Biliary contrast agents

Biliary Excretion. The effects of significant hepatic extraction as a result of biliary secretion, with or without metabolism, would be expected to follow the same principles just outlined for hepatic metabolism. In fact, a whole class of compounds that serve as biliary contrast agents for radiological examination depend on significant first-pass biliary secretion to be effective. 

Structural features such as electrical charge, hydroxyl groups in the side-chains and amino atoms of the amide groups reduce the lipophilicity (increase the hydrophilicity) whereas additional methyl groups sitting on amino moieties or free positions on the benzene ring increase the fipophilicity. Biliary contrast agents with a free position have > 5. 

In this section the first two approaches, modification of biliary contrast agents and iodination of nutritional lipids will be described. 

Fig. 7. Comparison of the two different types of extracellular and biliary contrast agents. One monomeric and one dimeric example are illustrated from each type, lopromide and iotrolan (extracellular type) are extremely hydrophilic whereas iopodinic acid and iotroxic acid (biliary type) are both ionic and very lipophilic... 

The mono-morpholide of tetraiodophthalic acid has been tested as biliary contrast agent (Fig. 11). It is easily synthesized by tetraiodination of phthalic acid using concentrated sulphuric acid and iodine. Although its diagnostic characteristics were quite satisfactory, the compound did not find clinical use due to toxicity [77]. 

Weichert also synthesized sterol esters of triiodinated biliary contrast agents [83], such as cholesteryl, pregnenolone and dehydroepandrosterone iopanoate (Fig. 13). Only the first compound accumulated in the liver. The reason probably was its resistance to hydrolysis. 

Fig.3.7a-d. MDCT in the arterial phase (a) and MRI with the hepato-biliary contrast agent Gd-EOB-DTPA in the arterial phase (b), portal venous phase (c) and liver-specific phase 20 min after injection (c) each examined with a Tl-w 3D GRE sequence with fat-saturation in a patient with a neuroendocrine tumor and liver metastases. The hypervascular metastases were not detected with MDCT. In the arterial phase after Gd-EOB-DTPA injection hyperintense lesions are demarcated. The hypervascular metastases show a wash-out to isointensity in the portal venous phase. With Gd-EOB-DTPA liver-specific phase imaging can be performed in addition to the early dynamic phase. Note the signal increase in the normal liver parenchyma in (d) caused by the physiological Gd-EOB-DTPA up-take whereas the suspected liver metastases are demarcated as areas spared from specific Gd-EOB-DTPA up-take... 

Zandrino E, Benzi L, Ferretti ML et al (2002) Multislice CT cholangiography without biliary contrast agents technique and initial clinical results in the assessment of patients with biliary obstruction. Eur Radiol 12 1055-1061... 

While conventional CT protocols do not allow for evaluation of the intrahepatic biliary system an all-in-one CT imaging protocol after administration of a biliary contrast agent (CT cholangiography) has been proposed (Schroeder et al. 2005). A superior visualization of the biliary system on CT cholangiography compared to conventional T2-weighted MRCP techniques has been reported (Schroeder et al. 2005 Yeh et al. 2004). Schroeder et al. (2005) were able to visualize at least up to the second intrahepatic branch in all their liver donor candidates with a CT cholangiography protocol (Schroeder et al. 2005). In this study, MRCP allowed reliable visu-... 

Endoscopic retrograde cholangiopancreatography (FRCP) is an important procedure that is used to evaluate and treat diseases of the biliary tree and pancreas. By injecting contrast agents throngh a catheter... 

A variety of liver contrast agents have been developed for contrast-enhanced MR imaging of the liver, which are designed to overcome the limitations of extracellular low molecular gadolinium chelates. The two main classes of liver-specific contrast agents are the superparamagnetic iron oxide (SPIO) with uptake via the reticuloendothelial system (RES) mainly into the liver and spleen, and the hepatobiliary contrast agents with uptake into hepatocytes followed by variable biliary excretion. 

Fig. 20.2. Hepatocyte-specific contrast agents such as Gd-EOB-DTPA (Primovist, Schering, Germany) may enhance the efficacy of lesion detection. Immediately after the injection of the contrast agent perfusion phenomena known from classic Gd-chelates can be seen such as ring-like enhancement (arrowhead). Delayed imaging, during the uptake and hepatocellular storage phase, may reveal additional findings such as a further metastasis. [Note the contrast within the biliary ducts based on the biliary elimination of the contrast agent (arrowhead)]...

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