Short name: Gd-DTPA, generic name: Gadopentetate dimeglumine, chemical compound: Gadolinium-diethylenetriaminepentaacetic acid
Gadopentetate dimeglumine was introduced in 1981, as the first paramagnetic MRI contrast agent (ionic). The Gd-induced dipole dipole interactions lead to shortening of T1, which results in contrast enhancement on T1 weighted images. The used metal ion Gd3+ (gadolinium) is toxic, and therefore bound in the renally excreted DTPA chelate, a very stable complex. The Gd-complex also induce susceptibility effects, as a result of the magnetic field gradient between the contrast agent in the blood vessels and the surrounding tissue, that lead to shortening of T2 or T2*.
Following intravenous administration, the compound is distributed rapidly in the extracellular space and is eliminated unchanged by glomerular filtration via the kidneys. Up to 6 hours, post injection an average of 83% of the dose is eliminated renal.

See also Magnevist®, Gadolinium and Contrast Agents.

Short name: Gd-EOB-DTPA, generic name: Gadoxetic acid, chemical compound: Gadolinium ethoxybenzyl diethylenetriaminepentaacetic acid
Primovist™ (formerly Eovist®) is a nonionic water-soluble liver tissue-specific MRI contrast agent with positive enhancement. The agent allows detection and differentiation of hepatic tumors due to specific uptake by healthy liver tissue, but not by tumors in the affected organ. Gd-EOB-DTPA has hepatocyte-specific properties, but provides earlier perfusion information acquired within first ten minutes. Hepatocyte uptake generally occurs beyond 10 minutes.

Liver imaging can be performed with sonography, computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound is, caused by the easy access, still the first-line imaging method of choice; CT and MRI are applied whenever ultrasound imaging yields vague results. Indications are the characterization of metastases and primary liver tumors e.g., benign lesions such as focal nodular hyperplasia (FNH), adenoma, hemangioma and malignant lesions (cancer) such as hepatocellular carcinomas (HCC). The decision, which medical imaging modality is more suitable, MRI or CT, is dependent on the different factors. CT is less costly and more widely available; modern multislice scanners provide high spatial resolution and short scan times but has the disadvantage of radiation exposure.
With the introduction of high performance MR systems and advanced sequences the image quality of MRI for the liver has gained substantially. Fast spin echo or single shot techniques, often combined with fat suppression, are the most common T2 weighted sequences used in liver MRI procedures. Spoiled gradient echo sequences are used as ideal T1 weighted sequences for evaluating of the liver. The repetition time (TR) can be sufficiently long to acquire enough sections covering the entire liver in one pass, and to provide good signal to noise. The TE should be the shortest in phase echo time (TE), which provides strong T1 weighting, minimizes magnetic susceptibility effects, and permits acquisition within one breath hold to cover the whole liver. A flip angle of 80° provides good T1 weighting and less of power deposition and tissue saturation than a larger flip angle that would provide comparable T1 weighting.
Liver MRI is very dependent on the administration of contrast agents, especially when detection and characterization of focal lesions are the issues. Liver MRI combined with MRCP is useful to evaluate patients with hepatic and biliary disease.
Gadolinium chelates are typical non-specific extracellular agents diffusing rapidly to the extravascular space of tissues being cleared by glomerular filtration at the kidney. These characteristics are somewhat problematic when a large organ with a huge interstitial space like the liver is imaged. These agents provide a small temporal imaging window (seconds), after which they begin to diffuse to the interstitial space not only of healthy liver cells but also of lesions, reducing the contrast gradient necessary for easy lesion detection. Dynamic MRI with multiple phases after i.v. contrast media (Gd chelates), with arterial, portal and late phase images (similar to CT) provides additional information.
An additional advantage of MRI is the availability of liver-specific contrast agents (see also Hepatobiliary Contrast Agents). Gd-EOB-DTPA (gadoxetate disodium, Gadolinium ethoxybenzyl dimeglumine, EOVIST Injection, brand name in other countries is Primovist) is a gadolinium-based MRI contrast agent approved by the FDA for the detection and characterization of known or suspected focal liver lesions.
Gd-EOB-DTPA provides dynamic phases after intravenous injection, similarly to non-specific gadolinium chelates, and distributes into the hepatocytes and bile ducts during the hepatobiliary phase. It has up to 50% hepatobiliary excretion in the normal liver.
Since ferumoxides are not eliminated by the kidney, they possess long plasmatic half-lives, allowing circulation for several minutes in the vascular space. The uptake process is dependent on the total size of the particle being quicker for larger particles with a size of the range of 150 nm (called superparamagnetic iron oxide). The smaller ones, possessing a total particle size in the order of 30 nm, are called ultrasmall superparamagnetic iron oxide particles and they suffer a slower uptake by RES cells. Intracellular contrast agents used in liver MRI are primarily targeted to the normal liver parenchyma and not to pathological cells. Currently, iron oxide based MRI contrast agents are not marketed.
Beyond contrast enhanced MRI, the detection of fatty liver disease and iron overload has clinical significance due to the potential for evolution into cirrhosis and hepatocellular carcinoma. Imaging-based liver fat quantification (see also Dixon) provides noninvasively information about fat metabolism; chemical shift imaging or T2*-weighted imaging allow the quantification of hepatic iron concentration.

See also Abdominal Imaging, Primovistâ„¢, Liver Acquisition with Volume Acquisition (LAVA), T1W High Resolution Isotropic Volume Examination (THRIVE) and Bolus Injection.

For Ultrasound Imaging (USI) see Liver Sonography at Medical-Ultrasound-Imaging.com.

Magnevist® is a paramagnetic ionic contrast agent for use in magnetic resonance imaging. Contrast enhanced MRI with Magnevist® allows additional diagnostic information of tumors, inflammation and vascular lesions and the determination or differentiation of such lesions.
The contrast enhancing effect is produced by the di-N-methylglucamine salt of gadopentetate (Gd-DTPA), the gadolinium complex of diethylenetriamine pentaacetic acid. Magnevist® has the strongest effect on T1 weighted images, by increasing T1 signal intensity in tissues where Magnevist® has accumulated.

WARNING: NEPHROGENIC SYSTEMIC FIBROSIS Gadolinium-based contrast agents increase the risk for nephrogenic systemic fibrosis (NSF) in patients with acute or chronic severe renal insufficiency (glomerular filtration rate less than 30 mL/min/1.73m²), or acute renal insufficiency of any severity due to the hepato-renal syndrome or in the perioperative liver transplantation period.

See also Ionic Intravenous Contrast Agents and Gadopentetate Dimeglumine.

Magnevist® enteral (Gadopentetate) is a Gd-DTPA solution for use as a gastrointestinal contrast agent to visualize the bowel in MRI scans.

See also Positive Oral Contrast Agents and Gadopentetate Gastrointestinal.