Magnetic resonance imaging (MRI) of the liver is performed to define focal liver lesions as well as to confirm and validate findings from other imaging studies. In contrast to CT, MRI has advantages of avoiding ionizing radiation and iodinated contrast, superior soft tissue contrast, ability to characterize smaller lesions (< 2 cm), and lesions with fat and iron. Disadvantages of MRI include its high cost, lower spatial resolution, increased imaging time, risk of nephrogenic systemic fibrosis associated with gadolinium, and poor image quality in uncooperative patients.

Several MRI protocols, modalities, and sequences are available, a combination of which allows for the evaluation of fat and iron content, detection and characterization of liver lesions, and also allows for accurate assessment of the biliary and vascular tree [21]. Accurate timing of the different vascular phases is essential for good quality imaging studies [22].

MRI can detect moderate to severe cirrhosis by identification of fibrotic septa, which are seen as subtle parenchymal reticulations with low T1 and high T2 signal intensity. These septa become more conspicuous on T1-weighted gadolinium-enhanced images acquired during the delayed phase due to delayed washout of fibrotic tissue compared to the surrounding liver. Enlargement of the hilar periportal space and the gallbladder fossa has been shown to be a helpful sign at MRI in the diagnosis of early cirrhosis [23]. Findings on MRI may also have prognostic value. A study of MRI in patients with compensated cirrhosis suggested that, a large spleen, the presence of varices or collaterals, and a higher volume index of caudate lobe to right lobe correlate well with clinical progression to decompensated cirrhosis [24].

Several iterations of MRI-based imaging have been investigated and aid in the diagnosis of cirrhosis. For example, diffusion-weighted imaging avoids the need for IV contrast and produces images of tissues weighted with the local structural properties of water diffusion. It can be used to characterize focal hepatic masses and assess liver fibrosis. Several studies have described lower apparent diffusion coefficient value in patients with varying degrees of cirrhosis as compared with healthy individuals undergoing diffusion-weighted MRI [25].

Elastography is a relatively novel method to assess the intrinsic property of the liver parenchyma, or liver stiffness , with elevated stiffness associated with presence of advanced fibrosis . Three specific US techniques include transient elastography (TE), acoustic radiation force impulse imaging (ARFI), and shear wave elastography (SWE) [26].

Ultrasound-based TE is a single dimension-based technique that measures the velocity of a low frequency 50-Hz elastic shear wave propagating through the liver. This propagation is directly related to tissue stiffness, called the elastic modulus (E). This is expressed as E = 3pv2, where v is the velocity and p is the density of tissue that is assumed to be constant. Shear waves propagate faster in stiffer tissue. The volume that is measured is 10 mm × 40 mm long, lying 26–65 mm below the skin surface. Results are expressed in kilopascals and range from 2.5 to 75 kPa with a normal value of approximately 5 kPa and a value generally above 12–14 kPa implying the presence of cirrhosis.

ARFI is based on the measurement of the velocity of short-duration acoustic pulses generated by mechanically exciting liver tissue through manual compression by the US probe. The shear wave velocity is measured in a smaller region. Mechanical excitation of tissue using short-duration acoustic pulses propagates shear waves and generates localized u-scale displacement in tissue. Displacement results in short shear-wave propagation away from the region of excitation are tracked. The shear wave velocity is examined and increases with increasing severity of fibrosis. The median value for a cirrhotic liver is ~ 1.8 m/s.

SWE, a newer technique has the ability to image liver stiffness in real time and allows the operator to choose the size and location of the region of interest. Series of push pulses create plane shear waves that propagate over a region of interest (ROI). The speed is estimated by Doppler-like acquisition. Color-coded two-dimensional quantitative SWE images of tissue stiffness are obtained in real time. A circular ROI is defined to measure stiffness. Values are approximately 12–14 kPa for cirrhosis (Fig. 6.2).

US-based techniques have the advantage of being an outpatient procedure that takes a few minutes. TE has excellent reproducibility for interobserver and intraobserver agreement. Limitations of this technique include incomplete examinations (approximately 20 %), mostly driven by obesity, ascites, and operator experience. Fibrosis may be overestimated in cases of acute hepatitis, cholestasis, and passive congestion.

Magnetic resonance elastography (MRE) is an MRI-based technique to evaluate the mechanical properties of tissue (Fig. 6.3) [27]. Mechanical shear waves are generated within the liver and the propagating waves are imaged using special MRI sequences. An active driver sends 60 Hz acoustic vibrations through a connected 7.6-m-long plastic tube to a passive pneumatic driver that is placed against the chest and upper abdomen of the patient. Measurements of liver stiffness are obtained. Based on previous studies, normal liver stiffness ranges between 1.5 and 2.9 kPa. Cirrhosis is often seen in persons with values greater than 5.2 kPa. Contraindications to performing MRE include cardiac pacemaker and severe claustrophobia. MRE performs well in pediatric patients, obese patients, and those with ascites as well as posttransplant patients. Other advantages include a lower rate of incomplete examinations as compared to US-based elastography. MRE allows for evaluation of morphological changes (MRI component) as well as provides a map of liver stiffness over the entire liver surface. Causes of elevated stiffness may include acute biliary obstruction and passive congestion due to congestive heart failure or elevated central venous pressure [27, 28]. Disadvantages of MRE include its inability to be successfully performed in patients with high iron overload because of signal-to-noise-limitations, cost, and longer examination time.