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Background
The use of quantitative MRI (qMRI) for assessment of hepatic fat, iron, and fibrosis has been increasing due to improvements in vendor techniques, noninvasive nature of the examination, and avoidance of sampling error seen with liver biopsy.

Educational Goals / Teaching Points
In this educational exhibit, we will discuss the indications, advantages, and limitations of qMRI. Techniques for fat quantitation, iron quantification, MRI elastography (MRE) will be described, including physics, optimization of images, and troubleshooting artifacts. Multiple examples of each technique will demonstrate the calculation of clinically relevant quantitative values, interpretation pearls, and avoidance of common pitfalls.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
qMRI is an effective tool for quantifying hepatic steatosis. In and out of phase imaging has been used to detect liver fat for many years and exploits the resonant frequency differences between water and fat proton signals. Four- or six-point quantitative DIXON techniques take this a step further and allow for the quantification of liver fat content with Proton Density Fat Fraction (PDFF). This technique accounts for confounding factors such as T2* signal decay. It is important for clinicians and radiologists to differentiate percentage fat content provided by PDFF from the histologic steatosis grading. Histologic grading measures the percentage of hepatocytes containing fat globules whereas the PDFF gives a ratio of hepatic fat signal to the total liver signal. As a result, the PDFF percent is usually less than half the histologic steatosis percent. Iron overload, whether from primary genetic causes or secondary exogenous causes, uniquely affects the liver, which stores the body's excess iron. T2*/R2* relaxometry takes advantage of susceptibility artifact produced by magnetic field inhomogeneities from iron deposition by allowing for quantification of iron deposition based on T2*/R2* signal decay. This signal decay correlates linearly with the iron content, as demonstrated by Wood et. al. [Fe] = 0.202 + 0.0254 x R2*. Noninvasive hepatic fibrosis quantification with MRE uses mechanical waves to evaluate the stiffness of hepatic tissue. Unlike biopsy, this technique samples a larger portion of the liver and is less susceptible to sampling error. Earlier techniques utilized Gradient Recalled Echo (GRE) based sequences and were susceptible to signal degradation from T2* effects when iron deposition is present. Newer Echo Planar Imaging (EPI) techniques are much more resistant to signal degradation.

Conclusion
qMRI is performed by altering the many variable parameters of magnetic resonance imaging to determine the specific physical makeup of a tissue. It can be clinically superior for patients with hepatic steatosis, iron deposition, and fibrosis/cirrhosis, by assessing the extent of disease, without invasive biopsy. In this way, qMRI is a noninvasive assessment of tissue histology, that is safer, easily reproducible, and less prone to sampling error.