed to serve as a good introduction to radiologists embarking on a career in thoracic imaging as well as a solid refresher for experienced thoracic radiologists.

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