HRCT in Diffuse Lung Diseases - I

Dr. Bhavin JankhariaJankharia Imaging

Techniques and Principles

A Good Quality Study Is An Absolute Must

HRCTTechnique

Most ImportantIn all cases•Breath-hold

A good number of cases turn out to be like this – blurred and then misinterpreted as ground-glass attenuation

HRCTTechnique

Most ImportantIn all cases•Breath-hold

In the same patient with good breath-hold, you can now see some air-trapping, but no interstitial lung disease

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration

So often, the images are in expiration, leading to a spurious diagnosis of ground-glass attenuation as was made in this case

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration

The images were repeated a week or so later. The end-inspiratory images show no significant abnormality

This is another example of the problems that expiratory images can create in interpretation

Insp Exp

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration

This is what expiratory images look like in normal patients – a gradient of increasing whiteness is seen from non-dependent to dependent – this is not

acceptable

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration

What we want is images like this – no gradient, pristine and clear blackness in end-inspiration

There is another way to tell when images are in expiration

The trachea in expiration has a posterior convexity and this helps in picking up expiratory images. Normally, in inspiration, the trachea should be round

or oval

Insp Exp

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration•Expiratory images

In most situations, except in the follow-up of known interstitial lung diseases, an expiratory set is also

required to assess the airways and air-trapping

Insp Exp

The left lower lobe in expiration shows air-trapping, suggesting lobar constrictive bronchiolitis

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration•Expiratory images•1mm or smaller slice thickness

In 16-slice and higher scanners, the current protocol is to do a

volume scan in 2-5 seconds and then retrospectively reconstruct the images as 1mm at 0.5mm

intervals and to review the stack on the workstation

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration•Expiratory images•1mm or smaller slice thickness

ImportantIn selected cases•Prone images

Prone images are required when there are reticular lesions or

opacities only in the dependent portions and we need to

differentiate between true interstitial lung disease and normal gravity-dependent

densities

This 30-years old lady with progressive systemic sclerosis came for an HRCT to rule out interstitial lung disease. Subtle disease (arrows) is seen in the

supine and prone positions

Supine Prone

In this patient the dependent densities (arrow) in supine disappear in the prone position – these are true gravity dependent densities and are of no significance

Supine Prone

Practically, these are the most important parameters to work with

when perfoming HRCT scans

To Repeat

HRCTTechnique

Most ImportantIn all cases•Breath-hold•Full inspiration•Expiratory images•1mm or smaller slice thickness

ImportantIn selected cases•Prone images

HRCTTechnique

Other ParametersThese used to be discussed extensively in the era of conventional scanners, but are not much relevant now

•kV – use the lowest acceptable•mAs – use the lowest acceptable•Scan time – the fastest possible•FoV – irrelevant•Interslice gap – irrelevant•Filming – relevant only where films are still an important means of communication

If providing films is still important, then the filming should be done

such that the pleural margins and ribs are seen with an optimum

grey-scale

Not acceptable

Correct window settings for filming

Please remember that the first step in HRCT interpretation of diffuse lung diseases is a good

quality scan

All possible efforts must be made to obtain high quality scans. The technologists, nurses, etc. should all be trained in making sure that

they understand how to elicit proper breath-hold in end-

inspiration, followed by an end-expiratory set as well

Thank You