High-resolution CT

High resolution CT is a scanning protocol in which thin sections (usually 0.625 to 1.25 mm) are aquired 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 aquired with an interval of 1 to 2 cms 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 aquired continuously using multidetector row CT scanners and in a single breath hold. It allows detection of all the abnormalities and has been considered easier and better for interpretation by providing the ability for coronal and sagital reformations. The patient dose though is higher as compared to axial imaging.

Fundamental technical protocols

• slice thickness: - 0 0.625-1.25 mm
• scan time:- 0 0.5-1 second
• kV: - 120
• mAs:- 100 100-200
• collimation: - 1 1.5-3 mm
• matrix size: - 768 768 x 768 or the largest available
• FOV: - 35 35 cm
• reconstruction algorithm: - high high spatial frequency
• window:- lung lung window
• patient position:- supine supine (routinely) or prone (if suspected ILD)
• level of inspiration: - full full inspiration ( routinely recommended) expiratory hrct scans at three or more levels in patients with obstructive lung diseases.