Improved understanding of common artifacts in liver MRI and methods to mitigate these artifacts can improve the robustness of MR examination and increase diagnostic yield of these studies.
Target Audience: Radiologists, MRI technologists, trainees and students
Objective: Understand various types of artifacts, causes of these artifacts, and how to mitigate these in clinical practice in liver (abdominal) MR imaging.
Purpose: Liver MRI is challenging due to lack of robustness and presence of image artifacts. These artifacts can result from involuntary patient motion, respiratory motion, bowel peristalsis, and bowel gas to name a few. In addition some of the artifacts can be specific to certain acquisition schemes. Understanding what these artifacts look like, what causes them, and subsequent mitigation of these artifacts can improve image quality and improve the diagnostic utility of liver MRI.
Method and Results: A case base evaluation will be performed of different types of artifacts commonly seen in liver MRI. Although numbers of artifacts impact the quality of liver MRI, following artifacts will be examined in detail:
Susceptibility artifact: Distortions in the image in areas where bulk magnetization has been altered due to paramagnetic substances such as metal or air.
Solutions: Lower field strength, decrease echo time, increase bandwidth, sequence modifications
Aliasing artifact: Occurs when field of view (FOV) is smaller than the object being imaged.
Solutions: Increase FOV and Oversampling
Chemical shift artifact: Fat and water protons resonate at different frequencies which are falsely interpreted as spatial differences. This occurs in frequency-encoding (kx) direction. There is increased chemical shift artifact at higher field strengths (i.e. more common at 3T).
Solution: Mitigated with imaging at lower field strength, increase sampling rates, change phase/frequency directions of gradients
Incomplete fat suppression: Fat and water protons resonate at different frequencies within a static magnetic field. Fat saturation is performed with number of techniques including frequency selective fat-saturation which is commonly used for abdominal imaging. Inhomogeneity within the magnetic field can cause changes in the Larmor frequency that is spatially dependent. Chemical shift sensitive fat saturation pulses will be ineffective in this region of inhomogeneous magnetic field. This is commonly observed in presence of hardware.
Solution: Improved shimming, Adiabatic B1 insensitive short inversion time inversion recovery (STIR), Spatial saturation, Use multi-echo Dixon Technique/IDEAL.
Motion related artifact: Abdomen (and liver) MR image quality routinely suffers from motion related artifact. 3D T1 weighted images are usually acquired in a breath-hold. Breath-hold constraints of the patients limit the spatial resolution achievable and the volumetric coverage. Suboptimal image quality results in patients who cannot adequately breath-hold.
Solution: Overcome motion related problems in dynamic T1-weighted imaging of the liver through: (A) Increase acquisition speed using methods such as parallel imaging, time resolved imaging, and compressed sensing to decrease breath-hold and (B) compensate for motion through free-breathing acquisition motion compensated schemes and novel reconstruction methods.
T2W and DWI are routinely acquired in 2D but these also suffer from motion related artifacts.
Solutions: Number of prospective and retrospective techniques for motion compensation exists. These include (A) Multiple acquisitions/excitation (B) External navigator (C) Self navigating techniques.
Discussion and Conclusion: Improved understanding of common artifacts in liver MRI and methods to mitigate these artifacts can improve the robustness of MR examination and increase diagnostic yield of these studies.