NAVLE Diagnostic Imaging Interpretation 5

Question 1

A 7-year-old Labrador Retriever presents with progressive hindlimb paresis. MRI of the thoracolumbar spine reveals a T2-hyperintense, T1-isointense intradural-extramedullary mass at T10-T11 that shows homogeneous contrast enhancement. The most likely diagnosis is:

Accepted Answer

Meningioma

Answer

Meningiomas are the most common intradural-extramedullary spinal tumors in dogs. Their MRI characteristics include T2 hyperintensity, T1 isointensity relative to spinal cord, homogeneous contrast enhancement, and a 'dural tail' sign. They are extramedullary (outside the cord but inside the dura), distinguishing them from intramedullary tumors like astrocytomas. Nerve sheath tumors are also intradural-extramedullary but more commonly affect nerve roots and show heterogeneous enhancement. Nephroblastoma occurs in the spinal cord of young dogs but is intramedullary.

Question 2

On thoracic radiographs of a 4-year-old DSH cat, you identify a soft tissue opacity in the cranial mediastinum that causes dorsal displacement of the trachea and is associated with pleural effusion. Ultrasonography reveals a cystic structure with internal echogenic debris. Which diagnosis is MOST consistent with these findings?

Accepted Answer

Branchial cyst

Answer

Branchial (or thymic) cysts are uncommon but produce a cranial mediastinal cystic mass with internal debris on ultrasound — a key differentiating feature. Lymphoma is the most common cranial mediastinal mass in cats but is typically solid (not cystic) on ultrasound and rarely shows the cystic-with-debris pattern. Thymoma can have cystic components but tends to be more heterogeneously solid. The purely cystic appearance with debris is the hallmark of a branchial/thymic retention cyst. This distinction is critical because management differs markedly.

Question 3

A 10-year-old Quarter Horse presents with recurrent episodes of forelimb lameness that worsen after exercise. Radiographs of the navicular region reveal radiolucent 'lollipop' lesions (synovial invaginations) in the distal border of the navicular bone with concurrent cortical thinning of the flexor cortex. Nuclear scintigraphy shows markedly increased radiopharmaceutical uptake in the palmar aspect of the foot. Which finding on MRI would MOST specifically confirm active navicular disease (podotracheritis) versus normal aging change?

Accepted Answer

Ill-defined T1 hypointensity in the navicular bone correlating with the scintigraphic hot spot

Answer

Ill-defined T1 hypointensity (bone marrow edema pattern) within the navicular bone that correlates spatially with the scintigraphic hot spot is the MRI finding most specifically linked to active pathology (edema, vascular compromise, remodeling) rather than incidental aging change. T2 hyperintensity alone can represent fluid or edema but is less specific. Navicular bursal fluid (T2 fluid signal) can be physiologic. DDFT thickening is a complication but not specific for active navicular bone disease itself. The T1 hypointensity + scintigraphy correlation confirms the lesion is metabolically active.

Question 4

Thoracic CT of an 8-year-old Golden Retriever reveals a 2.5 cm pulmonary nodule with a 'halo sign' (ground-glass opacity surrounding a solid core). No other nodules are present. Tracheal wash cytology is negative. What is the MOST appropriate next diagnostic interpretation of the halo sign in this species?

Accepted Answer

It most commonly represents hemorrhage around the lesion and warrants evaluation for a fungal or vascular etiology

Answer

The CT 'halo sign' (ground-glass opacity around a solid nodule) in dogs most commonly reflects perinodular hemorrhage, seen with angioinvasive organisms (Aspergillus, other fungi) or vascular lesions, rather than representing a specific neoplastic pattern as in human radiology. In people, the halo sign is associated with invasive aspergillosis in immunocompromised patients — the same pathophysiology applies in veterinary patients. It is not pathognomonic for carcinoma, does not indicate cavitation (that would be a lucency within the nodule), and does not specifically indicate lymphatic spread. Fungal serology and bronchoalveolar lavage should be prioritized.

Question 5

On abdominal ultrasound of a 6-year-old intact female Rottweiler, the adrenal glands are both enlarged (left: 1.8 cm, right: 2.1 cm in maximum width) with rounded poles and heterogeneous echogenicity. ACTH stimulation test is equivocal. Urine cortisol:creatinine ratio is elevated on two samples. Low-dose dexamethasone suppression test (LDDST) shows failure to suppress. Which additional imaging finding would MOST strongly support pituitary-dependent hyperadrenocorticism (PDH) over adrenal-dependent hyperadrenocorticism (ADH)?

Accepted Answer

A pituitary mass >6 mm on contrast-enhanced MRI (pituitary macroadenoma)

Answer

A pituitary macroadenoma (>6 mm) on contrast-enhanced MRI is the most definitive imaging finding confirming PDH, as it directly identifies the source of excess ACTH. Bilateral adrenal enlargement is consistent with PDH but does not distinguish it from other bilateral adrenal diseases. Preserved corticomedullary ratio supports hyperplasia (PDH) but is not definitive. Unilateral adrenal mass >2 cm with contralateral atrophy is the classic finding for ADH (functional adrenal cortical tumor), not PDH. Adrenal mineralization can occur in both functional and non-functional masses.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Diagnostic Imaging Interpretation 5