Cardiac and Pericardiac Imaging

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What forms the borders of the heart on the frontal posteroanterior (PA) or anteroposterior (AP) chest radiograph?

The chest radiograph represents an important first step in the imaging workup of cardiovascular disease. On a PA frontal view of the chest, a cardiothoracic ratio (cardiac width : thoracic width) of less than 1 : 2 is considered normal in adults. Analysis of the cardiac silhouette can yield valuable information about chamber enlargement. The right atrium comprises the right heart border. This border becomes bulbous when the right atrium is enlarged. Superiorly, three convexities (called the “three moguls”) comprise the left heart border, which, from superior to inferior, are the aortic knob, main pulmonary artery, and left atrial appendage. Inferiorly, the left ventricle (lateral wall and apex) comprises the left heart border.

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What forms the borders of the heart on the lateral chest radiograph?

Superiorly, the left atrium comprises the posterior heart border. Inferiorly, the left ventricle comprises the posterior heart border. The right ventricle comprises the anterior heart border; inferiorly, this abuts the posterior aspect of the sternum ( Figure 12-1 ).

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What are the signs of left atrial enlargement on the chest radiograph?

Cardiac chamber enlargement can generally be identified by increased prominence of convexity of the corresponding heart border. In particular, left atrial enlargement causes a posterior bulging of the superoposterior heart border. On the frontal chest radiograph, signs of left atrial enlargement include enlargement of the left atrial appendage, “splaying” of the carina and elevation of the left main stem bronchus, and a “double density” projecting over the central portions of the heart, sometimes extending toward the right ( Figure 12-2 ).

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What are the signs of right ventricular enlargement on the chest radiograph?

On the frontal chest radiograph, right ventricular enlargement can cause rounding of the left heart border (obscuring the normal shadow created by the left ventricle) and uplifting of the cardiac apex. On the lateral chest radiograph, right ventricular enlargement can displace normal retrosternal aerated lung (filling in the “anterior clear space”), although this is an unreliable finding ( Figure 12-3 ).

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What is Eisenmenger syndrome?

Small left-to-right shunts, such as with an atrial septal defect (ASD) or ventricular septal defect (VSD), are commonly asymptomatic early in life and may go undetected. Over time, the right side of the cardiopulmonary circulation responds to the increased volume with pulmonary arterial hyperplasia. This hyperplasia ultimately leads to elevated pulmonary vascular resistance and an increase in right-sided pressures (pulmonary hypertension). When right-sided pressures exceed left-sided pressures, the left-to-right shunt switches to become a right-to-left shunt. Dyspnea, fatigue, and cyanosis develop. This syndrome usually manifests in young adulthood.

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What are the advantages of MRI for imaging the heart?

Magnetic resonance imaging (MRI) does not use ionizing radiation, and therefore may be a more appropriate modality for radiation-sensitive populations such as children and pregnant women. The contrast material used for MRI, which contains gadolinium rather than iodine used for computed tomography (CT), is associated with fewer allergic reactions and a lower incidence of contrast-induced nephropathy. MRI can also be used for functional evaluation, with a temporal resolution that is superior to CT, although not as good as that of echocardiography. Unlike echocardiography, MRI provides more operator-independent image quality, without interference from bone and air, and allows superior visualization of the right ventricle as well as measurement of blood flow. MRI has much higher soft tissue contrast compared to other imaging modalities, allowing for visualization of edema and scar tissue.

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What are the disadvantages of MRI?

MRI examinations of the heart are lengthy, taking up to one hour, compared to minutes for CT. The examination also requires a good deal of patient cooperation with many breath holds of 15 to 20 seconds each. Claustrophobic patients may not be able to undergo the examination. MRI is costly and is generally not performed in patients with pacemakers or defibrillators, which limits its usage for many cardiac diseases. Also, CT has better spatial resolution than magnetic resonance angiography (MRA) in noninvasive coronary artery imaging, allowing for visualization of both calcified and noncalcified plaques as well as luminal obstruction. Furthermore, CT allows for simultaneous assessment of the lung parenchyma, which is more limited on MRI.

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What are the indications for cardiac MRI?

Indications for cardiac MRI presently include evaluation of known or suspected coronary artery disease, cardiac masses and thrombi, pericardial abnormalities, valvular heart disease, cardiomyopathy, congenital cardiac disease, dysrhythmia, and aortic disease.

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What are the contraindications to performing cardiac MRI?

Cardiac MRI is contraindicated in most patients with intracranial or intraocular metal or shrapnel, cochlear implants, most pacemakers or defibrillators, and some older prosthetic valves. Pregnancy is a relative contraindication, although it may still be performed when the benefit to the mother is greater than the unknown risk to the fetus. However, gadolinium-containing contrast material is contraindicated in pregnancy.

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What are the standard planes for cross-sectional cardiac imaging?

The oblique orientation of the heart with respect to the body axis requires special planes of imaging. The four-chamber view of the heart captures all four cardiac chambers in a single image and includes the tricuspid and mitral valves. The short-axis view of the heart is perpendicular to the four-chamber view and allows accurate measurement of septal thickness and ventricular free wall thickness. In the short-axis view, the left ventricle is depicted in cross section and appears almost circular. Standard axial images also are commonly used. Coronal images are sometimes used to evaluate the diaphragmatic surface of the heart ( Figure 12-4 ).

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How is bright blood MRI accomplished, and why is it used?

As its name implies, bright blood imaging is an MRI technique whereby flowing blood has high signal intensity compared with tissue. This technique is accomplished through the use of gradient echo MR imaging. Bright blood imaging techniques are useful because they are fast and have high spatial resolution; they are commonly used for depicting blood vessels, cardiac chambers, and valve leaflets. Turbulent blood flow causes dark “jets” in bright blood images, allowing the detection of valvular stenosis or incompetence.

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How is dark blood MRI accomplished, and why is it used?

Dark blood imaging is an MRI technique whereby the blood signal intensity is suppressed; this can be done with spin echo or inversion recovery imaging. By reducing the signal from blood, myocardial tissue and vessel walls stand out from the background and can be seen in great detail. Dark blood imaging techniques are useful for detailed analysis of myocardial morphology and composition, cardiac anatomy, and vascular anatomy.

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