Diagnostic Imaging Interpretation Flashcards

A 7-year-old German Shepherd presents with progressive rear limb ataxia. Spinal MRI reveals a well-defined, intradural extramedullary lesion at T10-T11 that is isointense on T1, hyperintense on T2, and demonstrates marked homogeneous contrast enhancement. The adjacent cord shows a 'cap sign' (hemosiderin deposition). Which diagnosis is MOST consistent with these findings?

Answer: Meningioma

Meningiomas are the most common intradural extramedullary spinal tumors in dogs, typically appearing isointense on T1, hyperintense on T2, with intense homogeneous contrast enhancement. The 'cap sign'—hemosiderin staining at the poles of the mass—is a specific MRI feature strongly associated with spinal meningiomas due to chronic microhemorrhage. Nephroblastomas occur in young dogs and are typically intramedullary. Peripheral nerve sheath tumors tend to extend through neural foramina. Arachnoid diverticula do not enhance with contrast.

On thoracic radiographs of a cat, you identify a soft tissue opacity causing ventral displacement of the trachea at the thoracic inlet and dorsal displacement of the thoracic esophagus. The opacity silhouettes with the cranial cardiac border. Which structure most likely accounts for this pattern?

Answer: Thymic lymphoma

In cats, cranial mediastinal masses that ventral-displace the trachea and dorsal-displace the esophagus while silhouetting with the cranial cardiac silhouette are classically thymic lymphoma. This is the most common cranial mediastinal mass in cats and frequently causes pleural effusion. Ectopic thyroid carcinoma is much rarer and tends to be more cranially positioned at the thoracic inlet. Cranial mediastinal cysts are uncommon and typically incidental; they would not show the described displacement pattern to the same degree. Branchial arch remnants are cervical, not thoracic.

Ultrasonography of a 9-year-old Labrador Retriever with suspected splenic disease reveals a large, heterogeneous splenic mass with multiple hyperechoic foci casting acoustic shadows, admixed with anechoic cavitations. The most likely diagnosis is:

Answer: Splenic myelolipoma

Splenic myelolipomas are rare benign tumors composed of mature adipose and hematopoietic tissue. On ultrasound, they appear as heterogeneous masses with hyperechoic foci (fat) causing acoustic shadowing and may contain cystic areas. This mixed echogenicity with shadowing foci is characteristic. Hemangiosarcoma is typically a large, heterogeneous cavitated mass but does not contain shadowing foci from calcification or fat. Nodular hyperplasia and extramedullary hematopoiesis are diffuse or multiple small nodular changes without the discrete mass architecture and shadowing foci described.

A radiograph of a 3-year-old Quarter Horse reveals periarticular bone proliferation and subchondral bone lucency at the distal interphalangeal joint with a linear radiolucent defect extending from the articular surface of the distal phalanx. This finding is MOST consistent with:

Answer: Subchondral bone cyst (osseous cyst-like lesion)

Osseous cyst-like lesions (subchondral bone cysts) of the distal phalanx in horses appear as radiolucent defects communicating with the articular surface, typically at the distal interphalangeal joint. The periarticular new bone production and the subchondral lucency with articular communication are hallmark findings. Palmar/plantar process fractures are linear lucencies at the palmar/plantar aspect of P3 not centered on the articular surface. Keratoma is a space-occupying soft tissue lesion within the hoof capsule causing focal pressure remodeling of the solar margin. Septic pedal osteitis produces irregular lysis without the discrete cyst-like architecture.

CT of the abdomen in a 5-year-old intact male dog reveals a retroperitoneal mass cranial to the kidney with heterogeneous contrast enhancement and central necrosis. The ipsilateral kidney is displaced ventrally. No primary tumor is identified. The MOST likely diagnosis is:

Answer: Paraganglioma (extra-adrenal pheochromocytoma)

Extra-adrenal paragangliomas (extra-adrenal pheochromocytomas) arise from chromaffin cells along the aorta and adjacent sympathetic ganglia, most commonly in the retroperitoneum cranial to or between the kidneys. On CT, they appear as large, heterogeneously enhancing masses with central necrosis that displace adjacent structures. Their location separate from the adrenal gland distinguishes them from adrenocortical carcinoma. Renal cell carcinoma would originate within the renal parenchyma. Retroperitoneal liposarcoma would contain macroscopic fat attenuation on CT. The intact male status and retroperitoneal location cranial to the kidney are classic for paraganglioma in dogs.

On a lateral skull radiograph of a rabbit, you observe bilateral tympanic bullae with significantly increased opacity and thickened walls. The petrous temporal bones appear sclerotic. This pattern is MOST consistent with:

Answer: Otitis media/interna with secondary bulla osteitis

In rabbits, bilateral tympanic bulla opacification with wall thickening and petrous bone sclerosis on skull radiographs is diagnostic of chronic otitis media/interna with bulla osteitis. Pasteurella multocida is the most common causative agent. This imaging triad—bulla opacity, bulla wall thickening, and petrosal sclerosis—represents chronic inflammatory and reactive bone change. Encephalitozoon cuniculi causes neurological signs but does not produce radiographic bulla changes. Thymoma is a thoracic disease. Cervical lymphoma would not produce intraosseous bulla changes visible radiographically.