Hyeeun Jo 1, Sang-Kwon Lee 1, Sooyoung Choi 2, Miori Kishimoto 3, Kija Lee 1
Background
Vicarious excretion refers to the nonrenal elimination of iodinated contrast media, historically seen as a pathological sign associated with renal impairment. Recent human studies suggest it can occur in healthy individuals, typically through the hepatobiliary system leading to gallbladder opacification (GBO) and small intestinal luminal opacification (SILO). This study aimed to assess the timing, patterns, and influencing factors of vicarious contrast medium excretion in healthy dogs using delayed-phase CT imaging.
Methods
This prospective, crossover study involved eight clinically healthy Beagle dogs scanned under fed and fasted conditions. Each dog underwent multiple CT scans at specified intervals (pre-contrast and 90 seconds, 10 minutes, 1, 4, 12, and 24 hours post-contrast). Iohexol was intravenously administered, and CT images were evaluated for GBO and SILO. GBO was graded on a 0–5 scale based on Hounsfield Unit differences and distribution, while SILO was assessed by intensity and distribution across intestinal segments.
Results
GBO began as early as 1 hour post-injection, with higher intensity and longer duration in fasted dogs, peaking at 12 hours. Feeding reduced GBO, likely due to gallbladder contraction and bile release. SILO appeared earlier, at 10 minutes, in 75% of cases, and persisted up to 24 hours with variable segmental distribution. GBO scores differed significantly within groups over time and between groups at 12 hours. The variability of SILO suggested a complex interplay of factors including sedation effects and individual anatomical or physiological differences.
Limitations
The small sample size limited generalizability, and medetomidine sedation might have influenced biliary dynamics, potentially affecting contrast distribution. The presence of intestinal content may have confounded SILO assessment. Additionally, individual variability was noted even under controlled conditions.
Conclusions
Both GBO and SILO are normal post-contrast phenomena observable on delayed CT in healthy dogs. Fasting enhances GBO intensity and duration, while SILO occurs earlier and is less influenced by feeding state. These findings highlight the need to recognize vicarious excretion as a non-pathologic process in delayed imaging, aiding in accurate interpretation of post-contrast CT scans in veterinary diagnostics.
Representative CT images of GBO scores in a soft tissue algorithm viewed in a soft tissue window (window width 400, window level 40). A, Grade 0: identical to noncontrast attenuation (HU). B, grade 1: differences in HU exceeding 100 within less than 10% of the gallbladder area, independent of gravity. C, Grade 1: mild differences (20–50 HU) and a gravity-dependent layered structure occupying less than 40% of the gallbladder (arrow). D, Grade 2: mild differences (20–50 HU) and a gravity-dependent layered structure occupying less than 40% of the gallbladder. E, Grade 3: ROI differences of 20–50 HU in most of the gallbladder, with localized areas (<20% of the gallbladder) showing differences of 50–100 HU. F, Grade 4: ROI differences of 20–50 HU in most of the gallbladder, with larger regions (20–60%) exhibiting differences of 50–100 HU. G, Grade 5: differences of 50–100 HU in most portions of the gallbladder. H, Grade 5: differences in HU exceeding 100 within more than 50% of the gallbladder area.
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