Elizabeth Huynh VCA West Coast Specialty and Emergency Animal Hospital, Fountain Valley, CA, USA The left and right adrenal glands are located in the cranial retroperitoneal space, ventral to T13 through L2 vertebrae, and are closely associated with their respective kidneys, aorta, and caudal vena cava [1]. The left adrenal gland is made up of cranial and caudal poles and the right adrenal gland is made up of cranial and caudal aspects. The phrenicoabdominal vein splits the adrenal glands into cranial and caudal halves. The left adrenal gland is located craniomedial to the cranial pole of the left kidney, lateral to the aorta, caudal to the celiac and cranial mesenteric arteries, dorsal to the phrenicoabdominal vein, and ventral to the phrenicoabdominal artery. The right adrenal gland is located further cranial than the left, craniomedial to the cranial pole of the right kidney, lateral to the caudal vena cava, dorsal to the phrenicoabdominal vein, and ventral to the phrenicoabdominal artery. In cats, the right adrenal gland can be found within the renal fossa of the caudate process of the caudate lobe of the liver [1]. Normal adrenal glands cannot be visualized on radiographs in dogs and cats. Ultrasonography and computed tomography are preferred modalities to evaluate the adrenal glands. Magnetic resonance imaging is another modality less commonly used to evaluate the adrenal glands. Radiographically, normal adrenal glands in dogs and cats cannot be visualized unless they are associated with a mass effect and/or mineralized. In some geriatric cats, incidental, likely clinically insignificant, adrenal gland mineralization (Figure 28.1) can be seen. Normal adrenal glands are soft tissue opaque and, therefore, are border effaced with the adjacent liver, surrounding musculature, and any other superimposed soft tissue structures. FIGURE 28.1 Left lateral (A) and ventrodorsal (B) abdominal radiographs of a geriatric cat with incidental adrenal gland mineralization (arrows). Ultrasonography is a preferred method of assessing the adrenal glands and entails real‐time scanning in parasagittal, dorsal, or oblique and transverse plane to depict the long and short axes of the gland. The highest frequency transducer that allows adequate penetration should be used. A 5 MHz transducer is usually required for larger dogs, although compression of the ventral abdominal wall with the transducer may allow use of a 7.5 MHz probe in compliant patients. Most animals are scanned in dorsal recumbency, after the hair is clipped and the skin is moistened with alcohol and acoustic coupling gel. A ventral abdominal or flank approach may be used. The left adrenal gland is identified by first locating the cranial pole of the left kidney in the parasagittal long‐axis plane, then sliding the transducer medially until the aorta is found. The left adrenal gland lies ventrolateral to the aorta, caudal to the celiac and cranial mesenteric arteries that branch from the aorta, and cranial to the renal artery that also branches from the aorta. The right adrenal gland is identified by locating the cranial pole of the right kidney in the parasagittal long‐axis plane, then sliding the transducer medially until the caudal vena cava and portal vein are found. The right adrenal gland is closely associated with the caudal vena cava and can be mistaken as a venous branch of the caudal vena cava. The right adrenal gland is located within the renal fossa of the caudate process of the caudate lobe of the liver and can be easily mistaken as a hepatic nodule (Figure 28.2). Color and power Doppler can assist in differentiating the right adrenal gland from the surrounding vasculature. FIGURE 28.2 Ultrasound image of the right adrenal gland (between the calipers) in a normal adult cat, closely located to the caudate process of the caudate lobe of the liver. The normal left adrenal glands (Figure 28.3a) in dogs are biconcave or oblong in shape and the right adrenal glands (Figure 28.3b) in dogs are biconcave, oblong, or triangular shaped. In cats, the adrenal glands are oval or bean shaped (Figure 28.4). The width and length of the adrenal glands are variable. Adrenal glands are hypoechoic to the surrounding retroperitoneal fat. In some instances, the corticomedullary distinction is well defined. FIGURE 28.3 Ultrasound images of the left (A) and right (B) adrenal glands (between the calipers) in a normal adult dog. Note the normal distinction of the cortex and the medulla of the adrenal glands. FIGURE 28.4 Ultrasound images of the left (A) and right (B) adrenal glands in a normal adult cat. Note the anatomic landmarks around the left adrenal gland and the proximity of the caudate lobe of the liver to the right adrenal gland. In dogs, the normal adrenal gland length and width are variable; however, the caudal pole width should measure between 6.0 and 7.4 mm [2–5]. Recent literature has suggested that the thickness of the caudal pole of the adrenal gland in the sagittal plane is correlated to body size: ≤5.4 mm for dogs <10 kg, ≤6.8 mm for dogs 10–30 kg, and ≤8.0 mm for dogs >30 kg [6]. Adrenal gland length has not been correlated with the age or size of dogs. Therefore, the caudal pole measurement is key to determining enlargement of the adrenal glands in dogs. In normal cats, the cranial height of the adrenal glands ranges between 3.0 and 4.8 mm and the caudal height of the adrenal glands ranges between 3.0 and 4.5 mm [7]. The adrenal gland height in cats weighing ≤4 kg ranges between 2.2 and 4.1 mm and in cats weighing >4 kg ranges between 2.4 and 5.1 mm [8]. Visualization of the adrenal glands with computed tomography (CT) is not restricted by patient size or gas in the overlying gastrointestinal tract. Typically, 3–5 mm thick transverse CT images of the abdomen in the region of the adrenal glands are obtained. Nonionic iodinated positive‐contrast medium may be administered intravenously to help distinguish vessels from the adrenal glands. A bolus injection of 400–800 mgI/kg of body weight may be given and the adrenal gland scan repeated immediately. The left adrenal gland is ventrolateral to the aorta and psoas minor muscle and medial to the cranial pole of the left kidney (Figure 28.5a). The right adrenal gland is located dorsolateral to the caudal vena cava, ventral to the right diaphragmatic crus, and medial to the cranial pole of the right kidney (Figure 28.5b). The adrenal glands are oval, triangular, or round soft tissue‐attenuating structures. Portions of the right adrenal gland may appear bipartite. FIGURE 28.5 Computed tomographic angiogram () image of the normal left adrenal gland (*) in a dog in the dorsal plane (A), soft tissue window (window width 400, window level 40) of the cranial abdomen during the portal phase of contrast; note the normal regional anatomy including the celiac (+) and cranial mesenteric (++) arteries. Computed tomographic image of the normal right adrenal gland in a dog in the parasagittal plane (B), soft tissue window (window width 400, window level 40) of the cranial abdomen; note the normal triangular to V‐shaped appearance of the adrenal gland. Orientation of each gland within the transverse plane also affects their measured width and thickness; therefore, these dimensions may not be comparable to those obtained with ultrasonography. The ventrolateral‐to‐dorsomedial dimension provides an estimation of the short axis of the gland. In a study of 10 healthy dogs, the maximum ventrolateral‐to‐dorsomedial dimension was 11.1 mm for the right gland and 14.6 mm for the left gland. In a study with 48 normal adult dogs, the mean CT volume of the left adrenal gland was 0.60 cm3 and the right adrenal gland was 0.55 cm3 [9]. Radiographically, marked adrenomegaly secondary to neoplasia may cause a soft tissue opaque mass effect in the craniodorsal retroperitoneal space, caudolaterally displacing the kidneys (Figure 28.6) [10, 11]. Diffuse retroperitoneal change may also be present. Right adrenomegaly is more difficult to assess due to its proximity to the caudate process of the caudate lobe of the liver, border effacing the adrenal gland mass with the liver. FIGURE 28.6 Left lateral abdominal radiograph of a geriatric dog with a large, partially mineralized right adrenal gland mass (arrows). This mass causes caudal displacement of the right kidney. Although radiographs are not considered the ideal modality to diagnose adrenal gland abnormalities, they are essential to diagnose metastatic disease in the thorax in addition to secondary signs of adrenal gland disease (i.e., pendulous abdomen, hepatomegaly, bronchial and pulmonary mineralization, calcinosis cutis, abdominal soft tissue mineralization, and osteopenia) as seen in adrenal adenomatous cortical hyperplasia. Occasionally, adrenal gland calcification can be seen radiographically in dogs with adrenal adenoma or carcinoma [10]. Differentials for adrenal gland masses include adenocarcinoma (cortical involvement), pheochromocytoma (medulla involvement), metastasis, hyperplasia, and rarely neuroblastoma, ganglioblastoma, myelolipoma, hemorrhage, inflammation or infection, or cyst. Adrenomegaly, adrenal gland nodules, and adrenal gland masses are better assessed on abdominal ultrasonography. Ultrasonography to detect adrenal lesions has high specificity (100%), but low sensitivity (63.7%) [12]. Differentials for adrenomegaly include hyperplasia or neoplasia such as adenocarcinoma, pheochromocytoma, metastatic disease, adenoma, or, less commonly seen, adrenalitis. An ultrasonographically normal‐appearing adrenal gland does not preclude disease. Small adrenal glands on ultrasonography, on the other hand, may also be seen. Considerations for small adrenal gland include unilateral adrenal gland atrophy secondary to functional contralateral adrenal tumor, exogenous corticosteroid administration, or hypoadrenocorticism. Adrenal carcinoma is the most common type of adrenal primary neoplasia and is generally seen unilaterally. Single or multiple nodules can be seen. The majority of adrenal carcinomas measure >10 mm in diameter. The echotexture is usually heterogeneous with small regions of mineralization (Figure 28.7); the echogenicity of these lesions may be hypo‐ to hyperechoic when compared to the renal cortex. Occasionally, vascular invasion of phrenicoabdominal veins can be seen in 23.5% of dogs with right‐sided adrenal gland tumors, given the proximity to the caudal vena cava [12]. If extensive vascular invasion is seen, CT may be helpful for further investigation. Vascular invasion on CT is a hypoattenuating contrast filling defect on the venous phase of contrast. FIGURE 28.7 Abdominal ultrasound images of a geriatric dog with a heterogeneous right adrenal gland mass (A) with focal intraluminal infiltration into the adjacent caudal vena cava (B) and a small left adrenal gland (C). Histopathologic diagnosis is consistent with a functional right adrenocortical carcinoma with secondary left adrenal gland atrophy. Pheochromocytoma is the second most frequent primary adrenal gland neoplasia. It is usually large, amorphous, and irregular in margination with loss of internal architecture. Pheochromocytomas are usually >10 mm in diameter. They are variable in echogenicity. Vascular invasion extending into the phrenicoabdominal vein and caudal vena cava (Figure 28.8) can be seen in 40% of dogs diagnosed with pheochromocytoma [12]. FIGURE 28.8 Abdominal ultrasound images of a geriatric dog with a heterogeneous right adrenal gland mass (A) with focal intraluminal infiltration into the adjacent caudal vena cava (B). Histopathologic diagnosis is consistent with a right pheochromocytoma. Adrenal cortical adenomas are rare adrenal primary neoplasms. They are solitary (Figure 28.9) or exhibit multifocal nodules and can affect both adrenal glands. The echogenicity and echotexture may be heterogeneous. There may be small regions of calcification. FIGURE 28.9 Abdominal ultrasound image of a geriatric dog with a well‐defined, round, homogeneously hyperechoic nodule (between the calipers) of the cranial aspect of the right adrenal gland. Considerations include adenoma or hyperplasia. No confirmed histopathologic diagnosis was made. Adrenal adenomatous cortical hyperplasia occurs in approximately 32% of dogs with adrenal gland lesions [12] and are secondary to pituitary disease. Adrenal cortical hyperplasia is smaller and more numerous than adrenocortical adenomas. Hyperplasia of both adrenal glands can be seen in 91% of dogs diagnosed with adrenal cortical hyperplasia and is often more hypoechoic [12]. The shape and margination of adrenal glands with hyperplasia are unaffected. This lesion usually affects the entire glandular parenchyma of the adrenal gland (Figure 28.9). Hyperplasia can also exhibit focal nodules that range from 3.1 to 10 mm in diameter [12]. Adrenal gland metastatic disease is usually bilaterally affected. There are usually multifocal, heterogeneous nodules, with irregular margins. Neoplasms that metastasize to adrenal glands include splenic hemangiosarcoma and pulmonary carcinoma. Lesions may measure up to 10 mm in diameter. The lymphatic system is a complex system of cellular and vascular components which is responsible for the immune response of the body. The component of the lymphatic system commonly assessed on computed tomography (CT) and ultrasonography are the lymph nodes. Special procedures using positive contrast medium can help assess the lymphatic ducts via lymphangiogram, which can be imaged on radiography or CT. On survey radiography, lymph nodes are difficult to assess as they are normally small and tend to border efface adjacent soft tissue structures. Anatomical localization of the lymph nodes is essential to understand abnormalities of the lymphatic system. Radiographic assessment of the lymph nodes of the skull and cervical region includes the mandibular and medial retropharyngeal lymph nodes, but these are border effaced by the surrounding soft tissue structures. Ultrasonography or CT are better modalities to assess these lymph nodes. Mandibular lymph nodes form a group of two or three nodes that lie dorsal to the linguofacial vein in the region of the ramus of the mandible, best appreciated on lateral radiographs (Figure 28.10). Afferent lymph vessels to the mandibular lymph nodes come from most parts of the head [1]. On ultrasound (Figure 28.11), the mandibular lymph nodes in dogs and cats are superficial/subcutaneous, oval, and predominantly hypoechoic [13]. Mandibular lymph nodes on ultrasound are considered enlarged with a length >2 cm, width >1.5 cm, and height >0.5 cm [13]. On CT (Figure 28.12), normal mandibular lymph nodes are oval, usually paired into lateral and medial portions, homogeneously soft tissue attenuating, and measure 10–25 mm in length [14]. FIGURE 28.10 Lateral skull radiographs of a normal middle‐aged brachycephalic dog (A), mesaticephalic dog (B), and dolichocephalic dog (C); note the regions of the mandibular (*) and medial retropharyngeal (**) lymph nodes. FIGURE 28.11 Transverse plane ultrasound image of a normal mandibular lymph node (between the calipers) in a middle‐aged dog. Note the lateral location of the mandibular salivary gland (*) in relation to the mandibular lymph node. FIGURE 28.12 Transverse (A) and parasagittal (B) plane, soft tissue window, CTA images of the skull of a middle‐aged dog with normal mandibular lymph nodes (*). The medial retropharyngeal lymph node (Figure 28.10) is the largest node found in the head and neck. It lies ventral to the wing of the atlas and dorsal to the pharynx and larynx. Afferent lymph vessels to the medial retropharyngeal lymph nodes come from the deep structures of the head such as the tongue, walls of the oral, nasal, and pharyngeal region, salivary glands, and deep parts of the external ear, larynx, and esophagus [1]. On ultrasonography (Figure 28.13), the medial retropharyngeal lymph nodes are oblong and predominantly isoechoic to the adjacent mandibular salivary glands. Normal medial retropharyngeal lymph nodes can be moderately heterogeneous in echotexture in cats [15]. The size of these lymph nodes is increased with higher body weight and younger age in dogs and decreases in size with increased age [13, 16, 17]. In dogs, medial retropharyngeal lymph nodes are considered enlarged on ultrasound with a length >3 cm, width >2 cm, and height >1 cm [13]. In cats, normal medial retropharyngeal lymph nodes on ultrasound measure 20.7 (L) × 12.4 (W) × 3.7 (H) mm [15]. FIGURE 28.13 Transverse ultrasound image of a normal medial retropharyngeal lymph node (between the calipers) in a dog. Note the mandibular salivary gland ventrolateral to the medial retropharyngeal lymph node and the anechoic external carotid artery adjacent to the medial retropharyngeal lymph node. On CT (Figure 28.14), the medial retropharyngeal lymph node is seen within a fatty triangular space, defined by the wing of the atlas dorsomedially, the mandibular salivary gland ventrolaterally, and the common carotid artery medially. These lymph nodes are homogeneous and irregular in shape or flattened in dogs, bilaterally symmetric, and are mildly heterogeneous in cats [15]. In dogs, normal medial retropharyngeal lymph nodes on CT measure 30–70 (L) × 10 (W) × 5–10 (H) mm [14] or 2.5–5.0 (L) × 1.0–2.0 (W) × 0.5–1.0 (H) cm [16]. FIGURE 28.14 Transverse (A) and parasagittal (B) plane, soft tissue window, CTA images of the skull of a normal middle‐aged dog depicting normal medial retropharyngeal lymph nodes (*). In recent studies including 161 presumed normal dogs, the median short‐axis transverse diameter of medial retropharyngeal lymph nodes measured 5.2 and 5.4 mm [18]. In cats, normal medial retropharyngeal lymph nodes on CT measured 20.7 (L) × 13.1 (W) × 4.7 (H) mm [15]. Radiographic assessment of thoracic lymph nodes includes sternal, cranial mediastinal, and bronchial lymph nodes (Figure 28.15). The sternal lymph node is usually a single node on each side in the dog and a single node in the cat. In the dog, there is occasionally only a single medial node. The sternal lymph node is located in the cranioventral aspect of the mediastinum, immediately cranial to the transversus thoracis muscle, and medial to the second costal cartilage or second interchondral space. On lateral radiographs, the sternal lymph node is located dorsal to the second sternebra in dogs and dorsal to the third sternebra in cats. Additionally, sternal lymph nodes can be evaluated on the ventrodorsal thoracic radiograph, over the cranial mediastinal reflection. The sternal lymph nodes drain a broad region of the thorax and abdomen, such as the ribs, sternum, thoracic and abdominal walls, thymus, abdominal and pelvic cavities, and cranial and caudal thoracic mammary glands [1, 19, 20]. Sternal lymph nodes, like other lymph nodes, are normally oval to round, isoechoic to slightly hypoechoic and can vary in size on ultrasound (Figure 28.16). FIGURE 28.15 Right lateral (A) and ventrodorsal (B) thoracic radiographs of a normal middle‐aged, medium‐sized dog. Note the region of the sternal lymph nodes (*), cranial mediastinal lymph nodes (**), and tracheobronchial lymph nodes (***). FIGURE 28.16 Sagittal ultrasound image of a normal sternal lymph node (between the calipers) in a geriatric cat. The sternal lymph node is isoechoic to the regional cranial mediastinal fat. No reported normal sternal lymph node size on ultrasound has been reported in the literature. Sternal lymph nodes on CT are similar to those seen on ultrasound, where they are paired, oval, soft tissue attenuating, homogeneously contrast enhancing, and located dorsal to the second sternebra in dogs (Figure 28.17); normal sternal lymph node size on CT can be measured using the ratio of the short‐axis dimension of the sternal lymph nodes to the thickness of the second sternebrae; the mean ratio is 0.457 (95% prediction interval ranging from 0.317 to 0.596), or 3.16 ± 1.09 mm in width and 5.15 ± 1.89 mm in height [21]. In cats, normal sternal lymph node size on CT measures 3.93 ± 0.74 mm [22]. FIGURE 28.17 Transverse (A) and parasagittal (B) plane, soft tissue window CTA images of the thorax of a normal middle‐aged dog depicting bilaterally symmetric, soft tissue‐attenuating, homogeneously contrast‐enhancing, paired sternal lymph nodes (*) within the ventral aspect of the cranial mediastinum, dorsal to the second sternebral segment. Similar to sternal lymph nodes, cranial mediastinal lymph nodes are also located over the cranial mediastinal reflection on the VD or DV thoracic radiograph and are adjacent to the great vessels of the cranial mediastinum (i.e., cranial vena cava, brachiocephalic trunk, left subclavian artery), and ventral to the trachea. However, on lateral thoracic radiographs, the cranial mediastinal lymph nodes are located within the craniodorsal mediastinum, ventral to the trachea (Figure 28.15). As such, cranial mediastinal lymphadenopathy usually causes dorsal deviation of the trachea on the lateral radiograph and rightward deviation of the trachea on the VD or DV radiograph (Figure 28.18). FIGURE 28.18 Right lateral (A) and ventrodorsal (B) radiographs of a geriatric dog with round cell neoplasm of the cranial mediastinum. There is a large, poorly defined soft tissue mass effect of the cranial mediastinum, causing an ill‐defined soft tissue mass effect (arrows) on the lateral radiograph and severe rightward displacement of the trachea (arrows) on the ventrodorsal radiograph. Given the superimposition of the cranial mediastinum over the lungs, differentiation of cranial mediastinal and pulmonary masses can be challenging on radiographs, and can be confirmed on thoracic CT [23]. Many lung masses can be distinguished from mediastinal masses because the lung mass is lateral to the mediastinum and is therefore more sharply marginated because of the surrounding air‐filled lung. Some mediastinal masses, however, may protrude laterally, or be in a thin portion of the mediastinum, and surrounded by air, making them sharply marginated and easily mistaken for a lung mass. Cranial mediastinal lymph nodes vary in number. Afferent cranial mediastinal lymph vessels come from the muscles and bones of the neck, thorax, abdomen, peritoneal cavity, thyroid lobes, cranial and middle mediastinal contents (i.e., trachea, esophagus, thymus, mediastinum, heart, and aorta) [1]. Normal cranial mediastinal lymph nodes are difficult to assess on ultrasonography due to reverberation artifact from aerated lung. Normal cranial mediastinal lymph nodes on CT are typically round but can be fusiform, homogeneous, and vary in measurement, depending on the size of the dog or cat (Figure 28.19). Size can be as large as 10 mm for large dogs but usually ranges from 1 to 3 mm in diameter or, more specifically, 3.16 ± 1.09 mm in width and 4.51 ± 1.66 mm in height [24]. In cats, normal cranial mediastinal lymph node size on CT measures 4.02 ± 0.65 mm in width [22]. FIGURE 28.19 Transverse (A) and parasagittal (B) plane, soft tissue window, CTA images of the thorax of a normal middle‐aged dog depicting a normal, fusiform, soft tissue‐attenuating, homogeneously contrast enhancing, cranial mediastinal lymph node (*) within the dorsal aspect of the cranial mediastinum. Lastly, the bronchial lymph nodes are split into two main groups: tracheobronchial and pulmonary. The pulmonary group is often absent. The tracheobronchial lymph nodes are further split into three parts: right, left, and middle (Figure 28.20). The middle tracheobronchial lymph node is the largest of the group when compared to the left and right tracheobronchial lymph nodes. The middle tracheobronchial lymph node lies between the tracheal bifurcation, slightly dorsal to the caudal aspect of the trachea and carina. The left and right tracheobronchial lymph nodes lie to the left and right side of the carina, respectively, best assessed on the VD or DV radiograph. The afferent bronchial lymph vessels come from the bronchi, lungs, cranial mediastinal and middle mediastinal contents (i.e., aorta, esophagus, trachea, heart), and diaphragm [1]. FIGURE 28.20 Anatomy of bronchial lymph nodes. End of the trachea a; left b and right c main bronchus; left d and right e cranial bronchus. Left tracheobronchial lymph node 1; middle tracheobronchial lymph node 2; right tracheobronchial lymph node 3; pulmonary lymph node 4. Source: Dr Hermann Baum (1918). (This work is in the public domain). On CT (Figure 28.21), normal left and right tracheobronchial lymph nodes are round and middle tracheobronchial lymph nodes are fusiform and all are homogeneous; the right tracheobronchial lymph node is not always detected on CT, possibly due to overlying vasculature (i.e., azygous vein and caudal vena cava) [24]. The average measurement of normal canine tracheobronchial lymph nodes is 3.17 ± 0.95 mm width of the left, 3.00 ± 1.02 mm width of the right, and 3.06 ± 1.05 mm width of the middle [24]. In cats, normal tracheobronchial lymph nodes on CT measure 3.51 ± 0.62 mm [22]. FIGURE 28.21 Transverse (A) and sagittal (B) plane, soft tissue window CTA images of the thorax of a normal dog centered over a normal middle tracheobronchial lymph node (*). Transverse (C) plane image of the same dog centered over the normal left and right tracheobronchial lymph nodes (*). Dorsal (D) plane image of the same dog centered over the normal left and middle tracheobronchial lymph nodes (*).
CHAPTER 28
Adrenal Glandsand LymphNodes
Overview of Adrenal Glands
Imaging Appearance of Normal Adrenal Glands
Radiography
Ultrasonography
Computed Tomography
Imaging Abnormalities of Adrenal Glands
Overview of Lymph Nodes
Imaging Characteristics of Normal Lymph Nodes
Head and Cervical Region
Thorax
Abdomen
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