High resolution computed tomography (HRCT) and new perspectives in functional radiology of the lung

Computed Tomography (CT) with the advent of new techniques as high resolution computed tomography (HRTC) and spiral CT with 3D reconstructions (3D CT) allows a new morphologic-qualitative as well as functional-quantitative evaluation of pulmonary perfusion and ventilation. HRCT allows the identification of secondary lobule and a detailed morphologic and comparative analysis of minute distal anatomical structures with combined densitometric evaluation of lung parenchyma on perfusion. In particular, a new, more specific significance could be attributed to changes in density of lung parenchyma (mosaic pattern) with associated assessment of the vessel number, caliber and distribution, and a comparative evaluation of vessels and density between healthy and impaired areas. The "optical" HRCT evaluation on serial axial scans in inspiration and expiration allows the functional assessment of compartments which require spirometry and tests of respiratory function. Spiral CT allows volumetric acquisitions in a single breath which can be reconstructed and processed according to single requirements. 3D tailored reconstruction of spiral CT exam in maximum inspiration and expiration with a dedicated densitometric window (-1024/+ 100 HU) allows the calculation of total lung volume (TLV), of both lungs, of a single lung or selected sections. With the "air" densitometric window (-1024/-400 HU) the total lung capacity (TLC) and residual volume (RV) are calculated. The ratio between these values and the corresponding TLV represents the lung aeration index (LAI). 3D reconstruction with fixed densitometric value corresponding to median air density (peak of histogram) allows the scintigraphic-like "alveolographic" reconstruction of lung ventilation. Combined 3D CT and HRCT evaluation possibly from a single spiral CT exam, is used in the morphologic-functional diagnosis of respiratory pathophysiology.