Recognizing Normal Cardiac Anatomy

Starting with conventional radiography, we’ll begin with an assessment of heart size, then describe the normal and abnormal contours of the heart on the frontal radiograph and, finally, discuss the normal anatomy of the heart as seen on computed tomography (CT) and magnetic resonance imaging (MRI).

Case Quiz 3 Question

The image is a frontal chest x-ray of a 52-year-old female with a history of episodes of fainting, angina, and congestive heart failure. Besides the history, there is an abnormality of the cardiac contour that suggests the diagnosis. The explanation appears in this chapter and the answer box is at the end of this chapter.

Evaluating the Heart on Chest Radiographs

Recognizing a Normal-Sized Heart

Important Points

You can estimate the size of the cardiac silhouette on the frontal chest radiograph using the cardiothoracic ratio, which is a measurement of the widest transverse diameter of the heart compared to the widest internal diameter of the rib cage (from inside of rib to inside of rib at the level of the diaphragm) ( Fig. 3.1 ).

In most normal adults at full inspiration , the cardiothoracic ratio is less than 50% . That is, the size of the heart is usually less than half of the internal diameter of the thoracic rib cage.

The Normal Cardiac Contours

The normal cardiac contours comprise a series of bumps and indentations visible on the frontal chest radiograph. They are demonstrated in Fig. 3.2 .

General Principles

As you interpret cardiac abnormalities, no matter what imaging modality is being used, the following principles hold true:
The ventricles respond to obstruction to their outflow by first undergoing hypertrophy rather than dilatation. Therefore, the heart may not appear enlarged at first with lesions like aortic stenosis, coarctation of the aorta, pulmonic stenosis, or systemic hypertension . When the ventricular wall becomes thicker, the lumen actually becomes smaller and it is only when the muscle begins to fail and the heart decompensates that the heart visibly enlarges on chest radiographs.
Cardiomegaly , as recognized on chest radiographs, refers to enlargement of the cardiac silhouette produced by ventricular enlargement , not by isolated enlargement of the atria. For example, the cardiac silhouette usually appears normal in size when there is isolated atrial enlargement, such as left atrial enlargement in early mitral stenosis.
In general, the most marked chamber enlargement will occur from volume overload rather than elevated pressure , so that the largest chambers are usually produced by regurgitant valves rather than stenotic valves . Therefore, the heart will usually be larger as a result of aortic regurgitation than aortic stenosis and the left atrium will usually be larger in mitral regurgitation than mitral stenosis ( Fig. 3.4 ).

Important Points

Key points about the cardiac contours

The ascending aorta should normally not project farther to the right than the right heart border (i.e., the right atrium).
The aortic knob is normally less than 35 mm (measured from the edge of the air-filled trachea) and will normally push the trachea slightly to the right.
The main pulmonary artery segment is usually concave or flat. In younger females it may normally be convex outward.
The normal-sized left atrium does not contribute to the border of the heart on a non-rotated frontal chest radiograph.
An enlarged left atrium and left atrial appendage “fills-in” and straightens the normal concavity just inferior to the main pulmonary artery segment and may sometimes be visible on the right side of the heart as well.
The lower portion of the left heart border is made up of the left ventricle . Remember that the left ventricle is really a posterior ventricle and the right ventricle is an anterior ventricle.
Normally, the descending aorta parallels the spine and is barely visible on the frontal radiograph of the chest. When it becomes tortuous or uncoiled , it swings farther away from the thoracic spine toward the patient’s left ( Fig. 3.3 ).

Evaluating the Heart on Cardiac CT

CT scanning of the heart is done using a fast, multi-slice CT scanner, usually with intravenous iodinated contrast and electrocardiographic (ECG)-gated acquisition to reduce motion artifacts.
Both cardiac CT and cardiac MRI use ECG-gating, which allows for a series of images to be obtained either prospectively or retrospectively only during a specified portion of the cardiac cycle when cardiac motion is at its least, usually during diastole. The images are then parsed together by powerful computer algorithms to produce images that are less degraded by the pulsations of the heart.
Cardiac CT can be used to evaluate the coronary arteries and valves and search for cardiac masses . By reconstructing multiple phases of the cardiac cycle, it is also possible to analyze wall motion and evaluate ejection fraction and myocardial perfusion .
The three standard planes for viewing CT images of the heart are the axial, sagittal, and coronal . Figs. 3.5 to 3.10 demonstrate the major normal CT anatomy of the heart and great vessels.