High-Resolution Computed Tomography (HRCT)

HRCT is computed tomography (CT) with high resolution. It is used in the diagnosis of various health problems. HRCT involves the use of special computed tomography scanning techniques to assess the lung parenchyma which enables it to diagnose certain Lung diseases better than conventional radiography.

Technique:
HRCT is performed using a conventional CT scanner. However, imaging parameters are chosen so as to maximize spatial resolution:

Protocol for High-Resolution CT Scanning of the Chest

• A narrow slice width is used (usually 1–2 mm).
• A high spatial resolution image reconstruction algorithm is used.
• Field of view is minimized, so as to minimize the size of each pixel.
• Other scan factors (e.g. focal spot) may be optimized for resolution at the expense of scan speed.

Intravenous contrast agents are not used for HRCT as the lung inherently has very high contrast (soft tissue against air), and the technique itself is unsuitable for assessment of the soft tissues and blood vessels, which are the major targets of contrast agents.

Use as a diagnostic test:

• HRCT is used for diagnosis and assessment of Interstitial lung disease, such as pulmonary fibrosis, interstitial pneumonitis and other generalized lung diseases such as emphysema, bronchiectasis.
• HRCT may be able to identify pulmonary fibrosis, but it may not always be able to further categorize the fibrosis to a specific pathological type. The major exception is usual interstitial pneumonitis (UIP) which has very characteristic features, and may be confidently diagnosed on HRCT alone.
•   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.
• Organ transplant patients, particularly lung, or heart-lung transplant recipients, are at relatively high risk of developing pulmonary complications. The major pulmonary complication is bronchiolitis obliterans which can be diagnosed by HRCT quite accurately.

Other miscellaneous conditions where HRCT is a useful diagnostic tool are :

• Lymphangitis carcinomatosa
• Fungal, or other atypical, infections
• Chronic pulmonary vascular disease
• Lymphangioleiomyomatosis
• Sarcoidosis