2598 Fig. 3 DW-SS-EPI images for b=50 s/mm2 (a) and b=600 s/mm2 (b). c ADC map. The hepatocellular carcinoma in segment IV (arrowhead) shows a merely moderate signal loss from the b=50 s/mm2 to the b=600 s/mm2 DW-SS-EPI image. The cyst in segment II (arrow) 2 displays markedly high signal intensity on the b=50 s/mm DW-SS-EPI image and becomes isointense on the b=600 s/mm2 DW-SS-EPI image. On the corresponding ADC map, the tumour shows a low ADC value (1.18×10−3 mm2/s), whereas the cyst has a high ADC value (3.09×10−3 mm2/s) (reproduced with permission from Bruegel et al. [33]) the lower end of the range, an effect attributed to perfusion- induced ADC increases in the normal liver measured at b< 400 s/mm2 in particular [35]. The ADC of segment II, anterior region, exceeds those measured in three other liver segments and is less reproducible, probably because of the increased exposure of segment II to cardiac motion artefacts and the use of respiratory triggering [33]. Kwee at al. recently reported that ADC measurements in respiratory- triggered DWI are significantly higher and less reproducible than in breath-hold and free-breathing DWI [36]. Another application of DWI is its use for control of treatment. Different authors demonstrated a significant increase in ADC from before to shortly after transcatheter arterial chemoembolisation in large groups of patients with hepa- tocellular carcinoma [37, 38], indicating that DWI could be used as a predictor of the effectivity of treatment. The location in liver can be a factor affecting the outcome of DWI, although a recent study revealed that the ADC values measured in segments V–VIII, right lobe, of the liver parenchyma of patients with benign focal lesions, malignant focal lesions and in patients without any lesions are very similar [39]. The ADC values measured in fasted livers tend to be lower than those in fed livers, probably because the portal vein bulk increases significantly in all cases [40]. Fasting could thus be of help in the discrimination of hepatic metastases (ADC slightly elevated) from liver parenchyma. Viscoelasticity Morphological criteria such as size, contour and shape are used in the diagnosis of diffuse liver disease. Changes in these measures occur late in the course of the disease, usually at an irreversible stage. Alterations in stiffness can be assessed by manual palpation, and more objectively by ultrasound (since 1990) [41] or MR elastography (MRE) (since 1995) [42, 43]. In MRE, the tissue’s constitutive parameters are visualised by using motion encoding for imaging harmonic shear waves propagating in organs [42]. Shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency as the mechanical vibration. Liver fibrosis can be detected by making wave images of a single frequency from which the elastic and viscous moduli can be calculated [44–47]. In conventional MRE experiments the wave propagation speed and the wave-damping coefficient can be deduced by repeatedly applying variable driving frequencies. In a very simple model assuming that the medium is nonviscous, the shear modulus or stiffness (μ) of the tissue examined can be defined as: μ ¼ ðf λÞ2ρ (3) The wavelength λ (in metres) visible on each phase difference image is used to calculate μ, where f is the driving frequency (in Hz) and ρ is the mass density in kilograms per