Introduction

The heart is a dynamic three‐dimensional structure requiring various imaging modalities with excellent temporal and spatial resolution for the diagnosis, staging, and monitoring of cardiovascular diseases. Current imaging modalities, such as radiography, echocardiography, nuclear scintigraphy, computed tomography (CT), and magnetic resonance imaging (MRI), provide a noninvasive way to assess the cardiovascular system in both a static and dynamic manner.

Radiography is most widely available and provides a practical way to screen for morphologic alterations of the cardiovascular structures and the presence of left‐ and/or right‐sided congestive heart failure. However, the absence of radiographic abnormalities does not rule out underlying cardiac disease. Echocardiography is considered the test of choice to comprehensively evaluate the structure and function of the heart. While echocardiography has many inherent safety and diagnostic advantages, it is limited in its capability of assessing the surrounding tissues within the thoracic cavity. Therefore, a more complete evaluation of the cardiovascular system is routinely achieved by combining radiographic and echocardiographic findings.

Many new technologic advances and developments have provided novel ways to assess the anatomy and function of the cardiovascular system, including new software applications and validated studies for echocardiography, nuclear scintigraphy (positron emission tomography [PET]), CT, and MRI. The use of contrast agents with routine or cross‐sectional imaging modalities aids in the detection of pathology. Original positive contrast selective angiographic techniques with fluoroscopy and digital subtraction angiography (DSA) have been, for the most part, replaced by noninvasive, higher resolution three‐dimensional imaging techniques. However, the use of positive contrast selective angiography with fluoroscopy and/or DSA maintains an important role in the guidance of interventional procedures and will be briefly discussed in this chapter.

Alterations in the movement of blood in the cardiovascular structures (heart, arteries, capillaries, veins) may lead to multiple Roentgen abnormalities that collectively can be used to synthesize an imaging diagnosis. For instance, variations in blood velocity, normal laminar flow, and direction can result in detectable dilation of cardiac chambers and vessels with third spacing of fluid (pleural effusion or pulmonary edema). Subsequently, the systemic delivery of oxygen and nutrients to various tissues may be impaired, resulting in clinical signs. An understanding of the normal three‐dimensional anatomy of the cardiovascular system and dynamics of blood circulation is important in the interpretation of all imaging modalities.

The following sections in this chapter will discuss the imaging characteristics of the normal canine and feline cardiovascular system, as well as abnormal variations for the identification of cardiovascular diseases.

Normal Anatomy

The heart is the driving force (pump) of the circulatory system. Through coordinated extrinsic and intrinsic neurogenic stimuli and muscular contractions, the heart pushes blood through a closed network of blood vessels to provide the body with a continuous supply of oxygen and nutrients. Blood circulates in the four chambers of the heart: the right atrium (atrium dextrum), right ventricle (ventriculus dexter), left atrium (atrium sinistrum), and left ventricle (ventriculus sinister). Deoxygenated blood from the systemic circulation returns to the right atrium via the cranial and caudal vena cava, coronary, and azygous veins. The right atrium pushes blood through the tricuspid valve during atrial systole (ventricular enddiastole) into the right ventricle. During ventricular systole, the right ventricle pushes the deoxygenated blood into the main pulmonary artery through the pulmonary valve. Blood continues through the pulmonary arteries into a fine network of capillary pulmonary beds surrounding the alveoli. Here, blood becomes oxygenated and enters the left atrium via the pulmonary veins. It then moves into the left ventricle through the mitral valve during ventricular diastole. During ventricular systole, the blood is pumped into the systemic and coronary circulation through the aorta. Each atrium has a small blind pouch, called the auricle.

One‐way passage of blood through the different chambers of the heart is controlled by four valves, all synchronized to open and close at distinct intervals during each cardiac cycle. The cardiac cycle and all the details are summarized in Appendix 18.1. The atrioventricular valves allow passage of blood from the atria to the ventricles during diastole and prevent backflow of blood during systole. Small fibrous bands, called the chordae tendineae, arise from papillary muscles along the ventricular wall and extend to the atrioventricular valves to prevent valvular prolapse during systole when the left and right ventricular pressures are increased. The left atrioventricular or mitral valve separates the left ventricle and left atrium. The mitral valve is larger than the right atrioventricular or tricuspid valve (separating the right ventricle and right atrium). As its name indicates, the tricuspid valve is composed of three leaflets, called the septal, parietal, and angular leaflets. In comparison, the mitral valve is bicuspid and includes two leaflets (septal and parietal). Blood exiting the right and left ventricles is controlled by the pulmonary and aortic semilunar valves, respectively. The pulmonary and aortic valves have a similar morphology and consist of three semilunar leaflets. The pulmonary valve has right, left, and septal semilunar leaflets, whereas the aortic valve has right, left, and intermediate (noncoronary) semilunar leaflets. The aortic and pulmonic sinuses represent a focal dilation of the proximal ascending aorta and main pulmonary artery, respectively, at the base of the heart. The aortic sinus contains opening into the right and left coronary arteries (right and left semilunar leaflets, respectively).

The pulmonary valve controls passage of blood from the right ventricle into the main pulmonary artery, which then divides into the left and right pulmonary arteries. The main pulmonary artery contacts the left lateral wall of the more centrally located aortic sinus and ascending aorta (Figure 18.1). A small ligamentous remnant of the ductus arteriosus (fetal embryology), called the ligamentum arteriosum, connects the aorta to the main pulmonary artery at the level of the bifurcation into the left and right pulmonary arteries. The left pulmonary artery is ventral to the left principal bronchus and divides into the left cranial and caudal lobar branches. The left cranial lobar branch further divides into smaller cranial and caudal segmental branches. The right pulmonary artery courses in a left to right direction ventral to the trachea and is ventral to the right principal bronchus and divides into right cranial, middle, and caudal branches. The branches of the caudal lobar pulmonary arteries are dorsolateral to the caudal lobar bronchi, which are then ventromedially bordered by the branches of the caudal lobar pulmonary veins, best seen on a ventrodorsal or dorsoventral projection (Figure 18.2). The cranial lobar arteries have a similar arrangement, respectively consisting of the arterial branch, cranial lobar bronchus, and branch of the pulmonary veins in a craniodorsal to caudoventral direction, best viewed on a lateral radiograph (Figure 18.3). Most of the pulmonary veins have separate openings for oxygenated blood to enter the left atrium. The right and left caudal lobar veins enter the left atrium in a caudodorsal position, with the right cranial vein being more upright. The right and left cranial lobar pulmonary veins enter the left atrium along its craniolateral border on the right and left sides respectively.

Oxygenated blood is distributed into the systemic circulation by the aorta. The aorta is the thickest blood vessel to withstand the higher pressure of blood (120 mmHg during systole; Table 18.1) entering the systemic circulation. The aorta consists of a short ascending portion, the aortic arch, and descending thoracic aorta that continues through the diaphragm as the abdominal aorta. Deoxygenated blood from the pelvic limbs, pelvis, and abdomen is returned to the heart by the caudal vena cava and azygos vein. Deoxygenated blood from the head and thoracic limbs is transported to the heart by the cranial vena cava. The azygos vein also drains the bronchial venous circulation in the right atrium.

Blood to the myocardium is supplied by the coronary arteries. The left and right coronary arteries arise from the supravalvular (sinus) region of the ascending aorta (Figure 18.4). The left coronary artery originates from the left sinus of the aorta and divides into two or, in many cases, three major branches called the circumflex, paraconal interventricular, and septal branches. The circumflex artery is in the coronary groove and can be traced along the caudodorsal aspect of the heart as it extends toward the apex of the heart. The circumflex artery principally supplies the left atrium, auricle, and ventricle. Small branches from the circumflex artery also supply blood to the right atrium. Part of the blood supply to the right and left ventricles is provided by the paraconal interventricular artery which extends ventrally and to the right of the heart in the paraconal interventricular groove. In many dogs and cats, the septal branch originates from the paraconal interventricular artery and runs deep into the interventricular musculature to supply the papillary muscles, as well as most of the interventricular septum. The right coronary artery is shorter than the left coronary artery and originates from the right sinus at the ascending aortic sinus. The right coronary extends along the right side of heart in a cranioventral direction into the coronary groove to supply blood to the right atrium and ventricle. All blood flow into the myocardium extends from an epicardial to endocardial direction and occurs during atrial and ventricular diastole.

The heart is surrounded by a thin fibrous and serous layer called the pericardium. These layers form the pericardial cavity which contains a small amount of physiologic fluid. Surrounding the pericardium is the pericardial mediastinal pleura. Within the mediastinum, the cardiac silhouette is centrally located with its apex positioned near the caudal sternum and cranial diaphragm. The long axis of the heart is slightly obliqued in a craniodorsal to caudoventral direction with the apex being rotated to the left of midline on a ventrodorsal or dorsoventral radiograph (called the normal levocardia position).

Variations in the size and positioning of the cardiac silhouette exist between the different dog and cat breeds. In older cats, the axis of the cardiac silhouette may be positioned in a more horizontal relationship relative to the sternum, also referred to as a “lazy heart” position [1]. Associated with this change in orientation, the aortic arch may be become elongated, redundant or tortuous (Figure 18.5). This change in cardiac silhouette orientation is caused by thickening of the endothelium and the tunica intima and media of the ascending aorta, aortic arch and proximal aspect of the descending aorta as the cat ages.

Radiographic Evaluation

Routine evaluation of the cardiac silhouette consists of three standard radiographic projections: left lateral, right lateral, ventrodorsal or dorsoventral (Figure 18.6). A dorsoventral projection should also be considered as part of the standard radiographic series to increase the conspicuity of the caudal lobar vessels and lesions in the caudodorsal lung lobes, due to magnification of these structures in the dorsal thoracic cavity, as well as better inflation of the caudal lung lobes with the patient in ventral recumbency.

Overall, straight patient positioning is essential for recognition of the normal cardiac anatomy. When possible, radiographs should be acquired before echocardiography to avoid atelectasis from prolonged lateral recumbency and wet‐hair artifact due to the application of acoustic gel or alcohol. Additionally, for appropriate evaluation of the ventrodorsal and dorsoventral projections, the positioning must be such that the sternum overlaps and is summated with the thoracic vertebrae and spinous processes. To reduce the superimposition of the appendicular musculature with the cranial thoracic region, the thoracic limbs should be pulled cranially, best with both thoracic limbs taped together (Figure 18.7).

Due to inherent high contrast within the thoracic cavity associated with the presence of air within the lungs, thoracic radiographs should be obtained using a high peak kilovoltage (kVp) and low milliampere‐second (mAs) technique. To maximize contrast and decrease atelectasis, the radiographic exposure should be made at peak inspiration. Due to border effacement of the myocardium with blood within the cardiac chambers and fluid within the pericardial space, interpretation of the heart relies on the recognition of morphologic changes to the size and shape of the cardiac silhouette. On the dorsoventral projection, there is increased contact of the cupula of the diaphragm with the apex of the cardiac silhouette, and the cardiac silhouette has a more upright position with a rounded appearance (Figure 18.8). The normal cardiac silhouette has a smooth contour or margin, and any alterations to the cardiac silhouette can be localized to specific cardiac chambers or the great vessels in the dog based on their relative positioning. On the ventrodorsal/dorsoventral projections, the clock face analogy is commonly used to illustrate the location of the different cardiac chambers in relation to the peripheral margins of the heart (Figure 18.9; Table 18.2). A similar approach can be applied to the cardiac silhouette on the lateral projections (Figure 18.10). This approach is not as readily applicable to the anatomic description of the feline cardiac silhouette.

The size of the cardiac silhouette in the absence of pathology varies greatly between different dog breeds [1–7]. Dogs with a barrel‐shaped chest may have a cardiac silhouette with a larger appearance than dogs with a narrow deep‐shaped chest. In large‐breed dogs, the cardiac silhouette may have an elongated contour on lateral radiographs and a rounded appearance on ventrodorsal or dorsoventral radiographs. As a subjective assessment of the cardiac silhouette size, the cranial to caudal length of the cardiac silhouette should not exceed 3.5 intercostal spaces on lateral projections. In addition, the height of the cardiac silhouette should not exceed two‐thirds of the height of the thoracic cavity on lateral projection. The width of the cardiac silhouette should not exceed one‐half the width of the thoracic cavity on the ventrodorsal or dorsoventral projection.

A more objective assessment of the cardiac silhouette uses the vertebral heart size or score (VHS) measurement, with some breed conformation variations having been reported. Using the VHS method, the long axis of the cardiac silhouette is measured from the ventral border of the carina to the apex of the cardiac silhouette at its longest distance on right lateral radiograph. The short axis is next measured at the widest point of the cardiac silhouette, perpendicular to the long axis. Both measurements are then individually scaled to the thoracic vertebral column, starting at the cranial endplate of T4. The number of vertebral bodies corresponding to the length of the short and long axes of the cardiac silhouette are summed together to provide the VHS (Figure 18.11). Out of 100 dogs with normal hearts, 98% of population had VHS less than 10.5 (9.7 ± 0.5).

The VHS of various dog breeds have been described and are listed in Table 18.3. This value may represent a clinically relevant maximum VHS to identify cardiomegaly in various dog breeds. However, a normal value does not exclude the possibility of cardiac abnormalities. A similar measurement method has been proposed in cats. In another study, a mean ± SD vertebral heart score of 7.5 ± 0.3 was measured in 100 healthy cats. In the same study, the long axis of the cardiac silhouette was scaled to the sternum, starting at the cranial endplate of S2, and equaled the length of three sternebrae. On right lateral and ventrodorsal radiographs, the length of the manubrium has been correlated to the size of the cardiac silhouette in dogs by expressing the sum of the cardiac short axis length and long axis length as a ratio over the length of the manubrium in both small‐breed (≤12 kg) and large‐breed (≥16 kg) dogs. In large‐breed dogs, ratios of 4.8 ± 0.5 and 5.4 ± 0.6 were measured on right lateral and ventrodorsal projections, respectively. In comparison, ratios of 5.3 ± 0.8 and 5.3 ± 0.9 were measured in small‐breed dogs on right lateral and ventrodorsal projections, respectively.

Fewer variations in heart size exist between breed of cats. However, obese patients may deposit fat around their pericardium, resulting in a nonpathologic increase in size of the cardiac silhouette, particularly on the ventrodorsal/dorsoventral views. The normal feline heart is ovoid to almond shaped and subjectively measures between 2 and 2.5 intercostal spaces in width on lateral projections. On ventrodorsal and dorsoventral projections, the cardiac silhouette should be less than 50% of the overall thoracic width.

Evaluation of the cranial and caudal lobar branches of the pulmonary arteries and veins is made by assessing their relative size and shape. Normal lobar pulmonary veins and arteries follow a generally straight path as they gradually taper in the periphery of the lung field. The size of the lobar pulmonary vessels may be assessed by comparing cranial lobar vessel diameter to the width of the fourth rib on lateral projections and caudal lobar vessels dimensions to the width of the ninth rib on ventrodorsal or dorsoventral projection. Normal lobar pulmonary arteries and veins should be approximately the same size as the rib used as a reference. Additionally, pulmonary lobar arteries and veins associated with a particular lung lobe should be similar in size (Figure 18.12).

Echocardiography

There are many excellent textbooks and articles related to veterinary echocardiography. In this text, a summary will be presented with the intention of providing a better understanding of the three‐dimensional nature of the cardiac silhouette and anatomic relationships between the cardiac structures. Two‐dimensional echocardiography is used for a dynamic structural and functional evaluation of the heart related to the electrocardiogram. A complete evaluation of the cardiovascular structures includes multiple standard echocardiographic views with the patient positioned in right or left lateral recumbency, while imaging the heart from the patient’s recumbent side (right or left parasternal locations, respectively). Routine echocardiographic views are listed in Table 18.4.

Time‐motion mode (M‐mode) echocardiography allows the continuous display of an “icepick” view through the heart such that structures within that view are assessed over time. Using M‐mode, the ventricular size can be measured during systole (LVIDs) and diastole (LVIDd) at the level of the chordae tendineae in dogs. From these measurements, the systolic function of the left ventricle can be extrapolated from a measurement known as fractional shortening (%), calculated as follows: ([LVIDd – LVIDs]/LVIDd) × 100. Normal fractional shortening values are 25–45% in dogs and 30–55% in cats. The disproportionate shape of the right ventricle limits reliable chamber measurements and is estimated to be one‐third the dimension of the left ventricle. Additionally, the interventricular septum (IVS) thickness can be measured in systole (IVSs) and diastole (IVSd), as well as the left ventricular free wall thickness in systole (LVWs) and diastole (LVWd). To assess the size of the left atrium, the diameters of the aorta and the left atrium are measured from the right‐sided short‐axis view at the level of the aortic cusps during early diastole (Figure 18.13). Calculation of the left atrium to aorta (LA:Ao) ratio may suggest left atrium enlargement is greater than 1.5. The size of the right atrium should be similar to the size of the left atrium.

Color flow Doppler is used to determine the direction and velocity of blood flow using a color‐coded map. The characteristic of the color signal can suggest normal laminar or turbulent blood flow based on the pulse‐repetition frequency (PRF) or the velocity scale. Quantitative measurements of blood velocities over time can be acquired using spectral Doppler. These velocity measurements are routinely measured at the mitral, tricuspid, pulmonic, and aortic valves (all velocities should be 1 m/s with the aortic valve outflow velocity being up to 1.6 m/s) and can be used to measure the pressure gradient between the chambers it separates using the modified Bernoulli equation (pressure gradient = 4 × [maximum or peak velocity][2]).

Alternative Imaging Modalities

The use of nuclear medicine to diagnose cardiac disease is small animals remains limited. Nuclear scintigraphy uses an intravenously administered radiopharmaceutical to trace blood flow through the cardiovascular structures to localize anatomic abnormalities, identify ventilation and pulmonary perfusion abnormalities, quantitate function, or assess for collateral blood flow. The combination of positron emission tomography with computed tomography (PET‐CT) provides excellent three‐dimensional spatial and temporal resolution for the localization of radiopharmaceutical uptake of PET metabolic radiotracers within the myocardium.

The greatest imaging advances have been in the development of new techniques and software in cardiac and vascular imaging with CT and MRI. Contrast agents can be used with CT and MRI to increase the conspicuity of blood flow within the cardiovascular structures. Cardiac‐gated MRI reduces motion artifacts with the continuous acquisition of a single slice over the duration of the cardiac cycle (Figure 18.14). The function and dynamic morphology of the heart can be assessed. The function of the heart may be evaluated with fluoroscopy and can be used to guide interventional procedures. The conspicuity of the cardiac chambers and great vessels is increased with intravenous or arterial catheterization for targeted injection of positive contrast medium at specific sites, also referred as selective angiography (Figure 18.15).