re: Comparison of Technical Failure of MR Elastography for Measuring Liver Stiffness Between Gradient‐Recalled Echo and Spin‐Echo Echo‐Planar Imaging: A Systematic Review and Meta‐Analysis

Magnetic resonance elastography (MRE) is an MRI-based technology for quantitatively imaging the mechanical properties of tissue. Published evidence has established that MRE has the highest diagnostic performance for noninvasively detecting and staging liver fibrosis. The technique is now widely available and used as a safer, less expensive alternative to liver biopsy in this application. A CPT code (76391) was assigned to MRE by the American Medical Association in 2017. The procedure was approved for reimbursement in the US under Medicare in 2019. In an MRE exam, low-frequency mechanical waves (typically 60 Hz) are generated in the liver with an external source of vibration. The waves are imaged with a phase contrast sequence with motion encoding synchronized to the applied vibrations. The resulting phase images, depicting propagating shear waves, are processed to generate quantitative images of tissue stiffness. Studies have demonstrated that MRE has a low technical failure rate (4–6%) compared with ultrasound-transient elastography, which has reported rates of unreliable or failed exams of 10–15% or more, especially in obese patients. In cases where MRE fails, the most common cause is abnormally low signal intensity in the liver, preventing visualization of waves. This phenomenon has been attributed to elevated hepatic iron levels, a condition that is not uncommon in patients with chronic liver disease. This failure mode has been reported to be more common with higher-field 3.0T MRI systems, presumably due to greater susceptibility effects. The pulse sequence most commonly used for MRE at 1.5T is a gradient echo (GRE) technique. With cyclic motion-encoding gradients to image shear waves in the liver, the sequence must have a relatively long echo time (TE) (20–28 msec), making it sensitive to T2* signal loss. An alternative approach for acquiring MRE data is to use a spinecho echo-planar (SE-EPI) sequence. An SE-EPI sequence optimized for MRE is less sensitive to T2* signal loss, since the acquired echo-planar train is centered on a spin echo and data are typically acquired over a range that extends less than 10 msec from the center of the spin echo. The new publication by Kim et al in this issue of JMRI is a systematic review and meta-analysis of published literature reporting the rates of technical failure of MRE. This aggregation of the existing literature shows that the pooled rate of technical failure with the SE-EPI MRE technique is only 2.0%, whereas the pooled failure rate for GRE MRE is 5.8% on all systems. The rate of technical failure of GRE MRE on 3.0T systems alone is even higher, at 10.7%. The study showed that there was no evidence of a significant difference in the low (2.0%) failure rate of the SE-EPI MRE sequence on 1.5T and 3.0T systems. This important meta-analysis confirms that the rate of technical failure in MRE exams at 3.0T is substantially reduced if an SE-EPI sequence is used rather than a GRE sequence. An SE-EPI MRE sequence has been provided as a standard by at least one of the MRI manufacturers on 3.0T scanners since 2012. Unfortunately, other commercial versions of the technology have not yet made this capability available. A remarkable feature of the evolution of MRI technology over the last 35 years has been the high speed with which innovations have been translated from laboratory to wide clinical application. There is perhaps no better embodiment of that phenomenon than the array of novel pulse sequences and data acquisition techniques that continue to transform MRI—enabling rapid exams like MRE. As noted by Kim et al, the need to stage liver fibrosis has become increasingly important in the age of antiviral and antifibrotic drugs. This demand is expected to grow significantly as therapies are approved for the treatment of nonalcoholic fatty liver disease, which affects up to one-third of