MR imaging ensures clarity in focal liver lesions

Differentiation between benign and malignant liver lesions is essential in patients with extrahepatic malignancies, in whom benign lesions are still more common than metastases. This classification is also crucial in otherwise healthy subjects with incidentally detected liver masses. Most such lesions can be characterized on ultrasound and CT in terms of their tissue character and resectability. Multislice CT, with its superb demonstration of vascular structures, is especially suitable for determining resectability. In some cases, however, the tissue character of a lesion remains problematic. As many as 30% of hemangiomas may demonstrate atypical contrast-enhancement patterns on CT. Additional tissue information that can confirm a lesion's status as malignant or benign may help avoid unnecessary biopsy or surgery. The characterization of lesions depicted in cirrhotic patients (regenerative nodule versus dysplastic nodule versus early hepatocellular carcinoma) is also important but, due to its complexity, beyond the scope of this article. MR imaging has already proved to be an efficient method of detecting focal liver lesions. The sensitivity of contrast-enhanced MRI using superparamagnetic iron oxide particles is reported to be at least equal to that of CT.1 MRI has also emerged as a powerful modality for the characterization of such lesions. Its main advantage lies in the availability of multiple techniques and sequences. Useful parameters include the signal intensity of lesions on moderately and heavily T2-weighted images and on T1-weighted images, including chemical shift (in-phase and opposed-phase gradient-echo) images. Patterns of contrast enhancement following administration of extracellular agents and agents specific to the hepatobiliary and reticuloendothelial systems can also assist diagnosis decision making.2 This opportunity to evaluate diverse and complementary data enables MRI to characterize liver lesions with better accuracy than CT in many instances. CT-based characterization relies mostly on dynamic enhancement patterns. T1AND T2-WEIGHTED IMAGING Evaluation of heavily T2-weighted images is an extremely accurate method of discriminating so-called nonsolid lesions from solid tumors. The most frequently encountered nonsolid lesions contain fluid (cysts, abscesses) or are composed of vascular channels filled with blood (hemangiomas). These features lead to a prolonged T2 relaxation time, and nonsolid lesions have markedly increased signal on T2-weighted MRI (Figure 1). Differences in signal intensity between solid and nonsolid tumors can be enhanced on heavily T2-weighted images (TE > 160 msec). Nonsolid lesions maintain high signal intensity, while the signal intensity of solid masses decreases noticeably. Quantitative analysis of T2 relaxation times, derived from spin-echo, turbo spin-echo, and echo-planar imaging, may also be used to discriminate solid from nonsolid liver lesions. Thresholds of 112 msec to 130 msec have been proposed for turbo spin-echo sequences. These have yielded high sensitivity (94% to 100%) and specificity (91% to 93%).3-7 We use the 116 msec threshold at our institution. The technique proved to be 96% sensitive and 93% specific when applied to a group of 168 patients with 292 focal lesions. Such quantitative analysis is slightly more time consuming than a qualitative assessment of signal intensity from heavily T2-weighted MRI. It is, however, more accurate in differentiating between solid and nonsolid lesions. All calculations can be performed easily on a PC with an Excel spreadsheet. Both quantitative and qualitative analyses enable confident diagnosis of hemangioma and cyst, the two most common benign liver lesions, as well as the less common hepatic abscess.4-7 Making a distinction between nonsolid and solid liver lesions is not the same as identifying a mass as benign or malignant, however. Nonsolid lesions may occasionally be malignant, and some solid tumors are