CHAPTER six Soft Tissues
Calcification is a process by which calcium salts are deposited in tissue. Dystrophic calcification is the deposition of calcium salts in abnormal tissue or tissue that is dead, degenerating, or damaged. Metastatic calcification is the deposition of calcium salts in tissue that is not the site of a disease process. It results from abnormalities in blood and tissue calcium and phosphate levels. It is associated with such metabolic disturbances as hyperparathyroidism (hypercalcemia) or hypovitaminosis D. Calcific deposits may be found in such soft tissues as the lungs, gastric mucosa, and kidneys (see Chapter 2, pp. 46, 58, and 136), as well as those around joints (see Chapter 4, pp. 420-422, 428, and Figure 4-45, A to C) and in the walls of blood vessels (Figure 6-1).
Figure 6-1 Calcification. A, Calcification (arrows) of the external ear canals in an old dog. B, Calcification in a chronically inflamed infraspinatus bursa on the lateral aspect of a shoulder joint. C, Calcification in a mammary tumor. The soft tissue swelling is well demonstrated. D, Calcified cataracts (arrows). Calcification. E, Calcification of the biliary tract, an incidental finding. F, This dog had a pendulous abdomen, polydipsia, and polyuria. The lateral radiograph shows a generalized mineralization of the soft tissues, particularly well seen in the soft tissues of the inguinal area. Diagnosis: Cushing’s syndrome—hyperadrenocorticism. G, A female 3-year-old cat with severe dyspnea, polydipsia, and harsh respiratory sounds on auscultation. Extensive mineralization of the soft tissues is seen. Note the prominent aorta, which is mineralized throughout its length. Diagnosis: dystrophic calcification caused by renal disease. Calcification. H, This 1-year-old Labrador Retriever presented with a left foreleg lameness of 2 months’ duration. The carpus was hyperextended, and tendon injury was suspected. Radiographs at the time of the injury were unremarkable. This subsequent study illustrates extensive mineralization of the soft tissues just proximal to the accessory carpal bone. This mineralized tissue extends distally to involve the body of the accessory carpal bone. Diagnosis: posttraumatic calcification. I and J, A 5-month-old kitten with a history of dyspnea. Lateral (I) and dorsoventral (J) views show widespread, symmetrical, pinpoint infiltrations throughout the lungs obscuring the normal thoracic structures. The diaphragm is flattened, and the gastric wall is mineralized. Rugal folds are visible as parallel radiopaque stripes. This was metastatic calcification caused by renal failure. Calcification. K to M, A 12-year-old Yorkshire Terrier had polydipsia, polyuria, and an enlarged abdomen. Lateral (K) and ventrodorsal (L) radiographs show hepatomegaly, a pendulous abdomen, and poor serosal detail. There is a vaguely circular soft tissue opacity in the left dorsal abdomen. It has a mineralized rim. It lies craniomedial to the kidney, with which it shows border effacement. M, An ultrasonogram shows a 5-cm-wide heterogeneous mass (M) invading the cranial pole of the left kidney. It has anechoic areas and hyperechoic foci (arrowhead) caused by calcification within the mass. This neoplastic mass could have arisen either from the adrenal gland or from the kidney.Calcification. N, A 9-year-old Kerry Blue Terrier presented with lethargy. There was proteinuria. Discrete areas of mineralization lie ventral to the thoracic vertebrae. Diagnosis: this was a mineralized adrenal gland. O, Coronary artery mineralization in a dog. A tortuous, mineralized linear structure is seen superimposed on the heart base on a lateral view of the thorax (arrows). This represents mineralization of vessels in the region of the coronary sinus. P, Several well-marginated mineral opacities are seen in the soft tissues distolateral to the great trochanter of the femur. They were an incidental finding. Diagnosis: dystrophic calcification. Q, This 2-year-old working German Shepherd presented with an unusual hindlimb action. On palpation both gastrocnemius muscles were hard distally. The longitudinal sonogram shows the normal muscle fibers (G) infiltrated by an echogenic infiltrate, causing acoustic shadowing (arrows) (proximal is to the left of the image). Diagnosis: myositis ossificans.
Calcification of the external ear canals may occur in old dogs (Figure 6-1, A). Hematomas or bursae may calcify, as may tumor tissue (Figure 6-1, B). Calcification of the medial meniscus of the stifle has been reported in dogs and cats (see Figure 4-5, B and C).
Calcium may be laid down in the skin and subcutis in Cushing’s syndrome (see Chapter 2, p. 123) and secondary to hyperparathyroidism (calcinosis cutis). The tracheal rings and bronchial walls may be affected and there may be diffuse pulmonary parenchymal mineralization (see Figures 3-12, E, and 6-1, F and G).
Mineralization of the coronary arteries may be seen as faintly radiopaque lines extending caudoventrally from the aortic root. Mineralization of the aortic bulb is occasionally seen at the level of the fourth intercostal space in the craniodorsal region of the cardiac silhouette (Figure 6-1, O).
Ultrasonography of calcified tissue can be unrewarding if a structure is only partially calcified. Deposits will be seen as hyperechoic foci scattered in the tissues (Figure 6-1, M). In myositis ossificans, bony plaques are deposited in muscle, or the muscle itself may become ossified. Trabeculated bone opacities may be seen. It may occur as a result of chronic trauma. Calcification cannot be distinguished from ossification unless a trabecular pattern can be identified (Figure 6-1, Q).
In calcinosis circumscripta (calcium gout, kalk gicht [chalk gout], tumoral calcinosis), deposits of amorphous calcified material are laid down in the subcutaneous tissue and skin. Lesions are usually found on the limbs, under the pads, or over bony prominences. Similar lesions have been described in the mouth. The etiology remains obscure. Chronic renal disease, hyperparathyroidism, and hypovitaminosis D have been suggested as possible causes. Approximately half the cases seen occur in young German Shepherds that are otherwise apparently normal (Figure 6-2).
An arteriovenous fistula is a direct communication between an artery and a vein without an interposed capillary bed. Numerous small vessels develop in the affected area. Such fistulas may be found centrally, as in patent ductus arteriosus or ventricular septal defect, or they may be peripheral.
Peripheral fistulas may be congenital or acquired as a result of injury. Peripheral arteriovenous fistulas have been reported in dogs and cats, but they are uncommon. The clinical signs are variable. They may appear as small, painless, warm swellings with a faintly palpable pulse, or they may be large and painful. Ulceration may occur. If pressure is exerted proximal to an arteriovenous fistula, the venous return to the heart is diminished and the heart rate falls. This is known as Branham’s bradycardia sign. Large fistulas, in time, produce compensatory cardiac changes.
Radiologically, arteriovenous fistulas on the limbs may cause alterations in the trabecular pattern of bones in the neighborhood. The trabecular pattern becomes coarse. The vascular bed can be demonstrated by arteriography (Figure 6-3; also see Figure 4-25 and Chapter 4, p. 398).
Figure 6-3 Arteriovenous fistula. A, Angiographic demonstration of the normal vascular pattern of the ear of a dog. B, Numerous abnormal vessels are present in the ear as a result of an arteriovenous fistula.
(From Kealy JK, Lucey M, Rhodes WH: Arteriovenous fistula in the ear of a dog, J Am Vet Radiol Soc 11:15, 1979.)
The fascial planes between muscles are frequently visible on radiographs because of the fat that is present in the connective tissue between the muscles. The use of a bright light assists in seeing them (see Chapter 4, p. 361, Figure 4-11, A). If fascial planes are of particular interest, a soft tissue technique is used to demonstrate them. Air may be injected into the subcutaneous fascia, where it will spread to the intermuscular fascial planes and be visible radiographically.
Displacement of fascial planes is of diagnostic significance. For example, displacement of the fascial plane usually visible caudal to the stifle joint indicates intraarticular swelling of the stifle joint. The infrapatellar fat pad may lose its radiolucency if intracapsular hemorrhage or edema is present (Figure 6-4).
Figure 6-4 A, Displacement of the fascial plane (arrows) caudal to the stifle joint. This is indicative of swelling within the joint capsule and caudal displacement of the gastrocnemius muscle. B, Displacement of the fascial planes (arrows) in the thigh resulting from swelling of the vastus lateralis muscle. The cause was an undifferentiated sarcoma in the muscle. C, Displacement of fascial planes on the lateral aspect of the thorax over the eighth rib by a soft tissue mass (arrows) originating from an early rib neoplasm. Fluid is present in the pleural cavity. D, Fascial planes (arrows) overlying the proximal femurs simulate fracture lines in this puppy with hip dysplasia.
Swelling of soft tissue or soft tissue masses are frequently seen on radiographs. More detailed information is usually obtained by clinical examination. Emphysema is seen as gas shadows within the soft tissues or under the skin. Gas opacities are seen within the soft tissues after puncture of the skin, such as with a compound (open) fracture. Air shadows are seen within the soft tissues after surgery. Radiopaque foreign bodies in soft tissues are visible radiographically (Figure 6-5, A to E).
Figure 6-5 A and B, A large soft tissue mass is visible on both the dorsopalmar and lateral views. The lesion was caused by a bite wound. C, Numerous shot grains are present in the shoulder region. Air has entered the soft tissues and is visible as radiolucent shadows in the ventral neck and cranial to the shoulder. There is a comminuted fracture of the humerus. D, This 6-year-old Jack Russell Terrier had a 3-week history of lameness. The lateral radiograph of the stifle shows a radiopaque structure overlying the cranial joint space. This proved to be a small stone lodged in the subcutaneous tissues. Diagnosis: foreign body. E, This dog had a swelling in the area of the thigh. A lateral radiograph shows a fat mass within the soft tissues caudal to the femur. Diagnosis: lipoma. F, This 4-year-old Yorkshire Terrier presented with a swelling on the dorsal aspect of the neck that had been present for 3 weeks. A mixed echogenic soft tissue mass (arrows) with hypoechoic foci is seen. The hyperechoic undulating line is the dorsal aspect of the cervical vertebrae. The histopathologic diagnosis was hemangiosarcoma. G, This 2-year-old German Shepherd presented with a firm, painful swelling over the proximal aspect of the twelfth rib. Tangential radiographic studies of the area were uninformative. A transverse sonogram shows a large hypoechoic mass (a) overlying the last three ribs (r), which cast acoustic shadows. The mass is closely adherent to the ribs, and its medial margin undulates between them. Some echogenic foci were evident within the mass. Gentle compression of the mass while performing the examination indicated that the contents were fluid and compressible. Diagnosis: abscess.
Soft tissue masses may be recognized because they displace adjacent structures. For example, a retropharyngeal mass will displace the larynx ventrally; a thyroid mass will displace the cervical trachea ventrally or laterally (see Figure 3-3, E). More specific details concerning soft tissue shadows are given in earlier chapters of this text.
A soft tissue mass in the subcutaneous tissues is easy to examine ultrasonographically with a high-frequency transducer. An assessment can be made regarding its echotexture and whether it contains fluid. The degree of infiltration or margination helps differentiate neoplastic disease, cyst, abscessation, or hemorrhage.
Hematomas have a variable echogenic pattern, depending on the age of the lesion and the degree of clot retraction. Abscesses are predominantly anechoic, with floccules representing cellular debris. Focal hyperechoic areas may indicate the presence of free gas or mineralization.
Foreign bodies may be located in the soft tissues. Nonmetallic foreign bodies cause variable degrees of acoustic shadowing, whereas metallic foreign bodies are highly echogenic, causing large acoustic shadows and multiple reverberation artifacts. Failure to demonstrate a foreign body does not preclude its presence (see Figure 6-5, G).
Soft tissue structures in the neck or limbs lend themselves to ultrasonographic examination, provided air-filled or bony structures are avoided. The tissue depths to be penetrated range from 1 to 10 cm. Depending on the tissue depth, a 10- to 15-MHz high-resolution transducer will be necessary. For structures lying close to the surface, a standoff is required if a lower resolution transducer is used.
The thyroid gland lies caudal to the larynx and adjacent to the trachea. It is a paired structure that lies on either side of the trachea, and each part lies medial to the carotid artery. The two halves may be connected ventral to the trachea. It is not seen on plain radiographs until it becomes enlarged. An enlarged thyroid displaces the trachea in a variety of ways, depending on the location of the enlargement.
The thyroid gland is found by locating a carotid artery as a linear anechoic, pulsing structure deep to the jugular furrow. The transducer angle is approximately 45 degrees between the lateral and ventral aspects of the neck. The lobes lie medial to the carotid arteries and are fusiform, well-defined, homogeneous structures that are contained within the carotid sheath. They are isoechoic or hypoechoic with a granular echotexture. Each lobe is 2.5 to 3.0 cm long and 0.4 to 0.6 cm wide in dogs. In cats the length is about 2 cm, and the width is approximately 0.2 cm (Figure 6-6, A).
Figure 6-6 A1 and A2, This is a transverse sonogram of the right (A1) and left (A2) lobes of the canine thyroid gland. The gland tissue (short arrows) is relatively hyperechoic, and each lobe is triangular in shape. The carotid (C) and trachea (T) are seen in cross-section. The esophagus is outlined by long arrows. B1, This dog had a firm swelling in the midcervical region. A mixed echogenic mass (arrows) with some hyperechoic foci is evident. Fine-needle aspirate confirmed a diagnosis of thyroid carcinoma. B2, In this case of thyroid carcinoma, the neoplastic tissue (star) is more uniform and has a slightly loose echotexture. Fine-needle aspiration or biopsy is required to make a definitive diagnosis. B3, Lateral thoracic radiograph showing widespread metastases from a thyroid carcinoma. C, Normal thyroid scintigraphy in a cat. There is uniform, symmetric uptake of the radiotracer in the thyroid glands. The uptake in the thyroid glands seen in the mid-neck is comparable to the zygomatic salivary glands seen in the skull area. D, Unilateral feline hyperthyroidism. A nuclear medicine thyroid scan in a cat. This ventral view of the head and neck shows a single, well-defined focus of intense increased radiotracer accumulation in the mid-neck, in the area of the thyroid gland. The uptake within the abnormal thyroid gland is much greater than in the zygomatic salivary glands. E,