Standard projections and shadow anatomy
For a comprehensive assessment of cardiac structures, radiography is performed in three basic positions (projections):
- Posteroanterior (PA) projection: a frontal radiograph that allows for the evaluation of transverse dimensions and configuration.
- Right anterior oblique (RAO) projection: the patient is positioned at a 45° angle with the right shoulder toward the detector.
- Left anterior oblique (LAO) projection: the patient is turned at a 45° angle with the left shoulder toward the detector.
In the PA projection, the cardiac shadow is formed by specific arches. The right contour is formed by two arches: the upper (aortic arch) and the lower (right atrium). Of greatest diagnostic significance is the left contour, which normally consists of four arches (from top to bottom):
- First arch: the aortic arch and the origin of its ascending part.
- Second arch: the pulmonary trunk.
- Third arch: the left atrial appendage.
- Fourth arch: the left ventricle.
Formation of contours in oblique projections
For topographical diagnosis, it is necessary to understand which structures form the contour in oblique positions:
- In the right anterior oblique (RAO) projection: the anterior contour of the shadow is formed by the ascending aorta, the pulmonary artery conus, and both ventricles (right and left). The posterior border is formed by the aorta and both atria.
- In the left anterior oblique (LAO) projection: the superior vena cava, ascending aorta, and right heart chambers (atrium and ventricle) are visualized anteriorly. The posterior contour in this position is formed by the descending aorta and the left chambers (atrium and ventricle).
Pathological cardiac configurations
Changes in the geometry of the arches due to chamber dilation (enlargement) lead to the formation of specific silhouettes:
- Mitral configuration: occurs in pulmonary hypertension due to enlargement of the left atrium and dilation of the pulmonary trunk. The radiograph shows elongation and bulging of the 2nd and 3rd arches of the left contour. Visually, this manifests as a smoothing of the angle between the arches (the cardiac "waist" disappears).
- Aortic configuration: forms with simultaneous dilation of the aorta (1st arch) and enlargement of the left ventricle (4th arch). A deep notch forms between them — an "accentuated waist" of the left contour.
- Trapezoidal (triangular) shape: characterized by a uniform enlargement of all chambers and the loss of clear demarcation between the arches. This is typical for diffuse myocardial lesions or the presence of fluid in the pericardial cavity (pericardial effusion).
Signs of enlargement of individual chambers
To objectify the dimensions, the cardiothoracic index (CTI) is used — the ratio of the maximum transverse diameter of the cardiac shadow to the maximum internal transverse diameter of the thorax. A CTI greater than 50% indicates the presence of cardiomegaly.
Enlargement of specific chambers manifests with the following signs:
- Right atrium: displacement of the right border of the cardiac shadow to the right in the PA projection.
- Left atrium: bulging of the left contour at the level of the second arch. To clarify this, barium esophagography in the RAO projection is used: isolated enlargement displaces the esophagus posteriorly along a small-radius arc (4–6 cm), while combined enlargement of the atrium and left ventricle displaces it along a large-radius arc (greater than 6 cm).
- Right ventricle: best visualized in oblique projections by the narrowing of the retrosternal space (between the heart and the sternum).
- Left ventricle: in the PA projection, the lower part of the left contour is displaced outward (to the left).
Limitations of the method and diagnosis of calcification
Radiography clearly reveals wall thickening only when it is accompanied by dilation. In concentric hypertrophy (reduction of chamber volume with wall thickening), the boundaries of the heart do not change. In addition, differential diagnosis can be complicated by the rotation of the heart around its vertical axis, which distorts the projection dimensions.
However, the method is extremely useful for detecting calcification, which is a marker of severe organic pathology:
- Coronary arteries: calcifications indicate pronounced atherosclerosis.
- Aortic valve: calcification occurs in 90% of patients with aortic stenosis (oblique projections are mandatory for visualization, as in the PA projection the valve is obscured by the spine).
- Pericardium: calcium deposits in the leaflets form the so-called "armored heart".
Briefly: answers to topical questions
Which structures form the right contour of the heart in the PA projection?
The right contour of the cardiac shadow in the PA projection is formed by the upper and lower arches, as well as the anatomical structures of the right border.
According to the radiologic picture, there are:
- Upper arch — the aortic arch.
- Lower arch — the right atrium.
Anatomically, the right border of the heart is formed by the superior vena cava and the margin of the right atrium. On the anterior chest wall, it projects from the upper edge of the 2nd rib cartilage (site of attachment to the sternum on the right) to the upper edge of the 3rd rib cartilage, and then runs as an arch from the 3rd to the 5th rib, standing 1–2 cm away from the right sternal margin. With pathological enlargement of the right atrium in the PA projection, a displacement of the right border of the cardiac shadow to the right is noted.
To remember the arches of the left contour from top to bottom in the PA projection, use the mnemonic: Aorta, Pulmonary trunk, Left atrium, Left ventricle (Aorta, Pulmonary trunk, Left atrium, Left ventricle).
Class and exam questions
Why might heart sizes on a radiograph be normal in aortic stenosis?
In aortic stenosis, concentric hypertrophy of the left ventricle often develops. The walls thicken inward toward the cavity, while the external configuration and boundaries of the cardiac shadow remain unchanged.
How to distinguish isolated left atrial enlargement from combined enlargement with the left ventricle?
For this purpose, an image is taken in the right oblique projection with esophageal contrast. Isolated enlargement deflects the esophagus along a small-radius arc (4–6 cm), while combined enlargement deflects it along a large-radius arc (greater than 6 cm).
What does the appearance of a "trapezoidal" cardiac configuration mean?
This is a sign of uniform expansion of all borders and smoothing of the arches. Most often, this picture indicates fluid accumulation in the pericardium (pericardial effusion) or severe diffuse myocardial damage.
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