MR Elastography in Medical Imaging
MR elastography is the most accurate noninvasive imaging technique for detecting and staging liver fibrosis, capable of analyzing almost the entire liver with superior performance in obese patients and those with ascites compared to ultrasound-based methods. 1
Technical Principles
MR elastography uses phase-contrast MRI to quantitatively measure liver stiffness by visualizing propagating shear waves through tissue. 1
How the Technique Works
An active driver generates shear waves at a fixed frequency of 60 Hz, transmitted through a passive driver (plastic tube) positioned on the right lobe of the liver at the intersection of the right midclavicular line and xiphoid process 1
The amplitude is typically set at 50% but can be adjusted based on abdominal wall thickness to optimize wave transmission and image quality 1
Magnitude and phase images are acquired at end-expiration across four axial levels, then processed using a multimodel direct inversion algorithm to produce grayscale elastograms, color elastograms, and wave propagation images 1
Results are expressed in kPa using a formula that determines the shear modulus (equivalent to one-third the Young's modulus used in transient elastography) 1
Measurement Protocol
Patients must fast for at least 4 hours before testing to avoid falsely elevated liver stiffness from postprandial blood flow 1
Liver stiffness is quantified by drawing regions of interest (ROIs) on grayscale elastogram images and calculating the weighted arithmetic mean: (m₁w₁ + m₂w₂ + m₃w₃ + m₄w₄) ÷ (w₁ + w₂ + w₃ + w₄), where m represents average stiffness values and w represents area sizes from each slice 1
Critical areas to avoid when drawing ROIs include: within 1 cm of liver capsule, gallbladder fossa, around major intrahepatic vessels, and "hot spots" (focal areas of higher stiffness near the liver dome or beneath the passive driver) 1
Areas covered by a 95% confidence grid must be carefully excluded to prevent measurement errors 1
Clinical Advantages Over Other Modalities
MR elastography demonstrates the highest diagnostic performance among all noninvasive methods for liver fibrosis assessment. 1
Superior Technical Capabilities
Can analyze almost the entire liver volume, unlike ultrasound-based methods that sample only small regions 1
Maintains excellent applicability in obese patients and those with ascites, where ultrasound elastography frequently fails 1
Has the fewest unreliable examinations compared to all other elastography techniques 1
Can simultaneously evaluate for hepatocellular carcinoma during the same examination 1
Distinguishes intermediate stages of fibrosis with good sensitivity (73%-91%) and specificity (79%-85%), which ultrasound methods cannot reliably achieve 1
Comparison to Ultrasound Elastography
Ultrasound elastography (transient elastography) shows unreliable measurements in 35.4% of obese patients, while MR elastography performance is unaffected by obesity 1
Point shear wave elastography (ARFI) demonstrates unreliable measurements in 17.6% of obese patients 1
In chronic HCV patients, MR elastography has little to no increased accuracy for ruling in cirrhosis compared to transient elastography, but has lower accuracy for ruling out cirrhosis (very low quality evidence) 1
Clinical Applications
Primary Role: Liver Fibrosis Staging
MR elastography serves as a safer, less expensive, and potentially more accurate alternative to invasive liver biopsy for diagnosis and staging of liver fibrosis 2
Multiple studies demonstrate strong correlation between MR elastography-measured hepatic stiffness and histologic fibrosis stage 2
Currently performed in more than 100 centers worldwide for liver fibrosis assessment 3
Risk Stratification in Chronic Liver Disease
Vibration-controlled transient elastography and MR elastography are the most commonly used imaging techniques to evaluate fibrosis in NAFLD/NASH patients 1
Vibration-controlled transient elastography identifies advanced fibrosis with 92% specificity 1
MR elastography identifies intermediate stages of fibrosis more readily than ultrasound methods, though it is not as widely available and is much more costly 1
Beyond Liver Applications
MR elastography is being investigated for brain, breast, blood vessels, heart, kidneys, lungs, and skeletal muscle pathologies 4
Animal studies demonstrate that MR elastography can detect inflammation and myofibroblast activation before extracellular matrix deposition, not just fibrosis 5
Critical Limitations and Contraindications
Technical Limitations
Cannot be performed in patients with hepatic iron overload due to signal-to-noise limitations 1
Failure rate of 4.3% has been reported 1
Currently too costly and time-consuming for routine practice use 1
Factors Causing False Elevation
Recent food intake (within 4 hours) artificially increases liver stiffness measurements 1, 6
Acute hepatitis and active inflammation overestimate fibrosis 6, 7
Congestive heart failure leading to liver congestion increases stiffness 6, 7
Alcohol consumption can transiently elevate readings 6
Amyloidosis causes falsely elevated measurements 6
Interpretation Requirements
MR elastography results must always be interpreted by specialists in liver disease according to clinical context, considering results of other biochemical, radiological, and endoscopic tests. 1
Quality Criteria
Automated software now exists that segments the liver, automatically draws ROIs avoiding major vessels, and transfers regions to elastogram images 1
Manual interpretation requires careful attention to avoid measurement error-prone areas 1
Clinical Decision-Making Algorithm
When to Use MR Elastography
First-line for obese patients where ultrasound elastography is unreliable 1
Preferred when intermediate fibrosis staging is clinically necessary 1
When ultrasound elastography yields indeterminate or conflicting results 1
When simultaneous HCC screening is needed 1