**2.3 Magnetic Resonance Imaging**

MDCT is usually preferred because it is more widely available and because it is a wellestablished technique for surveying the extrahepatic abdominal organs and tissues. However, MRI has an advantage in the characterization of focal lesions and is also preferred for patients who cannot receive intravenous iodinated contrast material or when concerns about the risk of radiation from repeated exposure to CT, as in children or young adults, exists. In general, MRI sensitivity varies from 85-90% and its specificity is up to 95%, although a comparison of the performance of MDCT vs. MRI needs to be reassessed periodically, considering the rapid evolution of both technologies and the increase in therapeutic options available.

Contrast media are of two types: the extracellular agents (gadolinium chelates) and the liver specific agents. Gadolinium is used for lesion detection and characterisation while liver specific agents are used as functional agents. The most commonly used substance in contrastenhanced dynamic MRI is extracellular gadolinium-chelate complex, which provides the greatest diagnostic sensitivity and specificity rates among cross-sectional techniques currently in use. The current standard MRI liver protocol includes a T2-weighted sequence, a T1 weighted sequence and a three-phase technique after administration of gadolinium (arterialdominant, portal venous and hepatic venous or interstitial). Like CT, the detection of CRC liver metastases using MRI is maximized during the portal venous phase.

The administration of organ-specific contrast agents with hepatocyte specificity (mangafodipir trisodium [MnDPDP], gadobenate dimeglumine [Gd-BOPTA]) or reticuloendothelial system specificity (superparamagnetic iron oxide [SPIO] particles, captured by Kupffer cells) allows an increase in the sensitivity and specificity of the method (Bluemke et al. 2000; Vidiri et al. 2004), but data about their benefits are controversial. Furthermore, these agents are generally costly and not widely available.

Diffusion-weighted MR imaging (DWI) is a recently introduced technique to depict differences in molecular diffusion caused by the random motion of molecules. It provides excellent tissue contrast based on molecular diffusion, which is different from ordinary T1 and T2-weighted images, without the need for a contrast agent (El Khodary et al. 2011). An additional benefit of DWI is the ability to derive quantitative indices, which may be important in the assessment of disease response to novel therapeutics, including antivascular and anti-angiogenic therapy, since conventional assessment based on measuring lesion size is insensitive to early, treatment-related changes (Koh et al. 2006). In summary, DWI is a simple and sensitive method for screening focal hepatic lesions and is useful for differential diagnosis (Koike et al. 2009).
