Overview of Basic HRCT Technique

High-resolution CT, also called thin-section CT, uses thin slice thickness and high-frequency reconstruction algorithms to create detailed images of the lungs.¹ HRCT is most often performed on a multidetector CT scanner using volumetric rather than axial incremental acquisition,¹ which captures the entire chest in near isotropic resolution.² A benefit of this type of volumetric HRCT is that the raw data can be utilized to reconstruct thin section images in the axial as well as other planes of the body, including the sagittal and coronal aspects.¹ It should be noted, however, that volumetric HRCT using conventional scanners and protocols can expose the patient to higher levels of radiation than gapped HRCT techniques.³ It is possible to use low-dose acquisition protocols to minimize the patient’s exposure.²

Blood, blood vessels, air, and lung tissue affect the physical density of the lung as it appears on CT.⁴ The components of the lung are not static nor are they distributed evenly throughout.⁴ Their distribution is affected by gravity, the degree of lung inflation, and the physical structure of the lung, which may vary between patients.⁴ When performing imaging, the area of the lung most affected by gravity is termed the dependent portion. For example, when standing upright, the dependent portion of the lung would be the bases as shown in the image to the left. When lying prone, the dependent portion would be the dorsal aspect of the lungs, as shown in the image to the right.⁴ Perfusion favors the dependent portion of the lung, while ventilation favors the non-dependent portions, which in turn affects lung density.