HRCT chest (protocol)

High-resolution CT (HRCT) of the chest, also referred to as HRCT chest or HRCT of the lungs, refers to a CT technique in which thin-slice chest images are obtained and post-processed in a high-spatial-frequency reconstruction algorithm. This technique obtains images with exquisite lung detail, which are ideal for the assessment of diffuse interstitial lung disease. A full HRCT protocol usually includes additional acquisitions:

• expiratory images
• prone images

Indications

HRCT is particularly useful in the assessment of diffuse lung conditions involving the interstitium such as:

• interstitial lung disease
• cystic lung disease
• small airways disease
• pulmonary micronodules
• bronchiectasis

HRCT may be used to evaluate extent of disease, monitor treatment response, and select biopsy sites in the setting of diffuse interstitial lung disease. Uninformed referrals commonly request for a "high-resolution" CT of the chest with the notion that HRCT represents a better version of a "standard" CT chest, however this is a misconception.  

Purpose

HRCT is performed in order to visualize small structures of the lung and detect subtle changes of disease that otherwise may be difficult to assess on conventional chest imaging ³. Prone HRCT imaging is useful in patients with basal disease, eliminating changes due to gravity or dependant atelectasis.

Technique

Sequential spaced acquisition 

HRCT was historically developed to improve spatial resolution in the era of the first-generation scanners (sequential image acquisition technique) and, later, was further used with similar parameters in the second generation scanners (helical scanners). Today, this technique is still used by some institutions, even when scanning in a multidetector row CT scanner (MDCT), as a way to reduce patient exposure to radiation. Also, when going for a full three acquisitions HRCT protocol, the additional expiratory and prone imaging can be performed as sequentially spaced in addition to a volumetric inspiratory acquisition.     

• thin sections defined as <1.5 mm (the original HRCT technique performed in first and second generation scanners, due to hardware limitations, had a thicker definition of thin slices defined as <3.0 mm)
• interval of 1-2 cm between the two sets of images
• the patient radiation dose is less compared to volumetric imaging

Volumetric acquisition 

Nowadays, MDCTs are ubiquitous and, therefore, volumetric thin sections have become standard. Volume imaging with thinner slices allows detection of a greater degree of pathology and also allow reconstruction in any plane.

• thin sections defined as <1.5 mm: usually 1 mm, but ranging between 0.625-1.25 mm
• postprocessing: high-spatial-frequency or sharpening algorithm (bone algorithm)  
  • reduces imaging smoothing 
  • increases spatial resolution: the structures are sharper and show more detail
  • streak artefacts are more apparent 
• the patient radiation dose is higher

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 in patients with obstructive lung diseases

Reducing radiation dose

Lower dose CT protocols may be used in follow-up scans of known or younger patients at the price of slightly lower image quality. Most patients undergo many studies for surveillance of their disease; hence, this can result in significant radiation dose reduction. Methods usually applied for that include:

• lower mAs (~40 mA)
• sequential spaced acquisition
  • commonly applied for the additional expiratory and prone scans 
  • some institutions also apply this technique for the full inspiration scan in patients that undergo imaging follow-up

Findings

• idiopathic interstitial pneumonias: HRCT chest approach

History and etymology

The term ''high-resolution computed tomography'' 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 et al., and Zerhouni et al. described the fundamental technique of HRCT and published the first report on HRCT.