Metatarsal pad iohexol administration in cats undergoing computed tomography lymphangiography yields superior thoracic duct enhancement versus perirectal administration

Objective: To compare the frequency of thoracic duct (TD) enhancement, time to enhancement, and quality after metatarsal pad (MTP) and perirectal region (PR) administration of iohexol in cats undergoing CT lymphangiography (CTLA). Methods: Purpose-bred, healthy cats underwent 2 sedated CTLA at a veterinary teaching hospital between August and October 2025, during which MTP or PR iohexol was administered in a cross-over, randomized design. Frequency of TD enhancement, time to enhancement, quality of enhancement, and Glasgow Feline Composite Measure Pain Scale scoring were performed upon recovery and at 1, 4, and 7 days after each scan. Results: Bilateral TDs were appreciated in all 7 cats. Seven out of 7 and 6 out of 7 cats after MTP and PR iohexol administration, respectively, achieved TD enhancement. Metatarsal pad administration resulted in superior quality TD enhancement by both subjective grading (median MTP, grade 2 [IQR, 2 to 3]; median PR, grade 1 [IQR, 0 to 2]) and measured HU at T10-T12 (MTP, 452.5 ± 130.4 HU; PR, 91.17 ± 19.5 HU). Mild discomfort, which resolved spontaneously within 24 hours of injection without additional treatment, was appreciated in 5 of 7 and 3 of 7 upon recovery after MTP and PR, respectively. Conclusions: Both MTP and PR iohexol administration are safe and effective in cats undergoing CTLA. However, MTP iohexol administration results in superior TD enhancement. Clinical Relevance: Although both MTP and PR iohexol administration effectively highlight individual TD morphology on CTLA in cats, MTP might represent a better choice for understanding the preoperative anatomy of the TD.