Hepatic kinetics and magnetic resonance imaging of gadolinium ethoxybenzyl diethylenetriaminepentacetic acid (Gd-EOB-DTPA) in dogs

Evaluation of hepatic contrast enhancement with a hepatocyte-specific magnetic resonance imaging contrast agent (gadoxetic acid) in healthy dogs

OBJECTIVE

To determine, by means of MRI, the time to maximal contrast enhancement in T1-weighted images following IV administration of gadoxetic acid in healthy dogs and assess the impact of gadoxetic acid on the signal intensity of T2-weighted images.

ANIMALS

7 healthy dogs.

PROCEDURES

No hepatic abnormalities were detected during ultrasonographic examination. Each dog was anesthetized and positioned in dorsal recumbency for MRI. Transverse T1- and T2-weighted images of the liver were acquired prior to and following (at 5-minute intervals) IV injection of 0.1 mL of gadoxetic acid/kg. Signal intensity of the liver parenchyma was measured in 3 regions of interest in the T1- and T2-weighted images before and at various times point after contrast agent administration. Time versus signal-to-noise ratio curves were plotted to determine time to maximal contrast enhancement and contrast agent-related changes in signal intensity in T2-weighted images.

RESULTS

Analysis of T1-weighted images revealed that mean ± SD time to maximal enhancement after gadoxetic acid injection was 10.5 ± 3.99 minutes. Signal intensity of T2-weighted images was not significantly affected by gadoxetic acid administration. No injection-related adverse effects were observed in any dog.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that gadoxetic acid can be used for hepatic MRI in healthy dogs and the resultant hepatic enhancement patterns are similar to those described for humans. Maximal contrast enhancement occurred between 10 and 15 minutes after contrast agent injection; thus, T2-weighted images may be obtained in the interval between injection and maximal enhancement for a more time-efficient clinical protocol.

Delivery of diagnostic agents in computed tomography

Computed tomography is a reliable, widely available imaging modality with high spatial resolution. Except for trauma detection, in practically all cases contrast agents are administered for improving image quality. The presently available agents are nonspecific compounds with distribution in the extracellular space. Specificity (i.e. high contrast in selected tissues) is only possible when the pharmacokinetic behavior (peak levels at the region of the interest) is followed on a time scale which counts in seconds. If the peak time is missed, a second injection of contrast material is necessary. The objective of extensive research efforts have therefore been the search for tissue-specific contrast agents which should result both in higher and in longer lasting local concentrations. For the tissue targets, liver, spleen, lymph nodes and the blood pool, efficient contrast enhancing compounds have been described utilizing different active transport systems or passive diffusion-limiting molecular sizes. The agents include water-soluble metal chelates, iodinated polymeric compounds and contrast agent-carrying liposomes. The remaining task for the future will be to improve the tolerability of the contrast agents to such an extent that the side-effect incidence and severity is low enough to allow for the use of these agents in patients.

Hepatic kinetics and magnetic resonance imaging of gadolinium-EOB-DTPA in dogs

RATIONALE AND OBJECTIVES

To measure the hepatic uptake and biliary elimination kinetics of gadolinium (Gd)-EOB-DTPA in dogs.

METHOD

Two groups of four beagles each were anesthetized and given an intravenous bolus of 25 mumol/kg or 250 mumol/kg of Gd-EOB-DTPA. Blood, hepatic bile, and urine were collected over 140 minutes, and liver samples were obtained immediately after the dogs were killed. Conventional T1-weighted spin echo sequences of the liver were performed on a 1.5-Tesla (T) magnetic resonance imager during sampling. A ninth beagle received a bolus of 25 mumol/kg followed 140 minutes later with a bolus of 250 mumol/kg of Gd-EOB-DTPA. Wedge liver biopsies were obtained for Gd estimation at various times after dosing, and Gd concentration was measured by inductively coupled plasma atomic emission spectroscopy.

RESULTS

The plasma concentration of Gd-EOB-DTPA decreased in a biexponential manner with half-lives of approximately 4 minutes and 60 minutes for the distribution and elimination phase independent of the dose given. Gadolinium bile concentration reached peak values between 80 and 140 minutes: 6.3 +/- 1.6 mmol/L for the low dose (LD) and 11.6 +/- mmol/L for the high dose (HD). Bile Gd output was 62.0 +/- 8.8 (LD) and 78.3 +/- 30.2 (HD) nmol/minute-kg 50 to 80 minutes after injection. Gadolinium-EOB-DTPA was excreted by the biliary route to 24.8 +/- 2.6 (LD) and 3.6 +/- 1.2 (HD) percent of the dose within 140 minutes. Liver Gd concentration was 0.43 +/- 0.14 (LD) and 4.3 +/- 0.5 (HD) mmol/kg liver tissue at the conclusion of the studies. Calculated concentrations in the hepatocyte were 60 (LD) and 15 (HD) times higher than in plasma at 25 minutes after dosing. Whereas the low dose exhibited excellent contrast enhancement for the whole period, the high dose displayed a biphasic signal enhancement with a decreasing signal caused by the too-high hepatic gadolinium accumulation.

CONCLUSIONS

Transport of the Gd-EOB-DTPA into the hepatocyte exceeded elimination from hepatocyte to bile. The high dose defined a biliary transport maximum for Gd-EOB-DTPA of 78.3 +/- 30.2 nmol/minute-kg. The liver accumulation results from fast transport into the hepatocyte and rate-limited slower transport from hepatocyte to bile. The accumulation occurs against a strong concentration gradient, suggesting energy-dependent active transport into the hepatocyte.