Lung disease
Airways diseases, such as emphysema or bronchiolitis obliterans, cause air trapping on expiration, even though they may cause only minor changes to lung structure in their early stages. To enhance sensitivity for these conditions, the scan may be performed in both inspiration and expiration.
HRCT may be diagnostic for conditions such as emphysema or bronchiectasis. While HRCT may be able to identify pulmonary fibrosis, it may not always be able to further categorize the fibrosis to a specific pathological type (e.g., non-specific interstitial pneumonitis or desquamative interstitial pneumonitis). The major exception is UIP, which has very characteristic features, and may be confidently diagnosed on HRCT alone.[10]
Where HRCT is unable to reach a definitive diagnosis, it helps locate an abnormality, and so helps planning a biopsy, which may provide the final diagnosis.
Other miscellaneous conditions where HRCT is useful include lymphangitis carcinomatosa, fungal, or other atypical, infections, chronic pulmonary vascular disease, lymphangioleiomyomatosis, and sarcoidosis.
Organ transplant patients, particularly lung, or heart-lung transplant recipients, are at relatively high risk of developing pulmonary complications of the long-term drug and immunosuppressive treatment. The major pulmonary complication is bronchiolitis obliterans, which may be a sign of lung graft rejection.
HRCT has better sensitivity for bronchiolitis obliterans than conventional radiography.[11][12][13] Some transplant centers may arrange annual HRCT to screen for this.
Diagnostic imaging, including HRCT, is one of the main diagnostic tools for COVID-19.[14] There is some debate about the usefulness of CT compared to other methods and imaging modalities for diagnosis.[15] Under HRCT scan, infected individuals generally showed a multifocal or unifocal involvement of ground-glass opacity (GGO).[16]
Nodularity
The presence of lung nodules on high resolution CT is a keystone in understanding the appropriate differential. Typically, the distribution of nodules is divided into perilymphatic, centrilobular and random categories. Furthermore, nodules can be ill-defined, implying they are in the alveoli, or well defined, suggesting an interstitial position. Distribution and appearance allow understanding of the disease process relative to the secondary lobule of the lung, the smallest anatomic unit with surrounding connective tissue, usually 1–2 cm across.[17]
Perilymphatic nodularity deposits at the periphery of the secondary lobule and tends to respect pleural surfaces and fissures. Sarcoidosis, lymphangitic spread of carcinoma, silicosis, coal worker's pneumoconiosis, and more rare diagnoses such as lymphoid interstitial pneumonitis and amyloidosis are included in the differential. Centrilobular nodularity deposits at the center of the secondary lobule, but spares pleural surfaces. Differential includes endobronchial tuberculosis, bronchopneumonia, endobronchial spread of tumor, and again silicosis or coal workers' pneumoconiosis. For randomly distributed nodules, the differential includes miliary tuberculosis, fungal pneumonia, hematogenous metastasis and diffuse sarcoidosis.[17]
Prone versus supine position
Because the bases of the lungs lie posteriorly in the chest, a mild degree of collapse under the lungs' own weight can occur when the patient lies on their back. As the very base of the lungs may be the first region affected in several lung diseases, most notably asbestosis or usual interstitial pneumonia (UIP), the patient may be asked to lie prone to improve sensitivity to early changes of these conditions.
The lung bases are often inconsistent in appearance in patients due to the potential for atelectasis causing positional ground glass or consolidative opacities. When the patient is positioned prone, or on their belly, the lung bases can expand further and help distinguish atelectasis from early fibrosis. In patients with normal chest radiographs, prone scans have been found useful in 17% of cases, particularly in excluding posterior lung abnormalities. In patients with abnormal findings on chest radiographs, prone scans are only useful in 4% of cases. The scans may be more useful in patients with basilar predominant disease processes, such as asbestosis and idiopathic pulmonary fibrosis.[17]
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