High-resolution CT

High resolution CT is a scanning protocol in which thin sections (usually 0.625 to 1.25 mm) are acquired and reconstructed using a sharp algorithm (e.g. bone algorithm).

It has been used for:

• lung imaging
• temporal bone imaging

Lung imaging

Two techniques have been used:

• spaced axial (non-helical)
• volumetric HRCT

Spaced Axial

Thin sections are acquired with an interval of 1-2 cm between the two sets of images. It has been considered sufficient to detect abnormalities in diffuse lung diseases. This technique is most useful when single detector CT is being used. The patient dose is less as compared to volumetric imaging.

Volumetric HRCT

Thin sections are acquired continuously using multidetector row CT scanners in a single breath hold. Volume imaging with thinner slices allows detection of a greater degree of pathology and also allow reconstruction in any plane. Patient dose is higher compared to axial imaging.

Fundamental technical protocols

• slice thickness: 0.625-1.25 mm
• scan time: 0.5-1 second
• kV: 120
• mAs: 100-200
• collimation: 1.5-3 mm
• matrix size: 768 x 768 or the largest available
• FOV: 35 cm
• reconstruction algorithm: high spatial frequency
• window: lung window
• patient position: supine (routinely) or prone (if suspected ILD)
• level of inspiration: full inspiration (routinely recommended) expiratory HRCT scans at three or more levels in patients with obstructive lung diseases.

History and etymology

• ''high resolution computed tomography'' term was first used by Todo et al.in 1982. He also described the usefulness of HRCT imaging in pulmonary diseases
• in 1985, Naidich et al., Nakata  Nakata et al. and Zerhouni et al.described the fundamental technique of HRCT and published first report on HRCT