California sea lion (Zalophus californianus) - THORAX

Northern elephant seal (Mirounga angustirostris) - THORAX

Thorax:

Radiographs should be taken on full inspiration to avoid over interpretation of expiratory induced interstitial patterns. Marine mammals in general have an increased interstitial pattern due to increased muscular cartilaginous support of the smaller airways compared to other species. This is a diving related adaptation. Multiple exposures may be required to allow evaluation of the entire lung field in large chested animals. In this case, radiographic views should overlap to ensure that no region is missed. Grids are usually required due to the thickness of the animals to reduce scatter and improve image quality.

Note the proximal bifurcation of the trachea in northern elephant seals and California sea lions. This is important to remember clinically when intubating as it is surprisingly easy to find the endotracheal tube disappearing down one bronchus if care is not taken.

Marine mammals have a more prominent aortic arch than terrestrial mammals called the aortic bulb. This  is an enlargement of the ascending aorta and aortic arch that extends from the aortic root to the ductus arteriosus or ligamentum arteriosus depending on the age of the animal. It is an adaptation that is thought to permit continued perfusion to the brain during the long diastolic periods during dive-associated bradycardia. The enlargement means that the aorta can be seen on the right and left of midline as a focal widening of the cranial mediastinum.

Superimposition of the pectoral flippers is common over the cranial thorax and virtually impossible to avoid on the lateral views particularly with the comparatively shorter, less mobile flippers of phocids. For otariids, every attempt should be made to abduct the flippers for VD views and to move the flippers cranially and ventrally for the lateral views. Orthogonal views are extremely important in ensuring that suspected lesions in the cranial lung fields are indeed that and not due to flipper superimposition.

CONTRAST RADIOGRAPHY:

Positive, negative or double contrast studies can be achieved using radiography. Exactly which contrast medium is best for evaluation of a region of interest depends on the region of interest and specific consideration for that individual animal. Advice should be sought prior to embarking on these special procedures.

Positive contrast media: iodinated contrast media, barium liquid or paste, BIPs

Negative contrast media: room air, carbon dioxide

Double contrast studies: combination of positive and negative contrast media

Contrast studies may be used to provide more information. Typical examples include evaluation of the anatomic position of structures (useful for diaphragmatic/body wall hernia or rupture), morphology of a structure (GI tract, kidneys, urinary bladder), motility (GI tract transit times), subjective/qualitative functionality (kidney nephrogram), extent of pathology (fistulous tracts), or identifying evidence of mechanical obstruction (GI tract, ureters, urethra, lacrimal duct, parotid salivary duct).

Specific contrast studies are usually named after the structures being evaluated and the type of contrast medium used eg. negative gastrogram (air is placed in the stomach), positive retrograde urethrogram (iodinated contrast medium injected in a retrograde direction through the urethra). Guidelines regarding type, concentration and volume of contrast medium needed for an individual study are not currently available for marine mammal species specifically, however doses may be extrapolated from other species based on body weight, or from personal experience.

For an example of a contrast study, go to ‘radiographic diagnosis of disease’.

California sea lion (Zalophus californianus) - SKULL

Northern elephant seal (Mirounga angustirostris) - SKULL

Alaskan sea otter (Enhydra lutris) - THORAX

Lateral and VD views of a normal immature Alaskan sea otter thorax. These radiographs were taken using computed radiography (CR). Note how the conformation of the thorax of the sea otter is different to other marine mammals with the thorax flattened in a dorsoventral direction. There is a mild diffuse bronchial pattern through the lungs, a normal finding in this species that is particularly evident when using digital radiography. This increased pattern is due to increased lower airway support compared to terrestrial mammals, similar to other marine mammals. In addition, the superior contrast resolution found when using digital radiography permits the fur of the animal to be seen adjacent to the body wall. Note the small volume of subcutaneous fat in this species. This is normal. Sea otters do not have a blubber layer like other marine mammal species.

Lateral and VD views of the normal Alaskan sea otter abdomen. These radiographs were taken using computed radiography (CR). Gas within the small intestines and the head of the spleen can be identified. A large urinary bladder is displacing the intestines craniad. Physes are evident within the distal femur and proximal tibia bilaterally, indicating an immature animal.

Many marine mammals have minimal intra-abdominal fat. Contrast within the abdomen in terrestrial animals is created by fat surrounding soft tissue structures, and where applicable, gas within lumens. So when the amount of fat within the abdomen is reduced, the detail within the abdomen also decreases. Sea otters have a small amount of intra-abdominal fat this there is some abdominal detail evident, for example serosal surfaces of intestinal loops and liver margins. However this is not the case in cetaceans and pinnipeds. Abdominal radiography in these species still has a place, but the limitations must be understood. Ultrasound may be preferable in many cases requiring evaluation of intra-abdominal organs (eg. liver, kidney, reproductive organs).

Musculoskeletal system:

Soft tissues are difficult to evaluate radiographically due to poor contrast between structures. The exceptions to this rule are when soft tissues are swollen resulting in variations in structure silhouette or margination, when there is calcification present or when gas is present. Soft tissues in general are best evaluated with MRI or ultrasound but can also be evaluated using CT.

Conversely, radiography is good for evaluation of the skeleton. Pinnipeds have a skeletal formula similar to other species: skull; cervical, thoracic, lumbar, sacral and coccygeal vertebrae; scapulae;  humerus; radius and ulna; carpus, metacarpi, phalanges; femur, tibia and fibula, tarsus, metatarsi, phalanges; os penis (males only).

Damage or alteration to complex structures such as the skull or vertebrae may benefit from evaluation with CT.

Alaskan sea otter (Enhydra lutris) - ABDOMEN