Chapter 143 Cardiovascular Radiography

Thoracic radiography and echocardiography are useful in the recognition and assessment of cardiovascular diseases. Other imaging modalities, such as cardiac magnetic resonance imaging, gated computerized tomography, and radionuclide studies, are uncommonly used in routine cardiology practice and have limited indications at this time. This chapter emphasizes the use of radiography in cardiac diagnosis.^*

When combined with the results of the clinical examination, thoracic radiographs may contribute to the specific cardiac diagnosis, verify the presence of congestive heart failure (CHF), and assist in the differential diagnosis of respiratory signs such as coughing and dyspnea. Analysis of thoracic radiographs in the diagnosis of respiratory diseases is discussed in more detail in Chapter 159. Essential principles in the application of echocardiography are considered at the end of this chapter. More specific applications of radiography and echocardiography are discussed in other chapters throughout this section.

GENERAL PRINCIPLES OF CARDIAC RADIOGRAPHY

Key Point

The importance of the technical quality of thoracic radiographs is often underestimated. A radiograph with inadequate penetration, exposure during expiration, or poor patient positioning can be very misleading.

Technical errors can be avoided by addressing the following technical points prior to radiographic evaluation for cardiac disease.

Technical Considerations

• The practice should develop routine protocols for thoracic radiography of the cardiac patient.

• At a minimum, two views, a right or left lateral projection and either a dorsoventral (DV) or a ventrodorsal (VD) projection, are needed for thoracic evaluation.

• In general, a DV and right lateral projection are usually recommended for cardiac evaluation.

• In some situations, both right and left lateral projections and VD and DV views are obtained.

• Radiographic technique (see Chapter 4):

• Expose films at the peak of inspiration.

• Remember that expiratory films are ineffective for evaluating the lung parenchyma for interstitial pulmonary edema.

• A long gray-scale (high kilovolt peak, low milliampere-second) technique is most helpful for assessing thoracic disease. Underexposure mimics pulmonary disease by increasing apparent lung density, whereas overexposure leads to an underestimation of the severity of pulmonary changes.

• Rotation often accentuates normal structures such as the main pulmonary artery on the VD projection or the left atrium or right ventricle on the lateral projection.

• False apex shifting due to patient rotation on the VD projection may cause overestimation of ventricular size.

• Consistent positioning and exposure techniques should be used when obtaining follow-up radiographs. This is particularly important when following patients with CHF.

Normal Lateral Radiograph (Fig. 143-1)

Dogs

The following are guidelines for understanding the normal canine lateral thoracic radiograph:

Figure 143-1 Three lateral views of the normal heart showing chamber location.

• The cardiac outline is roughly tear-shaped with a narrowed, ventral apex.

• Orientation—The long axis creates approximately a 45-degree angle with the sternum (varies with breed).

• Location—The cardiac silhouette extends from the third to sixth ribs, and the heart and diaphragm usually touch or overlap.

• Size—Ventricles occupy approximately three intercostal spaces. Using the vertebral score method (discussed below) the vertebral heart score (VHS) ranges from 8.5 to 10.6 (mean, 9.7), although studies in specific breeds have demonstrated breed-specific variation.

• The cardiac borders on the lateral projection represent the following:

• Cranial—Rounded border, right ventricle and auricle

• Caudal—Straight border, left ventricle and atrium

• Dorsal—Both atria, pulmonary arteries, cranial and caudal vena cava, and aorta

• Quadrant—The cardiac silhouette can be divided into sections on the lateral radiograph by drawing a line from the tracheal bifurcation to the apex parallel to the cardiac axis and a second horizontal line at the level of the vena cava.

• Atria are dorsal to the horizontal line.

• Ventricles are ventral to the horizontal line.

• Right-sided heart structures make up the cranial three-fifths of the heart shadow (remember that the two ventricles overlap owing to the curved ventricular septum).

• Left-sided heart structures make up the caudal two-fifths of the heart shadow.

• There are some differences between right and left lateral films. Importantly, the left atrial border is more prominent and the cardiac apex appears slightly more elevated on left lateral films.

Cats

The following are guidelines for understanding the normal feline lateral thoracic radiograph:

• The cardiac outline is more elongated and elliptical than that in the dog.

• Cardiac borders are similar to those in the dog.

• Orientation is more variable than that in the dog, with a more horizontal major cardiac axis.

• Location—The heart and diaphragm are separated by one to two intercostal spaces.

• Size—The ventricles occupy approximately 2 to 2.5 intercostal spaces and the heart extends about two-thirds of the VD height of the thorax. The VHS is typically 8.5 to 9.0 in cats.

Normal Ventrodorsal or Dorsoventral Radiograph (Fig. 143-2)

Dogs

The following are guidelines for understanding the normal canine VD or DV thoracic radiograph:

Figure 143-2 Four ventrodorsal views of the normal heart showing chamber location.

• The cardiac outline has a curved (convex) right border and a relatively straight left border.

• Orientation—The apex is slightly to the left of midline, approximately at a 30-degree angle with the spine. The cardiac apex appears slightly closer to the midline on the DV as opposed to the VD projection.

• Location—The cardiac silhouette extends from the third to the eighth vertebrae, often overlapping the diaphragm.

• Size—The cardiac-thoracic index is about 60% to 65% of the thorax as measured from right to left.

• Cardiac borders (using the clock face analogy):

• 11 to 1 o’clock—Aortic arch (seen better on the DV projection)

• 1 to 2 o’clock—Main pulmonary artery

• 2 to 3 o’clock—Left auricular appendage

• 3 to 6 o’clock—Left ventricle (includes the apex)

• 6 to 9 o’clock—Right ventricle

• 9 to 11 o’clock—Right atrium

Cats

The following are guidelines for understanding the normal feline VD or DV thoracic radiograph:

• The cardiac outline is slightly more elongated than that in the dog.

• Orientation—The apex is at or slightly to the left of midline.

• Location—The heart and diaphragm are separated by one to two intercostal spaces.

• Cardiac borders are similar to those in the dog, except that the main pulmonary artery does not contribute to the 1- to 2-o’clock border.

Normal Pulmonary Arteries and Veins

Location and Size

• The cranial lobe artery is dorsal to the bronchus which is dorsal to the vein on the lateral view.

• Arteries are lateral to the bronchus and vein, and originate cranial to the carina on the VD or DV view.

• Veins terminate in the left atrium caudal to the carina (veins are ventral and central).

• Vascular markings are often very prominent in healthy, deep-chested dogs, as well as in cats with hyperinflated lungs.

• Arteries and veins are approximately equal in size.

• Overlapping or superimposition of vessels from both cranial lung lobes can create a misleading appearance regarding vessel size on the lateral view.

• Normal, average vessel-to-rib ratio on the lateral projection is about 0.75 when comparing the artery with the proximal one-third of the fourth rib. However, the range of normal values is quite expansive (0.5–1.25).

• Pulmonary vessels on the VD or DV view should not exceed the width of the ninth rib at the point of intersection. Generally, this intersection creates a “square” as opposed to a rectangle.

• The caudal lobe arteries and veins appear larger on DV views because of distance from the cassette and subsequent magnification. In obese cats, magnification can be substantial and lead to overinterpretation of true vascular size.

Appearance

• Vessels are best seen on the lateral view as they extend into the cranial lobes.

• Vessels in the lung farthest from the cassette are magnified.

• Arteries are denser than veins, are slightly curved, and have dichotomous branching (branches of equal size).

• Veins are less visible, are straighter and blunter than arteries, and have branches of unequal size.

INTERPRETING THORACIC RADIOGRAPHS IN CARDIAC DISEASE

Once an adequately positioned and exposed radiograph has been obtained, the radiographs must be interpreted in a systematic manner. Optimally, the films would be assessed without knowledge of the patient and then reassessed after reading the signalment, clinical history, and examination findings. In most instances this is impractical, but it is clear that including the clinical findings will both assist the evaluation (by providing a clinical perspective) and hinder an objective assessment (by prejudicing the reader with the clinical diagnosis).

Initial Approach to Interpretation

The following general questions should be addressed during the radiographic assessment of cardiac disease:

• Are the radiographs of sufficient technical quality? Is the positioning straight, is the exposure appropriate, and was the film exposed during inspiration?

• Is cardiomegaly present or absent? If present, is this cardiac enlargement mild, moderate, or severe?

• Species and conformational variations are important in this assessment (e.g., barrel-chested dogs normally have wider and more rounded hearts, and the cardiac size appears large relative to the thoracic volume, whereas deep-chested dogs have a narrower, more vertically oriented heart shadow).

• Owing to marked breed variation, the subjective appearance of heart size may differ between projections. For example, cardiomegaly may be suspected on one view but may not be confirmed on the complementary projection.

• Concentric ventricular hypertrophy may be underestimated by cardiac size as viewed by radiography.

• It may be useful to calculate a VHS in dogs, particularly in cases of equivocal cardiomegaly. This score is especially helpful in toy-breed dogs, which often appear to have cardiomegaly by subjective evaluation. The technique is described more fully below.

• Which specific cardiac chambers are abnormal? Is more than one cardiac chamber affected? Is the abnormality restricted to one side of the heart, or is it generalized? (See below and Figs. 143-3–143-6.)

• Is the cardiac apex in its normal position? If not, is this shift due to poor patient positioning or true cardiomegaly?

• Left heart enlargement may shift the apex to the right (typical of cats) or to the left. Typically, the heart will also be elongated in the left heart enlargement.

• Right-sided enlargement may shift the apex to the left but in general does not elongate the heart.

• Rotation of the heart and shifting of the apex often leads to overestimation of the size of other cardiac chambers. Evaluating the heart on both views may prevent erroneous conclusions in the case of apex shifting.

• Are the great vessels (aorta and main pulmonary artery) normal or abnormal in appearance?

• Are the pulmonary vessels normal, small, or enlarged? (Also see Chapter 159.) If enlarged, are changes restricted to the arteries (e.g., heartworm disease) or veins (e.g., left heart failure), or are both altered in size (e.g., pulmonary undercirculation or overcirculation)?

• Is the radiographic opacity of the lung normal, decreased, or increased? Are areas of increased opacity generalized or focal? If diffuse, what is the distribution of the pulmonary infiltrates (generalized, dorsal-caudal, or cranial-ventral)? What is the radiographic pattern of abnormal density? (See Chapter 159.)

• Is there pleural effusion?

• Pleural effusion can stem from CHF or a noncardiogenic origin.

• Thoracocentesis is necessary to determine the type of fluid present (see Chapters 3 and 164).

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Disorders of Muscle and Neuromuscular Junction
2. Diseases of the Adrenal Gland
3. Respiratory Infections
4. Principles of Oncology

Stay updated, free articles. Join our Telegram channel

Join