Liver fibrosis imaging: A clinical review of ultrasound and magnetic resonance elastography

Abstract

Liver fibrosis is a histological hallmark of most chronic liver diseases, which can progress to cirrhosis and liver failure, and predisposes to hepatocellular carcinoma. Accurate diagnosis of liver fibrosis is necessary for prognosis, risk stratification, and treatment decision-making. Liver biopsy, the reference standard for assessing liver fibrosis, is invasive, costly, and impractical for surveillance and treatment response monitoring. Elastography offers a noninvasive, objective, and quantitative alternative to liver biopsy. This article discusses the need for noninvasive assessment of liver fibrosis and reviews the comparative advantages and limitations of ultrasound and magnetic resonance elastography techniques with respect to their basic concepts, acquisition, processing, and diagnostic performance. Variations in clinical contexts of use and common pitfalls associated with each technique are considered. In addition, current challenges and future directions to improve the diagnostic accuracy and clinical utility of elastography techniques are discussed. Level of Evidence: 5 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:25-42.

Figure 1.

Illustrations (a, b, and c; all by XXX and XXX) of current quantitative ultrasound-based elastography methods (not drawn to scale). Vibration-controlled transient elastography (VCTE, a) uses a probe to transmit mechanical vibrations through the skin surface and body wall (back and forth blue arrows). Point shear wave elastography (pSWE, b) and two-dimensional shear wave elastography (2D SWE, c) use a probe to generate acoustic radiation force impulse (ARFI) within the liver (blue dots). The wavy blue arrows indicate the induced shear waves and their direction. The rectangles outlined in white (a and b) indicate the interrogated cylindrical volume and the user-defined region of interest (ROI); the green-colored trapezoid and the white circle within (c) indicate the elastogram and the user-defined ROI, respectively.

Figure 2.

VCTE scanner (a) with the M probe (b) is used to acquire controlled attenuation parameter (CAP, c) and Time Motion (TM) and Amplitude (A) mode shear wave propagation images (d). The CAP estimate of attenuation in units of dB/m is shown in (c). The 5th measurement of liver stiffness in units of Young’s modulus (kPa) is shown in (d). The y-axis is distance from skin, x-axis is time. Slope of the dashed line represents shear wave speed. Median values of these 10 measurements are calculated for stiffness and attenuation assessment. CAP is an integrated technology that quantifies steatosis severity at the same time as liver stiffness assessment on VCTE.

Figure 3.

2D-SWE technology is available in multiple devices manufactured by various vendors. Common features are colored polygonal elastogram maps and circular ROIs. The user selects the location of the elastogram away from overlying vessels and rib shadows and then places an ROI within a representative portion of the elastogram as described below. Mean stiffness or SWS values from the ROIs are reported. a) General Electric LOGIQ E9 ultrasound device. b) Supersonic Aixplorer ultrasound device. Stiffness value in kPa is presented at left corner of the image. Standard gray scale image is used as a guide. Tissue stiffness (c) or shear wave speed (d) can be presented in the same scanning session using the same device; images from Toshiba APLIO500 are presented as examples. Manufacturers add new software into the systems to increase the accuracy of the SWE results and decrease variability. For example, Toshiba system calculates the e) Contour map, f) Variance map. Based on the manufacturer’s suggestions in contour map (e), operator located ROI in an area where propagation lines (colored lines) are parallel to each other. In variance map (f), operator located ROI in an area away from ‘extreme’ areas (yellow/red).

Figure 4.

Transverse colorized MR elastograms (3T GE 750 scanner using 2D GRE technique, top) and ultrasound-based two-dimensional shear wave elastography images with placement of regions of interest filled in with colorized elasticity (GE Logiq E9 with C1–6 transducer, bottom) demonstrate increasing stiffness estimates (kilopascals, kPa) or shear wave speed estimates (meters per second) with increasing liver fibrosis stage as determined on histology (Brunt system) in patients with nonalcoholic fatty liver disease. From left to right: stage 0 in 53-year-old man, stage 1 in 49-year-old man, stage 2 in 55-year-old woman, stage 3 in 68-year-old woman, and stage 4 in 72-year-old woman. Regions of interest, an automated 95% confidence grid, and estimated magnitude of complex modulus (“shear stiffness”) values in kPa are overlain on the MR elastograms. Shear wave speed estimates are overlain on the ultrasound images.

Figure 5.

MR elastography involves several steps. A driver system generates and transmits longitudinal waves into the patient. Some of the wave energy is converted into shear waves via “mode conversion” (not shown). A phase-contrast pulse sequence with motion-encoding gradients images the shear waves at several (typically three or four) “phase offsets” to capture different phases of the wave cycle; one phase cycle is shown. Four or more slices are usually acquired; one slice is shown. Raw phase and magnitude images are generated. Post-processing produces wave images and colorized magnitude-of-complex-shear-modulus (“shear stiffness”) maps known as elastograms for each acquired slice. An analyst places a region of interest on each elastogram. A mean “shear stiffness” value in units of kPa is calculated from all pixels contained in all ROIs in all slices.

Figure 6.

MR elastography performed on a 3T GE 750 scanner by using a two-dimensional (2D) gradient-recalled echo (GRE) sequence (top row) and a 2D spin-echo-based echo planar imaging (SE-EPI) sequence (bottom row) in a 54-year-old man with nonalcoholic fatty liver disease (NAFLD) and fibrosis stage 0 on histology (Brunt system). Shown from left to right for each sequence: magnitude image, transverse colorized wave image, and transverse colorized elastogram. Using magnitude images as reference, regions of interest (ROIs) are drawn over the right liver lobe by an analyst on the wave images. Areas with poor wave propagation, blood vessels, and inhomogeneous liver parenchyma are avoided. The ROIs are propagated to the elastograms for mean “shear stiffness” estimates (kPa). An automated 95% confidence grid, appearing as a cross-hatched pattern within the ROIs, is placed over areas of unreliable data that do not contribute to the mean stiffness estimates on the elastograms.