Colin B. Carrig, BVSc, PHD; Otto I. Lanz, DVM; Jacob Rohleder, DVM
Foreign bodies are associated with a wide range of clinical problems that can be difficult to define and manage. Foreign objects vary in opacity, which influences their detection on survey radiographs. They are often suspected because of accompanying clinical and/or secondary radiographic signs (e.g., vomiting, ileus, draining tracts, periosteal new bone formation on adjacent bone), and frequently only identified following radiographic contrast procedures, such as sinography (fistulography) and barium swallow.
The four basic subject densities that can be differentiated on a radiograph are metal and opaque contrast agents, fluid and fluid dense tissues, fat, and air or similar gases. Opaque foreign bodies composed of metal, glass or bone are usually seen on survey radiographs because they contrast with surrounding tissue. Similarly, if a foreign body contains air, or is surrounded by air, it will be seen. Foreign bodies that are similar in opacity to, and surrounded by, fluid opaque tissues (e.g., plastic, rubber, wood, grass awns), are difficult to see. In human patients wood foreign bodies are identified on survey radiographs in a low percentage of affected patients, and account for a high percentage of false negative reports. In dogs, conventional radiography failed to identify wood foreign bodies in all 6 patients in one study, sinography identified the foreign bodies in 2 of 4 patients in which it was performed, while diagnostic ultrasound identified the foreign body in 5 of 6 cases.
The ability to see wood foreign bodies in soft tissues is related to the physical density of the wood. Dense wood such as cedar and mahogany are less obvious on survey radiographs than less dense wood such as pine. Further, the longer the wood remains in tissue, the more water it absorbs, and the more difficult it will be to see on a radiograph. Imaging methods used to identify wood and other non-opaque foreign bodies include conventional radiographs, which are often negative, various radiographic contrast studies including sinography, diagnostic ultrasound, which has become the mainstay of identifying the presence of foreign bodies, xeroradiography, computed tomography and MRI.
Sinography, which is performed by injecting water-soluble contrast medium into a draining tract, is variably effective, and often associated with false positive and negative results. Success will depend on delivering the contrast medium to the site of the foreign body. The contrast medium will surround the foreign object, if present, which will appear as a filling defect. Frequently, the foreign body is identified because of well defined, straight or regularly curved margins. Sinography is less effective in areas where there is loose subcutaneous tissue, as this allows the contrast medium to dissect along planes other than the sinus tract associated with the foreign body.
Ultrasound findings seen with foreign bodies include hyperechoic surface echoes and distal acoustic shadowing, without reverberation. Most foreign bodies (e.g., wood, plastic and graphite) have similar appearances, differing only in the intensity of surface echoes produced. Other information that is provided by ultrasound evaluations includes location, relationship to visceral organs, depth, size and orientation of the foreign body and fistulous tract.
Foreign bodies affect many parts of the body, including the nasal cavity, eyes, pharynx, lungs, alimentary tract, vertebral column, urinary bladder, and soft tissues.
Nasal Cavity, Larynx and Pharynx
Opaque objects in the nasal cavity will usually be seen on survey radiographs, however objects with bone opacity can go unrecognized because of overlying skull bones. The radiographic identification of non-opaque foreign bodies in the nasal cavity can be difficult. However, when located in the pharyngeal/laryngeal region, they are more likely to be seen because they are partially surrounded by air. Endoscopy provides a direct way of locating a foreign body, however computed tomography allows better spatial orientation of the object and recognition of accompanying disease.
Ocular foreign bodies are commonly encountered in small animals, and appear to be most common in cats. Ultrasound evaluation of the orbit provides a direct method of imaging the retro-bulbar space, and identifying foreign objects not seen on survey radiographs. Ultrasound, CT and MRI provide spatial relationships between the location of the foreign body and the eye and orbit, gives information on the extent of accompanying pathology and is helpful in planning surgical approaches for removal.
Aspiration of foreign objects into the respiratory tract occurs occasionally, and produces clinical signs of coughing. Tracheal foreign bodies often lodge in the area of the tracheal bifurcation, while smaller objects can pass into a bronchus. If a foreign body is non-opaque and lodges in a bronchus, it detection becomes difficult on survey radiographs because of associated peribronchial infiltrate and atelectasis of adjacent lung. If bronchial obstruction is complete, atelectasis of entire lung lobes can result. This commences within hours of obstruction, and is recognized on survey radiographs by a shift of the mediastinum towards the side of the obstruction.
Introduction of opaque contrast agent into the bronchus (bronchography) can outline the presence of a foreign body, however endoscopy is more commonly used to diagnose and remove tracheal and bronchial foreign bodies
Young dogs and cats ingest a variety of foreign bodies that cause intestinal obstruction. The radiographic diagnosis of obstruction is usually based on localized dilation of intestinal loops and accumulation of fluid, gas and/or food proximal to the obstruction. Opaque objects are easily seen, however non-opaque foreign bodies are more difficult to identify. Common opaque gastrointestinal foreign bodies include stones, certain toys and bottle tops, and coins. When a coin is suspected based on shape and opacity, the possibility that it contains zinc should be considered. Non-opaque foreign bodies include hairballs, rubber balls, rubber or plastic objects, string or thread, corncobs, nuts and fruit pits. Linear foreign bodies caused by string-like objects may be recognized on survey radiographs because of their effect on the intestine (clumping, plication of intestine, eccentrically located gas bubbles). Radiographic demonstration of foreign bodies in the gastrointestinal tract often relies on the oral administration of barium sulfate, or ultrasound evaluation.
Non-opaque gastric foreign bodies (plastic, rubber) might be identified if part of their surface is in contact with gas. In the stomach, the distribution of fluid and gas will influence the detection of foreign bodies, e.g., an object located in the body or pyloric antrum will be more likely surrounded by gas, and so more easily seen, on left lateral and ventrodorsal survey radiographs. Following the administration of barium sulfate, right lateral and dorsoventral radiographs will position the contrast medium in the body and pyloric regions of the stomach and best demonstrate a foreign body in that location.
Foreign bodies in the stomach and intestine can be identified by ultrasound. The acoustic patterns generated from foreign bodies vary depending on their physical properties and interaction with the sound beam. Most will show hyperechoic linear or curvilinear surface echoes, have sharply defined margins and acoustic shadowing. With intestinal obstruction, dilated gas and fluid filled intestine with hyperperistalsis will be identified proximal to the foreign body. With linear foreign bodies, plication of the intestine is seen around the string, which will appear as an echogenic line.
Foreign objects in the peritoneal cavity can result from penetration of the body wall secondary to migration or trauma, or from iatrogenic causes such as instruments or surgical sponges left in the abdomen following celiotomy. Surgical sponges are available that contain radio-opaque markers, however these are not commonly used in veterinary medicine. When surgical sponges that do not have radiographic markers are left in the peritoneal cavity, they can be difficult to detect. Close inspection of survey radiographs will detect a surgical sponge because of small amounts of air between the mesh of the gauze. Ultrasound evaluation will identify an associated granuloma, which appears as a hypoechoeic mass. The surgical sponge shows as a hyperechoeic central layer. Acoustic shadowing is seen deep to the mass, primarily due to absorption of the ultrasound beam by the surgical sponge.
Vertebral Column and Musculature
Bullets and other projectiles will often cause severe disruption of vertebrae and result in severe clinical signs. These objects, because of their opacity, are easily seen on survey radiographs, however the extent of associated trauma can be difficult to appreciate. Techniques, such as linear tomography, myelography, computed tomography and MRI can be used to better define the extent of spinal cord and vertebral involvement, and facilitate management decisions.
Grass awn migration is commonly encountered and a wide range of body systems can be affected, including the external ear, paws, eyes, nose and thoracic cavity. Migration of grass awns can result in infection in the area of the vertebral attachment of the diaphragm and cause spondylitis of the rostral lumbar vertebrae. Vertebral lesions are characterized by increased opacity of the vertebral bodies, along with chronically-active periosteal new bone formation along the ventral margin of the vertebrae. While identification of grass awns in this location is unlikely, sinography can identify the origin of an associated sinus tract, and guide surgical exploration.
Wood foreign bodies can penetrate superficial tissues in any part of the body and often go unrecognized unless they are opaque. The paw is a common site. Ambiguous signs associated with these foreign bodies include swelling, masses and draining tracts, which can appear over a period of weeks, months or even years. Multiple medical and/or surgical treatments are often given before a definitive diagnosis is made. Sinography and ultrasound are methods that are used to locate and identify foreign bodies in these locations.