Written by: Elliott R. Carthy BMedSc (Hons)
School of Medicine, Imperial College London, UK
This text aims to cover the basic cardiac pathologies visible on a chest radiograph (CXR), and their associated diagnostic features. This will include not only the signs of gross pathologies such as cardiomegaly, but also those of the aortic and pulmonary vasculature.
Figure 1 demonstrates the basic cardiac contours of the heart as visible on a standard frontal chest radiograph.
Figure 1: Cardiac contours on a normal frontal chest radiograph. Image courtesy of Gaillard, Radiopaedia.org.
The cardiothoracic ratio is the maximum transverse diameter of the heart visible on the CXR, divided by the total width of the thoracic cage (inside of rib to inside of rib). Normally, the cardiothoracic ratio is <50%. Greater than 50% can be indicative of cardiomegaly, secondary to pathologies such as dilated or hypertrophic cardiomyopathy.
Figure 2: Cardiomegaly as shown by a raised cardiothoracic ratio. Image courtesy of Gaillard and Radswiki et al., Radiopaedia.org.
However, some individuals can have what appears to be cardiomegaly on the CXR yet have a structurally normal heart. For example, given the manner in which they are taken, anterior-posterior (AP) films are prone to falsely magnifying mediastinal structures such as the heart. It is therefore practice that the cardiac size cannot be accurately assessed on an AP film. Abnormalities of the cage wall as well as obesity, ascites and pregnancy can lead to a falsely enlarged heart on the CXR and should be interpreted with caution, in conjunction with clinical examination.
The aorta can be broadly divided into the ascending and descending aorta, each of which are associated with their own demarcations and own abnormalities identifiable on a CXR.
Figure 3: Normal ascending and descending aorta and the aortic arch. Image adapted from Gaillard, Radiopaedia.org.
The ascending aorta is the first right-sided contour of the heart on a CXR, visible as a straight edge just lateral to the trachea that should not deviate beyond the right heart border. The ascending aorta can either be smaller or larger than normal depending on the pressure encountered in the lumen. A small aorta is associated with an atrial septal defect, whereas enlarged ascending aorta reflects an increase in the pressure of flow such as in aortic stenosis or aortic regurgitation.
The descending aorta is usually barely visible on a normal CXR, through the heart and to the left of the thoracic spine. The descending aorta may appear tortuous or enlarged if atherosclerotic or in a hypertensive patient.
The aortic knob is the first left sided contour of the heart, comprised of a portion of the aortic arch and a proximal segment of descending aorta. The aortic knob should measure <35mm from the lateral border of the trachea to the most lateral border of the aortic knob. This structure may dilatate in response to increased pressure or reduced elasticity, such as in hypertensive patient or due to an aortic dissection (Fig. 4).
Figure 4: Thoracic aortic dissection. There is a prominent aortic arch and descending aorta, with overall widening of the mediastinum, consistent with a vascular event such as an aortic dissection. Image courtesy of the Royal Melbourne Hospital Respiratory, Radiopaedia.org.
The contour above that of the left ventricle is representative of the left atrium, shown as a slight indentation. Left atrial enlargement can cause subsequent straightening of this indentation, which with any left atrial enlargement may even cause convexity (for example in mitral stenosis).
The right atrium comprises the right heart border, and any enlargement is generally associated with hypertrophic changes of the right ventricle.
Intra-cardiac masses such as an atrial myxoma may manifest themselves via chamber enlargement. However, they are best visualsied on alternative imaging modalities such as echocardiography and computed tomography.
Figure 5: Left atrial enlargement. Image courtesy of Jones & Gaillard, Radiopaedia.org.
Below the aortic knob, the next left sided cardiac contour is that of the pulmonary trunk. Enlargement of the trunk may be secondary to pulmonary hypertension. Enlargement can be established by measuring its distance from what is known as the “tangent line” – a straight line drawn from the aortic knob to the apex of the left ventricle. Measuring the perpendicular distance between this line and the pulmonary trunk should be 0-15mm.
Pulmonary hypertension may manifest itself on the CXR by the pulmonary trunk distended past the tangent line. It is a condition that is very difficult to manage, with multiple aetiologies such as left heart failure, hypoxic lung disease and connective tissue disorders. The consequences include right ventricular failure with subsequent hepatosplenomegaly and peripheral oedema, and ultimately a reduced cardiac output and impaired end organ perfusion. Suspected pulmonary hypertension should be further invetsigated with other imaging modalities such as echocaridography and computed tomography.
An increase in the distance (>15mm) could be due to congenital hypoplasia or absence of the pulmonary trunk, or due to an enlargement of the left ventricle and/or aortic knob pushing the tangent line away from the pulmonary trunk.
Figure 6: Tangent line (white line) and the perpendicular measurement between the tangent line and pulmonary trunk to assess for enlargement. Image adapted from Gaillard, Radiopaedia.org.
The left ventricle makes up the last left cardiac contour. Enlargement of the left ventricle is seen with an enlarged cardiothoracic ratio and aorta. There are numerous causes of left ventricular hypertrophy including heart failure, hypertension and valvular disease.
Right ventricular hypertrophy is seen when the cardiothoracic ratio and pulmonary trunks are both enlarged. There is no cardiac contour for the right ventricle, which lies anteriorly within the mediastinum, and right ventricular hypertrophy is therefore difficult to determine on the CXR.
In acute pericarditis, the CXR is often normal. However, if there is a large fluid collection (pericardial effusion; >200mL) then this can manifest itself as cardiomegaly (see previous section). Although in many cases the CXR may be completely normal. In general, the CXR is not specific nor is it accurate enough in the diagnosis of pericardial disease including that of tamponade, and alternative imaging modalities should be explored such as echocardiography.
The CXR is not typically used to diagnose calcification of the coronary arteries. However, it can show the end organ consequences of poor metabolic supply such as ventricular hypertrophy associated with heart failure. For a more precise visualisation and quantification of calcific atheromas in the coronary vasculature, computed tomography scans are preferred to help in risk assessment for individual patients.
It is controverisal as to what the use vs. benefit is of routine lateral CXRs. In terms of cardiac pathology, lateral CXRs may be useful with the loss of the cardiac silhouette sign - the loss of normal borders between thoracic sturctures. For example, lobar pneumonia can lead to a loss of the cardiac silhouette, with the right lower lobe best visualised on the lateral CXR. Retrocardiac abnormalities can also be an indication for a lateral CXR, such as with an opacity in the left lower lobe.
Information obtained from: Herring, W. Learning Radiology: recognising the basics. 2007. Mosby Inc., USA.
Gaillard, F. Normal chest x-ray, Radiopaedia.org.
Jones, J. & Gaillard, F. Left atrial enlargement, Radiopaedia.org.
Gaillard, F. Cardiomediastinal outlines on chest x-ray, Radiopaedia.org.
Gaillard, F. & Radswiki et al. Cardiomegaly, Raidopaedia.org.
Royal Melbourne Hospital Respiratory. Thoracic aortic dissection – Type B. Radiopaedia.org.
Sharma, DK. 2011. Atrial Myxoma, Medscape Reference
Acute Pericarditis, Patient.co.uk.
Gabet et al., 2005. Lateral chest X-ray for physicians, JAMA. 98(7); 310-112.
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