Technical Aspects of HRCT

Routine Imaging

Transverse images of thin slices of lung (1-1.5 mm thick) are obtained at non-contiguous intervals, usually 1 to 2 cm apart, throughout the whole lung. The computer constructs the images to give high spatial resolution. This process results in images that show fine detail, but only 5 to 10% of the lung is sampled. While this sampling is appropriate for evaluating diffuse lung disease, focal lesions may require more images. (In contrast, in routine CT, slices 3 to 10 mm thick are obtained contiguously, imaging 100% of the lung (see diagram below).)

Images are usually obtained at end-inspiration in a supine patient. Air-trapping can be identified on images taken at the end of forced expiration (see later).

In the supine patient, crowding of dependent lung may obscure findings in the posterior, subpleural lung. In the image of normal lung shown here, note the increased opacity of dependent lung posteriorly (bottom) compared to that anteriorly (top). When more detail of dependent lung is needed, images may be obtained with the patient prone.

Images are usually displayed using "lung windows," as shown here, in which air appears black, aerated lung dark grey, and other structures white.

 

HRCT Sampling

Transverse images of thin slices of lung (1 to 1.5 mm thick) are obtained at non-contiguous intervals, usually 1 to 2 cm apart, throughout.

CT Sampling

In routine CT, slices 3 to 10 mm thick are obtained contiguously, imaging 100% of the lung.

 

Soft Tissue Windows

Images may also be displayed using "soft tissue windows" to evaluate the mediastinal structures, pleural fluid, and calcification. Aerated lung is black; bone and calcium are white; muscle, heart, and large vessels are light grey; and fat is dark grey.

Arrows on the right point to pleural calcifications. Also note a vertebra, ribs, the heart in the midline, and the top of the liver, which appears as a large, oval structure on the right.

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