Overhauser‐enhanced magnetic resonance elastography

Abstract

Magnetic resonance elastography (MRE) is a powerful technique to assess the mechanical properties of living tissue. However, it suffers from reduced sensitivity in regions with short T2 and T2 * such as in tissue with high concentrations of paramagnetic iron, or in regions surrounding implanted devices. In this work, we exploit the longer T2 * attainable at ultra-low magnetic fields in combination with Overhauser dynamic nuclear polarization (DNP) to enable rapid MRE at 0.0065 T. A 3D balanced steady-state free precession based MRE sequence with undersampling and fractional encoding was implemented on a 0.0065 T MRI scanner. A custom-built RF coil for DNP and a programmable vibration system for elastography were developed. Displacement fields and stiffness maps were reconstructed from data recorded in a polyvinyl alcohol gel phantom loaded with stable nitroxide radicals. A DNP enhancement of 25 was achieved during the MRE sequence, allowing the acquisition of 3D Overhauser-enhanced MRE (OMRE) images with (1.5 × 2.7 × 9) mm(3) resolution over eight temporal steps and 11 slices in 6 minutes. In conclusion, OMRE at ultra-low magnetic field can be used to detect mechanical waves over short acquisition times. This new modality shows promise to broaden the scope of conventional MRE applications, and may extend the utility of low-cost, portable MRI systems to detect elasticity changes in patients with implanted devices or iron overload.

Liver fibrosis: Review of current imaging and MRI quantification techniques

“…MRE is a highly accurate technique for the staging of liver fibrosis, but it is affected by a technical failure rate of 5.6%, especially in patients with iron deposition . Investigational sequences may improve coverage, robustness to iron deposition, and technical success …”

Magnetic resonance elastography of liver and spleen: Methods and applications

“…This other report also studied the differences in failure rates between the two field strengths, and indicated that the higher field strength was associated with significantly more technical failures (15% at 3.0 T versus 3.5% at 1.5 T). Overhauser‐enhanced MRE, by virtue of its operation at low magnetic field strength and theoretically benefiting from longer T 2 * in patients with iron overload, could prove beneficial …”

Encoding and readout strategies in magnetic resonance elastography

“…Most current volumetric MRE acquisitions are performed using sequential or interleaved stacked 2D scans. Multislab acquisition was recently developed and applied in conjunction with spiral MRE readout, whereas full 3D acquisition was also recently performed together with compressed sensing and dynamic nuclear polarization MRE …”

“…The randomness of the sampling pattern leads to noise‐like interference, which can be filtered in the sparse domain of the MRI images by non‐linear thresholding . CS has recently been used in ultra‐low field MRE in conjunction with dynamic nuclear polarization (DNP) . Here, Cartesian 3D acquisition with phase encoding in both phase and slice encoding directions was paired with variable density Gaussian undersampling patterns .…”