Chest

Published on 01/04/2015 by admin

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Last modified 01/04/2015

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Chest

The chest X-ray (CXR)

A comprehensive description of the information that can be provided by the CXR requires a textbook all of its own.

Our companion book The Chest X-Ray : A Survival Guide ¹ will assist you to get the very best from this, the commonest radiological investigation in an Emergency Department (ED).

In this chapter we focus on the ten most common clinical questions that are asked of a CXR in the ED.

The standard radiographs

PA CXR. A lateral CXR in selected cases.

Abbreviations

CTR, cardiothoracic ratio;

CXR, chest X-ray;

LLL, left lower lobe;

LUL, left upper lobe;

LVF, left ventricular failure;

ML, middle lobe;

PA, posterior-anterior;

RLL, right lower lobe;

RUL, right upper lobe;

T3, the 3rd thoracic vertebra.

Normal anatomy

Frontal CXR—the lungs

The tricky hidden areas.

The four areas where small (and large) lung lesions are overlooked.

Frontal CXR—lung markings

Normal lung markings are solely due to vessels and to nothing else. The normal bronchial walls, normal interstitium, and normal lymphatics are not visible.

There is a complete absence of any lung markings in the lung immediately adjacent to each costophrenic angle.

In this peripheral part of the lung the normal vessels are simply too small to be visualised.

If this area shows detectable markings, then interstitial disease, or oedema, is likely ¹.

Frontal CXR—hila

The hilum is the site at which the bronchi and vessels enter or leave the lung.

The two hila are approximately the same size and of very similar density.

The hilar shadows are due to pulmonary arteries and pulmonary veins. X marks the main pulmonary trunk.

Blue = pulmonary trunk and pulmonary arteries;

Red = pulmonary veins;

Lilac = part of left atrium.

Frontal CXR—cardiothoracic ratio

Most normal hearts have a cardiothoracic ratio (CTR) of less than 50% when assessed on a PA chest radiograph obtained in full inspiration².

Measuring the CTR.

Two lines are drawn tangential to the outermost aspects of the right and left sides of the heart and the maximum cardiac width is measured.

The transverse diameter of the thorax is measured as the maximum internal width of the thoracic cage (ie inner cortex of rib to inner cortex of rib).

a/b < 50%.

Cardiac silhouette. The left and right borders of the heart are well defined by the surrounding air in the lungs.

Lateral CXR—the lobes of the lung

The two fissures in the right lung divide the lung into three lobes.

The single fissure in the left lung divides the lung into two lobes.

The oblique fissure on each side is propeller shaped and consequently we only, and very occasionally, see a part of a fissure on a normal lateral CXR.

On the other hand, we will see most of the horizontal fissure on the lateral CXR because it is straight. We will often see this fissure on the frontal CXR for the same reason.

Lateral CXR—skeletal shadows

The scapula shadows (blue) are usually evident … but rarely create any difficulties.

The oblique shadows of the ribs are always evident.

The vertebrae are always seen.

Lateral CXR—three normal appearances

A normal lateral CXR will show three normal features relating to the two lungs (which project one over the other):

(1) The vertebral bodies gradually appear blacker as you look from T3 down to T12 …“grey at top; black at bottom”.

(2) Both domes of the diaphragm are clearly visualised. Note that the anterior aspect of one dome (the left dome) fades away because the heart is positioned on this part of the dome.

(3) There is no abrupt change in density over the cardiac shadow. (Of course, any overlying rib shadows need to be discounted).

Analysis: the checklists

The frontal CXR

Four steps underpin accurate analysis.

1. Check whether the CXR is a PA or an AP radiograph and whether it was obtained erect or supine. An AP CXR will magnify the heart shadow; a supine CXR will alter the position of pleural air or pleural fluid.

2. Check whether there is an adequate inspiration. A small inspiration alters the normal appearance of the heart and lung bases.

3. Direct your initial analysis to your primary clinical concern. Is your primary concern: a left sided pneumothorax? A right sided pleural effusion? A coin lodged in the oesophagus?

4. Now you can assess the radiograph in an organised and systematic manner. As follows:

□ Is the heart enlarged?
In an adult, the cardiothoracic ratio (CTR) should be < 50% on a PA CXR (see p. 309).

□ Are both domes of the diaphragm clearly seen and well defined?
If part of a dome is obscured, suspect pathology in the adjacent lower lobe.

□ Are both heart borders clearly seen and well defined?
If not, there is a high probability of pathology in the immediately adjacent lung.

□ Are the hila normal … position, size, density?

□ Check the tricky hidden areas of the lung: both apices, behind the heart shadow, around each hilum, below the diaphragm (see p. 308).

□ Are the bones normal?

□ Finally, ask yourself once again:

Have I addressed this patient’s particular clinical problem?

Normal PA CXR.

▪ Good inspiration.

▪ Heart size normal.

▪ Both domes of the diaphragm well defined.

▪ Both heart borders well defined.

▪ Hila unremarkable.

Is there an adequate inspiration? PA CXRs of the same patient: (a) is a good inspiration; (b) is a very poor inspiration. In (b) the heart appears enlarged and the lung bases hazy with crowding of vessels. These appearances are bogus; they are due to the extremely poor inspiration. CXR (a) is normal.

Are both domes of the diaphragm clearly seen and well defined?

Both domes of the diaphragm will be well defined if the adjacent lung is normal. In this patient, presenting with chest pain and fever, much of the left dome is obscured. This is due to pus in the adjacent alveoli.

Diagnosis: left lower lobe pneumonia.

Are both heart borders clearly seen and well defined?

Both heart borders will be well defined if the adjacent lung is normal. In this patient with a cough and fever the right heart border is obscured because the air in the adjacent lung has been replaced by pus.

Diagnosis: middle lobe pneumonia.