In a preliminary study, Venkatesh et al. [5] studied 44 focal tumors (13 benign and 31 malignant) and found that the mean stiffness of the malignant lesions was significantly higher than benign tumors (10.1 vs. 2.7 kPa, p < 0.001), normal liver parenchyma (vs. 2.3 kPa, p < 0.001), and fibrotic livers (vs. 5.9 kPa, p < 0.001) (Fig. 5.3). The mean stiffness of benign tumors was not significantly different from normal liver parenchyma. Fibrotic livers were significantly stiffer than benign liver tumors. In this study, a threshold value of 5 kPa was able to accurately distinguish malignant from benign liver tumors (Fig. 5.3) with 100 % accuracy. Garteiser et al. [6] studied 72 FLLs (37 benign and 35 malignant) and found that absolute shear modulus and loss modulus were significantly higher in malignant tumors as compared to benign tumors. The loss modulus was found to be significantly better than storage modulus and shear modulus (0.774 vs 0.578 and vs.0.718 respectively) for discriminating malignant from benign lesions. These preliminary studies need to be supported by larger studies. In a recent study, Venkatesh et al. showed that MRE performs significantly better than DWI in differentiating primary neoplasms of the liver (0.98 vs.0.85, p = 0.008) [33].

Malignant tumors tend to be larger than benign tumors. However the tissue stiffness is dependent on composition and therefore stiffness of the FLL is probably independent of the size of the lesion (Fig. 5.4). Among the benign tumors, hemangiomas and hepatocellular adenomas are softer than focal nodular hyperplasia. Cholangiocarcinomas are stiffer than HCCs and may be dependent on relative amount of fibrosis, solid tissue, and necrotic components. Characterization of individual types of FLL using stiffness values may be possible and future studies in this direction are awaited. An example is demonstrated in Fig. 5.5. In this example of epithelioid angiomyolipoma, the lesion shows a predominantly fatty core surrounded by an intensely enhancing region. The central core region is softer while the surrounding region is stiffer.

Cystic or necrotic regions of large tumors, or those which become necrotic following treatment, may be demonstrated as areas of lower stiffness (Fig. 5.6). In a recent study, Pepin et al. demonstrated a significant decrease in tumor shear stiffness within 4 days of chemotherapy treatment in an animal model. A similar study by Li et al. [34] also demonstrated that a reduction in tumor viscoelasticity occurs earlier than changes in tumor ADC. Demonstration of change in MRE-derived shear stiffness may be useful as an early and sensitive biomarker of tumor response. This early reduction in stiffness may represent reduced vascularity and decreased interstitial pressure that can occur before necrosis results.