Contrast and acquisition technique prove critical in liver imaging

The choice of a contrast agent for MR imaging of the liver is highly dependent on the clinical problem. Extracellular gadolinium can characterize most lesions and detect hypervascular tumors, while superparamagnetic iron oxide (SPIO) particles detect smaller metastases and characterize hepatocellular lesions, according to Prof. Philip J. Robinson, a clinical radiologist at St. James' Hospital in Leeds, U.K.

"Combined use of Gd-SPIO agents is the most effective technique for detecting hepatocellular carcinoma in cirrhosis, characterizing focal nodular hyperplasia and adenoma, preoperative mapping of cholangiocarcinoma, and problem-solving with difficult lesions," he told ECR delegates at a special session on contrast-enhanced liver imaging.

SPIO particles help increase the time window for imaging, improve lesion/liver contrast, and provide access to pathology at the cellular level, enabling characterization of hepatocellular lesions and hemangiomas. They are strongly paramagnetic and cause local field inhomogeneities and loss of signal from the normal liver. Particles are cleared from plasma by reticuloendothelial cells in the first few minutes after injection (blood half-life of three to five minutes). About 80% are taken up into the liver, 15% into the spleen, and the remainder into bone marrow, Robinson said.

The two SPIO agents available for liver MRI are ferumoxides (Endorem, Guerbet) and ferucarbotran (Resovist, Schering). Ferumoxides comprise iron oxide crystals coated with dextran. The mean particle size is 150 nm, and the recommended dose is 15 micronol/kg of body weight, or half that amount at 1T and 1.5T. The contrast is diluted in 100 mL of 5% glucose solution and infused over 30 minutes. Images are optimal 30 to 120 minutes postinjection.

The ferucarbotran iron oxide particles are coated with carboxydextran, and the mean particle size is 60 nm. The agent has a blood half-life of three to five minutes, and optimal T2-weighted images are acquired at 10 to 40 minutes after injection. The recommended dose is 7 to 11 micronol/kg of body weight, administered as a bolus injection, which allows dynamic T1 acquisitions that show transient enhancement.

"As is always the case, it is the choice of techniques and treatments that are critical to the status and well being of the patient. Whether or not to use SPIO or another process is of course very subjective. It wholly depends on the needs of the patient," Robinson said. "Acquisition technique is critical. The efficacy of post-SPIO imaging is determined by the choice of pulse sequence parameters."

The use of contrast agents dramatically improves ultrasound's ability to detect and characterize focal liver lesions, said Dr. Thomas Albrecht, a radiologist at Freie University in Berlin. Contrast-enhanced ultrasound is about as sensitive as CT for detecting metastases and is comparable to MRI in characterizing focal lesions.

In the liver, ultrasound contrast agents are used for dynamic assessment of enhancement during the arterial, portovenous, and delayed phases. In the delayed phase, which is of particular value for detecting tumors, the bubbles pool in normal liver and spare malignant focal lesions such as metastases.

Ultrasound contrast can also be used as a tracer for functional studies of liver perfusion. Hepatic transit time is shortened in certain diseases, including cirrhosis and metastases. Transit time analysis of a microbubble bolus is a promising research tool for early detection of these diseases, Albrecht said.